Nanoscale probing the electronic transport in transition metal … · 2020. 10. 22. · Filippo Giannazzo –NanoInnovation 2020, Roma, September 16, 2020 Bright-field TEM and diffraction

Filippo Giannazzo – NanoInnovation 2020, Roma, September 16, 2020

Filippo Giannazzo

Nanoscale probing the electronic transport in transition

metal dichalcogenides by conductive atomic force

microscopy

* [email protected]

CNR-Institute for Microelectronics and Microsystems,

Catania, Italy


The 2D materials family

A. Geim, I. Grigorieva, Nature 499, 419–425 (2013)

1.7 eV1.5 eV1.9 eV1.8 eVEg(1L), direct

1.2 eV1.1 eV1.4 eV1.2 eVEg(bulk), indirect

WSe2MoSe2WS2MoS2

1.7 eV1.5 eV1.9 eV1.8 eVEg(1L), direct

1.2 eV1.1 eV1.4 eV1.2 eVEg(bulk), indirect

WSe2MoSe2WS2MoS2

Layered materials with the formula MX2M transition metal: Mo, W,..

X chalcogen: S, Se,...

Q.H.Wang, K.Kalantar-Zadeh, A.Kis, J.N.Coleman and M.S.Strano, Nature Nanotechnology 7, 699–712 (2012).


Field effect transistors based on TMDs

B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nature Nanotechnology 6, 147 (2011)

-ultrathin, uniform channel thickness

for excellent gate control

-On/Off current ratio >107

-nearly ideal subthreshold swing

(SkT log(10))

μ=217 cm2V–1s-1

Production methods for electronic quality TMDs

Exfoliation from bulk crystals Chemical vapour deposition

• Direct deposition on insulating or semiconductor

substates (SiO2, sapphire, GaN)

• Growth temperature range: 700 – 900 °C

Y.-H. Lee, et al., Advanced Materials 24, 2320-2325 (2012)

• Micrometer size flakes

Electronic properties of state of the art TMDs strongly affected by electrically

active point and extended defects


Nanoscale defects in exfoliated MoS2

F. Giannazzo, E. Schilirò, G. Greco, F. Roccaforte, Conductive Atomic Force Microscopy of Semiconducting

Transition Metal Dichalcogenides and Heterostructures, Nanomaterials 10, 803 (2020)

P. Bampoulis, et al. Defect Dominated Charge Transport and Fermi Level Pinning in MoS2/Metal Contacts.

ACS Appl. Mater. Interfaces 9, 19278–19286 (2017).

S. Das, et al., High Performance Multi-layer MoS2 Transistors with Scandium Contacts, Nano Lett. 13, 100

(2013).

Fermi level pinning below Ec

cMoS2=4.0 eV

Ideal band alignment of common

elementary metals to MoS2

Ev

EC

MoS2 surface after exfoliation

C-AFM

STM of a nanometer defect and a S vacancy

LFM and AFM (insert)

Origin of the contact resistance typically

observed at the interface between metals

and MoS2


Dark Field-TEMBright-field TEM and diffraction

A. M. van der Zande, et al., Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide, Nature Materials

12, 554–561 (2013).

2D polycrystalline material: grain boundaries (GBs)

Extended defects in CVD grown MoS2

Transport model

T. H. Ly, et al., Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries,

Nature Communications 7, 10426 (2016).

Impact of GBs on the electronic transport in MoS2 transistors


Nanoscale resolution electrical investigation of exfoliated

and CVD MoS2 by conductive atomic force microscopy

(C-AFM)


Nanoscale Schottky barrier distribution on exfoliated MoS2 surface

CAFM

F. Giannazzo, G. Fisichella, A. Piazza, S. Agnello, F. Roccaforte, Phys. Rev. B 92, 081307(R) (2015).

-2 -1 0 1 2

-0.4

-0.2

0.0

0.2

0.4

Vtip

(V)

Curr

ent (

A)

