Semiconductor Nanostructures for Novel Device Applications · Semiconductor Nanostructures for Novel Device Applications Maria C. Tamargo The City College of New York 1 . Outline

Semiconductor Nanostructures for

Novel Device Applications

Maria C. Tamargo

The City College of New York

1

Outline

• Bandstructure Engineering

– Band Alignment

– Low Dimensionality

– Strain

– II-VI Materials

• Quantum wells

– R-G-B Emitters

– Intersubband devices

• Quantum dots

– QD-based Emitters

– Type-II QDs: Photovoltaics

• Summary

2

Semiconductor Nanostructures for Novel Device

Applications

Bandgap versus Lattice-Constant for Common Semiconductors

5.1 5.4 5.7 6.0 6.3 6.6-1

0

1

2

3

4

5

6

7

8

3.0 3.3 3.6-1

0

1

2

3

4

5

6

7

8

IVII-VI

III-V

BeSe

MgTe

HgTe

InSbInAs

GeGaSbSi

InPAlSb

GaAs

CdTeCdSeAlAsGaP

ZnTeAlPZnSe

ZnS MgSe

MgS

MgO

ZnO

AlN

GaN

Lattice Constant (A)

InN

Ban

dg

ap

en

erg

y a

t R

T (

eV

)


Applications

zinc blend

wurtzite

4

Bandstructure Engineering

Bulk semiconductor materials are artificially manipulated to

create a new material with a new “bandstructure” that is

useful for a particular application.

Examples:

• Semiconductor alloys

• Heterostructures

• Quantum wells

• Superlattices

• Quantum dots


Applications

5.1 5.4 5.7 6.0 6.3 6.6-1

0

1

2

3

4

5

6

7

8

3.0 3.3 3.6-1

0

1

2

3

4

5

6

7

8

BeSe

MgTe

HgTe

InSbInAs

GeGaSbSi

InPAlSb

GaAs

CdTeCdSeAlAsGaP

ZnTeAlPZnSe

ZnS MgSe

MgS

MgO

ZnO

AlN

GaN


InN

Ban

dg

ap

en

erg

y a

t R

T (

eV

)Bandgap versus Lattice-Constant for Many Semiconductors

InGaAs/InAlAs/InP

ZnCdMgSe/InP

InGaAs/InAsSb

InGaAlN

ZnMgO


Applications

Alloy Formation

Heterojunctions: Epitaxial Growth

Layer organizes in a

crystalline arrangement

by “mimicking” the

substrate crystal lattice.

The more similar the

substrate and the layer

crystalline structures are

the easier it is to

achieve good epitaxy.

Substrate

Crystalline Material A

Crystalline Material B

Layer


Applications

5.1 5.4 5.7 6.0 6.3 6.6-1

0

1

2

3

4

5

6

7

8

3.0 3.3 3.6-1

0

1

2

3

4

5

6

7

8

BeSe

MgTe

HgTe

InSbInAs

GeGaSbSi

InPAlSb

GaAs

CdTeCdSeAlAsGaP

ZnTeAlPZnSe

ZnS MgSe

MgS

MgO

ZnO

AlN

GaN


InN

Ban

dg

ap

en

erg

y a

t R

T (

eV

)Bandgap versus Lattice-Constant for Many Semiconductors

InGaAs/InAlAs/InP

ZnCdMgSe/InP

InGaAs/InAsSb

InGaAlN

ZnMgO


Applications

GaAs and InP are well developed substrate materials

Epitaxial growth technique

that takes place in ultra

high vacuum

“Growth” is produced by

the impingement of

elemental sources in

atomic or molecular form

on a heated substrate

surface.

Molecular Beam Epitaxy


Applications

Processes during Molecular Beam Epitaxy

9

Many different

processes compete at

the surface during

growth.

The growth conditions

determine which of

these dominate.

Ts, fluxes, substrate

surface orientation, etc.


Applications

Frank-van der Merwe Volmer-Weber

monolayer-by-monolayer island growth

Layer-by-layer mode is

preferred for high

quality ultrathin layers:

