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University of Groningen
Charge transport and trap states in lead sulfide quantum dot field-effect transistorsNugraha, Mohamad Insan
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Charge Transport and Trap States in
Lead Sulfide Quantum Dot
Field-Effect Transistors
Mohamad Insan Nugraha
Charge Transport and Trap States in Lead Sulfide
Quantum Dot Field-Effect Transistors
Mohamad Insan Nugraha
PhD thesis
University of Groningen
Zernike Institute PhD thesis series 2017-10
ISSN: 1570-1530
ISBN: 978-90-367-9746-7 (printed version)
ISBN: 978-90-367-9745-0 (electronic version)
The research presented in this thesis was performed in the research group
of Photophysics and Optoelectronics of the Zernike Institute for Advanced
Materials at the University of Groningen, The Netherlands. This work was
financially supported by Ubbo Emmius PhD Scholarship of University of
Groningen.
Cover design by Widianta Gomulya and Anggita Putri Wibowo
Printed by www.ipskampprinting.nl
Charge Transport and Trap States in Lead Sulfide
Quantum Dot Field-Effect Transistors
PhD Thesis
to obtain the degree of PhD at the
University of Groningen
on the authority of the
Rector Magnificus Prof. E. Sterken
and in accordance with
the decision by the College of Deans
This thesis will be defended in public on
Friday 16 June 2017 at 11.00 hours
by
Mohamad Insan Nugraha
born on October 31, 1989
in Tangerang, Indonesië
Supervisors
Prof. M. A. Loi
Prof. J. Takeya
Assessment Committee
Prof. J. Ye
Prof. C. R. Kagan
Prof. M. Law
i
Contents
CHAPTER 1 Introduction ............................................................... 1
1.1 Colloidal Quantum Dots ............................................................................. 2
1.2 Electronic structures of QDs ...................................................................... 3
1.2.1 Quantum confinement .................................................................... 3
1.2.2 Role of capping ligands ................................................................... 6
1.3 Optical properties of PbS QDs .................................................................... 9
1.3.1 Effect of QD size ............................................................................ 10
1.3.2 Effect of inter-QD distance ........................................................... 10
1.4 PbS QD field-effect transistors (FETs) ..................................................... 12
1.4.1 Basic operation of FETs ................................................................ 12
1.4.2 Research progress on PbS QD-FETs. ........................................... 17
1.5 Experimental techniques ......................................................................... 20
1.5.1 Electrical characteristic measurements ....................................... 20
1.5.2 Simulation for analysis of carrier traps ....................................... 20
1.5.3 Optical absorption spectroscopy .................................................. 21
1.6 Outline of the thesis .................................................................................. 22
1.7 References ................................................................................................. 24
CHAPTER 2 Tunable Doping in PbS Quantum Dot Field-Effect
Transistors using Surface Molecular Dipoles .................................. 29
2.1 Introduction .............................................................................................. 30
2.2 Doping mechanism using SAMs .............................................................. 31
2.3 Properties of PbS films on SAM-treated SiO2 .......................................... 32
2.3.1 Morphology of PbS films ............................................................... 32
2.3.2 Electrical properties of PbS QD-FETs .......................................... 33
2.3.3 SAM doping concentration ........................................................... 35
2.3.4 Mobility and density of traps ........................................................ 38
ii
2.4 Conclusion................................................................................................ 39
2.5 Methods ................................................................................................... 39
2.6 References ................................................................................................. 41
CHAPTER 3 Dielectric Surface Passivation and Hydroxyl-free
Polymer Gating in PbS Quantum Dot Field-Effect Transistors ......... 43
3.1 Introduction ............................................................................................. 44
3.2 Properties of PbS FETs with HMDS-treated SiO2.................................... 45
3.2.1 Morphology of PbS films ............................................................... 45
3.2.2 Electrical properties of PbS QD-FETs ......................................... 46
3.3 Cytop Gating in PbS QD-FETs ................................................................. 50
3.4 Distribution of Trap DOS in PbS QD-FETs .............................................. 52
3.5 Conclusion................................................................................................. 55
3.6 Methods .................................................................................................... 55
3.7 References ................................................................................................. 57
CHAPTER 4 Broadening of the Distribution of Trap States in PbS
Quantum Dot Field-Effect Transistors with High-k Dielectrics ........ 59
4.1 Introduction ............................................................................................. 60
4.2 Polarons at the semiconductor/insulator interface ................................ 60
4.3 PbS QD-FETs with polymer insulator gating .......................................... 62
4.3.1 Characteristics of polymer insulators .......................................... 62
4.3.2 Electrical properties of FETs ........................................................ 62
4.4 Electronic structures of traps with high-k ............................................... 64
4.4.1 Density of traps in subthreshold regime ...................................... 64
4.4.2 Distribution of trap DOS ............................................................... 65
4.5 Conclusion................................................................................................ 68
4.6 Methods ................................................................................................... 68
4.7 References ................................................................................................ 70
CHAPTER 5 Enabling Ambipolar to Heavy n-type Transport in PbS
Quantum Dot Solids through Doping with Organic Molecules ......... 71
5.1 Introduction .............................................................................................. 72
iii
5.2 Doping strategy using Benzyl Viologen (BV) ........................................... 73
5.3 Electrical characteristics of BV-doped PbS QD-FETs ............................. 74
5.4 4-terminal conductivity of BV-doped PbS QD-FETs ............................... 79
5.5 Conclusion ............................................................................................... 80
5.6 Methods .................................................................................................... 81
5.7 References ................................................................................................. 83
CHAPTER 6 Strain-Modulated Charge Transport in Flexible PbS
Quantum Dot Field-Effect Transistors ............................................ 85
6.1 Introduction .............................................................................................. 86
6.2 PbS QD-FETs on flexible substrates ........................................................ 87
6.3 Strain effect on flexible PbS QD-FETs ..................................................... 89
6.4 Traps and barrier potential in strained devices ....................................... 94
6.5 Conclusion ................................................................................................ 97
6.6 Methods .................................................................................................... 97
6.7 References ................................................................................................. 99
Summary ...................................................................................... 101
Samenvatting ................................................................................ 105
Acknowledgement ......................................................................... 109
List of Publications ........................................................................ 113
1
Chapter 1
Introduction
This chapter gives an overview of the physical properties (including charge
transport and optical properties) of semiconducting colloidal quantum dots
based on lead sulfide (PbS), and research progresses on the use of these colloidal
quantum dots for the fabrication of field-effect transistors (FETs). Some
potential methods for improving charge carrier mobility in PbS FETs will be
discussed. In the final section, the experimental techniques used and the outline
of this thesis will be provided.
Charge transport and trap states in PbS QD field-effect transistors
2
1.1 Colloidal Quantum Dots
Semiconductor colloidal quantum dots (QDs), often also called nanocrystals
(NCs), are a class of solution processable crystalline materials with size ranging
from 1 to 20 nm.[1] Due to their tiny dimensions, QDs consist of only few hundreds
to thousands atoms. With this finite number of atoms, the properties of QDs
resemble those of atoms showing quantization of their energy levels, for which
reason they are also often called artificial atoms.[2–5] Indeed, when the size of QDs
is smaller than their exciton Bohr radius, strong quantum confinement of charge
carrier wave-function, which leads to size-dependent electronic and optical
properties, is displayed.[6–8] This size-tunable phenomenon due to the quantum
confinement is commonly called as quantum size effect.
Among many types of QDs which have been introduced in the last years, lead
sulfide (PbS) QDs are one of the most interesting particularly because they have
large exciton Bohr radius (~20 nm) and at the same time possess straightforward
and well-established synthetic methods which give rise to high quality
materials.[9–12] As a result of the large exciton Bohr radius, strong quantum
confinement can be effectively achieved to control the band gap of the PbS QDs
from 0.4 to 2 eV by tuning their size from bulk to 2 nm.[13,14] Because of this size-
tunable band gap, PbS QDs have been used as promising semiconducting building
blocks for highly efficient optoelectronic devices such as solar cells,[15–25]
photodetectors,[26–28] and light-emitting devices.[8,29] More recently, the solution
processability of these QDs has also attracted great attention for the fabrication of
field-effect transistors.[30–42]
PbS QDs are synthesized using the so-called hot injection process by rapidly
injecting organometallic precursors into a hot solvent containing organic
surfactants.[10,12] The organic surfactants consist of long-chain hydrocarbon
compounds such as oleic acid,[43,44] n-octadecylphosphonic acid,[35]
trioctylphosphine oxide (TOPO),[45] and hexadecylamine.[46,47] When the
organometallic precursors are rapidly injected into the mixture of the hot solvent
and the organic surfactants, a decrease of temperature initiates the nucleation of
the compounds, which is followed by particle growth after reheating the final
mixture. The long-chain hydrocarbon surfactants, organic ligands, play an
important role in determining the kinetics of nucleation and growth in the
solution.[46] The key synthesis parameters need to be balanced in order to obtain
monodisperse colloidal QDs with controllable size. The monodisperse QDs which
should possess identical shape, structure, size, and surface chemical properties
result in samples with narrow emission bandwidth and high luminescence
efficiencies.[12] The high resolution transmission electron microscopy (HRTEM)
images of monodisperse PbS QDs (with diameter of 3.6 nm) are shown in Figure
1.1 (a) and (b).
Chapter 1
3
(a) (b)
(c)
50 nm
Figure 1.1 High resolution transmission electron microscopy (HRTEM) images of PbS QDs
at (a) low and (b) high magnification. (c) Schematic of PbS QDs with long organic ligands
on their surfaces.
The long ligands are also fundamental as stabilizers for the colloidal
dispersion by capping the QD surfaces, and preventing particle aggregation. The
ligands generally used provide good QD dispersibility in most organic solvents.[46]
These allow QD thin films to be deposited using low cost and low temperature
solution processed methods such as spin-coating,[27] blade-coating,[28] dip-
coating,[40], and roll-to-roll processes.[40] Figure 1.1 (c) presents the schematic of
PbS QDs with long ligands on their surfaces. When PbS colloidal QDs are
assembled onto thin film solids, the ligands behave as tunneling barriers, which
maintain the quantum confinement of the individual QDs.
1.2 Electronic structures of QDs
1.2.1 Quantum confinement
Due to complete quantum confinement of charge carrier wave function, the
electronic structures of QDs have unique characteristics with respect to their
parent bulk materials and other material structures such as quantum wells and
quantum wires. In bulk materials, charge carriers are fully delocalized. The
Charge transport and trap states in PbS QD field-effect transistors
4
carriers move through the energy bands formed due to the overlap of wave
functions between neighboring atoms. According to the nearly-free electron
model, charge carriers in bulk materials have almost continuous energy spectrum
(E) (vide infra) and the density of states in this system is proportional to E1/2.[48]
By reducing the material dimension, the bulk materials turn to quantum wells
(two-dimensional structure – confined in 1 direction) and quantum wire
structures (one-dimensional structure – confined in 2 directions), or to a
quantum dot (zero-dimensional structure – confined in 3 directions) as
demonstrated in Figure 1.2. As a consequence of the quantum confinement, the
density of states in quantum wells and quantum wires displays unique profiles
showing increment of the density of electronic states only at certain energy.[49]
Bulk Quantum Wells Quantum Wires Quantum Dots
DB
(E)
EnergyEG
D0D
(E)
EnergyEG
D2
D(E
)
EnergyEG
D1
D(E
)
EnergyEG
Figure 1.2 Quantum confinement and density of states in materials with different
dimensionality.
In QDs, charge carriers are confined in all directions of the materials as
demonstrated in Figure 1.2. With the quantum confinement, the density of states
in QDs appears as the delta Dirac function, which again displays the
characteristics of energy quantization. As mentioned, the quantum confinement
in QDs becomes active when the size or radius of the particles is smaller than the
exciton Bohr radius (RB),[50]
2
2
eRB
(1.1)
where ε, ħ, μ, and e are dielectric constant, reduced Planck constant, reduced mass
of holes and electrons, and elementary charge constant, respectively. In PbS, the
exciton Bohr radius can be as large as 20 nm which effectively allows strong
quantum confinement even in relatively large sized particles with respect to other
materials such as CdSe (RB=5.6 nm),[51] CdS (RB=2.8 nm),[52,53] and ZnO (RB=2.87
nm).[54]
Chapter 1
5
Ee1
Ee2
Ee3
Eh1
Eh2
Eh3
Ee1
Ee2
Eh1
Eh2
L1 L2
Figure 1.3 Quantum confinement and size effect in QDs. The dash lines show discrete
energy levels in QDs. The band gap (red lines) increases with decreasing the QD size.
Many studies have been performed to understand the electronic structures
of QDs.[55,56] A simple approach to study their electronic structures is based on the
strongly confined particle model in a square potential well as demonstrated in
Figure 1.3. The potential barrier is produced by the surface of QDs and the ligands
on the QD surface. By applying the Schrodinger equation, the energy levels (En)
of carriers in QDs are quantized as:
2
2
2
2
2
2
*
22
2 z
z
y
y
x
xn
L
n
L
n
L
n
mE
(1.2)
where m* is the effective mass of holes and electrons in QDs. Lx, Ly, and Lz are the
size of QDs in x, y, and z direction respectively. The three integer quantum
numbers (nx, ny, and nz) indicate the quantization of energy in the directions. The
first transition energy, thus the band gap, can be estimated from the difference of
the ground state energy of electrons and holes (EG=Ee1-Eh1) as demonstrated in
Figure 1.3. This ground state energy is inversely proportional to the QD size as
stated in equation (1.2). As a consequence, the band gap (EG) of QDs increases
with decreasing the particle size. The size-tunable band gap of QDs is shown in
Figure 1.3.
Another approximation for the electronic structures of QDs is using a
spherical potential model.[49,56] By solving the Schrodinger equation in spherical
coordinates, the energy levels of charge carriers in QDs with radius R can be
written as:[49]
2
,2*
22
,2
lnlnRm
E
(1.3)
where χn,l is the nth zero of the lth-order spherical Bessel function. The schematic
of energy levels in spherical QDs is shown in Figure 1.4.
Charge transport and trap states in PbS QD field-effect transistors
6
Figure 1.4 Energy levels of spherical QDs with radius R.
A more comprehensive analysis involving the effective mass model shows
that the energy of the first excited electronic states (band gap energy) in QDs with
radius R can be stated as,[57,58]
R
e
mmREE
ohe
bulkGG
4
8.111
2
2
**2
22
(1.4)
where EGbulk and εo are the bulk band gap energy and vacuum permittivity
constant, respectively. The third term in equation (1.4) corresponds to the
Coulomb interaction between holes and electrons in the QDs. In line with the
square-shaped QDs, the band gap energy of spherical QDs increases with
decreasing the radius of the QDs.
1.2.2 Role of capping ligands
The ligands have the double role of stabilizing the QD colloidal solution and
to guarantee the quantum confinement over long time. For this reason they are
chosen to be well insulating. This also means that when the QDs are organized in
a film, the ligands suppress charge transport within the QD film. To improve the
film conductivity, the long ligands need to be exchanged with shorter ones. This
ligand exchange can be done either in solution or in solid state.[59–61] Several short
ligands have been shown to enable charge transport while still maintaining
quantum confinement within the QD solids.[62,63] As this statement can appear
peculiar, it needs to be clarified in the sense that the short ligands allow carriers
to tunnel between QDs while the QD films still show quantum confinement in
terms of their band gap and optical properties.
The short ligands can be organic (e.g. 3-mercaptopropionic acids/3MPA,[64]
benzenethiols/BT,[65] benzene-dithiols/BDT,[66] 1,2-ethanedithiols/EDT,[67] 1,2-
ethylenediamine/EDA,[68] thiocyanates/SCN-,[33] etc) or inorganic molecules or
single atoms (e.q. metal chalcogenides,[46,47] halometallates,[62] halide anions,[60]
etc). The chemical structures of some of the most common ligands are shown in
Figure 1.5 (a). As a result of the ligand exchange, the exchange coupling energy
Chapter 1
7
(β), related to the tunneling rate of charge carriers between QDs, increases. The
exchange coupling energy can be written as:[1]
dEm b
21
2*22exp (1.5)
where Eb and d are the potential barrier height and the inter-QD distance,
respectively. From equation (1.5), it is obvious that the tunneling rate of charge
carriers increases with decreasing the inter-QD distance. In agreement with
equation (1.5), Law et al. reported that the charge carrier mobility in lead
chalcogenide QD systems increases exponentially with decreasing ligand length,
thus the inter-QD distance.[7]
(a)
(b)
3MPA
SHHO
O
BT
SH
BDT
SHHS
1,2-EDT
SHHS
NH2H2N
EDA
NH4+ SCN-
Ammonium Thiocyatanate
N(Bu)4+ F- Cl- Br- I-
Tetrabuthylammonium Halides
6.0
5.5
5.0
4.5
4.0
3.5
TB
AB
r
TB
AI
TB
AC
l
NH
4S
CN
1,4
-BD
T
1,3
-BD
T
TB
AF
ED
T
MP
A
ED
A
BT
1,2
-BD
T
En
erg
y (
eV
)
Figure 1.5 (a) Chemical structures of ligands and (b) energy levels of PbS QDs with different
ligands. The black and red lines correspond to the valence bands and Fermi levels of the
materials, respectively. The green and blue lines are the optical and transport conduction
bands of the materials, respectively. Tetrabuthylammonium halide ligands (TBAX, X=Br,
I, Cl) produce deeper conduction bands than those with thiol-based ligands. The data of
the energy levels are extracted from literature.[69]
Charge transport and trap states in PbS QD field-effect transistors
8
The short capping ligands on the QD surfaces also modify the electronic
structures of PbS QDs. As referred to Figure 1.5 (a), the short ligands may change
dielectric environment between neighboring QDs. The ligands with their different
dipole moments can influence the energy alignment inside QDs. As a result, the
electronic structures of QDs including the vacuum energy, Fermi level, valence
band, and conduction band can be affected. Brown et al. reported energy level
modification in PbS QD films using various ligands.[69] The same authors reported
a comprehensive energy level schematic of PbS QD films with different capping
ligands that is reproduced in Figure 1.5 (b). Although capped with similar
bidentate thiols (BT and 1,4-BDT), a significant shift of the conduction and
valence bands up to 0.7 eV is observed. With halide-based ligands, the conduction
bands of the PbS QD films can be as deep as -4.5 eV (TBABr). With these
significant shifts, the use of particular capping ligands can determine the
performance of electronic devices based on PbS QDs by controlling the charge
extraction between injection/extraction contacts and the semiconducting films.
(a) (b)
Ligand Exchange
X
X
Halide-filled
PbS QDs
Halide ligands
3MPA
Figure 1.6 (a) Schematic of ligand exchange. Due to incomplete ligand exchange reaction,
some dangling bonds on the QD surfaces are formed. (b) Schematic of PbS QDs passivated
with organic ligands (3MPA) and hybrid organic/inorganic (3MPA/halide) ligands. Due to
their large size, some 3MPA molecules are not able to penetrate into some inter-cation
trenches. The schematic is adapted from reference.[16]
The high surface area to volume ratio of QDs, typical of nano-objects, makes
them more prone to develop a high number of traps on the QD surfaces. These
carrier traps are extremely important as they limit the performance of the
Chapter 1
9
electronic devices based on QDs. The carrier traps may originate from structural
aperiodicity and off-stoichiometry composition of the QDs.[70] The ligands take
also an important role in providing surface passivation against carrier traps.
Nevertheless, after the exchange of the long ligands to shorter ones, some surface
dangling bonds may also be formed because of an incomplete ligand exchange
reaction. These dangling bonds can indeed act as additional carrier traps on the
QD surfaces as schematically shown in Figure 1.6 (a). Ip et al., found that steric
interactions prevent some short organic ligands such as 3MPA from penetrating
the inter-cation trenches on the QD surfaces as displayed in Figure 1.6 (b).[16] As
a result, some un-passivated QD surfaces remain which act as additional traps.
In comparison to organic ligands such as 3MPA and other thiol-based
ligands, halide ligands have been reported to provide an effective passivation
against carrier traps that results in high performance FETs and highly efficient
solar cells based on PbS colloidal QDs.[15,63] However, recently Balazs et al., have
also underlined the importance of counterions (ammonium, methylammonium,
tetrabuthylammonium in iodide salts) in the ligand-exchange mechanism.[63]
Since they have specific reactivity and acidity, the types of counterions can
determine the effectiveness of the ligand exchange reaction, thus the removal of
native long organic ligands. With the treatment of the first two ligands, efficient
charge transport in the PbS films is revealed, as indicated from the high film
conductivity. Particularly, the films treated with methylammonium iodide enable
good performing transistors with electron mobility of 0.05 cm2V-1s-1 and on/off
ratio of 106. Meanwhile, among those ligands, the films treated with
tetrabuthylammonium iodide show poor conductivity. The lower reactivity of
tetrabuthylammonium iodide than that of others results in incomplete removal of
the native long insulating ligands on the QD surfaces, suppressing the charge
transport between neighboring particles.
1.3 Optical properties of PbS QDs
Similar to their unique electronic structures, the optical properties of QDs
are also size-dependent as a result of the quantum confinement effect. In addition,
when colloidal QDs are assembled onto thin film solids, the distance between QDs
determines the electronic coupling between neighboring QDs. The change of the
electronic coupling between QDs also influences their optical properties. In this
section, the effect of QD size and inter-QD distance on the optical properties of
PbS QDs is reviewed.
Charge transport and trap states in PbS QD field-effect transistors
10
1.3.1 Effect of QD size
QDs are characterized by strong excitonic interactions that are displayed
from the strong (excitonic) peak in the lower energy region of the absorbtion
spectrum. Figure 1.7 shows the absorbance spectrum of PbS QDs with different
sizes. It is obvious that the excitonic peaks are red-shifted with increasing the QD
size, also indicating the decrease of the band gap. In 3.7 nm PbS QDs, the excitonic
wavelength is about 1100 nm, which corresponds to the band gap of 1.13 eV. This
band gap as explained earlier is larger than that of bulk PbS (EG=0.41 eV). By
increasing the QD size to 6.5 nm, the excitonic wavelength shifts to 1660 nm.
Meanwhile, when the QD size is reduced to 2 nm, a blue-shifted excitonic
wavelength results in the band gap of 2 eV.[14] With the size-tunable optimal band
gap properties, PbS QDs allow optimal utilization of the solar spectrum
particularly in the near-infrared region which is beneficial to realize highly
efficient solar cells.
Figure 1.7 Absorbance spectrum of PbS QDs of different sizes. The spectrum are modified
from reference.[14]
1.3.2 Effect of inter-QD distance
In QD films, the inter-QD distance determines the electronic coupling
between QDs. When they are at a large distance, the electronic coupling between
QDs is very small and charge transport does not occur. In this regime, the QD
arrays have approximately identical electronic and optical properties as individual
QDs. As the inter-QD distance is reduced, the electronic coupling between QDs
becomes important. Consequently, the carrier wave function can extend over the
films, which influences the electronic structures and the optical properties of the
QD films. Several groups have reported that the excitonic peaks of CdSe and InP
QD films in the absorbance spectrum shift to the red by reducing the inter-QD
distance.[71,72] Similarly, a red-shifted profile of the absorbance spectrum of PbS
1000 1200 1400 1600 1800
3.7 nm
4.3 nm
5.4 nm
6.5 nm
PbS QDs
Abso
rba
nce
(a.u
)
Wavelength (nm)
Chapter 1
11
films is also observed by reducing the inter-QD distance from the use of long oleic-
acid (OA) and short EDT ligands as presented in Figure 1.8.
Figure 1.8 Absorbance spectra of PbS QD films with OA and EDT ligands.
The decrease of the inter-QD distance can also be achieved by annealing the
QD films at higher temperature. However, the high annealing temperature may
result in sintering of the QDs leading to the vanishing of quantum confinement
within the QD films. Sintering the QD films is undesirable particularly for
photovoltaics, where the loss of quantum confinement results in lower open
circuit voltage and consequent reduction of the device performance.[73] Recently,
the use of different types of atomic ligands puts also in evidence for the formation
of necking, which is shown by the continuity of the PbS crystal lattice along a
particular facet.[63] In PbS QDs, three main facets define their crystal structure,
namely {100}, {110}, and {111}. The last two facets have more acidity than the
{100} facets, and at the same time have higher surface energy than un-passivated
{100} facets, which promote the removal of anionic ligand species from the {100}
facets.[63] This anionic ligand species removal is then followed by the diffusion of
Pb and S atoms along {100} facets, indicating the formation of necking along this
direction. Consequently, the PbS films show reduced band gap and weaker
excitonic signatures.
Recently, the application of external mechanical pressure has also been
reported to influence the inter-QD distance.[13,74] The effect of pressure on the
inter-QD distance is demonstrated by the red-shifted excitonic peaks in the
absorbance spectrum of the PbS films with increasing the pressure. The red-
shifted profiles indicate the decrease of the inter-QD distance, which is attributed
to the bending of the capping ligands on the QD surfaces. As the transport of
EDT
OA ligands
600 800 1000 1200 1400
Absorb
ance (
a.u
)
Wavelength (nm)
OA ligands
Charge transport and trap states in PbS QD field-effect transistors
12
charge carriers within the QD films is strongly influenced by the inter-QD distance,
this effect can also be used to tune the charge transport properties in the QD films.
1.4 PbS QD field-effect transistors (FETs)
Understanding charge transport in PbS QD solids has recently become an
important research interest in the QD community. One of effective tools for this
investigation is the fabrication of field-effect transistors (FETs).[75–78] The
fabrication and characterization of FETs are one of the most straightforward
methods to obtain information on charge carrier mobility and to study transport
mechanisms in semiconductors. This section is devoted to provide an overview
about the basic operational principles of FETs as well as some research progress
related to the methods to improve charge transport in PbS QD-FETs.