0.0 0.5 1.010

-10

10-9

10-8

10-7

Vtip

(V)

B,n

=0.32 eV

n=1.05

I (

A)

Array of 5x5 local I-V curves, 50 nm spacing

Evaluation of the local Schottky barrier

height by thermionic emission model

0.2 0.3 0.4 0.5 0.6 0.7 0.80

50

100

Counts

(%

)

B,n

(eV)

MoS2 Schottky barrier map and histogram

nkT

qV

kTTAAI

tipnB

tip expexp,2*


Soft O2 plasma functionalization of MoS2 surface

Remote O2 plasma source and no accelerating bias, to avoid physical

etching of MoS2

F. Giannazzo, et al., ACS Appl. Mater. Interfaces 9, 23164−23174 (2017).

-Partial oxidation of the MoS2 surface: Mo6+ and substoichiometric Mo(6-x)+ states

- Negative shift of S 2p, Mo 3d and S 2s peaks: Fermi level shift towards the valence band (MoS2 p-type doping)

XPS of pristine and O2 functionalized MoS2 surface

hν = 1486.6 eV


Inhomogeneous distribution of patches with low Schottky

barrier height for holes (high for electrons)MoS2MoS2

Si

SiO2

MoS2Metal

MoS2

APt tipO2 plasma

functionalization

O2 plasma, t=600 s

Nanoscale investigaton of Schottky barrier distribution: O2 plasma functionalized MoS2

Array of 5x5 local I-V curves,

50 nm spacingBoth I-V curve shapes of

Schottky contacts on n-type

and p-type semicondutors

0.0 0.5 1.010

-10

10-9

10-8

10-7

Vtip

(V)

B,n

=0.45 eV

n=1.5 eV

I (A

)

-1.0 -0.5 0.010

-10

10-9

10-8

10-7

Vtip

(V)

|I| (A

)

B,p

=0.49 eV

n=1.15 eV

0.2 0.3 0.4 0.5 0.6 0.7 0.80

50

100

Counts

(%

)

B,n

(eV)

Thermionic emission model

nkT

qV

kTTAAI

tipnB

tip expexp,2*

nkT

qV

kTTAAI

tippB

tip expexp,2*

F. Giannazzo, et al., ACS Appl. Mater. Interfaces 9, 23164−23174 (2017).


Pristine MoS2 field effect transistor

Transfer characteristics

F. Giannazzo, et al., Phys. Status Solidi –RRL 10, 797 (2016)

n-type behavior

MoS2 FET with selective O2 plasma functionalization under the contacts

VG>0: electron injection through n-type MoS2 regions

VG


Growth conditionsSulfur T1=170°C

T2 = 785°C

Flows= 3 sccm N2Powders: 50 mg MoS2 + 75 mg MoO3 powders were placed in the quartz boat

CVD growth of MoS2 onto a SiO2/Si substrate

Y. Okuno, O. Lancry, A. Tempez, C. Cairone, M. Bosi, F. Fabbri, M. Chaigneau, Nanoscale 10, 14055-14059 (2018).


360 390 420 450 480

0

2000

4000

6000(A

1g-E

2g)= 21 cm

-1

A1g

E2g

Inte

nsity

(a.

u.)

Raman shift (cm-1)

Morphological and spectroscopic characterization

Optical Microscopy Tapping mode AFM

1.2 1.4 1.6 1.8 2.0 2.2

0

20000

40000

60000

PL in

tensi

ty (

a.u

.)

Energy (eV)

PL Raman spectroscopy

Strong emission at 1.85 eV:

excitonic direct bandgap of MoS2monolayer

• E2g in-plane and A1g out-

of-plane phonon modes

of MoS2.

• Vertical dashed lines:

nominal E2g and A1g positions for bulk MoS2.

• Wavenumber difference

between the peaks’

positions =21 cm-1:

mono- or bilayer MoS2domains

• Large triangular

MoS2 domains (up

to 50 m).