Quantum Wells

MBE Growth Modes


Applications

Near lattice-matched heteroepitaxy

11

type-I band alignment type-II band alignment

Semiconductor 2

Semiconductor 1

Semiconductor 1

Band Alignment

conduction

band

valence

band


Applications

Quantum Wells

CB e2

e1

ΔEv VB

Band to band

Band-to-band

transition wavelength

depends on the QW

thickness ‘d’

quantum size effects h1 h2

ΔEc


Applications

Barrier Barrier QW

d

Eg barrier

Eg well

discrete energy

levels in the well

5.1 5.4 5.7 6.0 6.3 6.6-1

0

1

2

3

4

5

6

7

8

3.0 3.3 3.6-1

0

1

2

3

4

5

6

7

8

BeSe

MgTe

HgTe

InSbInAs

GeGaSbSi

InPAlSb

GaAs

CdTeCdSeAlAsGaP

ZnTeAlPZnSe

ZnS MgSe

MgS

MgO

ZnO

AlN

GaN


InN

Ban

dg

ap

en

erg

y a

t R

T (

eV

)Bandgap versus Lattice-Constant for Common Semiconductors

ZnCdMgSe/InP


Applications

14

- 4 - 2 0 2 41 .5

2 .0

2 .5

3 .0

3 .5

4 .0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

Z n T e

C d S e

Z n S e

M g S e

Ba

nd

ga

p E

ne

rg

y (

eV

)

L a t t ic e M is m a tc h to I n P ( % )

0 2 0 4 0 6 0 8 0 1 0 0

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0 Z n

0 .5C d

0 .5S e

Z n0 .3

C d0 .4

M g0 .3

S e

Z n0 .3

C d0 .7

S e ( s t r a i n e d )

T = 1 0 K

En

erg

y (

eV

)

W e l l T h i c k n e s s ( Å )

ZnxCdyMg1-x-ySe on InP substrates


Applications

• Bandgaps ranging from

2.1 – 3.2 eV (3.6 eV)

• Low defect densities

• n and p-type doping

• p+ contact layer

(ZnSeTe)

ZnxCd(1-x)Se/ZnxCdyMg(1-x-y)Se QWs on InP

0 20 40 60 80 100

2.0

2.2

2.4

2.6

2.8

3.0 Zn

0.5Cd

0.5Se

Zn0.3

Cd0.4

Mg0.3

Se

Zn0.3

Cd0.7

Se (strained)

T = 10K

Ene

rgy

(eV

)

Well Thickness ( Å)

Photoluminescence emission energy

as a function of QW thickness

InP substrate

InGaAs buffer

ZnCdMgSe

ZnCdMgSe ZnCdSe

QW

ZnCdSe

buffer

cap


Applications

Appl. Phys Letters, 66, 2742 (1995)

App. Phys. Letters, 68, 3446 (1996)

QW structure

400 500 600 700

RT

521 nm

FWHM=17 nm551 nm

FWHM=18 nm

628 nm

FWHM=27 nm500 nm

FWHM=19 nm

Inte

nsity

(a.u

.)

Wavelength (nm)

-6 -4 -2 0 2 4 6

0

20

40

60

80

100

InP substrate

III-V buffer

ZnCdSe buffer

ZnxCdyMg1-x-ySe

ZnSeTe contact

ZnCdSe QW

n+

n

p

p+

nn

ZnxCdyMg1-x-ySe

Cur

rent

(mA

)

Voltage (V)

Light Emitting Diodes (LEDs)

RT Electroluminescence

ZnCdMgSe-based R-G-B Light Emitters

I-V Characteristics


Applications

J. Crystal Growth 214/215, 1058 (2000)

Photopumped R-G-B laser diodes

604

nm

540

nm

497

nm

400 450 500 550 600 650

Pum

p

Inte

nsit

y (a

.u.)

Wavelength (nm)

Room temperature lasing spectra

for three DH laser structures

InP substrate

InGaAs buffer

ZnCdMgSe ZnCdSe

buffer

ZnCdMgSe

ZnCdSe

QW

cap

ZnCdMgSe

ZnCdMgSe

cladding

cladding

waveguide

waveguide

Double heterostructure (DH)

laser structure


Applications

Appl. Phys. Lett., 70, 1351, (1997)

Appl. Phys. Lett., 72, 3136 (1998)

Exploring new Vertical External Cavity

Surface Emitting Laser (VECSEL)

Intersubband transitions

CB e2

e1

h1 h2

ΔEv VB

Barrier Barrier QW

Band to band

Intersubband CBO (ΔEc)

ISB transition

short

wavelength

limit is given by

the CBO

Need new materials with large CBO for shorter wavelength


Applications

Intersubband

transition energy

depend ONLY on the

QW thickness ‘d’

Conduction band offset (CBO)

ZnMgSe (3.6eV, 0.1 µm)

50 Å ZnCdSe QW

ZnMgSe (3.6eV, 0.5 µm)

InP-SI substrate

InGaAs layer (0.1 µm)