1.4.1 Basic operation of FETs
In their structure, FETs use three materials including metal, insulator, and
semiconductor as main components. Figure 1.9 (a) shows the schematic of an FET
with channel length L and width W. FETs have 3-terminal configuration namely
gate, source, and drain electrodes. In general, the source electrode is connected to
the ground whereas the gate and drain electrodes are connected to a voltage
source. In FETs, the semiconductor and the gate electrode are separated by a thin
layer of insulator. By applying a voltage to the gate electrode, charge carriers are
accumulated at the semiconductor/insulator interface. These accumulated charge
carriers form a conducting path where charge carriers can flow from the injecting
electrode (source) to the collecting electrode (drain). The minimum gate voltage
(Vg) that is required to form the conducting channel is defined as threshold
voltage (Vth).
Depending on the semiconductor, FETs can show n- or p-type or ambipolar
properties. In n-type FETs, when a positive gate voltage is applied, the Fermi level
of the gate electrode is shifted down resulting in downward bending of the
conduction band (CB) and valence band (VB) of the semiconductor. Consequently,
electrons are accumulated at the semiconductor/insulator interface. Meanwhile,
in p-type FETs, when a negative gate voltage is applied, the Fermi level of the gate
electrode is shifted up. This condition results in upward bending of the CB and VB
leading to hole accumulation at the semiconductor/insulator interface. In
ambipolar FETs, both electrons and holes are accumulated at the
semiconductor/insulator interface depending on the polarity of the gate voltage.
Figure 1.9 (b)-(d) show the schematics of energy diagram in FETs with several
gate bias conditions.
Chapter 1
13
Gate
Insu
lato
r
Semiconductor
CB
VB
EFEF
- -- -- -
Gate
Insu
lato
r
Semiconductor
CB
VB
EFEF
Vg>0 Vg<0
+ ++ +
+ +
Insu
lato
rGate Semiconductor
CB
VB
EF
EF
(a) (b)
(c) (d)
GateInsulator
Semiconductor
Source Drain
LW
Figure 1.9 (a) Schematic of an FET device. Energy band diagram in an FET with (b) zero,
(c) positive, and (d) negative gate voltages.
The electrical behavior of an FET is determined by measuring two types of I-
V characteristics, namely the output and transfer characteristics. The output
characteristics of FETs are obtained by measuring the source-drain current (Ids)
as a function of source-drain voltage (Vds) at fixed Vg. The transfer characteristics
are obtained by measuring the source-drain current (Ids) while sweeping the Vg at
a fixed Vds. The transfer characteristics demonstrate the effectiveness of the gate
voltage in modulating the electrical current in the devices and in switching the
devices from the off to the on-state. When a Vds << (Vg – Vth) is applied on the
FETs, as mentioned already charge carriers are accumulated forming a
conducting path with homogeneous carrier density as demonstrated in Figure 1.10
(a). The conducting path forms a channel connecting source and drain which
allows the flow of electrical current. In this condition, the FETs behave like a
resistor where the current increases linearly with increasing Vds, which is referred
to as the linear regime. The source-drain current in the linear regime is written as,
2
2
dsdsthg
ids
VVVV
L
WCI
(1.6)
where μ and Ci are the charge carrier mobility in the devices and the capacitance
of the gate insulator. It is important to note that the above equation assumes the
Charge transport and trap states in PbS QD field-effect transistors
14
absence of contact resistances at the source and drain contacts, thus an Ohmic
contact. In the presence of a Schottky barrier between the source/drain work
function and CB/VB of the semiconductor, FETs show S-shaped output
characteristics in the linear regime at low VDS.
Gate
S D- - - - - - - - -Ids
Vds
Gate
S D- - - - - - -Ids
Vds
Gate
S D- - - -Vd,sat
Ids
VdsVd,sat
(a)
(b)
(c)
Figure 1.10 Schematic view of the basic working principles of FETs in (a) the linear, (b)
parabolic, and (c) saturation regime, with corresponding Ids-Vds output characteristics.
As the Vds increases, the voltage drop across the insulator near the drain
decreases resulting in non-uniformity of the density of accumulated carriers as
displayed in Figure 1.10 (b). As a consequence, the channel resistance of the
devices increases, leading to a decrease of the slope of the output characteristics.
At this point, the transistors operate in the parabolic regime. In this regime, Ids
will depend on the other parameters as expressed in equation (1.6). In the linear
regime, the second term in the equation (1.6) is much smaller than the first part
allowing neglecting the quadratic term, which preserves the linearity of the output
characteristics. However, as the Vds increases, the second part of the equation will
become important, resulting in the parabolic shape of the output characteristics.
When the Vds is further increased (Vds >> (Vg-Vth)), the accumulated charge
carriers move toward the source electrode. At a given Vds, the so-called Vd,sat, is
reached, then the density of the accumulated carriers is zero at the drain electrode.
In this condition, the charge carriers travel through the channel toward the drain
electrode. At the point where the carrier density goes to zero, so-called pinch-off
Chapter 1
15
point demonstrated as Vd,sat in Figure 1.10 (c), the charge carriers are swept by the
electric field between the pinch-off point and the drain electrode, and flowing
through the space-charge region. In the ideal condition, the increase of the
conductance is zero, thus the source-drain current is constant, since the electric
current is only governed by the carrier diffusion through the space-charge region
(between pinch-off point and the drain electrode). This condition is called
saturation regime since the source-drain current saturates (constant) even though
the Vds increases as demonstrated in Figure 1.10 (c). In the saturation regime, the
source-drain current can be expressed with the equation:
2
2thg
ids VV
L
WCI
(1.7)
Ids
VgVth
Slope=Ids/ Vg
Vds=constant
Von
Ids
Vg
Subthreshold region
(a) (b)
off
on
Figure 1.11 The Ids-Vg transfer characteristics of FETs in (a) the linear and (b) semi-
logarithmic scale.
The transfer characteristics are obtained by measuring the Ids and sweeping
Vg at a given Vds. As previously mentioned, when a Vg is applied, the charge
carriers are accumulated at the semiconductor/insulator interfaces forming a
conducting path (channel) between the source and drain electrodes. When the Vg
is lower than the threshold voltage (Vth), no current flows through the channel. As
the Vg gradually increases beyond the Vth, the density of the accumulated charge
carriers increases which results in the decrease of the channel resistance.
Consequently, the Ids increases with increasing the Vg. In the ideal condition, no
current flows to the gate because the gate electrode is separated by an insulating
layer. The typical Ids-Vg transfer characteristics of FETs in the linear scale are
shown in Figure 1.11 (a). By measuring the transfer characteristics, the charge
carrier mobility in the linear regime (μlin) can be derived as,
g
ds
dsi
linV
I
VWC
L
(1.8)
Charge transport and trap states in PbS QD field-effect transistors
16
where Ids/Vg is the transconductance that can be calculated from the slope of
the transfer characteristics of the devices. The mobility in the saturation regime
(μsat) is derived from equation (1.7) and stated as,
22/1
2
g
ds
i
satV
I
WC
L (1.9)
where Ids1/2/Vg is calculated from the slope of the square root of Ids versus Vg
characteristics.
At this point, it is important to consider the Ids-Vg transfer characteristics in
the semi-logarithmic scale as displayed in Figure 1.11 (b). The Vg where the current
starts increasing exponentially is defined as on-voltage (Von). The Von also
provides information on the doping level in FETs. Moreover, Figure 1.11 (b) also
shows the off- and on-state of the devices. The ratio of on-current and off-current
is defined as the on/off ratio, which indicates the ability of the devices to be
switched from the off- to on-state. Good performing FET devices are expected to
have high on-off ratio, the precise figure of merit will mostly depend on the
application. The semi-logarithmic profile of the transfer curve also reveals another
important regime in the FET devices.[79] The so-called subthreshold regime,
displayed in Figure 1.11 (b), occurs when the applied gate voltage is below the
threshold voltage of the devices. In this regime, the current is mainly governed by
the diffusion process, which is independent of the drain voltage (Vds<<kBT/q). The
diffusion current is a consequence of the large gradient of the charge carrier
concentration between the source and drain contact regions, and is thus
dependent on the gate voltage,
Tkn
eVI
B
g
ds *exp (1.10)
where e, kB, and T are charge constant, Boltzmann constant, and temperature,
respectively. The ideality parameter (n*) corresponds to the density of carrier
traps in the devices, which originates from the semiconductor/insulator interface
and the bulk of semiconductor.[79] The subthreshold swing (SS) and the ideality
parameter are defined as,
*10n
e
nTkSS B
(1.11)
i
sc
C
Cn 1* (1.12)
where Csc is the capacitance of semiconductor. The Csc is related to the density of
traps in the devices which influences the transfer characteristics of the devices in
the subthreshold regime, where
Chapter 1
17
trapsBulkscsc NeNeC 2 (1.13)
The parameter NBulk is the density of bulk traps per volume and energy,
whereas Ntraps corresponds to the density of interface traps per area and energy
and ϵsc is the permittivity of the semiconductor. From equation (1.13), the
maximum density of interface traps can be estimated as,
2
110ln e
C
Tk
eSSN i
B
traps
(1.14)
From the transfer characteristics of the FET devices, the subthreshold swing SS is
calculated from:
1
log
g
ds
V
ISS (1.15)
It is obvious from equation (1.14) and (1.15) that a low SS corresponds to a low
Ntraps which corresponds to a steeper profile of the semi-log transfer
characteristics in the subthreshold region. Conversely, a high SS indicates a high
Ntraps which is reflected in a shallower profile of the semi-log transfer
characteristics in the subthreshold region.
1.4.2 Research progress on PbS QD-FETs
Recently, PbS QDs have shown their potential as semiconducting active
materials for FETs.[30–42] FETs are one of the main components in several
advanced electronic devices such as complementary metal oxide semiconductor
(CMOS)-like integrated circuits,[80–83] current regulators for displays[84–86] and
themselves can be light-emitting transistors (LETs).[29,87] However, the use of PbS
QDs in FETs is still challenging because they still suffer from low carrier mobility.
The low carrier mobility is one of the main obstacles in understanding the charge
transport within QD films and further utilizing PbS QDs for diverse applications.
Therefore, improving carrier mobility in PbS QD-FETs is an important and a
necessary task to understand the properties of QD films.
The origin of low carrier mobility in the devices is mostly attributed to the
high number of charge carrier traps on the QD surfaces, formed during ligand
exchange.[16,88] Since the charge transport in FETs takes place very close to the
semiconductor/insulator interface, the interface traps on the insulator surfaces
may also determine the characteristics and the performance of the devices.[89]
These interface traps further reduce the carrier mobility in the devices.
Depending on the processing conditions, the stoichiometry of QDs, and
ligands, PbS QD-FETs can display n-type, p-type, or ambipolar properties.[1,34]
The deposition of PbS QD films is commonly performed using a layer-by-layer
Charge transport and trap states in PbS QD field-effect transistors
18
(LbL) method.[31] The exchange of the long ligands into the shorter ones is done
after the deposition of each oleic-acid capped film. The LbL film deposition and
ligand exchange ensure complete removal of the native oleic-acid ligands. In
addition, the LbL method also allows the deposition of the next PbS layer to fill
the cracks on the films that are often produced after the ligand exchange process,
which result in an overall shrinking of the films. Our group previously reported
the increase of the carrier mobility with increasing the number of PbS layers, thus
the film thickness.[31] The increase of the carrier mobility starts to saturate after
reaching certain film thickness indicating that a crack-free film has been obtained.
So far, charge transport improvement in PbS QD-FETs has been obtained
through the use of different capping ligands.[63] In fact, the nature of the capping
ligands on the QD surfaces strongly determines the transport characteristics of
the PbS QD-FETs.[69] Ambipolar properties can be obtained using short organic
ligands such as 3MPA, BDT, and EDT.[31] By crosslinking PbS QDs with 3MPA,
our group reported electron mobility as high as 0.03 cm2V-1s-1.[64] Meanwhile, the
hole mobility (5 x 10-5 cm2V-1s-1) is much lower than the electron mobility, as has
also been observed in the QD films treated with other organic ligands (EDT and
BDT).[31,64] In addition, a high on-off ratio of 105 is achieved in devices employing
SiO2 gate dielectric.[64]
Further efforts in improving charge transport in PbS QD-FETs were done by
using inorganic atomic ligands.[63] These ligands have been reported to provide a
more complete passivation against carrier traps on the QD surfaces than that with
organic ligands.[16,63] In addition, the films capped with inorganic ligands have
shorter inter-particle distance than those capped with organic ligands, resulting
in a strong increase of the film conductivity.[46,90] Despite intensive works using
inorganic ligands, the mobility in the devices (with SiO2 gate dielectric) is,
however, still limited to 0.07 cm2V-1s-1.[63] The limited charge transport is
attributed to the high number of traps at the semiconductor/insulator interfaces
as mentioned earlier. Passivation of the SiO2 dangling bonds is one of important
strategies that can be adopted to suppress the interface traps. To date, introducing
self-assembled monolayers (SAMs) on the SiO2 surface is one of the most effective
methods to passivate the dangling bonds.[91–93] In addition, the SAMs also
introduce molecular dipoles on the SiO2 surface which affect the device
properties.[91,92] However, the study of the use of SAMs in FETs has so far been
limited to organic semiconductor active layers.[91–94] This opens a question if PbS
QD-FETs would also benefit of the passivation of the SiO2 surface and the
introduction of molecular dipoles through the use of SAMs.
Another way to minimize the interface traps in the devices is by gating the
devices with hydroxyl-free insulators. Koh et al. reported symmetric ambipolar
transport in PbS QD-FETs with electron and hole mobility of 0.1 cm2V-1s-1 using
thiocyanate (SCN-) ligands employing parylene gate insulator.[33] Meanwhile,
Chapter 1
19
with high-k dielectrics (Al2O3), Jo et al. reported a significant improvement in the
electron mobility to 0.47 cm2V-1s-1.[73] Further improvement of the electron
mobility in PbS QD-FETs to 2 cm2V-1s-1 was achieved by gating the devices with a
new class of gate dielectric, namely ion gel.[31] The ion gel can accumulate high
carrier density due to its high capacitance (1-20 μF/cm2). The high carrier density
effectively fills the traps resulting in an increase of the film conductivity and the
charge carrier mobility. The above mentioned research progresses clearly show
that the properties of gate insulators strongly determine the charge transport in
PbS QD-FETs. As there are still many promising hydroxyl-free insulators,
investigation on the use of broad types of the insulators as gate dielectrics in PbS
QD-FETs still needs to be addressed. Furthermore, a comprehensive study on the
electronic structures of the carrier traps in PbS QD-FETs employing different gate
insulators is also important to provide understanding of the charge transport in
the QD films and to potentially improve the performance of devices.
Together with the use of hydroxyl-free insulators, doping of semiconductors
is one of other effective methods to improve charge transport in FETs.[95–97]
Stoichiometrically, the surfaces of PbS QDs are rich in Pb, which leads to the n-
type properties of the material.[34] Kagan et al reported n-type FET devices with
the excess of Pb atoms on the QD surfaces.[34,35] Despite these dominant n-type
properties, most literature instead reports strategies to turn them into p-type. The
p-type doping of PbS QDs can be achieved by exposing the PbS films to air.[64] As
mentioned earlier, the 3MPA-crosslinked devices show ambipolar properties with
electron-dominated characteristics. As the devices are exposed to air, the devices
display the quenching of the electron transport. Meanwhile, in the p-channel, an
increase of the hole current by one order of magnitude and a significant threshold
voltage shift towards positive value, indicating strong p-type doping, are displayed.
The strong p-type transport of the air-exposed PbS QD-FETs is also observed in
the films crosslinked with EDT ligands.[98] In line with the 3MPA-crosslinked
films, the air exposure of the films with EDT ligands results in the increase of the
film conductivity and hole density supporting the strong p-type doping. While the
p-type doping is researched intensively mainly because of interest for the
fabrication of efficient solar cells,[15,99] studies on n-type doping of PbS QDs are
still limited. Koh et al reported the n-type doping of PbS QDs using cobaltocene
(CoCp2) molecules.[32] Although they successfully investigated the doping effect
via optical absorption spectroscopy, the doped-QDs show a decrease of
conductivity as measured in FETs indicating inefficient electron doping. To
enable an efficient n-type doping of the semiconductors, the highest occupied
molecular orbital (HOMO) level of dopant should be shallower than the
conduction band (CB) of the semiconductors. Finding dopant molecules with
shallow HOMO level is, however, a challenging work. Recently, benzyl viologen
(BV) has been reported as promising strong n-type dopant with shallow HOMO
Charge transport and trap states in PbS QD field-effect transistors
20
level.[100,101] The BV molecules have also demonstrated efficient electron doping in
carbon nanotube and MoS2 systems.[100,101] As the conduction band of PbS QDs
can be tuned from shallow to deep level by using various capping ligands, doping
of the QD films with BV donor molecules is also promising in improving charge
transport in PbS QD-FETs.
1.5 Experimental techniques
In this section, some experimental techniques related to the electrical and
optical measurements, which are used in this thesis are presented.
1.5.1 Electrical characteristic measurements
The FET electrical characteristic measurements were performed using a
probe station inside an N2-filled glovebox at room temperature. The probe station
is connected to an Agilent B1500A semiconductor parameter analyzer. The Ids-Vg
transfer characteristics were measured by sweeping the gate voltage with some
given values of source-drain voltage. Meanwhile, the Ids-Vds output characteristics
were obtained by varying source-drain voltage with some fixed gate voltages. The
detailed values of the applied source-drain voltage and gate voltage are provided
in the corresponding chapters.
1.5.2 Simulation for analysis of carrier traps
In principle, the trap density of states (trap DOS) in FETs can be estimated
from equation (1.14). The equation (1.14), however, can quantify the trap DOS at
a certain energy level in the subthreshold regime only. Meanwhile, study on the
trap DOS in a broad energy range is important to understand charge transport in
the FET devices.
In this thesis, we use a computer simulation to determine the trap DOS in
PbS QD-FETs. The simulation quantifies the total density of trap states, which
consists of the density of bulk and interface trap states. In PbS QD-FETs, bulk
traps originate from the active materials whereas the interface traps come from
the gate dielectric surface. Therefore, the presence of the interface traps
determines the calculated total density of trap states in the devices. The computer
simulation is based on a numerical model developed by Oberhoff et al.[102] The
model uses the effective medium approach where it is assumed that the material
between the source and the drain electrodes is homogeneous. Therefore, the trap
states in the material are assumed to be homogeneously distributed over the
whole channel region. The computer simulation works by solving drift-diffusion
current (J) and Poisson equation,
Chapter 1
21
xe
TkEeJ B (1.16)
so
e
x
V
2
2
(1.17)
where E, ρ, T, and εs are electric field, carrier density, temperature, and
semiconductor permittivity, respectively. In the effective medium, the drift-
diffusion equation is solved self-consistently under the given boundary conditions
(Vds, Vg, channel length L, channel width W, etc). The input parameters are all
macroscopically measurable except the number of trap states in the band gap. The
trap DOS is determined by fitting the simulated curves to the measured ones. As
the only free input parameter is the density of states, the spectral distribution of
trap states can be unambiguously determined.
For every step in the simulation, the number of total charge carriers is
calculated, which directly determines the position of the Fermi level. Depending
on the energy, charge carriers are separated into free charge carriers, residing in
the transport level, and trapped charge carriers, energetically positioned in the
band gap. The energy, which divides these two states is the mobility edge. When
the gate voltage increases, the number of charge carriers is increased and
therefore also the Fermi level shifts closer to the respective transport level. This
changes not only the total number of charge carriers, but also the ratio of free vs.
trapped charges and thus the electric current. This enables us to determine the
trap DOS directly from the measured transfer curves.
1.5.3 Optical absorption spectroscopy
Optical absorption spectroscopy is a technique that characterizes the
absorption of electromagnetic radiation as a function of the wavelength. In this
thesis, optical absorption measurements are used to characterize the excitonic
wavelength and therefore the dimension of QDs and their level of quantum
confinement. When a photon is absorbed by QDs, an electron is excited to a higher
energy level. As mentioned in the previous section, charge carriers in QDs can
only occupy certain discrete energy levels. Therefore, QDs only absorb photons
with certain wavelengths. In PbS QDs, the measured excitonic peak in the
absorbance spectra corresponds to the absorbed photon energy to excite an
electron from Eh1 to Ee1, thus close to the band gap energy, as demonstrated in
Figure 1.3.
Absorption spectroscopy is based on the Beer Lambert law. When light with
initial intensity Io irradiates a material, it is partly absorbed and partly transmitted
by the material. For a solution of QDs, the number of particles determines the
amount of absorbed light, which is proportional to the concentration of solution c
Charge transport and trap states in PbS QD field-effect transistors
22
and the length of light’s path Δx in the sample. The transmitted light intensity is
written as,
xc
oeII (1.18)
where ε is absorption coefficient. From equation (1.18), the absorbance, A, is
defined as,
I
IA olog (1.19)
The optical absorption measurements were recorded by using a double beam
spectrometer (Shimadzu UV-3600). The spectrometer is able to precisely
measure transmittance or reflectance from the ultraviolet (UV) region to near the
infrared (NIR) region. The spectrometer uses InGaAs and cooled PbS detectors
which work in the NIR region. For the UV to visible region, the spectrometer uses
a photomultiplier tube (PMT). As the electromagnetic sources, the spectrometer
has two main lamps which operate in two spectral ranges namely a Deuterium
lamp for the UV range and a Tungsten lamp for the visible and NIR regions. A
single wavelength of the lamp spectrum is selected by a double-grating
monochromator.
1.6 Outline of the thesis
In this thesis, the study of charge carrier transport and the analysis of the
electronic structures of carrier traps in PbS QD-FETs are reported. Our focus is to
understand how the properties of the semiconductor and/or insulator interfaces
and the introduction of dopant molecules on the semiconductor films can
determine the charge carrier transport and the electronic structures of carrier
traps in field-effect transistors. In detail:
In chapter 2, the effect of various self-assembled monolayer (SAM)
treatments of the SiO2 surface on the electrical characteristics of the devices is
investigated. The use of SAMs induces molecular dipoles on the SiO2 surface
which control the type of doping in the devices. The threshold voltage of the
devices was found to be proportional to the SAM doping concentration, providing
an opportunity to tune the properties of PbS QD-FETs toward n- or p-type
depending on the type of SAM molecules used.
In chapter 3, the charge transport properties in PbS QD-FETs through the
passivation of dangling bonds on the SiO2 surface and the use of Cytop as
hydroxyl-free gate dielectric are investigated. We found that the SiO2 surface
passivation using hexamethyldisilazane self-assembled monolayers (HMDS
SAMs) promotes better organization of particle assembly and reduces interface
traps, resulting in the improvement of charge carrier mobility. We then found that
the charge carrier mobility is further improved by one order of magnitude with
Chapter 1
23
Cytop gating. Finally, the calculation of the trap density of states (trap DOS) using
a computer simulation method, explains that the reduced trap DOS is a
microscopic origin of the improved charge carrier mobility in the devices.
In chapter 4, the trap DOS in PbS QD-FETs employing several gate
insulators with a wide range of dielectric constants is investigated. The trap DOS
was found to increase with increasing the gate dielectric permittivity. In addition,
the broadening of the trap DOS with increasing the dielectric polarization strength
was observed. The increase and the broadening of the trap DOS originate from
dipolar disorder which appears at strong dielectric polarization (high gate
dielectric constant). Despite the increase of the trap DOS, we demonstrate the
absence of negative effect of high gate dielectric permittivity on the charge carrier
mobility at the highest applied gate voltages.
In chapter 5, an efficient n-type doping of PbS QD solids using benzyl
viologen (BV) donor molecules is demonstrated. The doping strategy relies on
combining the use of the donor molecules with the engineering of the QD energy
level through the use of appropriate capping ligands. The BV doping enabled
controlling the properties of PbS QD-FETs from ambipolar to heavy n-type
allowing improving the electron mobility in the devices by one order of magnitude.
In addition, the doping reduced the contact resistance of the devices to 0.77 kΩcm.
The 4-terminal conductivity transistor measurements confirmed the efficient
doping of the devices, which display electron mobility of 0.64 cm2V-1s-1 employing
SiO2 gate dielectric.
Finally, in chapter 6, the effect of mechanical strain on the electron transport
properties in PbS QD-FETs is investigated. An ion gel which is able to accumulate
high carrier concentration was used as gate dielectric, resulting in electron
mobility as high as 2.1 cm2V-1s-1. With the application of compressive strain, we
observed the improvement of the electron mobility up to 45% (mobility of 3 cm2V-
1s-1) at 2% strain. The improvement of the mobility is attributed to the reduced
inter-QD spacing due to the bending of crosslinking ligands. When strain was
applied in the opposite direction, we observed the decrease of the electron
mobility due to the increase of the distance between QDs. In addition, we found
that the application of strain influences the property of the trap states in PbS QD-
FETs as indicated by the modulated threshold voltages.
Charge transport and trap states in PbS QD field-effect transistors
24
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29
Chapter 2
Tunable Doping in PbS Quantum
Dot Field-Effect Transistors using
Surface Molecular Dipoles
This chapter presents the effect of various self-assembled monolayer (SAM)
treatments of the SiO2 dielectric on the electrical characteristics of PbS quantum
dot field-effect transistors (QD-FETs). With the use of SAMs, the threshold
voltage of the devices shows a linear relationship with the SAM doping
concentration. This study allows tuning the electrical properties of the devices
towards n- and p-type doping depending on the type of SAM molecules used.
Furthermore, the use of SAMs reduces the interface traps due to the passivation
of dangling bonds on the SiO2 surface. The reduced traps confirm that the
threshold voltage shifts and the mobility changes in the devices are attributed to
electron/hole doping process and are not caused by the traps induced by the
SAMs, as observed in some organic semiconductor systems.
M. I. Nugraha, H. Matsui, S. Z. Bisri, M. Sytnyk, W. Heiss, M. A. Loi, J. Takeya,
APL Mater. 2016, 4, 116105.
Charge transport and trap states in PbS QD field-effect transistors
30
2.1 Introduction
Increasing carrier concentration through doping has been used to improve
the film conductivity and carrier mobility in different types of semiconductors.
