Magneto-optical study of InP/InGaAs/InP quantum well

B. Karmakar, A.P. Shah, M.R. Gokhale and B.M. Arora

Tata Institute of Fundamental Research

Mumbai, India

Plan of the talk:

1. Introduction to surface photo voltage (SPV) spectroscopy

2. Experimental setup

3. Growth and characterization of sample

4. Experimental results

5. Summary

Introduction to surface photo voltage (SPV) spectroscopy

SPV: Optical + Transport Process

a) Photon absorption and electron hole pair generation

b) Charge separation due to surface field.

Motivation:

a) MQW studied by B. B. Goldberg et al [ PRL 63, 1102 (1989)]

b) Growth of MQW is not possible in highly strained system

c) Transport and SPV spectroscopy can be done on same single quantum well sample

d) Quantitative measurement of join density of states and their evolution with magnetic field

SPV on bulk sample:

The wavelength scan gives band edge

Ec

Ev

EF

e

h

Generation of SPV in bulk materials Schematic spectrum

Wavelength

SP

V

Eg

SPV in quantum well structure:

A single quantum well can be probed easily

e

h

EC

EF

EV

Generation of SPV from a QW

Wavelength

SP

V

Eee1-Ehh1

Eee1-Elh1

Eee2-Ehh2

Schematic spectrum

Advantage over absorption or transmissionspectroscopy

a) SPV is very sensitive to SQW

b) In MQW energy levels are broader compared to SQW

c) Electron density is not same in all well in MQW structure

e) Local measurement is possible

SPV spectroscopy in the presence of magnetic fieldand selection rules

There are inter band transition between Landau levels

Parity conservation in growth direction for sub-band transition n = 0, 2 etc

Parity conservation of the LL n = 0

Spin conservation mj = 1+1/2

-1/2

+3/2

-1/2-3/2

+1/2

hh statesn = 0

e statesn = 0

mj

Tunable DiodeLaser Optical Switch

Optical FiberSample

ITO Coatedglass

Buffer AmplifierLock-in amp

Super conducting magnet

Schematic diagram of measurement setup

Tunable diode laser

Tunable range: 1520-1570 nm & 1565-1625 nm

Optical switch

Power requirement

Photo voltage saturates logarithmically with intensity

Experiment is done in linear regime

Illuminated power is sub-micro Watt

Structure of the system under study

SI InP Substrate

1500 Å InP buffer

90 Å In0.64Ga0.36As QW

100 Å InP spacer

200 Å Si doped InP

100 Å InP capModulation doped quantum well structure InP/InGaAs/InP is used for the study.

The sample is grown by metalorganic vapor phase epitaxy (MOVPE) under optimized conditions .

Sample structure

Schematic band diagram

EF

EC

EV

Characterization of the sample:

Pl measurement

X-ray diffraction

Electrical measurement: ns = 1.4 1011/cm2; µ = 90,000 cm2/V-sec

Photoluminescence spectrum of the sample

1525 1550 1575 1600 16250.00

0.25

0.50

0.75

1.00

T = 15 K

T = 1593 nm

ET = 0.778 eV

FWHM = 9 meV

PL

(A.

U.)

Wavelength (nm)

Experimental conditions

1. T << /k

2. Tunneling should be possible

3. There should not be any relative vibration between sample and electrode

Experimental results

Zero field temperature dependence of SPV

15701580

15901600

16101620

0

20

40

60

2040

6080

100120

140

Temp (K)

spv

(nm)

Without magnetic field results

Optical process enhances with the lowering of temperature

A peak like features is seen. This isattribute to formation of exciton

At high temperature exciton doesnot form due to low binding energy

At low temperature exciton does notbrake, therefore exciton peak vanishes

At low temperature tunneling is the main mechanism of charge separationfrom the quantum well

1560 1575 1590 1605 1620

0

20

40

60

30

6090

120150 Tem

p (K)

spv

(nm)

Shift of band edge

Zero field temperature dependence of SPV

The shift of band edge is due toincrease of band gap with the lowering of temperature.

A comparison between PL and SPV

1525 1550 1575 1600 16250

2

4

6

8

10

12

SPV spectra PL spectra

SP

V &

Pl s

pect

ra (

A.

U.)

Wavelength (nm)

SPV spectrum at finite field

1520 1530 1540 1550 1560 1570 1580 1590 1600 1610 1620

0

2

4

6

8

10

1529 nm 1550 nm 1572 nm

B = 2.5 T

SP

V

(nm)

Finite field results

Magnetic field dependence of SPV spectrum

15201540

15601580

16001620

0

4

8

12

16

0

2

4

6

8

Temp: 2.12 K

Mag

. Fie

ld (

T)

SP

V

(nm)

Magnetic field dependence of SPV

0

2

4

6

81520

15401560

15801600

1620

0

4

8

12

16

SPV

(nm) B (T)

Evolution of energy levels with magneticfield

0 2 4 6 80.780

0.785

0.790

0.795

0.800

0.805

0.810

0.815

2nd Peak

1st Peak

Temp: 2.12 K

Pe

ak

Po

sitio

n (

eV

)

Mag. Field (T)

Shift of peaks at higher energy with magnetic field

SPV spectroscopy is suitable to detectinter band LL transition in single QW

To characterized the transitions, QHexperiment is necessary

The width of join density of statescan be measured J(h) = E gh(E)ge(E + h) dE

Summary

SPV is shown to be a suitable techniques to probe magneto-optics ofsingle quantum well.

SPV signal increases with the lowering of temperature and then decrease further lowering of temperature.

The excitonic peak is observed , this feature disappear at low temperature.

To characterized the transitions, quantum Hall experiment is necessary.