magneto-optical study of inp/ingaas/inp quantum well
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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.
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