This effort has been widely performed in organic field-effect transistors (FETs),
whereas doping in colloidal quantum dots (QDs) is generally attempted by using
different capping ligands.[1–7] Heavy doping in lead chalcogenide QDs using donor
molecules, such as cobaltocene (CoCp2) has also been investigated.[8] Doping with
these donor molecules has enabled strong n-type transport in FET devices.
However, these heavily-doped PbS QDs result in the FET devices always turned-
on with consequent low on/off ratio, which is an undesirable property for some
electronic device applications. At this point, light doping of the semiconducting
active material is preferred.
Molecular dipoles, well-known as self-assembled monolayers (SAMs), have
been widely used to achieve light doping in organic FETs. This light doping is a
consequence of band bending due to the modulation of interfacial carrier
concentration induced by the SAM molecules. Due to this modulation, the
molecules affect the threshold voltage of the devices, allowing the control of
charge carrier density as well as the device operation.[9–12] In addition, the use of
SAMs can modify the surface properties of the oxide gate dielectric, which
influences the morphology of the deposited films. Therefore, the SAM treatment
in FETs is very promising to realize high performance devices with controllable
operation. These effects have so far been widely studied in organic
semiconductors, while the investigation of SAMs in PbS QD-FETs is still limited.
In addition, there are still broad types of SAMs that have not been investigated in
PbS QD-FETs. This leaves many unsolved questions about the possibility to
improve the performance as well as to influence the electrical characteristics of
the QD-FETs using SAMs.
In this chapter, we present a study on the effect of SAM surface treatment of
the SiO2 gate dielectric on the electrical characteristics of PbS QD-FETs. Upon the
application of SAMs, we observed threshold voltage shifts indicating electron or
hole doping introduced by the molecular dipoles on the dielectric surface. We
observed an obvious relationship between the threshold voltage of the devices and
the doping concentration induced by the SAM molecules, giving us the
opportunity to tune the FET properties towards n- or p-type depending on the
SAM molecules used. The use of SAMs also allows reducing the density of traps in
the devices by passivating the dielectric surface, leading to improved electron
mobility up to a factor of three. This light doping strategy through SAM treatment
is promising in controlling the electrical characteristics and improving charge
carrier mobility in PbS QD-FET devices.
Chapter 2
31
2.2 Doping mechanism using SAMs
In this study, six types of SAM molecules were utilized to functionalize the
SiO2 surface namely hexamethyldisilazane/HMDS, hereinafter referred to as (A),
n-octyltriethoxysilane/OTS (B), trimethoxy(2-phenylethyl)silane/PEMS (C),
trichlorophenetylsilane/β-PTS (D), 3(mercaptopropyl)trimethoxy-silane/MPMS
(E), and dodecylthrichlorosilane/DTS (F). The molecular structures of the SAMs
are displayed in Figure 2.1 (a). To ensure that the SAM molecules influence the
semiconductor/insulator interface only, a top contact device structure was
fabricated as shown in Figure 2.1 (b). In this way, the effect of SAMs on the work
function of the source-drain electrodes is minimized. The insertion of the SAM
layer between the semiconductor and the dielectric films does not significantly
change the total series capacitance due to higher capacitance of the SAM layer
than that of the SiO2 gate dielectric. After the deposition of the semiconductor
films on the SiO2 dielectric, the Fermi level of the semiconductor is aligned with
the work function (Fermi level) of the gate electrode as described in Figure 2.1 (c).
The treatment of the SiO2 surface with SAMs introduces dipoles on the surface
which allow upward or downward bending of the energy levels of the
semiconductor, leading to hole or electron accumulation depending on the
polarity of the SAMs. This accumulation of carriers without applying a gate
voltage appears as a carrier doping in the devices, which therefore influences the
electrical characteristics of the devices.
CH3
SiH3C CH3
CH3
Si CH3H3C
HN
(a) (b)
SiO2
PbSSAMs
n-Si
Au
(c)
Gate
SiO2
Ec
Ev
Ef
OCH3
Si OCH3H3CO
CH2
(CH2)6
CH3
OCH3
Si OCH3H3CO
(CH2)2
Cl
Si ClCl
(CH2)2
OCH3
Si OCH3H3CO
CH2
(CH2)2
SH
Cl
Si ClCl
CH2
(CH2)10
CH3
A B C
D E F
Gate
SiO2
Ef
+-SAMs
-----Ec
Ev
Gate
SiO2
Ef
+ -SAMs
+++++
Ec
Ev
Figure 2.1 (a) Molecular structures of SAMs. (b) Configuration of PbS FETs. (c) Schematic
of energy level diagrams in PbS FETs with untreated SiO2 (left) and SAM-treated SiO2
(middle and right) at zero gate voltage.
Charge transport and trap states in PbS QD field-effect transistors
32
2.3 Properties of PbS films on SAM-treated SiO2
2.3.1 Morphology of PbS films
As a first step, AFM measurements on PbS films were performed to verify
the effect of SAMs on the morphology of the PbS films. Because the first PbS layer
has a direct contact with the SAM-treated SiO2 surface, the AFM images were only
taken for this first layer as shown in Figure 2.2 (a)-(f). We found that the use of
SAMs on the SiO2 surface generally promotes a better particle organization as
demonstrated by the reduced surface roughness in all films, with the exception of
the DTS-treated surface. In DTS-treated SiO2, the surface becomes very
hydrophobic which drastically reduces the wettability of the QD solution in
chloroform, producing some particle aggregates during the film deposition.
Figure 2.2 AFM images of the first PbS layer on different SAM treated-SiO2 surfaces with
given surface roughness (R).
Chapter 2
33
2.3.2 Electrical Properties of PbS QD-FETs
To investigate the effect of SAM surface treatment of the gate insulator on
the electrical characteristics of PbS QD-FETs, we first performed the electrical
characterization of the devices without any surface treatment in an N2-filled glove
box. The transfer characteristics of the PbS QD-FETs in the n-channel operation
without surface treatment are shown in Figure 2.3 (a). The devices show
pronounced n-type properties with extracted electron linear mobility of 0.02
cm2V−1s−1 and a high current modulation of 104. The electron mobility is calculated
from the forward sweep of the transfer curve, which means that the extracted
values do not overestimate the real value of the mobility. In the semi-logarithmic
profile of the transfer characteristics, we also observed a slight hole current. Here
we focus on the electron transport properties to investigate the effect of SAM
surface treatment of the SiO2 gate dielectric, since the hole current in the devices
is much lower than the electron current. The hole transport in the devices is also
influenced by the much higher hole trap density with respect to the electron trap
density, which may result in uncertainty regarding the effect of doping.[13–15]
-20 0 20 40 60
0.0
0.5
1.0
1.5
10-11
10-10
10-9
10-8
10-7
10-6
Dra
in C
urr
ent
(A)
Dra
in C
urr
ent
(A)
Gate Voltage (V)
Vds
=5 V
x10-6(a)
5 10 15 200.0
0.5
1.0
1.5x10
-6
40 V
Dra
in C
urr
ent
(A)
Drain Voltage (V)
0,10,20,30 V
50 V
0
Dra
in C
urr
ent
(A)
(b)(a) (b)
Figure 2.3 (a) Ids-Vg transfer and (b) Ids-Vds output characteristics of pristine PbS FETs.
The FET devices utilize a SiO2 gate dielectric with thickness of 200 nm
defining the oxide capacitance (Ci) of 17.3 nF/cm2. By estimating the intercept of
the current voltage curve in the x-axis, the threshold voltage of the devices is
calculated to be 29 V. To confirm that the devices are in the linear regime at the
applied source-drain voltage (Vds), the output characteristics of the devices are
displayed in Figure 2.3 (b). From the output characteristics, it is also clear that the
devices show both clear linear and saturation regime with no indication of contact
limitation.
Charge transport and trap states in PbS QD field-effect transistors
34
-20 0 20 40 60
0
1
2
3
4
5
C
B
A
P
x10-6
Dra
in C
urr
ent
(A)
Gate Voltage (V)
Vds
=5 V
-20 0 20 40 60
0.0
0.5
1.0
1.5
F
E
D
P
Vds
=5 V
Dra
in C
urr
ent
(A)
Gate Voltage (V)
x10-6
0 20 40
0.0
0.2
0.4
I ds(A
)
Vg (V)
x10-6
0 20 40
0.0
0.2
0.4
x10-6
I ds (
A)
Vg (V)
(b)
(a)
Figure 2.4 Ids-Vg transfer characteristics of PbS FETs after various SAM treatments on the
SiO2 surface indicating (a) electron and (b) hole doping.
The transfer characteristics of PbS QD-FETs on different SAM-treated SiO2
surfaces are shown in Figure 2.4 (a) and (b). The characteristics of the devices
fabricated on the pristine SiO2 are shown in black line and indicated by ‘P’. Using
the SAM treatment, we are able to improve as well as to effectively tune the source-
drain current of the devices. The treatment of the SiO2 gate insulator using SAM
A shows the highest current and the highest mobility among the tested SAMs. In
addition, the SAM surface treatment of the SiO2 gate dielectric also influences the
threshold voltage characteristics of the devices. By treating the SiO2 surface with
SAM A-C, the transfer characteristics shift to negative gate voltages indicating
reduced threshold voltage and electron doping in the devices, as demonstrated in
Figure 2.4 (a). The inset of Figure 2.4 (a) displays the detailed profiles of the
negatively shifted transfer characteristics of the devices with SAM A-C treated
SiO2 dielectric. Conversely, Figure 2.4 (b) shows that the threshold voltage shifts
towards more positive voltages when the SiO2 surface is treated with SAM D-F
Chapter 2
35
indicating hole doping in the devices. A clearer image of the positively shifted
transfer characteristics of the devices with SAM D-F treated SiO2 surface is
displayed in the inset of Figure 2.4 (b). With these results, the SAM treatment
strategy allows tuning the electrical characteristics of PbS QD-FETs through
electron or hole doping, depending on the type of SAM molecules utilized.
2.3.3 SAM doping concentration
At this point, it is interesting to quantify the doping strength induced by the
SAM molecules, which corresponds to the concentration of electron and hole
doping in the devices. The concentration of electron and hole doping can be
estimated from the semi-logarithmic profile of the device transfer characteristics.
Figure 2.5 shows the transfer characteristics of PbS QD-FETs in semi-logarithmic
scale fabricated on several SAM-treated SiO2 surfaces. In line with Figure 2.4, a
clear shift of the transfer characteristics to negative and positive voltages was
demonstrated, depending on the type of SAMs used. In addition, an obvious
variation of the on-voltage (Von) of the devices using different SAM treatments
was also observed. The Von can be calculated from the gate voltage where the
source-drain current starts increasing exponentially. This voltage corresponds to
the concentration of electron and hole doping in the devices following the
equation eVCN oniSAMs / where ΔVon is the shift of the on-voltage in SAM-
treated devices with respect to that in pristine devices.[10] Table 2.1 shows the
doping concentration of electron and hole in the devices with different SAM
treatments of SiO2 gate dielectric. The negative values of the doping concentration
correspond to electron doping, while the positive values indicate hole doping.
Figure 2.5 Ids-Vg transfer characteristics of PbS FETs in semi-logarithmic scale after various
SAM treatments on the SiO2 surface.
(b)
-20 0 20 40 6010
-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
F
E
P
B
A
Dra
in C
urr
ent
(A)
Gate Voltage (V)
Vds
=5 V
Charge transport and trap states in PbS QD field-effect transistors
36
Table 2.1 Carrier doping concentration and trap density in the devices with different SAM
treatments.
SAMs Molecules NSAMs
(1012 cm-2)
Dipole
Moment (D)
Ntraps
(1012 eV-1 cm-2)
P Pristine - - 11.6
A HMDS -1.1 -0.98 4.9
B OTS -0.54 3.03 10.2
C PEMS -0.33 -1.74 9.0
D β-PTS 0.76 0.41 9.8
E MPMS 0.97 1.21 10
F DTS 1.5 0.91 7.4
Figure 2.6 Schematic of dipole moments on the SiO2 surface after treatment with SAM A-
F. One of end groups is terminated with H atom to simplify the calculation.
Chapter 2
37
The doping concentration is related to the dipole moments induced by the
SAM molecules. The SAM dipole moments and their direction are given in Table
2.1 and displayed in Figure 2.6. These dipole moments are calculated using
Gaussian program by terminating the end group of the SAM molecules with
hydrogen atom to simplify the calculation. The magnitude and direction of the
dipole moments might be influenced by the model used for the calculation using
Gaussian program. In Figure 2.6, the negative dipole moments correspond to
their downward direction, which favor more electron doping, thus negative
concentration values. Meanwhile, upward direction is shown by the positive
values of dipole moments which promote hole doping. Interestingly, a peculiar
result with SAM B is observed, in which electron doping is produced by a positive
dipole moment. In addition, we also did not find a monotonic trend between the
magnitude of dipole moment and the number of carrier concentration at a specific
type of carrier doping. These results can be attributed to the effective arrangement
of the SAM molecules on the SiO2 surface. It is important to note that the
arrangement of the SAMs and their orientation on the SiO2 surface are very crucial
to determine the effective magnitude of dipole moments and the type of charge
carrier doping.
Figure 2.7 The characteristics of threshold voltage for electron accumulation as a function
of doping concentration induced by SAMs.
As we compare Figure 2.4 and 2.5, the variation in the Von, thus in doping
concentration, is interestingly followed by the threshold voltage shift in the same
direction. It further shows that the charges induced by the SAMs are the origin of
the shifted transfer characteristics. Figure 2.7 presents a linear relationship
between the threshold voltage of the devices and the SAM doping concentration.
The deposition of the SAM thin layer builds a series capacitance with respect to
-1 0 1 210
20
30
40
50
FE
D
CB
Thre
shold
Voltage (
V)
NSAMs
(1012
cm-2)
AP
(a)
Charge transport and trap states in PbS QD field-effect transistors
38
the capacitance of the oxide gate dielectrics (Ci), which induces a threshold voltage
shift that can be described as follows,
i
SAMsSAMs
SAMsi
SAMsith
C
eNeN
CC
CCV
(2.1)
where CSAMS and NSAMs are the capacitance of SAMs and the SAM doping
concentration, respectively. According to equation (2.1), the inverse gradient of
the curve in Figure 2.7 corresponds to the capacitance of the SiO2 gate dielectric.
From Figure 2.7, the calculated gradient of the curve is 5.2 x 10-12 V cm2, which
corresponds to the gate dielectric capacitance of 31.1 nF/cm2. This capacitance
value shows a substantial deviation from the one of the actual gate dielectric used
in the devices, 17.3 nF/cm2. This deviation may come from an underestimation of
the calculated Vth, particularly in the devices with the characteristics shifted
towards the right direction, as we refer to Figure 2.4. The Vth value can be
extracted at different charge carrier densities, which may result in an error in the
Vth extraction.
2.3.4 Mobility and density of traps
The doping strategy using SAMs also allows determining the charge carrier
mobility in the devices. Figure 2.8 (a) displays the electron mobility in the linear
regime obtained using different SAM surface treatments. The electron mobility
increases with increasing electron doping concentration. We also observed that
the electron mobility tends to decrease as the hole doping concentration increases
with the exception of the devices treated with SAM D. In this condition, holes in
the devices reduce the film conductivity, which result in the decrease of the
electron mobility. This could be explained with the recombination of holes and
electrons, which gives rise to light emission.[16,17]
(b)
-1 0 1 20.00
0.02
0.04
0.06
0.08
P
Mobili
ty (
cm
2V
-1s
-1)
B
NSAMs
(1012
cm-2)
A
F
E
D
C
(a)
0
5
10
15
20
N
traps (
10
12 c
m-2eV
-1)
P FEDCBA
SAM
(b) SAMs
Figure 2.8 (a) Electron mobility as a function of doping density induced by SAMs. (b) The
density of traps (Ntraps) in PbS FETs with various SAM-treated SiO2 surfaces.
Chapter 2
39
In organic FETs, organosilanes have been indicated to induce additional
traps which further reduce the charge carrier mobility.[18,19] Therefore, to confirm
that the decrease of the electron mobility is caused by hole doping and not by the
additional traps, we quantify the trap density (Ntraps) in our devices using equation
(1.14). The density of traps in the devices using different surface treatments is
shown in Figure 2.8 (b) and given in Table 2.1. Obviously, the use of SAMs on the
SiO2 surface reduces the density of electron traps in the devices, which is in
agreement with the fact that the SAMs passivate the dangling bonds on the SiO2
surface. These results further confirm that the decrease of the electron mobility in
Figure 2.8 (a), particularly in the devices treated with SAM E and F, originates
from the hole doping, not from the SAM-induced traps as observed in some
organic FETs.[18,19] In addition, the reduction of the density of traps by almost a
factor of three using SAM A contributes to the highest electron mobility in the
devices (0.07 cm2V-1s-1). The improvement of the carrier mobility in the devices is
not as high as in the case of organic single crystals such as rubrene.[20,21] In rubrene
FETs, the trap density in the material is very low. Hence, a small doping level
induced by SAMs will have a great impact on the mobility of the charge carriers.
Meanwhile, the trap density in PbS FETs is rather high (1.2 x 1013 eV-1cm-2).
Because of this high trap density, the improvement of the mobility in the devices
is very reasonable considering the doping level given by the SAMs.
2.4 Conclusion
The effect of various SAM treatments of SiO2 gate dielectric on the electrical
characteristics of PbS QD-FETs has been studied. The use of SAMs resulted in the
threshold voltage shifts by inducing electron or hole doping in the devices. The
threshold voltage of the devices was found to have a linear relationship with the
SAM doping concentration, allowing tuning the electrical characteristics of PbS
QD-FETs depending on the type of the SAM molecules used. The use of particular
SAMs was also able to improve the electron mobility by a factor of three, which is
attributed to higher electron doping as well as lower interface traps than in the
devices fabricated on untreated SiO2 surface. Meanwhile, hole doping results in a
decrease of the electron mobility, probably due to the recombination of electron
and hole.
2.5 Methods
SAM treatment of SiO2 dielectric. Heavily n-doped silicon covered with
a thermally grown 200 nm thick SiO2 layer was used as substrates. Before use, the
substrates were cleaned with acetone and isopropanol for 10 min, respectively.
The substrates were then dried at 120oC to remove residual solvents. Six different
Charge transport and trap states in PbS QD field-effect transistors
40
types of SAM molecules, namely, hexamethyldisilazane/HMDS, hereinafter
referred to as (A), n-octyltri-ethoxysilane/OTS (B), trimethoxy(2-
phenylethyl)silane/PEMS (C), trichloro-phenetylsilane/β-PTS (D), 3-
(mercaptopropyl)trimethoxysilane/MPMS (E), and dodecyltrichlorosilane/DTS
(F) were used to functionalize the SiO2 surface. The treatment with SAM A was
done by soaking the substrates in SAM A for 1 min followed by spin-coating at
1000 rpm for 1 min (drying). For the treatment with SAM B-E, the substrates were
immersed in the corresponding SAM solution (20 µL of SAM molecules in 10 mL
of toluene) for 48 h. The last SAM F treatment was done using vapor deposition
at atmospheric pressure obtained by placing the SAMs and substrates inside a
Teflon container at 120oC for 3 h. To remove the excess of unbound SAM
molecules, the substrates were cleaned with acetone and isopropanol for 5 min,
respectively. Finally, the substrates were dried at 120oC to remove the residual
solvents as well as to promote stronger bonding between the SAM molecules and
the SiO2 surface.
Device fabrication. The deposition of PbS QD films (diameter of 3.6 nm)
from 10 mg/mL of oleic acid stabilized-QD solution in chloroform on the
previously SAM-treated substrates was done using spin-casting. To improve the
conductivity of the films, the long-alkyl chain organic ligands were exchanged
with shorter organic molecules, namely 1,2-ethanedithiol (EDT), with
concentration of 1% (v/v) in acetonitrile. The film deposition and the ligand
exchange (LE) were done using layer-by-layer (LbL) spin-coating method
repeated for five times to ensure complete LE process, as previously reported.[22–
25] After each LE step, the samples were cleaned with acetonitrile to favor the
removal of the exchanged native oleic acid ligands. The devices were then
annealed at 120oC for 20 min to remove residual solvents. Finally, Au with
thickness of 50 nm was evaporated as source-drain electrode defining the channel
length and width of 50 µm and 2 mm, respectively. All device fabrication was
performed in an N2-filled glove box.
Device measurements. The electrical characteristics of FETs were
measured using an electrical probe station (placed in an N2-filled glove box) that
is connected to an Agilent B1500A semiconductor parameter analyzer.
Chapter 2
41
2.6 References
[1] M. H. Zarghami, Y. Liu, M. Gibbs, E. Gebremichael, C. Webster, M. Law, ACS Nano
2010, 4, 2475.
[2] P. R. Brown, D. Kim, R. R. Lunt, N. Zhao, M. G. Bawendi, J. C. Grossman, V.
Bulovic, ACS Nano 2014, 8, 5863.
[3] M. V Kovalenko, M. Scheele, D. V Talapin, Science 2009, 324, 1417.
[4] D. M. Balazs, D. N. Dirin, H.-H. Fang, L. Protesescu, G. H. ten Brink, B. J. Kooi, M.
V. Kovalenko, M. A. Loi, ACS Nano 2015, 9, 11951.
[5] S. M. Thon, A. H. Ip, O. Voznyy, L. Levina, K. W. Kemp, G. H. Carey, S. Masala, E.
H. Sargent, ACS Nano 2013, 7, 7680.
[6] D. Zhitomirsky, M. Furukawa, J. Tang, P. Stadler, S. Hoogland, O. Voznyy, H. Liu,
E. H. Sargent, Adv. Mater. 2012, 24, 6181.
[7] M. J. Greaney, R. L. Brutchey, Mater. Today 2015, 18, 31.
[8] W. Koh, A. Y. Koposov, J. T. Stewart, B. N. Pal, I. Robel, J. M. Pietryga, V. I. Klimov,
Sci. Rep. 2013, 3, 2004.
[9] C. Celle, C. Suspène, M. Ternisien, S. Lenfant, D. Guérin, K. Smaali, K. Lmimouni,
J. P. Simonato, D. Vuillaume, Org. Electron. 2014, 15, 729.
[10] K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, B. Batlogg, A.
N. Rashid, G. Schitter, J. Appl. Phys. 2004, 96, 6431.
[11] J. E. McDermott, M. McDowell, I. G. Hill, J. Hwang, A. Kahn, S. L. Bernasek, J.
Schwartz, J. Phys. Chem. A 2007, 111, 12333.
[12] M. F. Calhoun, J. Sanchez, D. Olaya, M. E. Gershenson, V. Podzorov, Nat. Mater.
2008, 7, 84.
[13] S. J. Oh, N. E. Berry, J.-H. Choi, E. A. Gaulding, T. Paik, S.-H. Hong, C. B. Murray,
C. R. Kagan, ACS Nano 2013, 7, 2413.
[14] S. J. Oh, N. E. Berry, J.-H. Choi, E. A. Gaulding, H. Lin, T. Paik, B. T. Diroll, S.
Muramoto, C. B. Murray, C. R. Kagan, Nano Lett. 2014, 14, 1559.
[15] M. I. Nugraha, R. Häusermann, S. Z. Bisri, H. Matsui, M. Sytnyk, W. Heiss, J.
Takeya, M. A. Loi, Adv. Mater. 2015, 27, 2107.
[16] C. Rost, S. Karg, W. Riess, M. A. Loi, M. Murgia, M. Muccini, Appl. Phys. Lett.
2004, 85, 1613.
[17] M. A. Loi, C. Rost-Bietsch, M. Murgia, S. Karg, W. Riess, M. Muccini, Adv. Funct.
Mater. 2006, 16, 41.
[18] K. P. Pernstich, A. N. Rashid, S. Haas, G. Schitter, D. Oberhoff, C. Goldmann, D. J.
Gundlach, B. Batlogg, J. Appl. Phys. 2004, 109, 84510.
[19] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H.
Shimotani, N. Yoshimoto, S. Ogawa, Y. Iwasa, Nat. Mater. 2004, 3, 317.
[20] J. Takeya, M. Yamagishi, Y. Tominari, R. Hirahara, Y. Nakazawa, T. Nishikawa, T.
Kawase, T. Shimoda, S. Ogawa, Appl. Phys. Lett. 2007, 90, 102120.
[21] J. Takeya, T. Nishikawa, T. Takenobu, S. Kobayashi, Y. Iwasa, T. Mitani, C.
Goldmann, C. Krellner, B. Batlogg, Appl. Phys. Lett. 2004, 85, 5078.
[22] A. G. Shulga, L. Piveteau, S. Z. Bisri, M. V. Kovalenko, M. A. Loi, Adv. Electron.
Mater. 2016, 2, 1500467.
[23] D. M. Balazs, M. I. Nugraha, S. Z. Bisri, M. Sytnyk, W. Heiss, M. A. Loi, Appl. Phys.
Charge transport and trap states in PbS QD field-effect transistors
42
Lett. 2014, 104, 112104.
[24] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss, M. A. Loi, Adv. Mater. 2013, 25, 4309.
[25] M. J. Speirs, D. M. Balazs, H.-H. Fang, L.-H. Lai, L. Protesescu, M. V. Kovalenko,
M. A. Loi, J. Mater. Chem. A 2015, 3, 1450.
43
Chapter 3
Dielectric Surface Passivation and
Hydroxyl-free Polymer Gating in
PbS Quantum Dot Field-Effect
Transistors
In this chapter, the effect of gate dielectric surface passivation and
hydroxyl-free Cytop polymer gating in PbS quantum dot field-effect transistors
(QD-FETs) are studied. The passivation of the gate dielectric surface through the
use of self-assembled monolayers (SAMs) results in a significant improvement
of electron mobility due to reduced interface traps and improved particle
assembly organization. With hydroxyl-free Cytop polymer gating, the charge
carrier mobility in the devices is further improved by one order of magnitude.