ZnCdSe cap layer

LT-ZnCdSe buffer

∆Eg (Eg barrier – Eg well) = 1.5 eV

CBO = 0.8 ∆Eg ~1.12 eV

- 4 - 2 0 2 41 .5

2 .0

2 .5

3 .0

3 .5

4 .0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

Z n T e

C d S e

Z n S e

M g S e

Ba

nd

ga

p E

ne

rg

y (

eV

)


0 2 0 4 0 6 0 8 0 1 0 0

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0 Z n

0 .5C d

0 .5S e

Z n0 .3

C d0 .4

M g0 .3

S e

Z n0 .3

C d0 .7


T = 1 0 K

En

erg

y (

eV

)


o

ZnxCdyMg(1-x-y)Se

Appl. Phys. Lett. 83, 1995 (2003)


Applications

Measured the CBO using

Contactless electroreflectance

Growth of doped MQW samples for FT-IR

Zn0.20Cd0.19Mg0.61Se, 0.1m

10X MQW

Zn0.20Cd0.19Mg0.61Se, 0.5m

SI-InP substrate

Zn0.20Cd0.19Mg0.61Se, 14nm

Zn0.5Cd0.5Se (n~1x1018) 5nm

Eg (barrier) = 3.0 eV

Eg (well) = 2.1 eV

CBO = 0.7 eV

- 4 - 2 0 2 41 .5

2 .0

2 .5

3 .0

3 .5

4 .0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

Z n T e

C d S e

Z n S e

M g S e

Ba

nd

ga

p E

ne

rg

y (

eV

)


0 2 0 4 0 6 0 8 0 1 0 0

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0 Z n

0 .5C d

0 .5S e

Z n0 .3

C d0 .4

M g0 .3

S e

Z n0 .3

C d0 .7


T = 1 0 K

En

erg

y (

eV

)


Zn0.5Cd0.5Se (n~1x1018) 5nm

Zn0.5Cd0.5Se (n~1x1018) 5nm


Applications

ZnxCdyMg(1-x-y)Se

21

0.1 0.2 0.3 0.4 0.5 0.6

12 9 6 4 3

No

rma

lize

d A

bs

orb

an

ce

Energy (eV)

Sample A

Sample B

Sample C

RT

Wavelength (m)

15 20 25 30 35 40 45 50

3

4

5

6

7C

B

Ab

so

rpti

on

(m)

QW width (A)

Qc=0.70

Qc=0.75

Qc=0.80

Exp.

A

RT

ISB absorption in 4-7 m mid-IR

Bound-to-bound

Appl. Phys. Lett. 89, 131903 (2006)

A = 50Å, B = 40 Å, C = 30 Å

ISB absorption in MQWs: FT-IR spectroscopy


Applications

High resolution

x-ray diffraction

Wu, et al, APL (2009) FT-IR Absorption

Transmission Electron

Microscopy

Good material quality

Absorption in the mid-IR region

J. Crystal Growth, 310, 5380 (2008)

ZnCdMgSe Material Quality


Applications

3-well active region graded “digital alloy” injector region 40 repeats of active/injector Waveguide layers for optical confinement

I

I

E=130 kV/cm

(>800 layers)

InP substrate:n–

InGaAs:n+

ZnCdSe n

ZnCdSe contact:n+

Active core

X 30

Injector (n doped)

Injector (n doped)

ZnCdSe n

ZnCdMgSe n

ZnCdMgSe n

Active region

ZnCdSe (LT)

waveguide

waveguide

Conduction band profile of active/injector region

QC Laser Structures


Applications

Active

region

QC laser electroluminescence

Narrow EL FWHM (270 cm-1 to 220 cm-1)

Room temperature EL

App. Phys. Lett. 99, 041113 (2011) J. Electron. Mat. 41, 944 (2012)

• Fabricated QC emitters with Pt/Au metal contacts


Applications

Dimensionality

QUANTUM

DOT


Applications

d3

2-D structures:

quantum wells

d2

a

1-D structures:

quantum wires

a

b

0-D structures

a

b

c

lower dimensional structures exhibit more efficient

radiative recombination (i.e., light emission)

Frank-van der Merwe Volmer-Weber Stransky-Kranstanov

monolayer-by-monolayer island growth

MBE Growth Modes

self-assembled QDs

Strain-driven QD formation

Wetting

layer


Applications

27

- 4 - 2 0 2 41 .5

2 .0

2 .5

3 .0

3 .5

4 .0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

Z n T e

C d S e

Z n S e

M g S e

Ba

nd

ga

p E

ne

rg

y (

eV

)


0 2 0 4 0 6 0 8 0 1 0 0

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0 Z n

0 .5C d

0 .5S e

Z n0 .3

C d0 .4

M g0 .3

S e

Z n0 .3

C d0 .7


T = 1 0 K

En

erg

y (

eV

)