• Domains

coalescence:

presence of GBs


-40 -20 0 20 40

0

50

100

150

200

I D

(n

A)

VG (V)

VDS

=3V

-40 -20 0 20 40

0

1

2

3g

m,peak

Vth

g

m (

nS

)

0 2 4 6 8 10

0

100

200

300

400

500

600

700

I D (

nA

)

VDS

(V)

VG (V)

-50

-40

-30

-20

-10

0

10

20

30

40

Monolayer MoS2 field effect transistor

L7 m, W5 m

tox

consistent with typical

values =0.1 - 2 cm2V-1s-1

for 1L MoS2 onto a SiO2substrate, without high-k

dielectric capping.

Field effect mobility

B. Radisavljevic, et al. Nature

Nanotechnol. 6, 147–150 (2011).

F. Giannazzo, M. Bosi, F. Fabbri, E. Schilirò, G. Greco, F. Roccaforte, Phys. Status Solidi RRL 1900393 (2019)

Output characteristics

Transfer characteristic

DSox

peak,m

VC

g

W

L0.12 cm2V-1s-1

•Linear behavior at intermediate

VDS values, followed by saturation

at higher VDS.

•Non-linear current onset at low

bias (VDS


D1

D2

D1

D2

D1 D2

D1

D2

Nanoscale current mapping by conductive atomic force microscopy (CAFM)

CAFM powerful tool to probe local current injection in 2D materials

F. Giannazzo, G. Greco, F. Roccaforte, C. Mahata, M. Lanza, Conductive AFM of 2D Materials and Heterostructures for

Nanoelectronics, Chapter 10 of “Electrical Atomic Force Microscopy for Nanoelectronics“. Editor U. Celano. Springer Nature,2019.

Morphology Current map

F. Giannazzo, M. Bosi, F. Fabbri, E. Schilirò, G. Greco, F. Roccaforte, Phys. Status Solidi RRL 14, 1900393

(2019)


D1

D2

2 4 6 8 101.5

2.0

2.5

3.0

3.5

Rs2

=559+/-5 M

I (nA)

RS1

=70+/-1 M

H(V

)

D1

D2

Local Schottky barrier height and resistance within individual MoS2 domains

Local current-voltage analyses CAFM tip on the MoS2 domains D1 and D2

D1

D2

•Rectifying behavior of the tip/MoS2 contact

•For Vtip>0 at low bias (from 0 to 0.5 V):

•H function from the I-Vtip characteristics

at higher current:

nkT

qVexp

kT

qexpT*AAI

tipB2

tip

Similar values of B on domains D1 and D2

Evaluation of local resistance on individual domains

2*tip TAA

Iln

q

nkTVH

• The H function depends on Rs as

H= nB+IRs

Rs2>>RS1 due to the grain

boundary resistance RGB

F. Giannazzo, M. Bosi, F. Fabbri, E.

Schilirò, G. Greco, F. Roccaforte,

Phys. Status Solidi RRL 14, 1900393

(2019)

SummaryElectronic properties of TMDs strongly affected by electrically active point and

extended defect

Acknowledgments

Conductive Atomic Force Microscopy powerful technique to investigate current

injection mechanism in TMDs and lateral conductivity:

• Local Schottky barrier height mapping

• Resistance contribution of grain boundaries in monolayer CVD MoS2

http://etmos.imm.cnr.it

ETMOS

JTC-2019

E. Schilirò, S. E. Panasci, G. Greco, I. Deretzis, G. Nicotra, A. La Magna,

R. Lo Nigro, P. Fiorenza, F. Roccaforte, C. Spinella

Parma, Italy

M. Bosi

Pisa, Italy

F. Fabbri

Catania, Italy

Univ. Palermo, Italy

S. Agnello, F. M. Gelardi

Documents

Nanoscale probing the electronic transport in transition metal … · 2020. 10. 22. · Filippo Giannazzo –NanoInnovation 2020, Roma, September 16, 2020 Bright-field TEM and diffraction