The calculation of the trap density of states (trap DOS) using a computer
simulation shows that a significant reduction of the trap DOS over broad energy
distribution is the origin of the improved charge carrier mobility in the devices.
M. I. Nugraha, R. Häusermann, S. Z. Bisri, H. Matsui, M. Sytnyk, W. Heiss,
J. Takeya, M. A. Loi, Adv. Mater. 2015, 27, 2107
Charge transport and trap states in PbS QD field-effect transistors
44
3.1 Introduction
The fabrication of field-effect transistors (FETs) is one of the best methods
to evaluate charge transport properties in semiconductors, including QD
assemblies.[1,2] Since FETs are interface-based devices, charge trapping is not only
influenced by the properties of the active layer (the QD assembly) but also by the
nature of the gate dielectric and its surface. Many efforts have been done to
enhance the carrier mobility in PbS QD transistors. These attempts include
variation of crosslinking ligands,[3–6] chemical post-deposition treatments to vary
doping levels or to fill carrier traps,[7–9] increasing the chemical purity,[10] and
controlling the effect of oxygen/moisture during fabrication.[4,5] Nevertheless, the
typical mobility is still only up to 10−2 cm2V−1s−1, with the exception of sintered
PbS QDs, which however often give rise to unipolar devices with limited on/off
ratio,[8,11] and the devices which utilize ionic-liquid gating that allows
accumulating a higher carrier density than trap density.[4,12,13] In devices using
conventional dielectrics such as SiO2, the transport characteristics are still trap
dominated.
In this chapter, we demonstrate a high electron mobility and a very low trap
density in ambipolar PbS QD-FETs through the improvement of the QD assembly
and the utilization of an amorphous fluoropolymer (Cytop) thin film as gate
dielectric. Cytop is a hydroxyl-free and transparent polymer dielectric which has
a dielectric constant of 2–2.3. We first improved the assembly organization of the
3-mercaptopropionicacid (3MPA) cross-linked PbS nanocrystals on the SiO2
surface through the utilization of hexamethyldisilazane self-assembled
monolayers (HMDS SAMs). This SAM treatment passivates the silanol on the SiO2
surface that may act as electron trapping site. Cytop was deposited on top of the
PbS nanocrystal assembly as second gate structure. The dual-gated FET
structures using Cytop as a top gate and SiO2 as bottom gate dielectric (Figure 3.1),
were utilized to compare the influence of the two different dielectrics on the same
PbS nanocrystal assembly. Finally, from the obtained transport characteristics,
the simulation and numerical fitting to quantify the trap density of states (trap
DOS), we observed for both holes and electrons in the FETs a sheet trap density
lower than 1012 cm−2, which explains the high electron mobility of 0.2 cm2V−1s−1.
Figure 3.1 Configuration of PbS QD-FETs with SiO2 bottom and Cytop top gate dielectrics.
Chapter 3
45
3.2 Properties of PbS FETs with HMDS-treated SiO2
3.2.1 Morphology of PbS films
The deposition of PbS QD films for FETs as well as for solar cells, is
commonly performed using a layer-by-layer method to ensure effective ligand
exchange. Therefore, the assembly organization of the first QD monolayer is the
most crucial factor that determines the morphology of the successive layers. A
conventional method for the deposition of QDs on a clean SiO2, which has been
used to successfully demonstrate well-performing devices,[4,5] produces clusters
instead of large scale homogeneous films. The clustering is observed from the first
monolayer after the ligand exchange of oleic acid with 3MPA as shown in Figure
3.2 (a). The formed clusters can be as thick as the equivalent of 3 monolayers,
resulting in a root mean square (RMS) roughness of about 2.4 nm.
Figure 3.2 AFM images of (a) the first PbS QD layer on bare SiO2 and (b) on HMDS-
functionalized SiO2. The inset shows the water contact angle of untreated and HMDS-
treated SiO2 surfaces.
To increase the hydrophobicity of the substrate, the SiO2 surface was
functionalized using hexamethyldisilazane self-assembled monolayers (HMDS
SAMs). The HMDS-functionalized SiO2 shows a water contact angle of 60o, which
is increased significantly with respect to the contact angle measured for pristine
SiO2 (30o), indicated in Figure 3.2 (a) and (b). From the AFM micrographs, the
surface of HMDS-treated SiO2 is comparably smooth as that of the pristine SiO2
as shown in Figure 3.3 (a) and (b). The smoothness of the surface and the high
water contact angle clearly show that HMDS forms a well packed self-assembled
monolayer and passivates the silanol (hydroxyl) groups on the SiO2 surface. This
functionalization significantly improves the assembly-order of the deposited PbS
QDs. The RMS roughness of 0.8 nm indicates that no significant clustering
observed in the first monolayer of the nanocrystal assembly after ligand exchange
as displayed in Figure 3.2 (b). This indicates that the use of HMDS-SAMs
Charge transport and trap states in PbS QD field-effect transistors
46
homogeneously reduces the surface energy for the deposition of PbS QDs from
chloroform-based solutions.
4.01 nm0.0
1 μm
(a)
0.0 4.5 nm
1 μm
(b)
Figure 3.3 AFM images of (a) bare SiO2 and (b) HMDS-treated SiO2.
3.2.2 Electrical properties of PbS QD-FETs
PbS QD-FETs prepared on HMDS-treated SiO2 were first operated as
bottom-gate bottom-contact FETs with SiO2 as gate dielectric. The configuration
of the devices is displayed in Figure 3.1. The Ids-Vds output characteristics of the
FETs with SiO2 accumulation are shown in Figure 3.4 (a). The FETs show
ambipolar properties with more electron-dominated characteristics, consistent
with previous reports on PbS QD-FETs using 3MPA ligands.[4,5] Both, n- and p-
channel output, show clear current saturation behaviors. In the small drain
voltage operation, it is obvious that the devices exhibit nearly ohmic-like injection,
both, for holes and electrons.
Figure 3.4 (b) shows the Ids-Vg transfer characteristics of electron and hole
modulations in the corresponding transistors. The electron on/off ratio achieves
a value as high as 106. The fabricated devices have a 20 µm channel length and a
10 mm channel width, the capacitance of SiO2 is 15 nF/cm2. The maximum
extracted mobility value in the linear regime is 0.07 cm2V-1s-1 for electrons and
5×10-5 cm2V-1s-1 for holes. The electron mobility is reasonably high for non-
sintered ambipolar PbS QD-FETs that show high on/off ratio of 106 using SiO2
gate dielectric and 3MPA ligands. The non-sintered PbS QD films are revealed
from the absorbance spectrum that still maintain excitonic peak even after
annealing the PbS films at 140oC as displayed in Figure 3.5.
Chapter 3
47
Figure 3.4 (a) Ids-Vds output and (b) Ids-Vg transfer characteristics of PbS QD-FETs with
HMDS-treated SiO2. The applied Vds in the transfer characteristics is ±5 V.
600 800 1000 1200 1400 1600
Absorb
ance (
a.u
)
Wavelength (nm)
PbS-OA films PbS-3MPA RT
PbS-3MPA 140oC
Figure 3.5 Absorbance spectrum of PbS QD films with oleic acid (black), 3MPA without
annealing (red), and 3MPA ligands after annealing at 140oC (blue).
(b)
-60 -40 -20 00.0
-0.2
-0.4
-0.6
-0.8
-1.0
0 20 40 60
Dra
in C
urr
ent
(A
)
Dra
in C
urr
ent
( A
)
Drain Voltage (V)
0
50
100
150
0 V
Vg -80 V Vg 80 V
0 V
-80 -40 00.00
0.05
0.10
0.15
0.20
0.25-8 -4 0
Carrier Density (1012
cm-2)
Gate Voltage (V)
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 40 80
0 4 8
|Dra
in C
urr
ent| (A
)
Dra
in C
urr
ent
(A
)
0
5
10
15
20
25
(a)
Charge transport and trap states in PbS QD field-effect transistors
48
We compare the above-mentioned results with those from reference devices,
in which the PbS layers were deposited on the top of bare SiO2 (without HMDS
treatment) and where the clustering of QDs occurs. From the parameters
extracted from the device characteristics of the reference samples in Figure 3.6
(a), it can be deduced that the improvement of the QD assembly organization
leads to an increase of the carrier mobility of about a factor of 3 for electrons.
There is, however, no significant change in the hole accumulation. Two main
differences can be observed from the comparison of the two types of devices. In
the reference samples, the source-drain current in the n-channel saturation
regime shown in Figure 3.6 (b) decreases with the increase of Vds, which is not
observed in the case of the HMDS-treated samples. This decrease of the current
in the saturation regime can be attributed to the trapping of electrons.
(a)
(b)
-60 -40 -20 00.0
-0.5
-1.0
-1.5
0 20 40 600
40
80
120
Dra
in C
urr
ent (
A)
Dra
in C
urr
ent (
A)
Drain Voltage (V)
Vg=-80 V
0 V
0 V
Vg=80 V
-80 -40 0
0.00
0.05
0.10
0.15
10-11
10-10
10-9
10-8
10-7
10-6
10-5
Vds=5 VVds=-5 V
0 40 80
|Dra
in C
urr
ent| (A
)
Gate Voltage (V)
Dra
in C
urr
ent (
A)
0
5
10
15
20
Figure 3.6 (a) The transfer and (b) output characteristics of PbS QD-FETs with bare SiO2.
The nature of charge trapping in FET devices can also be estimated from the
subthreshold swing in the transfer characteristics of the devices. Figure 3.7 (a)
shows the comparison of the n-channel Ids-Vg transfer characteristics between the
HMDS-treated devices and the reference devices fabricated on bare SiO2.
Obviously, the devices with HMDS-treated SiO2 show a lower subthreshold swing
(SS = 2.71 V.dec-1) than the reference devices (SS = 6.77 V.dec-1). The lower
subthreshold swing corresponds to a reduced interface trap density due to the
passivation of silanol groups after HMDS treatment.
Chapter 3
49
Figure 3.7 Comparison of the transfer characteristics in the devices with bare and HMDS-
treated SiO2 in (a) semi-logarithmic and (b) linear scale.
At this point, it is also interesting to investigate the effect of HMDS surface
treatment on the doping behavior in the devices as indicated from the threshold
voltage values. By using the intercept linear extrapolation of source-drain current
to the gate voltage axis in the linear scale of transfer characteristics shown in
Figure 3.7 (b), we obtained an average Vth for bare SiO2 of 41.3 V and 12.6 V for
electrons and holes, respectively. The devices that were treated with HMDS SAMs
demonstrate an average threshold voltage of 27.2 V and -11 V for electrons and
holes, respectively. This indicates that the HMDS SAM treatment shifts the
electron threshold voltage as much as -14.1 V and the hole threshold voltage as
much as -23.6 V. The transistors that undergo the HMDS treatment show electron
doping, indicating less trapped electrons and the effect of the introduction of
interface dipoles. The influence of the attached HMDS molecules on the SiO2
surface dipole moment was then characterized using Gaussian program. After
HMDS treatment, we found that the dipole moment on the SiO2 surface was
-80 -40 00.00
0.05
0.10
0.15
0.20
0.25
0 40 80
SiO2
HMDS-SiO2
|Dra
in C
urr
ent| (A
) Vds=5 V
Dra
in C
urr
ent
( A
)
Vds=-5 V
Gate Voltage (V)
0
5
10
15
20
25
-20 0 20 40 60 80
10-11
10-10
10-9
10-8
10-7
10-6
10-5
Dra
in C
urr
en
t (A
)
Gate Voltage (V)
SiO2
HMDS-SiO2
Vds=5 V
(b)
(a)
Charge transport and trap states in PbS QD field-effect transistors
50
changed by 0.61-0.98 D. It has been reported that the change of the dipole
moment at the dielectric interface can influence the threshold voltage values.[14,15]
The threshold voltage shift can be estimated from,[16,17]
SAMs
doxdo
soxsth
AtC
tCCV
(3.1)
where Cs, is the capacitance of semiconductor, εo is the vacuum permittivity, and
µSAMs is the SAM dipole moment. The thickness ts of the QD layer is about 30 nm
and the diameter of PbS QDs is around 3.6 nm which corresponds to a molecular
area A of about 13 nm2. The dielectric coefficient of PbS QDs has been reported as
εs = 22.5, obtained from capacitance measurement.[18] In equation (3.1), the
dielectric constant εd and the SAM thickness td are 2.27 and 0.45 nm, respectively.
The threshold voltage shift was calculated to be -23.5 V, which is very close to the
experimental result of -23.6 V for holes. The threshold voltage shift for electrons
according to our experiment is -14.1 V, which shows a little deviation with respect
to the result of the calculation. This deviation may originate either from the
approximated model used in the equation (3.1) or the magnitude of
semiconductor capacitance Cs, which is influenced by traps thus overestimating
the calculated value. These results explain in addition by the improvement of the
QD assembly organization, the use of HMDS SAMs is able to reduce the threshold
voltage for electron accumulation.
3.3 Cytop Gating in PbS QD-FETs
After having studied the devices using bottom gate structure, the FETs using
Cytop as top gate dielectric were operated (Figure 3.1). Cytop is a
perfluoropolymer with chemical structure displayed in Figure 3.8 (a). The water
contact angle of 115o demonstrated in Figure 3.8 (b) reveals that Cytop has
hydrophobic properties.
Figure 3.8 (a) Chemical structure and (b) water contact angle of Cytop.
Chapter 3
51
-80 -40 00.00
0.05
0.10
0.15
10-10
10-9
10-8
10-7
10-6
10-5
0 40 80
|Dra
in C
urr
ent| (A
)
Dra
in C
urr
ent
( A
)Gate Voltage (V)
Vds=5 VVds=-5 V
0
5
10
15
-60 -40 -20 00.0
-0.2
-0.4
-0.6
-0.8
-1.0
0 20 40 60
Dra
in C
urr
ent
(A
)
Dra
in C
urr
ent
(A
)
Vg 80 V
0 V
Vg -80 V
0 V
Drain Voltage (V)
0
20
40
60
80
100
(b)
(a)
-8 -4 00.0
0.2
0.4
0.6
0.8
1.0
0 4 80.0
0.2
0.4
0.6
0.8
1.0
Vds=5 V
Conductivity (
10
-8 S
)
Conductivity (
10
-10 S
)
Carrier Density (1012
cm-2)
Vds=-5 V
(a)
(b)
(c)
Figure 3.9 (a) Ids-Vds output and (b) Ids-Vg transfer characteristics of PbS QD-FETs
employing Cytop gate dielectric. (c) Comparison of conductivity versus carrier density in
the devices with HMDS-treated SiO2 (red) and Cytop (blue).
Figure 3.9 (a) and (b) show the Ids-Vds and Ids-Vg characteristics of the FETs
with Cytop gating. The transistors display ambipolar characteristics dominated by
electron transport. In general, the I-V characteristics look very similar to the one
measured by operating the devices with SiO2 gate dielectric. This indicates that
the deposition of the fluoropolymer does not introduce any unwanted chemical
Charge transport and trap states in PbS QD field-effect transistors
52
reaction with the active layer. The devices show electron on/off ratio up to 105 and
a subthreshold swing of 6.4 V∙dec-1. The threshold voltages for electron and hole
accumulations are rather high, 37.2 V and -20.7 V, respectively. However, it is
important to mention that the Cytop gate dielectric is overwhelmingly thick (2
µm) and makes the capacitance rather small (0.86 nF/cm2). As a result, the field-
induced sheet carrier density was only n = 4×1011 cm-2 at Vg = 80 V. This value is
much smaller than the carrier density that could be induced with the SiO2 gating
(n = 8×1012 cm-2 at Vg = 80 V). This is the reason behind the smaller on/off ratio
and the higher threshold voltage in the Cytop gate operation.
Figure 3.9 (c) compares conductivity versus carrier density induced with
Cytop and HMDS-treated SiO2. This plot shows that the threshold carrier density
for electron accumulation using Cytop gate dielectric is much smaller than the
accumulation using HMDS-treated SiO2. The electron and hole mobility in the
linear regime of the transistor operation is as high as 0.2 cm2V-1s-1 and 8×10-4
cm2V-1s-1, respectively. Importantly, this high mobility is achieved in PbS QD
layers that still maintain the original band gap of the particles as confirmed in
Figure 3.5 and therefore the transistors have an on/off ratio higher than 105.
3.4 Distribution of Trap DOS in PbS QD-FETs
The significant mobility improvement is attributed to the lower density of
trap states at the PbS/Cytop interface. Despite the lower induced carrier density,
the Cytop gated FETs can maintain a conductivity as high as the SiO2-gated and
HMDS-treated SiO2-gated FETs as shown in Figure 3.9 (c). Figure 3.10 shows the
analyzed trap density of states (trap DOS) for the ambipolar transistors shown in
Figure 3.4 (b), Figure 3.6 (a) (with HMDS-treated and bare SiO2 gate) and Figure
3.9 (b) (with Cytop gating). The analysis uses a numerical model developed by
Oberhoff et al., which solves the coupled drift diffusion equations governing the
charge transport in semiconductors, incorporating an arbitrary distribution of
trap states in the band gap.[19] The analysis has been done for the n- as well as the
p-type transport. The range of validity of the trap DOS analysis (solid lines) has
been estimated from the position of the Fermi level at the lowest and highest
measured source-drain current. Therefore, the range of validity is narrower on the
HOMO (highest occupied molecular orbital) side, where the p-type charge
transport occurs. After treatment of the bare SiO2 surface with HMDS there is a
clear reduction of the trap DOS close to the LUMO (lowest unoccupied molecular
orbital), whereas on the HOMO side there is no reduction of the trap DOS visible,
a slight increase has even been calculated. This is in line with the measurement of
the mobility: for the n-type transport the mobility increases by more than a factor
of 3 (0.02 cm2V-1s-1 → 0.07 cm2V-1s-1), whereas for the p-type transport the
mobility is constant at 5×10-5 cm2V-1s-1. This leads to a conclusion that HMDS can
Chapter 3
53
reduce the trap DOS on the LUMO side only by passivating the SiO2 surface. The
calculated trap DOS for the bottom gate structures is comparable to results
obtained for polycrystalline organic thin-films.[20–22]
Figure 3.10 Comparison of trap DOS in PbS QD-FETs with SiO2, HMDS/SiO2, and Cytop
gate dielectrics. The use of Cytop leads to a drastically reduced density of trap states close
to both transport levels.
When the top gate configuration with Cytop gate dielectric was operated, a
drastic reduction of the trap DOS over the whole measured energy range by more
than one order of magnitude for both HOMO and LUMO sides was observed. This
is in line with the improvement of the p-type as well as n-type mobility by one
order of magnitude in this configuration. The Cytop top gate structure reduces the
density of trap states clearly below the level of organic polycrystalline thin-films,
therefore the trap density is closer to pentacene single crystals.[23] Quantitatively,
by integrating the trap DOS over the energy, the trap density of electrons and
holes in PbS QD ambipolar FETs with Cytop gating are 2.49×1017 cm-3 and
1.20×1018 cm-3, respectively. The values correspond to 3.96×1011 cm-2 and
1.13×1012 cm-2 sheet trap densities for electrons and holes, respectively. These
values are comparable to the free carrier density for electrons about 4×1011 cm-2.
These results demonstrate a high potential of solution processed PbS QDs when
they are combined with appropriate materials, which reduce the density of trap
states.
0.0 0.1 0.2 0.3
1017
1018
1019
1020
-0.4 -0.3 -0.2 -0.1 0.0
1017
1018
1019
1020
PbS on SiO 2PbS on SiO
2
PbS under Cytop
PbS on HMDS/SiO
2
Energy above HOMO (eV)
PbS on HMDS/SiO 2
PbS under CytopHO
MO Tr
ap D
OS
(eV
-1cm
-3)
Energy below LUMO (eV)
Trap
DO
S (e
V-1
cm-3
)
LUM
O
Charge transport and trap states in PbS QD field-effect transistors
54
Figure 3.11 The measured and simulated transfer curves for all investigated FETs.
The reliability of the calculated trap DOS results is shown by a good fitting
of the simulated transfer characteristics to the measured ones as displayed in
Figure 3.11. In addition, these results are compared with the trap DOS extraction
done using transient photovoltage and thermal admittance spectroscopy (TAS)
measurements on quantum dot solar cell structures.[24,25] The measurements done
on solar cells are sensitive to the trap DOS in the bulk of the semiconductor
whereas the FETs probe the trap DOS at the interface to the dielectric as well as
in the bulk. Energy wise, transient photovoltage measurements can only measure
trap states in the middle of the band gap, whereas the data from FETs cover an
energy range which is closer to the respective transport level. This means, the
reported trap DOS values for 3MPA-crosslinked PbS QDs cover the middle of the
band gap. On the other hand, the TAS measured trap DOS values for EDT-
crosslinked PbS QDs cover the trap states close to the LUMO level only.[24] In
contrast, these results show the complete picture of the trap DOS for both
transport levels. The comparison of the top gate configuration (Cytop) with the
reported bulk measurements reveals the trap DOS at 0.3-0.4 eV away from the
LUMO to be in the same order of magnitude. This leads to the conclusion that the
use of Cytop results in a trap DOS which is limited by the number of trap states in
the bulk only and therefore the number of trap states at the interface to the Cytop
dielectric is rather low. Thus, this very low density of trap states in the top gate
configuration using Cytop is the microscopic origin of the improved mobility for
n- as well as p-type charge carriers. Further reduction of the number of carrier
traps will allow the achievement of the mobility values close to the case where
almost all carrier traps are completely filled.[4]
-80 -40 0 40 8010
-11
10-10
10-9
10-8
10-7
10-6
10-5
Vds=5 V
Cytop HMDS SiO2
Dra
in C
urr
ent
(A)
Gate Voltage (V)
Vds=-5 V
Chapter 3
55
3.5 Conclusion
Ambipolar PbS QD-FETs with improved particle assembly organization were
fabricated. Through the utilization of HMDS SAMs, the interface trap density was
reduced and better QD arrays on the SiO2 surface were achieved which led to the
high on/off ratio for PbS QD-FETs. The use of hydroxyl-free Cytop as gate
dielectric allowed obtaining high electron mobility up to 0.2 cm2V-1s-1 in
ambipolar FETs of PbS QDs. The high carrier mobility is attributed to a very low
density of trap states at the interface, which is almost 2 orders of magnitude lower
than that with conventional oxide dielectrics. The high carrier mobility and the
high on/off ratio results show that the controlled assembly of PbS QDs as well as
the trap density reduction is crucial for the utilization of this material system for
diverse optoelectronic applications.
3.6 Methods
HMDS SAM treatment on SiO2 substrate. 230 nm SiO2/Si with pre-
patterned interdigitated Au electrodes were used as substrates. The channel
length and width of the devices were 20 µm and 10 mm, respectively. HMDS was
spin-coated on the substrates, the samples were then cleaned in acetone and
isopropanol before they were dried at 120oC for 10 min. Contact angles were
measured to investigate the coverage of the SAMs and the hydrophobicity of the
substrates. The morphology of the substrates and SAMs was measured using
atomic force microscopy (Seiko Instrument Inc.) in tapping mode.
TEM and HRTEM measurement. TEM and HRTEM images were
obtained using a JEOL 2011 FasTEM microscope operating at an accelerated
voltage of 200 kV. The QDs were deposited on the carbon side of a holey carbon-
copper TEM grid by drop-casting from a 2 mg/ml toluene solution.
Device fabrication. On the substrates that were modified by HMDS SAMs,
the PbS QD films were deposited and cross-linked with 3-mercaptopropionic acid
(3MPA) via a layer-by-layer (LbL) sequential spin coating technique, following
previously reported procedures.[4,5] The Cytop thin film has hydrophobic
properties as shown by its water contact angle of 115o in Figure 3.8 (b). Cytop is
utilized as top gate dielectric, since the QD films are difficult to be deposited on
very hydrophobic surfaces. Cytop (CT-809M) was spin-coated onto the fabricated
PbS QD devices. The samples were then annealed at 100oC for 1 h. Finally, 30 nm
Al was evaporated to fabricate the top gate electrode. All device fabrication
processes were performed in an N2-filled glove box.
Device measurements and trap DOS analysis. The transistor
electrical characterizations were performed using an electrical probe station
Charge transport and trap states in PbS QD field-effect transistors
56
(placed in an N2-filled glove box) that was connected to an Agilent B1500A
semiconductor parameter analyzer. To analyze the density of trap states (trap
DOS), we used a numerical simulation software which was developed to calculate
the trap DOS from the measured transfer curves.[19] The software is based on the
mobility edge model, which assumes the existence of a specific energy level
(mobility edge) separating the mobile from trapped states. This model has been
used to analyze a wide range of organic semiconductors.[20–22] Comparison of
various trap DOS extraction methods demonstrated that the numerical simulation
used here gives the most accurate results.[20,22]
Chapter 3
57
3.7 References
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2011, 6, 348.
[2] D. V Talapin, C. B. Murray, Science 2005, 310, 86.
[3] W. Koh, S. R. Saudari, A. T. Fafarman, C. R. Kagan, C. B. Murray, Nano Lett. 2011,
11, 4764.
[4] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss, M. A. Loi, Adv. Mater. 2013, 25, 4309.
[5] D. M. Balazs, M. I. Nugraha, S. Z. Bisri, M. Sytnyk, W. Heiss, M. A. Loi, Appl. Phys.
Lett. 2014, 104, 112104.
[6] S. J. Oh, N. E. Berry, J.-H. Choi, E. A. Gaulding, H. Lin, T. Paik, B. T. Diroll, S.
Muramoto, C. B. Murray, C. R. Kagan, Nano Lett. 2014, 14, 1559.
[7] C. M. Chuang, P. R. Brown, V. Bulović, M. G. Bawendi, Nat. Mater. 2014, 13, 796.