ZnxCdyMg1-x-ySe on InP substrates


Applications

• Bandgaps ranging from

2.1 – 3.2 eV (3.6 eV)

• Low defect densities

• n and p-type doping

• p+ contact layer

(ZnSeTe)

CdSe SAQDs on ZnCdMgSe

CdSe tD =10” (2 ML)

Height: 1.6 nm

Density: 1.02x109 cm-2

Radius: 30 nm

Ec

Ev

ZnCdMgSe CdSe

Type-I band alignment

Stranski-Krastanov Growth


Applications

J. Cryst. Growth 294, 296–303 (2006)

QD emission energy (i.e.

size) varies regularly with

CdSe deposition time

Can achieve emission

throughout the visible

spectrum range

CdSe SAQDs on ZnCdMgSe grown with different CdSe

deposition times


Applications

InP substrate

InGaAs buffer

ZnCdMgSe

ZnCdMgSe

cap

CdSe

QDs wetting

layer

Appl. Phys. Lett. 85, 6395 (2004)

Stacked-QD LED structure

(white-light source)

Curr

ent

(A)

Voltage (V)

Room

temperature

PL emission

I-V


Applications

J. Vac. and Sci. Technol. B 23, 1236 (2005)

Stacked multi-QD structures

ZnCdSe (70 Å)

ZnCdSe (60 Å)

ZnCdMgSe (1300 Å)

InP (SI)

ZnCdMgSe

InGaAs (1500 Å)

ZnCdSe

ZnCdMgSe ZnCdMgSe

spacer

CdSe QDs (tD=10”) 29x

Stacked or multi-QD

structure

5 µm

2.5 µm

0 µm

5 µm

2.5 µm

0 µm

7.77 nm

3.89 nm

0 nm

5 µm

2.5 µm

0 µm 0 µm

2.5 µm

5 µm

7.77 nm

AFM image of an uncapped

multi-QD structure:

Lateral alignment: quantum

wires?


Applications

J. Vac. Sci. Technol. B 24, 1649 (2006)

Stacked QD layers are

vertically aligned

32

type-I band alignment type-II band alignment

Semiconductor 2

Semiconductor 1

Semiconductor 1

Band Alignment

conduction

band

valence

band


Applications

GaAs Substrate

ZnSe Spacer

ZnSe Spacer

ZnSe Spacer

ZnSe

Zn

Te

Zn

Te

Zn

Te

Zn

ZnSe Buffer

× Nperiods

ZnSe Spacer

Shutter sequence

< 1ML ZnTe:N

Submonolayer type-II QDs


Applications

Migration Enhanced Epitaxy

< 1ML ZnTe:N

Shutter sequence

Zn

Te

Zn

Te

Zn

Te

Zn

GaAs Substrate

ZnSe Spacer

ZnSe Spacer

ZnSe Spacer

ZnSe

ZnSe Buffer

× Nperiods

ZnSe Spacer

New composite

material made

up of ZnTe

nanoislands

embedded in a

matrix of ZnSe

Submonolayer type-II QDs


Applications

Phys. Rev. B 77, 15534 (2008)

Migration Enhanced Epitaxy

Frank-van der Merwe Volmer-Weber Stransky-Kranstanov

monolayer-by-monolayer

MBE Growth Modes

self-assembled QDs

Wetting

layer


Applications

island growth

No wetting layer

Evidence for type-II alignment:

Energy shift of the “green band”

“blue band” “green band”

Ec

Ev

Type I

+

_ A B

Ec

Ev

Type II

+

_


Applications

Spatial separation of carriers

“blue band” “green band” weak excitation strong excitation

hweak < hstrong


Applications



“blue band” “green band”

Figure 2.2: The probability densities of the

ground electron and hole states in type-II

ZnTe/ZnSe QDs.

Spatial separation of electrons and holes and other interesting phenomena

May result in enhanced materials properties for Photovoltaics


Applications

Phys. Rev. B. 71, 045340 (2005)



Evidence for QDs:

Structural characterization & magneto optics

TEM studies indicate the presence of stacked nanoislands of high Te content

Magneto optical studies show characteristic signature of 0-d structures (Aharonov-Bohm Effect)

TEM Characterization

J. Appl. Phys. 99, 064913 (2006)


Applications

Intermediate band solar cell (IBSC)

• An IBSC has an intermediate band (IB)

material with a band of states within the band

gap of the host semiconductor.1,2

• Absorption of below band gap photons is

possible without significant reduction in open

circuit voltage.

