Transcript No Slide Title

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:
e
Ec
Eg
EF
h
Ev
Wavelength
Generation of SPV in bulk materials
The wavelength scan gives band edge
Schematic spectrum
SPV in quantum well structure:
e
EC
EF
Eee2-Ehh2
Eee1-Elh1
Eee1-Ehh1
h
EV
Wavelength
Generation of SPV from a QW
Schematic spectrum
A single quantum well can be probed easily
Advantage over absorption or transmission
spectroscopy
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 field
and selection rules
mj
e states
n=0
-1/2
+1/2
Parity conservation in growth direction for
sub-band transition n = 0, 2 etc
Parity conservation of the LL n = 0
hh states
n=0
+3/2
+1/2
-1/2
-3/2
Spin conservation mj = 1
There are inter band transition between Landau levels
Schematic diagram of measurement setup
Tunable Diode
Laser
Optical Switch
Optical Fiber
Super conducting magnet
Lock-in amp
Buffer Amplifier
ITO Coated
glass
Sample
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
Modulation doped quantum well
structure InP/InGaAs/InP is used
for the study.
100 Å InP cap
200 Å Si doped InP
100 Å InP spacer
The sample is grown by metalorganic
vapor phase epitaxy (MOVPE) under
optimized conditions .
90 Å In0.64Ga0.36As QW
1500 Å InP buffer
SI InP Substrate
Sample structure
Schematic band diagram
EC
EF
EV
Characterization of the sample:
Pl measurement
1.00
T = 15 K
T = 1593 nm
ET = 0.778 eV
FWHM = 9 meV
PL (A. U.)
0.75
0.50
0.25
0.00
1525
1550
1575
1600
1625
Wavelength (nm)
Photoluminescence spectrum of the sample
X-ray diffraction
Electrical measurement: ns = 1.4  1011/cm2; µ = 90,000 cm2/V-sec
Experimental conditions
1. T << /k
2. Tunneling should be possible
3. There should not be any relative vibration between
sample and electrode
Experimental results
Without magnetic field results
Optical process enhances with the
lowering of temperature
60
A peak like features is seen. This is
attribute to formation of exciton
spv
40
At high temperature exciton does
not form due to low binding energy
20
0
1570
1580
1590
 (n 1600
1610
m
)
60
40
1620
20
80
140
120
100
Te
(
mp
K)
Zero field temperature dependence of SPV
At low temperature exciton does not
brake, therefore exciton peak vanishes
At low temperature tunneling is the
main mechanism of charge separation
from the quantum well
Shift of band edge
60
The shift of band edge is due to
increase of band gap with the
lowering of temperature.
spv
40
Te
150
120
90
60
30
20
1575
1590
1605
(K)
1560
mp
0
1620
 (nm)
Zero field temperature dependence of SPV
A comparison between PL and SPV
12
SPV & Pl spectra (A. U.)
10
8
SPV spectra
PL spectra
6
4
2
0
1525
1550
1575
Wavelength (nm)
1600
1625
Finite field results
10
B = 2.5 T
8
SPV
6
4
2
0
1529 nm
1550 nm
1572 nm
1520 1530 1540 1550 1560 1570 1580 1590 1600 1610 1620
 (nm)
SPV spectrum at finite field
Magnetic field dependence of SPV spectrum
Temp: 2.12 K
16
12
SPV
16
8
12
6
8
2
1540
1560
 (n
0
1580
m)
1600
)
ld (T
0
1520
. F ie
V
4
Mag
SP
4
8
4
1620
1600
0
1580
0
2
4
1560
 (nm)
1620
Magnetic field dependence of SPV
6
1540
1520
8
B (T)
Evolution of energy levels with magnetic
field
0.815
SPV spectroscopy is suitable to detect
inter band LL transition in single QW
Temp: 2.12 K
Peak Position (eV)
0.810
To characterized the transitions, QH
experiment is necessary
2nd Peak
0.805
0.800
The width of join density of states
can be measured
J(h) = E gh(E)ge(E + h) dE
0.795
1st Peak
0.790
0.785
0.780
0
2
4
6
8
Mag. Field (T)
Shift of peaks at higher energy with magnetic field
Summary
SPV is shown to be a suitable techniques to probe magneto-optics of
single 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.