[8] S. J. Oh, N. E. Berry, J.-H. Choi, E. A. Gaulding, T. Paik, S.-H. Hong, C. B. Murray,
C. R. Kagan, ACS Nano 2013, 7, 2413.
[9] P. R. Brown, D. Kim, R. R. Lunt, N. Zhao, M. G. Bawendi, J. C. Grossman, V.
Bulovic, ACS Nano 2014, 8, 5863.
[10] C. Piliego, L. Protesescu, S. Z. Bisri, M. V. Kovalenko, M. A. Loi, Energy Environ.
Sci. 2013, 6, 3054.
[11] Y. Liu, J. Tolentino, M. Gibbs, R. Ihly, C. L. Perkins, Y. Liu, N. Crawford, J. C.
Hemminger, M. Law, Nano Lett. 2013, 13, 1578.
[12] S. Z. Bisri, E. Degoli, N. Spallanzani, G. Krishnan, B. J. Kooi, C. Ghica, M. Yarema,
W. Heiss, O. Pulci, S. Ossicini, M. A. Loi, Adv. Mater. 2014, 26, 5639.
[13] S. Z. Bisri, C. Piliego, J. Gao, M. A. Loi, Adv. Mater. 2014, 26, 1176.
[14] C. Celle, C. Suspène, M. Ternisien, S. Lenfant, D. Guérin, K. Smaali, K. Lmimouni,
J. P. Simonato, D. Vuillaume, Org. Electron. 2014, 15, 729.
[15] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H.
Shimotani, N. Yoshimoto, S. Ogawa, Y. Iwasa, Nat. Mater. 2004, 3, 317.
[16] W. Ou-Yang, X. Chen, M. Weis, T. Manaka, M. Iwamoto, Jpn. J. Appl. Phys. 2010,
49, 04DK04.
[17] D. M. Taylor, Adv. Colloid Interface Sci. 2000, 87, 183.
[18] M. J. Speirs, D. M. Balazs, H.-H. Fang, L.-H. Lai, L. Protesescu, M. V. Kovalenko,
M. A. Loi, J. Mater. Chem. A 2015, 3, 1450.
[19] D. Oberhoff, K. P. Pernstich, S. Member, D. J. Gundlach, B. Batlogg, IEEE Trans.
Electron. Devices 2007, 54, 17.
[20] W. L. Kalb, S. Haas, C. Krellner, T. Mathis, B. Batlogg, Phys. Rev. B 2010, 81,
155315.
[21] W. Xie, K. Willa, Y. Wu, R. Häusermann, K. Takimiya, B. Batlogg, C. D. Frisbie,
Adv. Mater. 2013, 25, 3478.
[22] W. L. Kalb, B. Batlogg, Phys. Rev. B 2010, 81, 35327.
[23] R. Häusermann, B. Batlogg, Appl. Phys. Lett. 2011, 99, 83303.
[24] D. Bozyigit, S. Volk, O. Yarema, V. Wood, Nano Lett. 2013, 13, 5284.
[25] A. H. Ip, S. M. Thon, S. Hoogland, O. Voznyy, D. Zhitomirsky, R. Debnath, L.
Levina, L. R. Rollny, G. H. Carey, A. Fischer, K. W. Kemp, I. J. Kramer, Z. Ning, A.
J. Labelle, K. W. Chou, A. Amassian, E. H. Sargent, Nat. Nanotechnol. 2012, 7, 577.
59
Chapter 4
Broadening of the Distribution of
Trap States in PbS Quantum Dot
Field-Effect Transistors with
High-k Dielectrics
This chapter presents a quantitative analysis of the trap density of states
(trap DOS) in PbS quantum dot field-effect transistors (QD-FETs), which utilize
several polymer gate insulators with a wide range of dielectric constants. With
increasing the dielectric constant of the gate insulators, the increase and the
broadening of the trap DOS are observed which are attributed to the dipolar
disorder and polaronic interactions appearing at strong dielectric polarization.
Despite the increase of these polaron-induced traps, no negative effect is
observed on the charge carrier mobility at the highest applied gate voltage. A
proposed model related to the large electron-polaron separation distance
explains the non-monotonic effect on the charge carrier mobility in high-k gated
devices.
M. I. Nugraha, R. Häusermann, S. Watanabe, H. Matsui, M. Sytnyk, W. Heiss,
J. Takeya, M. A. Loi, ACS Appl. Mater. Interfaces 2017, 9, 4719.
Charge transport and trap states in PbS QD field-effect transistors
60
4.1 Introduction
Reducing the operating voltage of FETs is a necessary step to use them for a
broader range of applications. With conventional SiO2 gate dielectric, the FET
devices suffer from very high operating voltage due to a low gate dielectric
capacitance, which is not compatible with practical applications. A lower
operating voltage can be achieved through the use of gate insulators with high
dielectric constant (high-k).[1–3] In organic semiconductor FETs, however,
energetic disorder and polaron relaxation are enhanced at the
semiconductor/insulator interface when utilizing high-k insulators (k>3).[4–6]
This disorder and polaronic-related interaction may modify the electronic
structure, such as the nature of trap states, at the semiconductor/insulator
interface and are responsible for the reduced charge carrier mobility in some
reported high-k gated organic FETs.[4] While the study on the localized (trap)
states with the use of high-k insulators has been intensively done in organic FETs,
only little information is available for FETs based on PbS QDs. Since the trap
states can strongly determine the performance of QD-FET devices and at the same
time dielectric insulators with higher-k are important in particular for low
operating voltage, understanding the nature of trap states in PbS QD-FETs with
increasing gate dielectric constant is crucial.
In this chapter, we present an analysis of the trap density of states (trap DOS)
in PbS QD-FETs employing several polymer gate dielectrics. These solution-
deposited polymer gate insulators display dielectric constants ranging from 2 to
41. Using these insulators, we first found that the number of deep traps extracted
from the subthreshold swing of the devices increases with increasing the dielectric
constant of the insulators. To understand this behavior, we quantified the
distribution of the trap DOS versus energy in the devices by simulating the device
working mechanism. In agreement to the subthreshold swing result, we observed
the increase and broadening of the trap DOS with increasing the dielectric
constant of the insulators. These results are rationalized in terms of an increased
disorder due to polaronic interaction at the semiconductor/insulator interface in
PbS QD-FETs with increased dielectric polarization strength.
4.2 Polarons at the semiconductor/insulator interface
When a gate voltage is applied on FET devices, charge carriers are
accumulated in the semiconductor at the interface with the insulator. The
microscopic nature of this interface has great influence on the accumulated charge
carriers as well as on their transport. Sirringhaus et al., reported that dipolar
disorder at the dielectric interface leads to the broadening of interface density of
states (DOS) in polymer FETs.[5,6] This dipolar disorder originates from Fröhlich
Chapter 4
61
polarons, which are caused by the interaction of the charges with induced dipole
moments and polarization of the dielectric, as demonstrated in Figure 4.1 (a).
With increasing the dielectric constant of gate insulators, strong polaronic
interaction can be responsible for the increased disorder and carrier activation
energy in FETs.[5,6] In agreement with this, Morpurgo et al., found that the polaron
binding energy increases with increasing the dielectric constant of gate
insulators.[4] These polaronic interaction and increased disorder also explain the
origin of the reduced charge carrier mobility in some reported high-k gated
organic semiconductor FETs.[4,6]
Figure 4.1 (a) Formation of polaron (black circle) due to the interaction between charges
and dielectric polarization at the semiconductor/insulator interface. (b) Configuration of
FETs with given chemical structures of polymer dielectrics.
To understand the nature of trap DOS in PbS QD-FETs with increased gate
dielectric constant, a series of polymeric insulators with dielectric constant
ranging from 2 till 41 were utilized. The polymer insulators were used in a bottom-
contact top-gate structure of FETs as shown in Figure 4.1 (b). In addition, the used
polymers are hydroxyl-group free, which allow excluding the effect of the interface
traps due to dangling bonds on the dielectric surface. The interface traps given by
these chemical groups have been known to influence the properties of the devices,
as mentioned in the previous chapters, which may affect the analysis of results.[7]
For this reason, we did not perform analysis on the charge accumulation using
bottom SiO2 gating. Furthermore, the bottom surface of the PbS films might have
CF2
CFCF
O O
CF2
CF2
n
H2C C
C
O
CH3
O
CH3
n
Cytop PMMA
F F
F F
F CF3
a b
PVDF-HFP
PVDF-trFE-CFE
C
F F
C
H H
C
F F
C
F Ha b
C
F Cl
C
F Fc
Al
PbSAu
SiO2
n-Si
(b)
(a)
e e e
e e e
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
Charge transport and trap states in PbS QD field-effect transistors
62
different properties with respect to the top surface, which may influence our
analysis.
4.3 PbS QD-FETs with polymer insulator gating
4.3.1 Characteristics of polymer insulators
As a first step to study the device properties, the characterization of the
polymer insulator films including the film thickness (d) and the capacitance (C)
was performed. The data of the film thickness and capacitance are presented in
Table 4.1. The film thickness was characterized by using a Dektak Profilometer
whereas the capacitance was measured by using Electrical Impedance
Spectroscopy (EIS). To measure the film capacitance, a metal insulator metal
(MIM) structure was used by sandwiching the polymer films with indium tin oxide
(ITO)-coated glass substrates and thermally-evaporated thin aluminum (Al) layer.
The dielectric constant (k) of the polymer insulator films was extracted using a
standard parallel plate capacitor model. In Table 4.1, the terpolymer (PVDF-trFE-
CFE) shows the highest dielectric constant among others with capacitance as high
as 0.126 µF/cm2, which enables to induce high carrier density in the devices.
Table 4.1 The characteristics of polymer insulator films.
Polymer Thickness
(nm)
Capacitance
(nF/cm2)
Dielectric
constant
Cytop 650 2.7 2
PMMA 468 6.5 2.6
PVDF-HFP 308 26 10.5
PVDF-trFE-CFE 257 126 40.5
4.3.2 Electrical properties of FETs
The Ids-Vg transfer characteristics in the linear and semi-logarithmic scale of
the devices gated with different polymer insulators are shown in Figure 4.2.
Generally, the devices show normally-off operation and good current modulation
in the n-channel operation with on/off ratio of 104-105. In the p-channel operation,
a small hole current is observed which indicates the ambipolarity of the devices.
However, because of the weak hole current, the analysis on this study is focused
on the n-channel operation of the devices. With increasing the dielectric constant
of the polymer insulators, the on-current of the devices significantly increases. In
addition, it is obvious that, with the use of high-k polymer insulators, the
Chapter 4
63
operation voltage of the devices is reduced. The operation voltage can be further
suppressed by optimizing the film thickness of the high-k insulators. Moreover,
the devices show also some hysteresis in the transfer characteristics as shown in
Figure 4.2. From the measurements, the devices employing high-k insulators
show less hysteresis than those with low-k insulators (Cytop and PMMA).
Figure 4.2 Ids-Vg transfer characteristics of the devices with (a) Cytop, (b) PMMA, (c)
PVDF-HFP, and (d) PVDF-trFE-CFE polymer gate dielectrics.
Table 4.2 Mobility and trap density in the devices with several gate insulators.
Polymer Mobility (cm2V-1s-1) Trap density
(1012 cm-2eV-1) similar n high Vg
Cytop 0.12 0.12 1.3
PMMA 0.11 0.11 5.3
PVDF-HFP 0.09 0.14 9.2
PVDF-trFE-CFE 0.05 0.15 17
The average electron linear mobility in the devices using Cytop is 0.12 cm2V-
1s-1 at the highest applied gate voltage as given in Table 4.2. With this low-k
-20 0 20 40 6010
-10
10-9
10-8
10-7
10-6
10-5
Dra
in C
urr
ent
(A)
Cytop
Gate Voltage (V)
Dra
in C
urr
ent
(A
)
Vds=5 V
0
5
10
15
20
-20 0 20 40 60 8010
-9
10-8
10-7
10-6
10-5
10-4
Dra
in C
urr
ent
(A)
PMMA
Gate Voltage (V)
Dra
in C
urr
ent
( A
)
Vds=5 V
0
10
20
30
40
50
-10 0 10 20 3010
-9
10-8
10-7
10-6
10-5
10-4
0
20
40
60
Dra
in C
urr
ent
(A)
PVDF-HFP
Gate Voltage (V)
Dra
in C
urr
ent
(A
)
Vds=2 V
0 4 8 12 1610
-10
10-9
10-8
10-7
10-6
10-5
10-4
0
40
80
120
Dra
in C
urr
ent
(A)
PVDF-trfE-CFE
Gate Voltage (V)D
rain
Curr
ent
( A
)
Vds=2 V
(d)(c)
(b)(a)
Charge transport and trap states in PbS QD field-effect transistors
64
insulator, we are only able to accumulate charge carrier concentration up to 4.7 x
1011 cm-2. By using high-k PVDF-trFE-CFE insulator, the carrier concentration can
be improved by one order of magnitude (4.6 x 1012 cm-2) at relatively low gate
voltage (15 V). In PbS QDs, carrier traps mainly come from dangling bonds on the
QD surfaces, which have strong influence on the charge carrier transport.[8–12] In
contrast to organic semiconductor FETs, the increase of carrier density in PbS
QD-FETs is expected to have great impact on charge carrier mobility particularly
due to the filling of the surface traps.[13–15] However, no significant change in the
charge carrier mobility is observed with this increased carrier density, as given in
Table 4.2. Using PVDF-trFE-CFE insulator, the average mobility is estimated to
be 0.15 cm2V-1s-1 at maximum applied gate voltage, which is only a few percent
higher than that measured with Cytop gating. Furthermore, a reduction in the
carrier mobility with the high-k dielectrics is even observed when comparing the
mobility values at similar carrier density (~4.2 x 1011 cm-2). At this point, different
phenomena are occurring when high-k dielectrics are used. The increase of
interface disorder in the devices with high-k dielectric gating, which suppresses
carrier transport, may be evoked.
4.4 Electronic structures of traps with high-k
4.4.1 Density of traps in subthreshold regime
To further investigate the reduced charge carrier mobility when comparing
the devices at similar carrier density, the number of traps (Ntraps) in the devices is
estimated using the subthreshold swing formula as given in equation 1.14. From
our measurements in Figure 4.2, it is obvious that the subthreshold swing
decreases with increasing the gate dielectric constant as expected from,[16,17]
i
trapsB
C
Ne
e
TkSS
2
110ln
(4.1)
where kB, T, and e are Boltzmann constant, temperature, and elementary charge
constant, respectively. However, if the subthreshold swing is analyzed in more
detail, the actual values do not decrease as fast as we would expect when
considering the effect only of the dielectric, meaning that an additional effect
comes into play at higher dielectric permittivity. The only unknown parameter in
equation (4.1) is the number of traps, which might increase due to an increase of
the dielectric permittivity. Figure 4.3 presents the number of electron traps
(Ntraps) extracted from subthreshold regime using equation 1.14. It is obvious that
the number of electron traps in the devices increases with increasing the dielectric
constant of gate insulators. In the devices with Cytop gating, the number of
electron traps is as low as 1.28 x 1012 cm-2eV-1, which is comparable to a previous
Chapter 4
65
report.[7] At increased gate dielectric constant with the use of PVDF-trFE-CFE
terpolymer, a significant increase in the electron traps by one order of magnitude
(1.82 x 1013 cm-2eV-1) is observed. These increased carrier traps can explain the
reduction in the mobility when comparing the mobility values at similar carrier
density. In addition, this result can also be responsible for the mobility
characteristics at maximum applied gate voltages with the high-k dielectrics.
Furthermore, the increase of carrier traps and the mobility characteristics in the
devices with the high-k dielectrics are an indication of the existence of polaronic-
related interaction at the semiconductor/insulator interface, as observed in
organic semiconductor FETs.[4–6]
Figure 4.3 The number of electron traps in the devices as a function of dielectric constant
extracted from subthreshold regime. The inset displays the number of traps in semi-
logarithmic scale.
4.4.2 Distribution of trap DOS
To have further understanding on the nature of the trap DOS in the devices
with increasing the dielectric constant of polymers, we performed a computer
simulation on the Ids-Vg transfer characteristics of the fabricated devices. The
simulation is performed by solving the drift-diffusion and Poisson equation on the
Ids-Vg transfer characteristics of the devices following a numerical model
developed by Oberhoff et al.[18] This simulator is based on general principles and
can be applied to study a wide range of semiconducting materials. Although the
subthreshold swing equation (4.1) can estimate the number of carrier traps in the
devices, it can only quantify the traps at a certain energy level in the subthreshold
0 10 20 30 40
0
6
12
18
24
Ntr
aps (
10
12 c
m-2eV
-1)
Dielectric Constant
0 20 4010
12
1013
Ntr
aps (
cm
-2eV
-1)
Charge transport and trap states in PbS QD field-effect transistors
66
regime, whereas the simulation can analyze the number of traps in a broad energy
range.[19–22] Furthermore, this simulation can explain the origin of a significant
improvement of the charge carrier mobility in PbS QD-FETs with hydroxyl-free
dielectric gating as previously mentioned in chapter 3.[7]
(a)
(b)
-0.4 -0.3 -0.2 -0.1 0.0
1018
1019
1020
1021
CFE (=41)
PVDF-HFP (=11)PMMA (=2.6)
Tra
p D
OS
(cm
-3eV
-1)
Energy below LUMO (eV)
Cytop (=2)
Figure 4.4 (a) Distribution of trap density of states (trap DOS) close to LUMO in the devices
with different dielectric constants of gate insulators. (b) Schematic of broadening of tail
trap states with increasing gate dielectric constant where the transport level corresponds
to the LUMO of QDs.
Figure 4.4 (a) shows the analyzed density of trap states close to the LUMO
level in the devices, fabricated with different polymer gate dielectrics. In
agreement to the result in Figure 4.3, an increase of the carrier traps over a wide
energy range close to the LUMO energy (electron traps) is also observed with
increasing the dielectric constant of gate insulator. The analyzed density of trap
Chapter 4
67
states is about 0.4 eV below the transport level (LUMO) of the charge carriers. As
PbS QD-FET devices show slight ambipolar characteristics, the analysis of the
traps is limited to this energy level to minimize the error due to the minority
charge carriers (holes), which may exist close to the off-state of the devices. In
addition to the increase of the trap DOS, the broadening of the trap DOS with
increasing the gate dielectric constant is also observed as displayed in Figure 4.4
(a). The schematic of the broadening of the trap DOS with high-k dielectrics is
shown in Figure 4.4 (b). The origin of the increase and of the broadening is most
likely a consequence of the polaronic interaction at the semiconductor/dielectric
interface which modifies the already-present disorder. This analysis is supported
by the fact that the polaron relaxation has great impact on the density of states in
polymer semiconductor FETs employing gate insulator with dielectric constant
(k) > 3, as confirmed by charge-modulation spectroscopy (CMS).[5,6] These results
confirm that the polaronic interaction at the semiconductor/insulator interface
also has important effects on the behavior of PbS QD-FETs.
Finally, as we analyze the extracted mobility and the calculated trap DOS,
the increase of charge trapping is expected to greatly suppress carrier transport,
thus charge carrier mobility, in the devices. In line with this, the mobility
decreases with increasing the dielectric constant of the gate insulators when we
compare the mobility values at similar carrier density as displayed in Table 4.2.
Meanwhile, at respective maximum applied gate voltages with different dielectrics,
the mobility slightly increases with increasing the gate dielectric constant (see
Table 4.2). In contrast to these results, the mobility in organic semiconductor
FETs monotonically decreases with increasing the dielectric constant of gate
insulators at all range of the applied gate voltages.[4] To understand these results,
we propose that the interplay between charge trapping and trap filling process
might take place in the devices. With increasing the dielectric constant of gate
insulators, the increased carrier density is expected to fill the traps in the devices.
Meanwhile, because the polaron-induced traps are increased at the same time,
many gate-induced charge carriers are used to fill the trap states. Therefore, the
effective free carriers with the high-k dielectrics will be only slightly higher than
in the devices with Cytop, leading to a small improvement of the charge carrier
mobility. Another possible explanation for the less affected charge carrier mobility
than in organic transistors at high gate voltage is a large separation between
polaron and the accumulated charge carriers in PbS QDs. In PbS QD-FETs, the
top surface of the dielectrics is separated by the crosslinking EDT ligands which
have length about 0.4 nm, while the radius of QDs is around 1.8 nm. As the
accumulation of charge carriers in PbS QDs is assumed to be in the center of mass
of the QDs, therefore, the total separation distance between polaron and the
accumulated charge carriers is around 2.2 nm. Moreover, some residual oleic acid
ligands on the bottom side of QDs may also exist which can increase the
Charge transport and trap states in PbS QD field-effect transistors
68
separation distance. This larger polaron separation distance than in the case of
polymer semiconductors (~0.3 nm) may minimize the effect of polaron on charge
carrier mobility in our devices.[5,6] However, despite the large polaron separation
distance, the trap states are still greatly influenced by the polaronic interaction.
Nevertheless, the charge carrier mobility at all range of the applied gate voltages
is expected to be greatly affected by the polaronic interaction at low temperature.
4.5 Conclusion
We have performed a study on the analysis of the trap DOS in PbS QD-FETs
employing several high-k polymer gate insulators. With increasing the gate
dielectric constant, we observed the increase of the electron traps in the devices
as extracted from subthreshold regime. By using a computer simulation, we
further analyzed the detailed distribution of trap DOS in the devices close to the
LUMO level of PbS QDs. We found a general broadening of the trap DOS which is
attributed to the increased disorder due to polaronic-related interactions at the
semiconductor/dielectric interface. While the increased trap states effectively
influence the mobility compared at similar carrier density, the extracted mobility
at maximum applied gate voltages was not significantly affected by the polaronic
interaction which is likely due to the interplay between trap filling and increased
charge trapping as well as the large separation distance between polarons and free
carriers in PbS QDs. Our results clearly show that by careful choice of dielectrics,
PbS QD FETs with low operating voltage can be built without reducing the charge
carrier mobility, which is a fundamental insight to further utilize colloidal QD
systems in optoelectronic applications.
4.6 Methods
Device fabrication. The deposition of PbS semiconducting thin films was
performed by spin-coating 10 mg/mL of oleic acid-capped PbS solution in
chloroform on SiO2/Si substrates. To improve the film conductivity, we exchanged
the long oleic acid ligands with shorter molecules, namely 1,2 ethanedithiol (EDT).
The concentration of the EDT solution was 1%v/v with acetonitrile as a solvent.
The deposition of the PbS thin films, as well as, the ligand exchange, was
performed using a layer-by-layer (LbL) spin-coating procedure. After each ligand
exchange, pure acetonitrile was dropped on the films to remove unbound EDT
and native oleic acid ligands. The PbS layer deposition and the ligand exchange
were repeated for 5 times until desired thickness was reached. After the deposition,
the devices were annealed at 120oC for 20 min to remove residual solvent and to
promote coupling between QDs, thus improving the conductivity without
sintering the QDs. As gate dielectrics, we used four different polymer insulators
Chapter 4
69
namely Cytop, polymethylmethacrylate (PMMA), polyvinylidene fluoride
hexafluoro-propylene (PVDF-HFP), and polyvinylidene fluoride-
trifluoroethylene-chlorofluoroethylene (PVDF-trFE-CFE). In this study, pure
Cytop (CT-809M) was used without further dilution. PMMA was dissolved in
ethyl acetate with concentration of 80 mg/mL. To prepare PVDF-HFP solutions,
we dissolved the polymer in dimethylformamide (80 mg/mL). The last dielectric,
PVDF-trFE-CFE, was dissolved in cyclohexanone to form a solution with
concentration of 60 mg/mL. All these polymers, were used to vary the dielectric
constant of the gate insulators in the devices in the range between 2 and 41. The
deposition of the polymer insulators was done by using spin-coating on the
previously-deposited active layer of PbS QDs. The devices were then annealed at
95oC for 1 h to remove residual solvent. Finally, a thin layer of aluminum (50 nm)
was evaporated as top gate electrode. All device fabrication was performed in an
N2-filled glove box.
FET electrical characteristic measurements. The electrical
characteristics of FETs were measured using an electrical probe station (placed in
an N2-filled glove box) that is connected to an Agilent B1500A semiconductor
parameter analyzer.
Charge transport and trap states in PbS QD field-effect transistors
70
4.7 References
[1] J.-H. Choi, H. Wang, S. J. Oh, T. Paik, P. Sung, J. Sung, X. Ye, T. Zhao, B. T. Diroll,
C. B. Murray, C. R. Kagan, Science. 2016, 352, 205.
[2] C.-Y. Wang, C. Fuentes-Hernandez, J.-C. Liu, A. Dindar, S. Choi, J. P. Youngblood,
R. J. Moon, B. Kippelen, ACS Appl. Mater. Interfaces 2015, 7, 4804.
[3] J. Zaumseil, H. Sirringhaus, Chem. Rev. 2007, 107, 1296.
[4] I. N. Hulea, S. Fratini, H. Xie, C. L. Mulder, N. N. Iossad, G. Rastelli, S. Ciuchi, A. F.
Morpurgo, Nat. Mater. 2006, 5, 982.
[5] N. Zhao, Y.-Y. Noh, J.-F. Chang, M. Heeney, I. McCulloch, H. Sirringhaus, Adv.
Mater. 2009, 21, 3759.
[6] H. Sirringhaus, M. Bird, N. Zhao, Adv. Mater. 2010, 22, 3893.
[7] M. I. Nugraha, R. Häusermann, S. Z. Bisri, H. Matsui, M. Sytnyk, W. Heiss, J.
Takeya, M. A. Loi, Adv. Mater. 2015, 27, 2107.
[8] D. V Talapin, C. B. Murray, Science 2005, 310, 86.
[9] L.-H. Lai, L. Protesescu, M. V Kovalenko, M. A. Loi, Phys. Chem. Chem. Phys.
2014, 16, 736.
[10] S. M. Thon, A. H. Ip, O. Voznyy, L. Levina, K. W. Kemp, G. H. Carey, S. Masala, E.