• An optimal IBSC with an efficiency of about

63%, has a host band gap of 1.95 eV, and an

IB at 0.71 eV. 1,2

• Can be fabricated using semiconductor QDs

embedded in a host semiconductor matrix.

• Challenges: Unavailability of the appropriate

material systems.3

1Luque, A. & Martí, Phys. Rev. Lett. 78, 5014-5017 (1997). 2Luque, A., Martí, A. & Stanley, C. Nature Photon. 6, 146-152 (2012). 3Luque, A. & Martí, A. Adv. Mater. 22, 160-174 (2010).


Applications

- 4 - 2 0 2 41 .5

2 .0

2 .5

3 .0

3 .5

4 .0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

Z n T e

C d S e

Z n S e

M g S e

Ba

nd

ga

p E

ne

rgy

( e

V )


0 2 0 4 0 6 0 8 0 1 0 0

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0 Z n

0 .5C d

0 .5S e

Z n0 .3

C d0 .4

M g0 .3

S e

Z n0 .3

C d0 .7


T = 1 0 K

En

erg

y (

eV

)


o

ZnxCdyMg(1-x-y)Se

ZnCdSe

Eg ~2.1 eV


Applications

ZnTe

QDs

Proposed (ideal) material for IBSCs

ZnTe QDs in ZnCdSe

• ZnCdSe has a band gap of ~ 2.1 eV

when lattice matched to InP.

• ZnTe-ZnCdSe valence band offset is

0.8 – 1.0 eV.

• Intermediate band at ~ 0.7 eV can

be engineered from the hole

confinement energy level within

ZnTe QDs.

• Type-II band alignment is expected

to suppress both radiative and Auger

recombination and improve the

carrier extraction process

significantly.

Parameters for an ideal IBSC:

Host semiconductor band gap ≈ 1.95 eV

IB at ≈ 0.71 eV


Applications

• Samples were grown by combination of molecular

beam epitaxy (MBE) and migration enhanced

epitaxy (MEE)

• During MEE, only submonolayer quantities of Zn,

and Te were used along with a specific shutter

sequence.

• .

ZnTe type-II QDs in ZnCdSe

InP Substrate

ZnCdSe Spacer

ZnCdSe Spacer

ZnCdSe

ZnCdSe Buffer

× Nperiods

ZnCdSe Spacer

ZnCdSe Spacer

Expected structure


Applications

J. Vac. Sci. Technol. B 31(3) C119 (2013).

Optical properties: type-II band alignment

• The HRXRD confirms the high structural quality of the sample

• The excitation intensity dependent PL at 10K showed a shift in PL energy,

demonstrating the type-II nature of the multilayered structure.

Blue

band Green

band


Applications

HRXRD Photoluminescence (PL)

Vertical correlation and miniband formation

ensure sufficient overlap of the QD confined hole wave functions to facilitate the

miniband formation.

Formation of miniband needed for successful operation of an IBSC to increase

below band gap absorption


Applications

Exploring growth of larger QDs to achieve vertical correlation

46

• Band structure engineering of semiconductor materials gives rise to a

myriad of novel physical phenomena and device functionalities

• Alloying, heterojunctions, low dimensionality, strain phenomena and

band alignment are some of the parameters that can be manipulated

for bandstructure engineering and device design

• R-G-B light emitters based on band-to-band emission in QW-based

structures of ZnCdMgSe have been demonstrated

• Devices based on intersubband transitions, such as QC lasers and

detectors, and QW infrared photodetetctors (QWIPs) are being

pursued for shorter wavelength applications in the mid-IR

• Strain effects are exploited to fabricate self-assembled QDs in

ZnCdMgSe structures that have shown potential for light emitting

devices

• Type-II QDs and the resultant spatial separation of carriers are being

explored for photovoltaic applications in IBSC


Applications

Summary

47


Applications

Acknowledgements: Linfei Zeng, Ning Dai, Abdullah Cavus, Wilson Lin, Catherine

Luo, Oleg Maksimov, Mohammad Sohel, Zuecong Zhou, Noemi

Perez-Paz, Hong Lu, Martin Muñoz, Shiping Guo, William

Charles, Kale Franz, Adrian Alfaro-Martinez, Yu Yao, Qiang

Zhang, Richard Moug, Joel de Jesus, Thor Axtmann Garcia,

Vasilios Deligiannakis, Arvind Ravikumar, Siddarth Dhombar

Aidong Shen, Igor Kuskovsky, I. Cevdet Noyan, Claire Gmachl

Documents

Semiconductor Nanostructures for Novel Device Applications · Semiconductor Nanostructures for Novel Device Applications Maria C. Tamargo The City College of New York 1 . Outline