inas on gaas self assembled quantum dots by kh. zakeri sharif university of technology, spring 2003

InAs on GaAs self InAs on GaAs self assembled Quantum Dotsassembled Quantum Dots

By KH. Zakeri sharif University of technology, Spring

2003

OutlineOutline

I. introductionII. Method of fabricationIII. Physical properties IV. characterizationV. Modeling & Simulation VI. Application

I. IntroductionI. Introduction

At first time Reed & coworkers in Texas

Method of formation Method of formation

I. Etching & lithography II. Selective growthIII. Self assembled ~ self organized

Growth System– Molecular Beam Epitaxy (MBE)– Metal-Organic Chemical Vapor Deposition (MOCVD)– Chemical Beam Epitaxy (CBE)

Influence of deposition conditions– Deposition mode and misorientation– Growth rate– Group V pressure– High index substrate– Capping layer– Stacking of QDs

QD size and densityLuminescence

StabilityGain saturation

QD size and densityLuminescence

StabilityGain saturation

III. Self assembled

Quantum Dot GrowthQuantum Dot Growth

• Growth modes depends on

1. Interface energy2. Lattice mismatch

FdvM VW SK

Placing monolayers of different lattice constants on top of each other can result in a deformation of the deposited layers

Essentially a way of relieving stress on the material

Under a certain set of parameters, you can create certain deformations called Quantum Dots

~2

0-3

0 n

m

Quantum Dot GrowthQuantum Dot Growth

Physical properties Physical properties I. Density of state

II. Optical absorption & quantum transition

)(2

nmlzyxnlm

EEttt

DOS

),()(3

41(( 5

.

RRPRdRV QD

DQav

j j

jQD

jjQD RRE

RRf

22 ]2/))(())([(

2/))(()(

•III. Energy state

)](9

[2

)2()2(2

0

1 JPPm

HH BLh

esource drain

gate

N= 2 1

0

N= 2 1

v on

offR1 R2

C1 C2

IV. Coulomb Blockade Effect & coulomb oscillation

Quantum tunneling of electron between source and drain can be blocked If the charging energy

Ec = e2/2C >> kT

c=4R

E = eV –Ec <0: blocked

2Ec =e2/(C1+C2)

PhotoluminescencePhotoluminescence We can use a laser to excite electrons into the

conduction band Recombination will often produce a photon – the energy

of this photon tells us what state the electron and hole were in

E

EC

EV

laser emitted lightEG emitted light

hν=2.4 eV hν=1.5 eV

(GaAs)

hν≈1.2 eV

(QDs)

Experimental SetupExperimental Setup

Laser – Argon or HeNe

diffraction grating

Sample dewar

77 Kelvin

Spectrometer

photo-multiplier tube

77 Kelvin

amplifier

I

E / λ

computer

diffraction grating

InAs Quantum Dots in GaAs InAs Quantum Dots in GaAs [100][100]

229C2a - HeNe

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1.911.841.781.731.681.631.581.541.491.451.421.381.351.321.281.261.231.21.171.151.131.11.08

Photon energy (eV)

Phot

on c

ount

s pe

r 3 s

econ

ds

GaAs

GaAsInAs

Modeling & simulationModeling & simulation Growth simulation1. Quasi particle 2. Poison – Schrödinger eq.3. Slater transition Energy

Model calculation's

Probability density isosurfaces

for an electron confined in a QD

• Kinetic Mont Carlo

• Molecular Dynamic • Random Deposition

• Structural properties 1. Quantum confined2. Tight-Bonding 3. Effective mass4. D.F.T

Modeling & simulationModeling & simulation

Squared pyramid S. Ruvimov et al, PRB 51, 14766 (1995)

Truncated pyramid N. Liu et al, PRL 84, 334 (2000)

Lens J. Zou et al, PRB 59, 12279 (1999)

JM Moisson et al, APL 64, 196 (1994)

Ring RJ Warburton et al, Nature 405, 926 (2000)

Elongated pyramidW. Yang et al, PRB 61, 2784 (2000)

Numerical solution

• Infra-red detector 1. Military 2. Communication 3. Multistage detector4. High Optical gain

• QD laser Diode

• QD transistors & Devices

• QD nano oscillator

Applications Applications

Thanks for your attention Thanks for your attention