H. Sargent, ACS Nano 2013, 7, 7680.
[11] M. V Kovalenko, M. Scheele, D. V Talapin, Science 2009, 324, 1417.
[12] D. Zhitomirsky, M. Furukawa, J. Tang, P. Stadler, S. Hoogland, O. Voznyy, H. Liu,
E. H. Sargent, Adv. Mater. 2012, 24, 6181.
[13] S. Z. Bisri, C. Piliego, J. Gao, M. A. Loi, Adv. Mater. 2014, 26, 1176.
[14] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss, M. A. Loi, Adv. Mater. 2013, 25, 4309.
[15] S. Z. Bisri, E. Degoli, N. Spallanzani, G. Krishnan, B. J. Kooi, C. Ghica, M. Yarema,
W. Heiss, O. Pulci, S. Ossicini, M. A. Loi, Adv. Mater. 2014, 26, 5639.
[16] B. Blülle, R. Häusermann, B. Batlogg, Phys. Rev. Appl. 2014, 1, 1.
[17] M. I. Nugraha, H. Matsui, S. Z. Bisri, M. Sytnyk, W. Heiss, M. A. Loi, J. Takeya, APL
Mater. 2016, 4, 116105.
[18] D. Oberhoff, K. P. Pernstich, S. Member, D. J. Gundlach, B. Batlogg, IEEE Trans.
Electron. Devices 2007, 54, 17.
[19] W. L. Kalb, B. Batlogg, Phys. Rev. B 2010, 81, 35327.
[20] K. Willa, R. Häusermann, T. Mathis, A. Facchetti, Z. Chen, B. Batlogg, J. Appl. Phys.
2013, 113, 133707.
[21] W. L. Kalb, S. Haas, C. Krellner, T. Mathis, B. Batlogg, Phys. Rev. B 2010, 81,
155315.
[22] R. Häusermann, B. Batlogg, Appl. Phys. Lett. 2011, 99, 83303.
71
Chapter 5
Enabling Ambipolar to Heavy n-
type Transport in PbS Quantum
Dot Solids through Doping with
Organic Molecules
This chapter discusses doping of PbS quantum dot (QD) films using benzyl
viologen (BV) donor molecules. By engineering the energy level of the dopant
and the PbS QDs through the use of particular capping ligands, the electrical
characteristics of PbS QD field-effect transistors (FETs) can be tuned from
ambipolar to strongly n-type. The doping significantly improves the charge
carrier mobility by one order of magnitude to 0.64 cm2V-1s-1 as revealed from
four terminal conductivity measurements. In addition, the doping also reduces
the contact resistance of the devices and assists the filling of carrier traps, which
can explain the origin of the increased mobility.
M. I. Nugraha, S. Kumagai, S. Watanabe, M. Sytnyk, W. Heiss, M. A, Loi,
J. Takeya, ACS. Appl. Mater. Interfaces 2017, Article ASAP.
Charge transport and trap states in PbS QD field-effect transistors
72
5.1 Introduction
PbS colloidal quantum dots (QDs) have been shown to be promising as
semiconducting building blocks for optoelectronic devices, such as solar cells,[1–6]
photodetectors,[7–10] and light-emitting devices.[11–13] This class of materials gives
the possibility to fabricate electronic devices using solution processable methods
such as blade-coating, dip-coating, printing, and roll-to-roll processes.[14–19]
Recently, many efforts have been devoted to exploit PbS QDs as active material
for field-effect transistors (FETs).[16,17,20,21] The fabrication of FETs offers the
possibility to integrate them in more advanced electronic devices such as
complementary metal oxide semiconductor (CMOS) devices - like inverters,
integrated logic circuits, radio frequency identification (RFID) systems, etc.[22,23]
Their use in FETs, however, is still challenging because they suffer from low
charge carrier mobility due to the high number of carrier traps on their surface.
Therefore, improving charge carrier mobility in FETs based on PbS QDs is crucial
to enhance their potential for diverse applications.
Doping is an effective tool to improve charge carrier mobility in
semiconductors.[23–26] Although PbS QDs are n-type on the basis of their
stoichiometry,[4,21,27] many published studies show strategies to turn them into p-
type semiconductors.[28,29] The p-type doping is mainly achieved by exposing
samples to air, which gives rise to a significant increase of the hole mobility and
density in the films.[28,29] A better control was achieved by evaporation of sulphur
and selenium, or acting on the surface of the QDs with specific ligands.[4,21,30] On
the other hand, studies on heavy n-type doping of PbS QD films are still limited.
The energy offset often built at the interface of semiconductor and dopant is
responsible for inefficient electron transfer from the HOMO of the dopant to the
LUMO of the semiconductor.[31] Recently, benzyl viologen (BV) has been reported
as a promising n-type dopant for the carbon nanotubes and MoS2 systems.[25,32]
Due to its shallow HOMO level, BV molecule treatment induces carrier doping in
samples which favours electron transfer to the semiconducting films. To date, the
use of BV as n-type dopant in PbS QD films has not been investigated yet. This
leaves a question on the possibility to use BV for obtaining strong n-type doping
of the PbS films and improving the performance of FET devices based on these
QDs.
In this chapter, we report a strategy for doping PbS quantum dot solids using
BV as the donor molecules. By combining the use of BV with engineering of the
quantum dot energy levels through the use of several capping ligands, we are able
to effectively tune the electrical properties of PbS QD-FETs from ambipolar to
heavily n-type. With this BV treatment, we improved the electron mobility by one
order of magnitude and we reduced the contact resistance down to 0.77 kΩcm.
Furthermore, the four-terminal (4T) conductivity transistor measurements
Chapter 5
73
confirmed the efficient doping of PbS QD films after BV treatment leading to
electron mobility as high as 0.64 cm2V-1s-1, the highest value reported for the low-
temperature processed PbS QD-FETs employing SiO2 as a gate dielectric.
5.2 Doping strategy using Benzyl Viologen (BV)
The successful doping of a semiconductor device is determined by the
effective charge transfer from the dopant molecules to the semiconducting films
or vice versa. Several methods have been used to introduce efficient doping of QD
films including mixing a solution of dopants and QDs prior to film deposition,
infiltrating a semi-metallic material into the active channels, and dipping the
deposited semiconducting films into the dopant solution.[22,31,33,34] Among others,
the dipping method has been shown to enable highly efficient optoelectronic
devices fabricated with heavily-doped PbS QD films.[34] Furthermore, this method
has also been used on a broad range of semiconductors leading to significant
increase of the film conductivity in the fabricated transistors.[25,32,35–36]
To heavily dope the PbS QD films, we treated the semiconducting films with
BV donor molecules, which have a redox potential of -0.79 V (BV0/BV+) relative
to standard hydrogen electrode (SHE).[25,32] This redox potential corresponds to a
HOMO level of -3.65 eV with respect to the vacuum level. Kiriya et al. reported
that the BV molecules have a second redox potential after releasing an electron,
which results in the BV+ state.[25] In this state, the redox potential (BV+/BV2+)
turns to -0.33 V relative to SHE, which corresponds to a HOMO level of -4.11 eV
with respect to the vacuum level. The chemical structure of BV is shown in Figure
5.1 (a). In this study, doping of PbS films is done by dipping the deposited films
into the BV solution for a specific time. The configuration of the BV-treated PbS
QD-FETs is displayed in Figure 5.1 (b). To promote the transport of electrons from
the dopant to the semiconductor, the LUMO level of the PbS films should be
deeper than the HOMO level of the dopant. Recently, it has been reported that the
HOMO/LUMO level of PbS QDs is strongly influenced by the cross-linking
ligands.[37] Therefore, by varying the ligands, we are able to potentially tune the
doping strength of the PbS QD films. Here, we use three kinds of ligands, namely
3-mercaptopropionic acid (3MPA), tetrabuthylammonium iodide (TBAI), and
methylammonium iodide (MAI). The chemical structures of the ligands are shown
in Figure 5.1 (a). These ligands have been reported to enable good performing FET
devices based on PbS QDs.[12,14,28] By crosslinking PbS QDs with 3MPA ligands,
the LUMO level of the materials is estimated to be around –3.6 and –3.7 eV, which
are quite close to the HOMO level of BV.[37] With this energy level offset, some
electrons are inefficiently transferred from BV to PbS films as shown in the
schematics of Figure 5.1 (c). After releasing an electron, the BV0 turns to BV+ state
which has HOMO of -4.11 eV. This HOMO value is significantly deeper than the
Charge transport and trap states in PbS QD field-effect transistors
74
LUMO of 3MPA-capped PbS films, which blocks electron transfer from BV+ to
PbS, leading to an inefficient n-type doping of the PbS films after the BV layer
deposition. This unfavourable energy offset could also explain the previously-
reported inefficient n-type doping of lead chalcogenide films with cobaltocene.[31]
With cobaltocene doping, the transport of charge carriers turns to n-type from
ambipolar with a reduced current.
Other types of ligands, such as TBAI and MAI have been reported to result
in a deep LUMO level for PbS QDs. The reported LUMO level using this class of
ligands is as deep as –4.4 eV.[37] This LUMO level is importantly deeper than both
the HOMO levels of BV0 and of BV+. Therefore, it opens the possibility to
successfully obtain n-type doping in PbS QD films.
(b)
(c)
(a)
O
OHHS N+H3C
H
H
HI-
BV
3MPA
TBAI
MAI
N+
H3C CH3
I-
H3C CH3
-3.6 eV
HOMO
PbS(3MPA)
BV
e-X
-4.4 eV
PbS(TBAI,MAI)
BV
e--3.65 eV (BV0/BV+)
-4.11 eV (BV+/BV2+)e-X e-
HOMO
Figure 5.1 (a) Chemical structure of the BV molecules and the capping ligands used in this
study; (b) device structure, and (c) schematic of the electron transfer mechanism from BV
to PbS QDs with different capping ligands. After transferring an electron, BV0 (black) turns
to BV+ state (orange) with deeper HOMO level.
5.3 Electrical characteristics of BV-doped PbS QD-FETs
The transfer characteristics of the pristine devices (before BV doping
treatment) with different capping ligands are shown in Figure 5.2 (a)–(c).
Obviously, the devices show ambipolar properties with more electron-dominated
transport using all the three ligands. A pronounced difference is observed in the
threshold voltages (Vth) of the devices. The devices with iodide ligands clearly
show lower Vth than those with 3MPA ligands, which indicates a good passivation
against carrier traps on the QD surfaces. The lower number of carrier traps in the
PbS QD films with iodide ligands than those with 3MPA is also demonstrated by
steeper transfer characteristics of the devices in the semi logarithmic scale as
Chapter 5
75
displayed in Figure 5.2 (a)–(c). Meanwhile, in the p-channel characteristics, we
also observe a slight hole current. However, in this study, we limit our analysis to
the n-type transport because the hole current is much weaker than the electron
current.
d
0 40 8010
-10
10-9
10-8
10-7
10-6
10-5
Dra
in C
urr
ent
( A
)
Gate Voltage (V)
Dra
in C
urr
ent
(A)
Vds
=5 V3MPA undoped
0
1
2
3
4a
0 40 8010
-11
10-10
10-9
10-8
10-7
10-6
10-5
TBAI undoped
Dra
in C
urr
ent
(A
)
Gate Voltage (V)D
rain
Curr
ent
(A)
Vds
=5 V
0
2
4
6
8
10(a) (b)
f
0 40 8010
-10
10-9
10-8
10-7
10-6
10-5
10-4
0
5
10
15
20
MAI undoped
Dra
in C
urr
ent
( A
)
Vds
=5 V
Gate Voltage (V)
Dra
in C
urr
ent
(A)
c(c)
Figure 5.2 The transfer characteristics of the pristine devices with (a) 3MPA, (b) TBAI, and
(c) MAI ligands.
TABLE 5.1 Electrical properties of PbS FETs with different capping ligands before and after
BV treatment.
Ligands 2T Mobility (cm2V-1s-1) Vth (V) Doping
concentration
(cm-2) Pristine BV-treated Pristine BV-treated
3MPA 2.6 x 10-3 5.1 x 10-3 36 18.8 1.6 x 1012
TBAI 5.3 x 10-3 1.4 x 10-2 16.5 -17.1 3.2 x 1012
MAI 0.03 0.32 12.2 -35 4.4 x 1012
The extracted electron linear mobility, μ, in the pristine devices with
different capping ligands is shown in Table 5.1. Obviously, the devices with MAI
ligands show the highest μ among the other capping ligands, which is comparable
to a previous report.[14] The lower μ in the devices with TBAI respect to the MAI
Charge transport and trap states in PbS QD field-effect transistors
76
ones is attributed to the presence of remaining oleic acid ligands on the PbS QD
surface which suppresses charge transport within the QD films. The remaining
oleic acid is a result of the poor reactivity of TBAI in the ligand exchange process,
due to non-acidity of the cation of TBAI ligands.[14]
-80 -40 0 40 80
10-6
10-5
10-4
10-3
0.0
0.2
0.4
0.6
MAI doped
Dra
in C
urr
ent
(mA
)
Vds
=5 V
Gate Voltage (V)
Dra
in C
urr
ent
(A)
f
-80 -40 0 40 8010
-9
10-8
10-7
10-6
10-5
10-4
TBAI doped
Dra
in C
urr
ent
(A
)
Gate Voltage (V)D
rain
Curr
ent
(A)
Vds
=5 V
0
20
40
60(b)
-80 -40 0 40 80
10-10
10-9
10-8
10-7
10-6
10-5
0
2
4
6
8
3MPA doped
Dra
in C
urr
ent
(A
)
Vds
=5 V
Gate Voltage (V)
Dra
in C
urr
ent
(A)
d
(c)
(a)
Figure 5.3 The transfer characteristics of the devices after BV doping treatment with (a)
3MPA, (b) TBAI, and (c) MAI ligands.
The transfer characteristics of the devices after BV treatment are shown in
Figure 5.3 (a)–(c). In the devices with 3MPA ligands, we observe that the devices
still show ambipolar characteristics with a slight increase of the source–drain
current (Ids) even after BV doping treatment, as shown in Figure 5.3 (a). The
unaffected ambipolar properties are explained by our previous hypothesis that an
unfavourable offset between the HOMO level of BV and the LUMO level of 3MPA-
crosslinked PbS QDs leads to a blockade of the electron transfer from the BV layer
to the PbS films. Nevertheless, some weak electron transfer from the BV0 HOMO
to the PbS LUMO may result in a shifting of the transfer characteristics to a
negative direction, indicating the presence of a certain level of n-type doping.
Instead, with TBAI and MAI ligands that enable the shift of the LUMO level of
PbS QDs, we observe that the device characteristics turn to heavily n-type from
ambipolar after BV doping treatment, as displayed in Figure 5.3 (b)–(c). In
addition, the devices show a normally “on” operation as indicated by the transfer
Chapter 5
77
characteristics in linear scale, which demonstrates the strong electron doping of
the active material.
Figure 5.4 Comparison of on-current of the devices before and after BV doping treatment.
By analysing the transfer characteristics further, the devices with MAI
ligands show the highest on-current after BV treatment among the other ligands
as given in Figure 5.4. The on-current in the MAI-capped PbS FET devices is
improved by more than one order of magnitude, whereas an improvement by only
a factor of six is observed in the 3MPA-crosslinked devices after BV treatment.
The extracted linear μ in all of the fabricated devices with different ligands before
and after BV treatment is shown in Table 5.1. In the devices capped with 3MPA
ligands, the μ is improved only by a factor of two. Importantly, a significant
improvement in the μ (the maximum μ is 0.45 cm2V-1s-1; the average μ is 0.32
cm2V-1s-1) is observed in the devices with MAI ligands after treatment with BV,
indicating a more efficient doping with respect to the case of PbS decorated by the
other ligands. The devices with TBAI, which are suggested to have a doping
mechanism similar to that of MAI, have a lower increase of the μ than that of the
devices with MAI. This result can be associated to the more effective removal of
oleic acid ligands with MAI than that with TBAI due to the acidity of the
methylammonium cation.[15] In addition, among the other ligands, we observe
that the BV doping treatment reduces the bias stress in devices with MAI ligands.
This result is supported by the smaller hysteresis observed in devices with MAI
after BV treatment with respect to the ones treated with the other ligands, as
10-6
10-5
10-4
10-3
MAITBAI
undopeddoped
on-c
urr
ent
(A)
Ligands3MPA
Charge transport and trap states in PbS QD field-effect transistors
78
shown in Figure 5.3 (c). Therefore, the proper choice of ligands combined with the
use of BV results in n-type doping and more stable PbS QD transistors.
Figure 5.5 Threshold voltages of the devices before and after BV doping treatment.
We are then interested in estimating the doping concentration in devices
after BV treatment. The doping concentration in FETs can be calculated by
measuring the difference in Vth before and after doping (ndoping=CiΔVth/e) using
equation (1.6). The Vth values of the devices with different capping ligands are
given in Table 5.1. All devices show n-type doping, indicated by the shifting of Vth
into the negative direction as is displayed in Figure 5.5. The doping concentration
in all fabricated devices is shown in Table 5.1. The highest doping concentration
is achieved in the devices with MAI ligands after BV treatment and is estimated to
be about 4.4 1012 cm–2, which is comparable to the accumulated charge carriers
induced by the gate voltage (4.3 1012 cm–2 at Vg = 60 V). This high doping
concentration can be the origin of the significant increase of the electron mobility
in the devices with MAI, as it allows a more efficient filling of carrier traps. It is
also worth noting that the doping concentration in the TBAI-capped devices is
quite close to the one obtained in the MAI-capped devices. However, the degree
of mobility improvement is remarkably different: in the MAI-capped devices, the
μ is higher by one order of magnitude after BV doping treatment, whereas it is
only a factor of two in the TBAI-capped devices. Although the doping
concentration is quite close in the devices with both iodide ligands after BV
treatment, some remaining oleic acid ligands on the PbS surfaces due to the
incomplete removal of the native oleic acids with TBAI ligands might supress the
charge transport between neighbouring QDs, as previously suggested.[14]
-60
-30
0
30
60
MAITBAI
undopeddoped
Thre
shold
Voltage (
V)
Ligands3MPA
b
Chapter 5
79
0 20 40 600
1
2
3
4
5D
rain
Curr
ent
(mA
)
Drain Voltage (V)
Vg=-40;-20;...;60 V
(a) (b)
0 5 10 15 200
6
12
18
RC
hW
(k
cm
)
Channel Length (m)
Vg=60 V
0 5 10 15 200.0
0.4
0.8
1.2
RC
hW
(M
cm
)
L (m)
7.7 kcm
Figure 5.6 (a) Output characteristics and (b) channel resistances of the BV-doped devices.
The inset shows the channel resistances of the pristine devices.
At this point, it is also interesting to measure the contact resistance (Rc) of
the devices after BV treatment. Since the devices with MAI ligands show the most
efficient n-type doping, we choose these systems for our next investigation. At a
first sight, we did not observe a contact-limited transport in the output
characteristics of the devices at low Vds as shown in Figure 5.6 (a), suggesting low
Rc in these devices. To estimate Rc, we fabricated FET devices to apply
transmission-line method (TLM) with channel lengths of 2.5, 5, 10 and 20 μm.
The measured channel resistance dependent on the channel length is shown in
Figure 5.6 (b). The Rc is estimated from the intercept of the curve at the y-axis
(zero channel length). With the BV doping treatment, the extracted contact
resistance (normalized by W; RcW) of the devices is 0.77 kΩcm, one order of
magnitude lower than in the pristine devices as displayed in the inset of Figure
5.6 (b). This low Rc can also have a great impact on the improvement of the charge
carrier mobility in the devices. It is expected that further improvement of the
charge carrier mobility can be achieved by reducing the contact resistance; for
instance, by the use of other metals with shallow work function, for the fabrication
of source and drain electrodes.
5.4 4-terminal conductivity of BV-doped PbS QD-FETs
We then performed 4-terminal conductivity measurements to investigate the
charge carrier mobility, μ in our devices. With these measurements, we are able
to exclude the effect of contact resistance, which can limit the charge transport in
the PbS FETs. The configuration of the devices for 4-terminal conductivity
measurements is displayed in Figure 5.7 (a). In order to measure the 4-terminal
voltage properly, we performed laser-etching treatment on the PbS films as
Charge transport and trap states in PbS QD field-effect transistors
80
displayed in the schematic of Figure 5.7 (a). L and W of the 4-terminal transistor
are 150 μm and 30 μm, respectively.
Figure 5.7 (a) Device configuration for 4T conductivity measurements. (b) 4T conductivity
characteristics of BV-doped devices.
The 4-terminal conductivity in the devices is displayed in Figure 5.7 (b) and
the mobility (μ4T) is calculated using the equation,
g
T
i
TVC
4
4
1 (5.1)
where σ4T is the 4-terminal conductivity. The maximum μ4T among the devices
with MAI ligands after BV doping is 0.64 cm2V-1s-1 (the average mobility = 0.58
cm2V-1s-1). To our knowledge, this value is the highest among the electron mobility
reported so far in PbS QD-FETs with SiO2 gate dielectric. Moreover, the high
mobility after the BV treatment can be further improved by optimizing the
interface quality of the PbS semiconducting films and the gate dielectric, for
instance by using hydroxyl-free dielectrics, as in our previous report, opening
great opportunities for the fabrication of highly performing solution processable
electronic devices with PbS QDs.[16,39]
5.5 Conclusion
We have demonstrated n-type doping of PbS QD-FETs with the use of
electron-donating BV molecules. By engineering the LUMO level of PbS
semiconducting films through the use of different capping ligands, we successfully
controlled the electrical characteristics of the PbS QD-FETs from ambipolar to
heavily n-type. In the devices with MAI ligands, we observed a significant
improvement of the electron mobility by more than one order of magnitude after
the BV doping treatment. This high mobility is associated with an effective filling
of carrier traps and a reduced contact resistance of the devices. The 4-terminal
(a)
-80 -40 0 40 800
200
400
600
4T
-conductivity (
nS
)Gate Voltage (V)
Vds
=5 V
(b)
n-Si
AuBV
SiO2
S
G
V2V1
V3 V4
D
Chapter 5
81
conductivity transistor measurements revealed mobility as high as 0.64 cm2V-1s-1
in the devices after BV treatment. These doping results open great opportunities
for the exploitation of PbS QDs as n-type materials for broad electronic and
optoelectronic applications.
5.6 Methods
Material preparation. PbS colloidal quantum dots of 3.6 nm in diameter
were synthesized following a previously reported method.[39,40] For ligand
exchange, we used three kinds of molecules, namely 3-mercaptopropionic acid
(3MPA), tetrabuthylammonium iodide (TBAI), and methylammonium iodide
(MAI). 3MPA ligand solution was prepared by dissolving 3MPA in methanol (115
mM). TBAI and MAI powders were dissolved in methanol (30 mM) to form TBAI
and MAI ligand solution, respectively. The solutions were then filtered by PTFE
filter (0.45 μm) before use.
The benzyl viologen (BV) solution was prepared according to previous
reports with some modifications for the use in glove box.[25,32] 51 mg (0.12 mmol)
of 1,1-dibenzyl-4,4-bipyridinium dichloride hydrate (TCI) was dissolved in
distilled water (4.5 mL) inside a vial. Toluene (9.0 mL) was carefully layered on
the aqueous layer, and 97 mg (2.5 mmol) of NaBH4 was added into the bilayer
system. The colourless aqueous layer immediately started to turn into deep violet
with a generation of hydrogen gas. After 12 hours, the aqueous layer turned to
colourless, and the top toluene layer became yellow. The toluene layer was
separated and dried over MgSO4, and filtered into a glass Schlenck flask. The
solvent was removed under vacuum and heating, and then dry hexane/toluene
(1:1 (v/v); 32 mL) was added into the yellow oily residue, affording a yellow
solution. The BV solution was transferred into a glove box after argon bubbling
for 15 minutes.
Device fabrication. We used SiO2/n-Si substrates with lithographically-
defined pre-patterned interdigitated Au electrodes. The thickness of the SiO2
dielectric was 230 nm. The channel length and width of the devices were 20 μm
and 1 cm, respectively. Before use, the substrates were cleaned with acetone and
isopropanol using ultrasonicator for 10 min. The substrates were then dried at
120oC for 10 min to remove residual organic solvents.
On the clean substrates, PbS films were then deposited by spin-coating the
oleic acid-stabilized PbS solution in chloroform (10 mg/mL). To improve the
conductivity of the films, the long oleic acid ligands were replaced by shorter
molecules or halide atoms. The deposition of semiconducting thin film and ligand
exchange (LE) were performed using layer-by-layer (LbL) for seven times to
ensure complete LE process. For the LE, the ligand solution was dropped onto the
previously-deposited oleic-acid capped PbS films for 30 s and then spin-casted for
Charge transport and trap states in PbS QD field-effect transistors
82
60 s. After each LE process, pure methanol was dropped for another 30 s and then
spin-casted for 60 s to remove native oleic acid ligands. The devices were then
annealed at 120oC for 20 min to remove residual solvents and to enhance coupling
between quantum dots. The BV doping treatment was done by dipping the
previously-fabricated devices into BV solution for 3 minutes. The samples were
then dried at room temperature under argon atmosphere. All device fabrication
was done in glove box.
For the fabrication of 4-terminal (4T) FETs, we used bare SiO2/n-Si
substrates. Before use, the substrates were cleaned following the cleaning
procedure mentioned above. As source-drain electrode, thermally-grown bottom
contact Au (30 nm) with Cr (10 nm) as buffer layer was patterned using 4T
transistor shadow mask. The deposition of PbS semiconducting thin films, ligand
exchange, and BV doping treatment were done following the previously
mentioned procedures.
Electrical measurement. The electrical characteristics of the fabricated
devices were measured using an Agilent B1500A semiconductor parameter
analyser connected to a probe station in a glove box.
Chapter 5
83
5.7 References
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85
Chapter 6
Strain-Modulated Charge
Transport in Flexible PbS
Quantum Dot Field-Effect
Transistors
This chapter presents a study on the effect of mechanical strain on the
charge transport properties in flexible PbS quantum dot field-effect transistors
(QD-FETs). By applying compressive strain, the mobility is improved up to 45%
at 2% strain, whereas a decrease of the mobility is observed with the application
of tensile strain. The change of the mobility in the strained devices is attributed
to the decrease/increase of the inter-QD distance due to the bending of the
capping ligands. Moreover, strain is also found to affect the carrier traps in the
devices as indicated from the modulated threshold voltages, because of the
change of inter-QD transfer integral. From the analysis of the applied strain
versus ligand length, a certain strain direction results in the activation of the
ligand chains leading to the increase of the tunnelling barrier potential in the QD
films.
M. I. Nugraha, H. Matsui, S. Watanabe, T. Kubo, R. Hausermann, S. Z. Bisri, M.
Sytnyk, W. Heiss, M. A. Loi, J. Takeya, Adv. Electron. Mater. 2017, 3, 1600360.
Charge transport and trap states in PbS QD field-effect transistors
86
6.1 Introduction
Extensive progresses in understanding charge carrier transport in PbS
semiconducting colloidal quantum dots (QDs) have provided hope to exploit this
class of materials for low-temperature processed electronic and optoelectronic
devices. Due to their solution processability, PbS QDs are compatible with
solution processable fabrication methods such as spin-coating, dip-coating, ink-
jet printing, etc.[1–6] This solution processability, combined with low temperature
processing, opens up opportunities to use this material to fabricate electronic and
optoelectronic devices on flexible plastic substrates.[7,8]
As the active materials are deposited onto flexible substrates, bending,
folding, or stretching of the substrates is able to induce mechanical strain on the
active layer which influences the characteristics of the fabricated devices. Recently,
mechanical strain effects have been intensively investigated on organic
semiconductor thin film transistors (TFTs).[9–11] The application of compressive
strain on pentacene films has allowed improving the film conductivity and carrier
mobility in the TFT devices as a consequence of reduced molecular spacing.
Oppositely applied strain produces increased molecular spacing which leads to
the decrease of the carrier mobility.[10–13] Meanwhile, the effect of mechanical
strain on lead chalcogenide QD devices, particularly in field-effect transistors
(FETs), is still poorly investigated. As the bulk moduli of organic ligands has been
reported to be lower than the QD cores,[14,15] the introduction of mechanical strain
is expected to have a great impact on the properties of the devices, with the
potential to improve carrier mobility in this system. Therefore, the effect of strain
on the electrical properties of QD-FETs needs to be addressed to further
understand their physical properties and to strengthen the applicability of QDs
for diverse applications.
In this chapter, we report a study of the effect of mechanical strain on the
electrical characteristics of PbS FETs. As a gate dielectric, we use an ion gel, which
is able to accumulate high carrier concentration leading to electron mobility as
high as 2.1 cm2V-1s-1. Upon the application of compressive strain, we observed an
improvement in the source-drain current, which leads to an increase in the
electron mobility up to 45% at 2% strain. This improvement is associated to the
reduced barrier length between QDs due to the bending of crosslinking ligands.
In the opposite strain direction, we observed a reduction in the electron mobility
which is an indication of the increase of the QD distance. Interestingly, we found
that the change in the electron mobility is followed by the variation in the
threshold voltages of the devices depending on the direction of the strain. The
decreased threshold voltages as the compressive strain is applied can be
attributed to the increase of the transfer integral between the QD arrays, which
results in a more efficient trap filling, thus reducing carrier trapping. Meanwhile,
Chapter 6
87
the increased carrier traps can be responsible for the increase of the threshold
voltages with the application of tensile strain. Furthermore, the observed larger
effect during the application of tensile strain than that with compressive strain
can be associated to the increase of barrier potential due to the activation of one
or more ligand chains.
6.2 PbS QD-FETs on flexible substrates
To perform strain effect measurement on the PbS QD films, flexible PbS QD-
FETs were fabricated using polyethylene naphthalate (PEN) as a substrate. As
crosslinking ligands, 1,2-ethanedithiol (EDT) was used to replace the native oleic
acid ligands on the QD surfaces, which improves the conductivity of the
semiconducting films. In the devices, an ion gel derived from a mixture of the ionic
liquid: 1-hexyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)imide
(HMIM-TFSI) and the gelling polymer, poly(vinylidene fluoridehexafluoro
propylene) (PVDF-HFP) was employed as the gate dielectric. This mixture allows
obtaining a solid gate dielectric film which is easy to handle and suitable for
electronic devices fabricated on plastic substrate. The ion gel is used to
accumulate high charge carrier concentration at the dielectric/semiconductor
interface, which can effectively fill the carrier traps in the QD films. As a result of
the effective trap filling, ion gel has been demonstrated as a promising gate
dielectric that enables good performing PbS QD-FETs with high electron mobility
of 2 cm2V-1s-1.[16] Figure 6.1 (a) shows the final configuration of bottom contact top
gate PbS QD-FETs fabricated on PEN substrate. In the devices, Au is used as
source, drain, and gate electrode. The flexibility of the fabricated devices is
displayed in Figure 6.1 (b).
Figure 6.1 (a) Device structure, (b) photograph of PbS QD-FETs on flexible substrate.
The transfer characteristics of the fabricated devices in pristine condition
(without applying strain) are shown in Figure 6.2 (a). The devices show a small
hysteresis (< 1.3 V) in the transfer curves, indicating that the ion gel provides good
Charge transport and trap states in PbS QD field-effect transistors
88
protection for the devices against bias stress and an effective filling of the traps.[17]
Importantly, the devices show clear current modulation with very low operation
voltage (<2 V), which is promising for future low energy consuming solution
processable electronic devices. In addition, a weak hole current in the negative
gate voltage is also measured indicating the ambipolarity of the devices. However,
since the hole current is much lower than the electron current and quite close to
the level of the gate leakage current, the investigation of the strain effect in this
study is focused on the electron transport.
Figure 6.2 (a) Ids-Vg transfer and (b) Ids-Vds output characteristics of PbS QD-FETs with
HMIM-TFSI ion gel gating on PEN substrate.
The capacitance of the ion gel film (Ci) is as high as 2.7 µF/cm2 at 10 Hz,
allowing a very low threshold voltage of 0.9 V in the devices. The sweeping speed
used for the FET measurements was 100 mV/s. In the devices, the channel length
-0.3 0.0 0.3 0.6 0.9 1.2
0.00
0.05
0.10
0.15
Dra
in C
urr
ent
(A
)
Gate Voltage (V)
Vds
=0.2 V
(a)
0.0 0.2 0.4 0.60.00
0.04
0.08
0.12
0.16
Vg=0,0.2,...,1.2 V
Dra
in C
urr
ent
(A
)
Drain Voltage (V)
(b)
Chapter 6
89
was designed to be 150 µm to minimize the effect of contact resistance. Moreover,
a channel width of 70 µm was used to avoid localized surface morphology defects
formed during film deposition as well as during ligand exchange, which may
obscure the effect of mechanical strain. The electron linear mobility extracted
from the transfer characteristics in Figure 6.2 (a) is as high as 2.1 cm2V-1s-1. It is
important to note that this very high electron mobility is achieved in the devices
fabricated on plastic substrate. In addition, the output characteristics shown in
Figure 6.2 (b) indicate that the devices do not show any contact limitation at low
drain voltage. Moreover, the clear saturation profiles confirm that the fabricated
FETs are well performing.
6.3 Strain effect on flexible PbS QD-FETs
The fabrication of PbS QD-FETs on flexible plastic substrate allows applying
compressive and tensile strain on the devices, continuously up to 2% without
breaking the devices. The schematic of the application of compressive and tensile
strain is shown in Figure 6.3. The radius of curvature of the samples upon the
application of strain is given as,[18,19]
2
22
122
l
h
l
dl
lR
(6.1)
where h, l, and dl are the total thickness, the initial length, and the change of
length of the samples, respectively. The strain can be simply estimated from
ε=(h/2R)·100%.[18,19]
Figure 6.3 Schematic of (a) compressive (bent downward) and (b) tensile strain (bent
upward) applied on the devices.
At first, the surface morphology characterization of the PbS QD films was
performed to investigate the presence of cracks on the films after the application
of strain. Figure 6.4 shows the AFM images of the PbS QD films on PEN substrate
with different strain magnitudes. Obviously, no significant change is observed on
(a) (b)
PEN substrate
Strain Direction
Compressive
PEN substrate
Strain Direction
Tensile
Charge transport and trap states in PbS QD field-effect transistors
90
the PbS films indicating the absence of cracks after applying compressive and
tensile strain up to 2%.
Figure 6.4 Surface morphology of the PbS films with (a) no strain and compressive strain
of (b) 1%, (c) 1.5%, and (d) 2%. AFM images of the PbS films with tensile strain of (e) 1.5%
and (f) 1.8%.
Chapter 6
91
Figure 6.5 Ids-Vg transfer characteristics with the application of (a) compressive and (b)
tensile strain.
Figure 6.5 (a) presents the transfer characteristics of the devices under
several static compressive strains. The applied compressive strain is started from
0.75% up to 2% with a strain interval of 0.25%. An improvement in the on-current
of the devices is measured when the compressive strain is applied on the devices.
Meanwhile, no significant change is observed in the off-current of the devices with
the application of the strain as shown in Figure 6.6 (a). Furthermore, the
improvement of the on-current is also followed by the shifting of the transfer
curve towards more negative voltages, i.e, showing a reduced threshold voltage as
displayed in Figure 6.5 (a) and 6.6 (a). Importantly, the variation of the transfer
curve upon the application of compressive strain is reversible, demonstrating that
the devices are still elastic after the strain is applied and that the current increase
is not caused by its drifting. When the opposite strain direction (tensile strain) is
applied on the devices, the decrease of the on-current is observed. The transfer
0.3 0.6 0.9 1.2
0.00
0.05
0.10
0.15
0% 0.75% 1% 1.25% 1.5% 1.8%
Vds
=0.2 V
|Dra
in C
urr
ent| (A
)
Gate Voltage (V)
0.3 0.6 0.9 1.2
0.00
0.05
0.10
0.15
0.20
0.25
0% 0.75% 1% 1.25% 1.5% 1.75% 2%
Vds
=0.2 V|Dra
in C
urr
ent| (A
)
Gate Voltage (V)(b)
(a)(a)
Charge transport and trap states in PbS QD field-effect transistors
92
characteristics measured after the application of the tensile strain are displayed
in Figure 6.5 (b) and 6.6 (b). In addition, we also observed the shift of the transfer
curve towards more positive voltages, thus an increased threshold voltage.
(a)
-0.4 0.0 0.4 0.8 1.2
10-11
10-10
10-9
10-8
10-7
2%
Vds
=0.2 V
Dra
in C
urr
ent
(A)
Gate Voltage (V)
0%
-0.4 0.0 0.4 0.8 1.2
10-11
10-10
10-9
10-8
0% 0.75% 1% 1.25% 1.5% 1.75% 2%
Vds
=0.2V
|Gate
Curr
ent| (
A)
Gate Voltage (V)
(b)
(a)
-8
Gate Voltage (V)
(b)
-0.4 0.0 0.4 0.8 1.210
-11
10-10
10-9
10-8
0% 0.75% 1% 1.25% 1.5% 1.8%V
ds=0.2V
|Gate
Curr
ent| (
A)
Gate Voltage (V)
(a) (b)
(c) (d)
-0.4 0.0 0.4 0.8 1.210
-11
10-10
10-9
10-8
10-7
1.8%
0%
Vds
=0.2 V
Dra
in C
urr
ent
(A)
Gate Voltage (V)
Figure 6.6 Semi-logarithmic scale of Ids-Vg transfer characteristics in the devices after the
application of (a) compressive and (b) tensile strain. Semi-logarithmic scale of gate leakage
Ig-Vg characteristics in the devices after the application of (c) compressive and (d) tensile
strain.
To have a confirmation that the variation of the on-current is not caused by
changes in the dielectric properties of ion gel, the gate leakage Ig-Vg characteristics
of the devices under several applied mechanical strain were measured, as
displayed in Figure 6.6 (c) and (d). The measurements confirm the absence of
significant changes in the gate leakage characteristics indicating that the carrier
density, thus the capacitance of the ion gel, is not influenced by the application of
strain.
To have better description about the influence of the mechanical strain on
the electrical characteristics of the devices, the electron mobility in the devices is
extracted for each strain condition. Figure 6.7 shows the electron mobility versus
strain. Under compressive strain (grey region), the electron mobility increases
Chapter 6
93
with increasing strain. These results are attributed to the reduced barrier length,
thus the reduction of the QD-interspacing. At 2% strain (R=3.1 mm), an
improvement in the electron mobility up to 45% is observed resulting in the
mobility value of 3 cm2V-1s-1. To date, the electron mobility is the highest reported
value achieved in PbS QD-FETs fabricated on flexible plastic substrate. Similar to
the current-voltage characteristics, the variation of mobility is reversible in the
range of the applied strain, which confirms that the devices are still elastic. With
given mechanical strain up to 2%, no cracks are observed on the PbS films as
referred to Figure 6.4.
Figure 6.7 Electron mobility as compressive and tensile strain are applied.
In the region of the applied tensile strain, the decrease of the electron
mobility is observed with increasing the strain magnitude as shown in Figure 6.7
(white region). These results can be ascribed to the increase of the barrier length
(longer inter-QD distance) when the tensile strain is applied. The variation of the
inter-QD distance by applying external pressure was reported in PbS colloidal
quantum dots.[14,15] In this system, uniaxial compressive pressure results in a red
shift of the excitonic peak absorption. Due to higher bulk modulus of the QD cores
than that of the ligands, this shift can be mainly associated to the reduced particle
spacing due to the bending of the ligands. The application of compressive strain
on organic semiconductor FETs has also been reported to lead to improvement of
carrier mobility due to reduced molecular spacing. The introduction of tensile
strain in organic FETs results in an increased molecular spacing leading to
reduced charge carrier mobility.[11,13] For organic TFTs, it has also been proposed
that suppressed molecular vibration can be a possible origin of the increased
charge carrier mobility.[20]
(c)
-2 -1 0 1 2
0
1
2
3
4
Forward Backward
CompressiveMobili
ty (
cm
2V
-1s
-1)
Strain (%)
Tensile
(c)
(a)
4
Charge transport and trap states in PbS QD field-effect transistors
94
6.4 Traps and barrier potential in strained devices
The increase of the electron mobility by applying compressive strain in
Figure 6.7 can also be associated with the reduction of the trap density in the
devices. In quantum dot solids, the charge carrier transport mechanism is
generally governed by quantum tunnelling of charge carriers between QDs. This
process strongly depends on the exchange coupling energy (β), which can be
written as,[3]
dEm b2
12*22exp (6.2)
where ħ, m*, and Eb are reduced Planck constant, the effective mass of carriers,
and barrier potential height, respectively. The distance between QDs, which is
proportional to the length of the ligands, is given by d. When compressive strain
is applied, the bending of the crosslinking ligands leads to the reduced inter-QD
distance, thus improving the exchange coupling energy. This increased exchange
coupling energy will, in turn, reduce the carrier traps near the QD band-edge as
indicated in the schematics reported in Figure 6.8.
Figure 6.8 Schematics of the density of states (DOS) (a) without strain and (b) with
compressive strain. The bandwidth increases due to the increased transfer integral.
Conduction band
trap states
Valence band
trap states
Energy
DOS
(a)
trap states
trap states
Energy
DOS
(b)
Conduction band
Valence band
Chapter 6
95
Conversely, the application of tensile strain will result in the reduction of the
exchange coupling energy. As a consequence, some carrier traps will be
introduced in the band edge of quantum dots. The analysis on this trap
modulation is also supported by the observed threshold voltage shifts upon the
application of the strain as shown in Figure 6.9. As compressive strain increases,
the threshold voltage decreases indicating a lower energy for electron
accumulation, which is related to the reduction of the traps in the devices. When
tensile strain is applied, the increase of the threshold voltage in the devices is
observed. This threshold voltage shift explains the increase of energy for electron
accumulation, which can be attributed to the increased number of carrier traps.
This fluctuation of density of states due to the change of the exchange coupling
bandwidth was previously observed in organic semiconductors. [21–24]
-2 -1 0 1 20.75
0.80
0.85
0.90
0.95
1.00
Forward Backward
Compressive
Thre
shold
Voltage (
V)
Strain (%)
Tensile
Figure 6.9 Threshold voltage of the devices with the application of compressive and tensile
strain.
Interestingly, a non-fully-symmetrical profile in the variation of the device
characteristics by comparing the conditions between compressive and tensile
strain is observed, as referred to Figure 6.7 and 6.9. At this point, the effect of
tensile strain on the device characteristics is larger than that with compressive
strain. This result can be explained by the induction of a barrier potential with the
introduction of tensile strain. To understand this effect, the change of the inter-
QD distance due to the applied strain (ε) is firstly estimated using equation Δx =
xε, where x is the distance between the center of mass of QDs. When compressive
strain is applied, the inter-QD distance is the only parameter changed due to the
bending of the ligands. When the tensile strain is applied on the QD films, the
inter-QD distance is increased by bending the crosslinking organic ligands. Due
Charge transport and trap states in PbS QD field-effect transistors
96
to this bending, at some points, one of thiol end groups on the QD surfaces can
also become active as demonstrated in Figure 6.10 (a). These active thiols may
increase the barrier potential between QDs and result in the reduced number of
the ligands interconnecting the PbS QDs, thus reducing the tunnelling probability
between QDs as indicated in equation (6.2). In addition, the change of the ligand
dielectric constant when the ligands are stretched can be another possible origin
for the change of barrier potential height.
(c)Strain (%)
(c)
0.35 0.40 0.45 0.50
0
1
2
3
4
CompressiveMobili
ty (
cm
2V
-1s
-1)
Estimated Ligand Length (Å)
Tensile
(b)
(a) Activex
d
Figure 6.10 (a) Unbound (active) ligand chains during the application of tensile strain. (b)
The electron mobility as a function of the ligand length when compressive and tensile strain
are applied.
To estimate the change of the tunnelling barrier potential (Eb) in the PbS QD
films, we use Wentzel–Kramers–Brillouin (WKB) approximation
βe=(2m*Eb/ħ2)1/2.[25] The tunnelling decay constant (βe) can be calculated from the
gradient of the curve of the calculated mobility versus estimated ligand length
(d+Δx). From Figure 6.10 (b), βe is a factor of 5 higher in the regime with tensile
Chapter 6
97
strain application (βe=4.3/Å) than that with compressive strain (βe=0.86/Å)
indicating the increase of barrier potential by referring to the WKB approximation.
These results are in line with the previous reports showing that HOMO-LUMO
gap for unbound (active) alkane chains is as high as 8-10 eV which is about 2 eV
higher than that for bound (non-active) ligands. The lower barrier potential in the
ligands bound on the QD surfaces can be attributed to the hybridization of the
organic ligand chains and the orbitals of the QD surface atoms, which can
introduce some density of states in the molecular junction.[25]
6.5 Conclusion
We have conducted a study of the effect of mechanical compressive and
tensile strain on the electrical characteristics of PbS QD-FETs. After the
application of the mechanical strain, the electron mobility was improved up to
45% (mobility of 3 cm2V-1s-1) at 2% compressive strain, which is attributed to the
reduced inter-QD distance due to the bending of ligands. The inter-QD distance
can be increased by applying tensile strain, which results in a reduced electron
mobility. The variation of the electron mobility by applying mechanical strain was
followed by a variation of the threshold voltage of the devices. With the
application of compressive strain, the threshold voltage increases due to the
increased transfer integral between the QD arrays and reduced carrier traps. In
the opposite strain direction, carrier traps are induced by reducing the exchange
coupling energy, which in turn leads to a decreased transfer integral of the QD
arrays. The application of tensile strain gave rise to a larger effect than that of
compressive strain. This finding can be associated to an increase of the barrier
potential due to the activation of some of the thiol ligands. These results provide
fundamental understanding of the effect of mechanical forces on PbS films which
can enhance and broaden the applicability of these materials for flexible cheap
solution processable electronic devices and also open new prospective towards the
fabrication of mechanical force sensors.
6.6 Methods
Device Fabrication. Polyethylene naphthalate (PEN) with thickness of
125 µm was used as a substrate. Before use, the substrate was annealed at 150oC
for 3 h on a hot plate to remove residual monomer on the PEN surfaces. The
substrate was cleaned with isopropanol for 10 min and dried with nitrogen. Cr and
Au (10 nm/30 nm) were then evaporated on the clean PEN substrate as source-
drain contacts.
PbS quantum dots were synthesized following a method previously reported
in literature.[26] The deposition of the PbS semiconducting thin film was done by
Charge transport and trap states in PbS QD field-effect transistors
98
spin coating 10 mg/mL of the oleic acid-capped QD solution in chloroform. To
improve the film conductivity, the long-alkyl-chain oleic acid ligands were
replaced by shorter organic molecules, 1,2-ethanedithiol (EDT) with
concentration of 1% v/v in acetonitrile. The film deposition and ligand exchange
(LE) were done using layer-by-layer spin coating procedures for 7 times to ensure
complete LE process. After each LE process, the film was cleaned with pure
acetonitrile to remove old native ligands. The devices were annealed at 120oC for
20 min to remove residual solvents and promote stronger coupling between QDs.
Ion gel based on 1-hexyl-3-methyl-imidazolium bis(trifluoromethane
sulfonyl) imide (HMIM-TFSI) ionic liquid was used as gate dielectric. To prepare
the solution of ion gel, 80 mg of poly(vinylidene fluoride hexa- fluoropropylene)
(PVDF-HFP) pellets was first dissolved in 1 mL dimethylformamide. The solution
was stirred at 60oC overnight. 20 mg of HMIM-TFSI ionic liquid was then added
into the PVDF-HFP solution and stirred at room temperature for 3 h. The ion gel
solution was then spin-coated on the PbS film and annealed at 90oC for 2 h.
Finally, Au with thickness of 50 nm was evaporated on the top of the ion gel film
as top gate electrode. All device fabrication was performed in an N2-filled glove
box
Device Measurement and Strain Effect Experiment. The FET
electrical characteristics were measured using an Agilent B1500A semiconductor
parameter analyser connected to a probe station in an N2-filled glove box. For
strain effect measurement, the devices were stressed using strain measurement
apparatus. The change of the device length was measured by using a calliper and
then used in equation (6.1) to further calculate the strain. The initial ligand length
(without strain) was estimated using the Gaussian software. Before performing
the electrical transport measurement, we waited for 10 minutes after changing
each level of strain to relax some slow ionic migration of the ion gel.
Chapter 6
99
6.7 References
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[8] J.-H. Choi, H. Wang, S. J. Oh, T. Paik, P. Sung, J. Sung, X. Ye, T. Zhao, B. T. Diroll,
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[14] K. Bian, B. T. Richards, H. Yang, W. Bassett, F. W. Wise, Z. Wang, T. Hanrath, Phys.
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[15] K. Bian, A. K. Singh, R. G. Hennig, Z. Wang, T. Hanrath, Nano Lett. 2014, 14, 4763.
[16] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss, M. A. Loi, Adv. Mater. 2013, 25, 4309.
[17] Y. Zhang, Q. Chen, A. P. Alivisatos, M. Salmeron, Nano Lett. 2015,
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[18] S. Il Park, J. H. Ahn, X. Feng, S. Wang, Y. Huang, J. A. Rogers, Adv. Funct. Mater.
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[19] S. P. Timoshenko, J. M. Gere, in Theory Elastic Stab., McGraw Hill, New York,
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[20] T. Kubo, R. Häusermann, J. Tsurumi, J. Soeda, Y. Okada, Y. Yamashita, N.
Akamatsu, A. Shishido, C. Mitsui, T. Okamoto, S. Yanagisawa, H. Matsui, J. Takeya,
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101
Summary
Electronic devices nowadays have become an integral part of our day-to-day
needs. Their great advancement has given remarkable impact in our lives, which
covers many sectors including energy, healthcare, sensor, light industry, etc. To
date, most of the electronic devices available in the markets are constructed from
silicon and other highly crystalline semiconductors. These materials are mainly
deposited using epitaxial growth method which requires ultra-high vacuum
environment, high temperature, and energy intensive process resulting in high
manufacturing cost.
In the past few years, colloidal quantum dots (QDs) have emerged as
promising future semiconducting building blocks for electronic devices. These
materials can potentially compete with epitaxial-based semiconductors as they
can be processed by solution methods, which are compatible with low
temperature processes and with device fabrication on plastic substrate. Thanks to
the quantum confinement, their band gap is size-tunable ranging from ultraviolet
to near-infrared, making them prospective candidates for well-controllable
electronics and highly efficient optoelectronic devices. Among other types of QDs,
lead sulfide (PbS) is one of the most promising because they can show strong
quantum confinement in relatively large particle size (<20 nm). As the research
on the PbS QD synthesis has been well developed, the size range can be achieved
together with the precise control of shape, chemical composition, and surface
properties.
QDs have a large surface to volume ratio, a typical characteristic of nano-
objects. As a consequence, QDs have defect states on their surfaces which are
responsible for charge carrier trapping within these materials. This charge
trapping is one of main obstacles in further utilizing these materials for broad
electronic and optoelectronic applications. For instance, in the application of
field-effect transistors (FETs) based on PbS QDs, the charge transport is still
limited. A problem further arises as some additional defects appear on the surface
of gate insulators, one of main components in FET devices, lowering the carrier
mobility in the fabricated devices. Several methods to improve the surface
properties of insulators and to modify the properties of semiconducting films have
been reported. These will be key strategies to understand and further improve
charge transport in the FETs based on PbS QDs. The studies of these strategies
are the focus of this thesis.
The surface properties of gate insulators are crucial in determining the
electrical properties and the performance of FETs. In chapter two, we
Charge transport and trap states in PbS QD field-effect transistors
102
introduced molecular dipoles on the surface of SiO2 gate dielectric using self-
assembled monolayers (SAMs). Using SAMs, the threshold voltages of the devices
are shifted and show a linear relationship with the SAM doping concentration,
providing an opportunity to tune the FET properties towards n-type or p-type
depending on the used SAMs. The SAMs are also found to reduce the trap density
in the devices, leading to the improvement of the electron mobility up to a factor
of three.
The use of gate dielectrics with better surface properties together with
comprehensive understanding on the nature of charge trapping in FETs is a
necessary step to improve charge transport in PbS QD-FETs. In chapter three,
we passivated the SiO2 surface using hexamethyldisilazane (HMDS) and utilized
hydroxyl-free Cytop dielectric. The HMDS treatment is found to improve the QD
assembly organization and indeed passivate the dangling bonds on the SiO2
surface, leading to the improvement of electron mobility by a factor of three. The
use of Cytop gate dielectric further improves the mobility by one order of
magnitude. Using a simulation, we showed that a significant decrease of the trap
density of states (trap DOS) by almost two orders of magnitude is responsible for
the improvement of the mobility in the devices.
In chapter four, we presented an analysis of the trap DOS in PbS QD-FETs
utilizing several polymer gate dielectrics with a wide range of dielectric constants
(2-41). We observed that the number of traps in the subthreshold regime increases
with increasing the dielectric constant of the gate insulators. A consistent result
was obtained from analysis using a computer simulation, where the increase and
broadening of the trap DOS in the devices were observed with increasing the
dielectric constant of the insulators. These results suggest the presence of
increased disorder due to polaronic interaction at the semiconductor/insulator
interface in PbS QD-FETs with increased dielectric polarization strength.
Doping is an effective tool to improve carrier mobility in semiconductors. In
chapter five, we reported a strategy to heavily dope PbS QDs combining the use
of benzyl viologen (BV) dopant with the engineering of the QD energy levels
through the use of several capping ligands. With this doping, the electron mobility
is improved by one order of magnitude. From the 4-terminal conductivity
transistor measurement, the BV doping of the PbS QD films resulted in an
electron mobility as high as 0.64 cm2V-1s-1 in devices employing SiO2 gate
dielectric.
The modulation of the inter-QD distances using mechanical strain is
expected to determine the charge carrier transport in PbS QD-FETs. In chapter
six, we demonstrated the effect of mechanical strain on the electrical properties
of flexible ion gel-gated PbS QD-FETs. Using ion gel gating, a high electron
mobility of 2.1 cm2V-1s-1 in the devices was achieved. The application of
compressive strain results in the bending of the capping ligands and reduced
Summary
103
inter-QD distances, leading to the improvement of the mobility up to 45% (3
cm2V-1s-1) at 2% strain. Meanwhile, the oppositely applied strain leads to an
increase of the inter-QD distances resulting in the reduction of the electron
mobility in the devices. In addition, we also found that the compressive strain
reduces the threshold voltage of the devices, which explains an efficient trap filling.
Instead, the increase of the threshold voltage is observed with the application of
tensile strain, which increases the carrier traps in the devices. Furthermore, we
found that the tensile strain also results in the activation of the ligand chains,
increasing the tunnelling barrier potential between QD solids.
To conclude, we investigated and improved charge transport in PbS QD-
FETs by modifying the surface properties of gate insulators, doping of PbS QDs
using organic molecules, and introducing mechanical strain on the devices. We
demonstrated high mobility FETs based on PbS QDs, showing the importance of
these materials as colloidal inks for future low cost and high performance plastic
electronics.
105
Samenvatting
Elektronica is niet meer weg te denken uit ons dagelijks leven. Grote
technologische vooruitgangen hebben een opmerkelijke invloed gehad op veel
gebieden, zoals energie, gezondheidszorg, sensor- en verlichtingsindustrie, etc.
Tot op heden zijn de meeste elektronische apparaten op de markt gemaakt van
silicium en andere hoogkristallijne halfgeleiders. Deze materialen zijn
hoofdzakelijk gefabriceerd met behulp van epitaxiale groei, wat een ultrahoog
vacuüm, hoge temperatuur en veel energie vereist. Dit resulteert weer in hoge
productiekosten.
In de afgelopen jaren zijn colloïdale Quantum Dots (QDs) ontsproten tot
veelbelovende toekomstige halfgeleidende elementen voor elektronische
apparatuur. Deze materialen hebben het potentieel om te kunnen concurreren
met epitaxiaal-gebaseerde halfgeleiders, omdat ze vanuit oplossing als vloeistof
verwerkt kunnen worden. Dit is verenigbaar met een lage
verwerkingstemperatuur en met verwerking op plastic substraten. Dankzij
kwantumopsluiting is hun bandkloof in grootte te variëren, met een bereik van
ultraviolet tot nabij-infrarood waardoor ze potentiele kandidaten zijn voor goed
afgestemde elektronica en hoog-efficiënte opto-elektronische apparaten. Onder
de verschillende types QDs, is loodsulfide (PbS) een van de meest veelbelovende
omdat het een sterke kwantumopsluiting vertoont in een relatief groot deeltje
(<20 nm). Het onderzoek naar PbS QD synthese is goed ontwikkeld; het bereik in
deeltjesgrootte kan geregeld worden alsmede precieze controle over vorm,
chemische samenstelling en oppervlakte-eigenschappen.
QDs hebben een groot oppervlak-tot-volume ratio, een typisch kenmerk van
nano-objecten. Daarom hebben QDs defecttoestanden op hun oppervlak, die
verantwoordelijk zijn voor gevangen ladingsdragers in deze materialen. Deze
ladingsvallen zijn één van de voornaamste hindernissen voor verder gebruik van
deze materialen voor brede elektronische en opto-elektronische toepassingen.
Bijvoorbeeld bij gebruik van veldeffecttransistoren (FETs) gebaseerd op QDs is
het ladingstransport nog begrensd. Een ander probleem doet zich voor als enkele
additionele defecten op het oppervlak van gate-isolatoren verschijnen, een van de
hoofdcomponenten in FET apparaten, waardoor de mobiliteit van de
ladingsdragers in de gefabriceerde apparaten afneemt. Er zijn verschillende
methodes gepubliceerd om de oppervlakte-eigenschappen van isolatoren en de
eigenschappen van halfgeleidende laagjes te verbeteren. Dit zijn belangrijke
strategieën om ladingtransport in FETs gebaseerd op PbS QDs te begrijpen en
verder te verbeteren. De studie van deze strategieën is de focus van dit proefschrift.
Ladingstransport en ladingsvallen in PbS QD veld-effect transistors
106
De oppervlakte-eigenschappen van gate-isolatoren zijn doorslaggevend voor
de elektrische eigenschappen en prestaties van de FETs. In hoofdstuk twee
introduceren we moleculaire dipolen op het oppervlak van SiO2 gate-diëlektrica
met behulp van zelf-geassembleerde monolagen (SAMs). Met behulp van SAMs
worden de drempelspanningen van de apparaten verschoven en wordt een
lineaire relatie vertoond met de SAM doteringsconcentratie, hetgeen een
mogelijkheid geeft om de eigenschappen van de FET richting n-type of p-type te
modificeren afhankelijk van de gebruikte SAM. De SAMs blijken ook de
ladingsval-dichtheid te verminderen, wat weer leidt tot een elektron mobiliteit die
tot wel drie keer zo hoog is.
Het gebruik van gate-diëlektrica met betere oppervlakte-eigenschappen
samen met een uitgebreide kennis over de aard van ladingsvallen in de FETs is
een noodzakelijke stap om het ladingstransport in PbS QD-FETs te verbeteren. In
hoofdstuk drie neutraliseren we het SiO2 oppervlak door middel van
hexamethyldisilazaan (HMDS) en gebruik te maken van het hydroxyl-vrije Cytop
diëlektricum. De HMDS behandeling blijkt de QD-assemblageorganisatie te
verbeteren en neutraliseert inderdaad de bungelende bindingen op het SiO2
oppervlak, wat leidt tot een verbetering van elektronmobiliteit met een factor van
drie. Het gebruik van Cytop-gate-diëlektricum verbetert de mobiliteit verder met
een ordegrootte. Met behulp van een simulatie hebben we laten zien dat een
significante vermindering van de toestandsdichtheid van ladingsvallen (trap
DOS), met bijna twee orders van grootte, verantwoordelijk is voor de verbetering
van de mobiliteit in de apparaten.
In hoofdstuk vier presenteren we een analyse van de toestandsdichtheid
van ladingsvallen in PbS QD-FETs gebruikmakend van verscheidene polymeer
gate-diëlektrica met een breed scala aan permittiviteiten (2-41). We merken op
dat het aantal ladingsvallen in het sub-drempelregime afneemt bij vergroting van
de permittiviteit van de gate-isolatoren. Een consistent resultaat is verkregen
door analyse met een computersimulatie, waar het vermeerderen en verbreden
van de ladingsvaltoestandsdichtheid in de apparaten is opgemerkt bij vergroting
van de permittiviteit van de isolatoren. Deze resultaten suggereren aanwezigheid
van verhoogde wanorde door polaronische interactie op de halfgeleider/isolator
interface in PbS QD-FETs met verhoogde diëlektrische polarisatiesterkte.
Dotering is een effectief middel om de mobiliteit van ladingsdragers in
halfgeleiders te verbeteren. In hoofdstuk vijf beschrijven we een strategie om
PbS QDs zwaar te doteren, door gecombineerd gebruik van benzyl-viologen (BV)
doteermiddel en de aanpassing van QD energieniveaus door verscheidene
afdekkende liganden. Met deze dotering is de elektron mobiliteit verbeterd met
een ordegrootte. Uit geleidbaarheidsmetingen, in een vier-terminal
transistorconfiguratie, is gebleken dat de BV dotering van de PbS QD films
Samenvatting
107
resulteert in een elektronmobiliteit zo groot als 0.64 cm2V-1s-1 in apparaten
gebruikmakend van SiO2 gate-diëlektricum.
De modulatie van inter-QD afstanden met behulp van mechanische
belasting zal naar verwachting het transport van ladingsdragers in PbS QD-FETs
bepalen. In hoofdstuk zes demonstreren we het effect van mechanische
belasting op de elektrische eigenschappen van flexibele ion-gel gate PbS QD-FETs.
Gebruikmakend van ion gel gates is een elektronmobiliteit van 2.1 cm2V-1s-1 in de
apparaten verkregen. De toepassing van drukspanning leidt tot het buigen van de
afdekkende liganden en reduceert inter-QD afstanden, wat leidt tot een
verbetering van de mobiliteit tot 45% (3 cm2V-1s-1) bij 2% rek. Ondertussen leidt
de tegengesteld aangebrachte spanning tot een verhoging van inter-QD afstanden,
wat resulteert in een verlaging van elektronmobiliteit in de apparaten. Daarnaast
merken we op dat de drukspanning de drempelspanning van de apparaten
vermindert, wat een efficiënte vulling van ladingsvallen verklaard. In plaats
daarvan wordt een verhoging van drempelspanning geobserveerd met de
aanbrenging van trekbelasting, hetgeen het aantal ladingsvallen in de apparaten
vergroot. Verder hebben we gevonden dat de trekbelasting ook resulteert in de
activatie van ligandketens, hetgeen het tunnelbarrièrepotentiaal tussen QD vaste
stoffen verhoogd.
Ter samenvatting: we hebben ladingstransport in PbS QD-FETs onderzocht
en verbeterd door het modificeren van oppervlakte-eigenschappen van gate-
isolatoren, dotering van PbS QDs door gebruik van organische moleculen en
aanbrenging van mechanische belasting op de apparaten. We hebben FETs
gebaseerd op PbS QDs met hoge mobiliteit gedemonstreerd, die het belang van
deze materialen demonstreren als colloïdale inkten voor toekomstige elektronica
met lage kosten en hoge prestaties.
109
Acknowledgement
Alhamdulillah, all praises to Allah Subhanahu Wata’ala for the strengths,
His guidance, and His blessings in completing this thesis.
A 4-year PhD is indeed a tough part of life, yet is very wonderful because of
many amazing people surrounding. They have made my PhD life more colorful
and full of happiness. From the deepest part of my heart, I would like to dedicate
this thesis to them because of their support, motivation, and prayer during this 4-
year long PhD journey.
First, I would like to send my gratitude to my supervisor, Prof. Maria
Antonietta Loi, for her kind support, supervision, and guidance during my PhD.
Maria, thank you for giving me an opportunity to study abroad and to join your
lab, the place where I developed my knowledge and research experiences. Thank
you for helping me with the paper writing, presentations, and some opportunities
to attend international conferences. Thank you for providing me a great
opportunity to have a collaboration in high quality research group at the
University of Tokyo, Japan.
Second, I would like to sincerely acknowledge Prof. Jun Takeya at the
University of Tokyo for giving me few years of life and research collaboration in
Japan. The place where I have been dreaming of visiting since I was a kid. I really
enjoyed living in Japan and learned a lot from the good Japanese culture in life
and work activities. Thank you for sharing me your idea and for discussions
during the collaboration. All of those as well as your typical Japanese pressure a
bit are indeed very helpful to finish this PhD.
I would like to thank Prof. Justin Ye from University of Groningen, Prof.
Cherie R. Kagan from University of Pennsylvania, and Prof. Matt Law from
University of California Irvine for accepting to be the members of the reading
committee for my PhD thesis.
I would like to specially thank Matsui-san, my great daily supervisor, for your
patience in supervising me and arranging everything during my first visit in Japan.
When I had a problem in research, you never blamed students. You always came
up with solutions and brilliant ideas. You always have positive thoughts to
students so that they can get highly motivated afterwards. Those what I can learn
from you when I become a supervisor in the future. I cannot say anything but
thank you very much for your valuable supports and kindness. Shun Watanabe, I
also would like to express my gratitude to you for your supervision during few
months of my last stay in Japan. Thank you for your great ideas and motivations.
It is a great honor to be supervised by one of great material scientists like you.
Charge transport and trap states in PbS QD field-effect transistors
110
During my research, I collaborated with many people. I would like to
acknowledge Prof. Wolfgang Heiss and Dr. Mykhailo Sytnyk for providing
quantum dot solutions; Prof. Chen and Prof. Wu for other side projects and
fruitful discussions. This thesis would never have been completed without your
valuable works and help.
A special acknowledgement goes to my both paranymphs: Wytse Talsma and
Abdurrahman Ali. Thank you for accepting to be my paranymphs and preparing
everything for my defense. Wytse, a funny Dutch friend and a good physicist with
additional expertise in farming, we shared everything in the lab and you also had
helped me a lot. Thank you also for translating the summary of my thesis into
Dutch. Thanks for allowing me to have a picture with your beautiful horse and
giving me a nice sunflower. Please let me know if you have a plan to visit Indonesia.
Ali, we come from the same university in Indonesia and now we meet in
Groningen. I hope we can have a strong research collaboration when we build a
laboratory in Indonesia, in the future.
I would like to thank Renate Hekkema, our group secretary, for helping me
with all paper and administrative works. I also would like to thank our research
technicians: Arjen Kamp, Frans van der Horst, and Jan Harkema for your
significant contribution to my research. Without your help, my research would
never have run smoothly.
I would like to thank the members of photophysics and optoelectronics
group at University of Groningen. Satria, thank you very much for giving me
information about PhD position in Maria lab and supervision in Groningen. Widi,
a friendly and nice Indonesian lab mate, thank you for everything and the
discussion in the lab. We discussed everything including research, life, gadget, etc.
Thank you also for your valuable inputs on the cover of my PhD thesis. Good luck
with your postdoctoral research in RIKEN. It was great to meet you! Lai Hung, a
funny Taiwanese and a food lover, I am really pleased to have met you in
Groningen. I hope we can meet again somewhere and some time. I wish you the
best in your careers. Musty, it was very fun to meet you, we frequently discussed
about Indonesia and Nigeria. It was very surprising that you could spell some
Indonesian words almost perfectly. Thank you also for your help in checking the
English of my thesis. Nisrina, we met only in a short time but we discussed a lot.
I am sure you can be a great person and I wish you a successful (love and) life.
Furthermore, I would like to thank the other group members: Marianna, Sampson,
Matthijs, Vlad, Jorge, Hong Hua, Shuyan, Artem, Mark, Daniel, and Simon.
Thank you also for Jan Anton, Davide, Niels, Tejas, and Solmaz.
As part of my PhD research, I have a great chance to do research
collaboration in organic electronics research group, the University of Tokyo,
Japan. Therefore, I would like to thank the members of the group. Roger: it was
my best luck to meet you in Tokyo because you provided me many great ideas and
Acknowledgement
111
did some successful simulation. Without your help, it is indeed very difficult for
me to finish my thesis. I hope I can visit Switzerland some time. Yu Yamashita:
thank you for discussion and sharing all about Japanese things. Fujishima: my
very best Japanese friend, the most open-minded Japanese I have ever met. You
have helped me a lot. I am really pleased to meet you and please let me know if
you visit Indonesia. Hikari is asking about you. Thank you also for the other group
members especially Kumagai, Balthasar, Sybille, Miyake, Kishi, Kubo, and
Yamamura (I am so sorry I cannot mention all of other members here, but thank
you very much for your hospitality and help).
I would have never been at this stage without genuine prayer and support
from two greatest persons in my life, Mamah dan Bapak. Mamah, terima kasih
ya atas doa, motivasi, dan dukungannya selama ini. Terima kasih udah ngasih
pendidikan yang sangat baik dari kecil sampai sekarang ini. Dengan ridho dan
doa Mamah, alhamdulilah selesai juga PhD ini. Semoga ini bisa membuat
Mamah bangga dan semoga Mamah selalu sehat, panjang umur, dan ikut
merasakan kesuksesan selanjutnya. Bapak, mungkin Bapak ga baca tulisan ini.
Tulisan ini untuk nunjukin betapa besar peran Bapak selama ini. Bapak, orang
yang keras untuk masalah pendidikan khususnya pendidikan Agama. Bapak
selalu ngajarin berprestasi di sekolah itu penting tapi tidak ada gunanya kalau
meninggalkan sholat dan tidak bisa baca Alquran. Walau Bapak sekarang udah
di sana, Bapak, selalu bisa membuat semangat dan termotivasi untuk
menyelesaikan pendidikan ini. Semoga pencapaian ini bisa membuat Bapak
bangga dan senyum di sana.
For my two beloved and beautiful angels: Prawistin and Ara. Bunda, terima
kasih ya atas doa dan cinta tulusnya selama ini. Makasih ya udah selalu sabar,
selalu ada di samping, dan selalu kasih semangat pas lagi mumet riset dan nulis.
Sering kasih masukan juga untuk paper dan tesis, untungnya riset kita sebidang
ya. Mau lanjut PhD ga nih? :P Makasih juga ya udah ngasih semangat baru dan
kado terbaik di akhir-akhir PhD, jadinya makin semangat ngerjain eksperimen
dan nulis tesisnya. Mohon maaf selama ini belum jadi imam yang baik. Maafin
juga ya selama PhD sering ditinggal sendirian malem-malem ke lab. :D Emang
bener, dibalik suksesnya pria ada wanita hebat disampingnya. Ayo kita sukses
selanjutnya! Abis dari Belanda dan Jepang, kemana lagi nih kita? Untuk Ara,
ini dia nih kadonya dari Bunda. Makasih ya udah jadi spirit booster-nya Papah.
Hadiahnya ikut jalan-jalan ke Belanda ya. Insya Allah Ara, Hikari Nazafarin
Humaira, bisa jauh lebih baik dari Papah dan Bunda. Sehat, pinter, dan jadi
anak sholihah ya.
Untuk keluarga besar di Tangerang: Teteh Leli, Teteh Rita, Aa Ari, Bang
Boy, Om Fajar, Ka Ana, dan semua keluarga di Tangerang (maaf ga bisa
disebut satu-satu). Terima kasih ya atas doa dan dukungannya. Mohon maaf
selama merantau banyak ngerepotin. Makasih ya udah bantu jagain Mamah.
Charge transport and trap states in PbS QD field-effect transistors
112
Jangan lupa doain terus untuk kesuksesan selanjutnya ya. Untuk Bapak dan Ibu
di Semarang, terima kasih ya atas doa dan dukungannya untuk kelancaran
pendidikan S3 dan selesainya tesis ini. Terima kasih juga udah nitipin Wistin,
jadinya lebih semangat kuliah dan ngerjain tesisnya. Terima kasih juga atas
bantuannya selama tinggal di Semarang dan jalan-jalannya juga. Terima kasih
juga untuk Hanif, Mbah, semua keluarga Boyolali dan Sragen atas doa dan
dukungannya selama hidup merantau.
My life in Groningen is unforgettable and I cannot forget to thank my
Indonesian friends in Groningen: Om Pi’I, Mba Erlin, Idrus “Biber”, Mas Edy,
Mas Zaky-Mba Sella, Pak Tatang-Bu Rohmah, Pak-Bu Asmoro, Pak Taufik-Mba
Tina, Om Lutfi, Almas, Mas Doni, Guntur, Adhyat-Mba Nuri, Mas Didik-Mba
Rosel, Mas Kus-Mba Fitri, Om Hegar, Mas Zainal-Mba Ayu, Mas Fean, Om Fikri,
Radit-Nia, Didin, Mba Susan-Mas Bino, Mas Yusuf, Salma, Shiddiq, Cacha, Mbak
Adel, Mas Bintoro, Mba Shofi, Mba Tiur, Mba Nur, Mas Azis-Amalina, grup
sandwich ITB-Groningen (Felix, Ali, Liany, Ulfa, Reren, Anis, Resti, Raisa,
Mamay), Mas Krisna-Mba Icha, temen-temen deGromiest and PPI Groningen
that are not mentioned here. Being far away from family is not a big problem,
because I have met all of you. Thank you for making my life in Groningen more
colorful and full of happiness.
Some parts of my PhD life were done in Japan. Therefore, I would not forget
to give special thanks to my Indonesian friends in Tokyo, Japan especially Windy,
Ronald, Mbak Ety, Helbert, Mbak Ambar, Mbak Dian, Ikhlas, and Mas Ardhi. It
is well-known that living in Japan as researcher/student is full of pressure,
especially in Kashiwa where almost nothing around. But, it is not a big problem
when I meet all of you. I hope we can meet again in Tokyo and Indonesia, in the
future. See you on top.
I also would like to thank Anggita Putri Wibowo (Ghe) for your valuable time
in discussing and giving some initial ideas about the cover of my PhD thesis. I
wish you the best in your career and life.
For my best friends in Tangerang: Candra, Indra, Muklis, Avan, Boby, Tyo,
Adit, Budi, Alin, dan yang lainnya. Terima kasih ya atas doa dan sharingnya
selama ini. Terima kasih juga udah mengisi hari-hari nulis tesis ini sambil
diskusi Bahasa Inggris dan kerja bakti. Kapok ga? :P
I also would like to acknowledge all of my teachers and supervisors during
my study especially my lecturers and supervisors in Department of Physics,
Institut Teknologi Bandung (ITB): Pak Yudi, Pak Ferry, Pak Mikra, and Bu Inge.
Last but not least, also for my great teachers in SMA Negeri 7 Tangerang. Thank
you for your great support, contribution, and prayer during my study which
brought me at this stage.
~For all readers of this thesis, thank you very much for reading~
113
List of Publications
1. Daniel M. Balazs, Mohamad Insan Nugraha, Satria Zulkarnaen Bisri,
Mykhailo Sytnyk, Wolfgang Heiss, Maria Antonietta Loi, Reducing charge
trapping in PbS colloidal quantum dot solids, Appl. Phys. Lett. 2014, 104,
112104.
2. Mohamad Insan Nugraha, Roger Hausermann, Satria Zulkarnaen Bisri,
Hiroyuki Matsui, Mykhailo Sytnyk, Wolgang Heiss, Jun Takeya, Maria
Antonietta Loi, High Mobility and Low Density of Trap States in Dual-Solid-
Gated PbS Nanocrystal Field-Effect Transistors, Adv. Mater. 2015, 27, 2107-
2112.
3. Mohamad Insan Nugraha, Hiroyuki Matsui, Satria Zulkarnaen Bisri,
Mykhailo Sytnyk, Wolgang Heiss, Maria Antonietta Loi, Jun Takeya, Tunable
Doping in PbS Nanocrystal Field-Effect Transistors using Surface Molecular
Dipole, APL Materials, 2016, 4, 116105.
4. Mohamad Insan Nugraha, Hiroyuki Matsui, Roger Hausermann, Shun Watanabe, Takayoshi Kubo, Satria Zulkarnaen Bisri, Mykhailo Sytnyk, Wolgang Heiss, Maria Antonietta Loi, Jun Takeya, Strain-Modulated Charge Transport in Flexible PbS Nanocrystal Field-Effect Transistors, Adv. Electron. Mater. 2017, 3, 1600360.
5. Jeng-Ting Li, Li-Chih Liu, Jen-Sue Chen, Jiann-Shing Jeng, Po-Yung Liao,
Hsiao-Cheng Chiang, Ting-Chang Chang, Mohamad Insan Nugraha,
Maria Antonietta Loi, Localized tail state distribution and hopping transport
in ultrathin zinc-tin-oxide thin film transistor, Appl. Phys. Lett. 2017, 110,
023504.
6. Mohamad Insan Nugraha, Roger Hausermann, Hiroyuki Matsui, Shun
Watanabe, Mykhailo Sytnyk, Wolgang Heiss, Maria Antonietta Loi, Jun
Takeya, Broadening of the Distribution of Trap States in PbS Quantum Dot
Field-Effect Transistors with High-k Dielectrics, ACS Appl. Mater. Interfaces
2017, 9, 4719.
7. Mohamad Insan Nugraha, Shohei Kumagai, Shun Watanabe, Mykhailo
Sytnyk, Wolfgang Heiss, Maria Antonietta Loi, Jun Takeya, Enabling
ambipolar to heavy n-type transport in PbS quantum dot solids through
doping with organic molecules, ACS Appl. Mater. Interfaces 2017, Article
ASAP.