1. 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

2. 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

3. 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.

4. 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

5. SPV on bulk sample: The wavelength scan gives band edge EcEc EvEv EFEF e h Generation of SPV in bulk materialsSchematic spectrum Wavelength SPV EgEg

6. SPV in quantum well structure: A single quantum well can be probed easily e h ECEC EFEF EVEV Generation of SPV from a QW Wavelength SPV Eee1-Ehh1 Eee1-Elh1 Eee2-Ehh2 Schematic spectrum

7. 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

8. SPV spectroscopy in the presence of magnetic field and 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  m j =  1 +1/2 -1/2 +3/2 -1/2 -3/2 +1/2 hh states n = 0 e states n = 0 mjmj

9. Tunable Diode Laser Optical Switch Optical Fiber Sample ITO Coated glass Buffer Amplifier Lock-in amp Super conducting magnet Schematic diagram of measurement setup

10. Tunable diode laser Tunable range: 1520-1570 nm & 1565-1625 nm

11. Optical switch

12. Power requirement Photo voltage saturates logarithmically with intensity Experiment is done in linear regime Illuminated power is sub-micro Watt

13. Structure of the system under study SI InP Substrate 1500 Å InP buffer 90 Å In 0.64 Ga 0.36 As QW 100 Å InP spacer 200 Å Si doped InP 100 Å InP cap Modulation 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

14. Schematic band diagram EFEF ECEC EVEV

15. Characterization of the sample: Pl measurement X-ray diffraction Electrical measurement: n s = 1.4  10 11 /cm 2 ; µ = 90,000 cm 2 /V-sec Photoluminescence spectrum of the sample

16. Experimental conditions 1. T <<   /k 2. Tunneling should be possible 3. There should not be any relative vibration between sample and electrode

17. Experimental results Zero field temperature dependence of SPV Without magnetic field results Optical process enhances with the lowering of temperature A peak like features is seen. This is attribute to formation of exciton At high temperature exciton does not form due to low binding energy 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

18. Shift of band edge Zero field temperature dependence of SPV The shift of band edge is due to increase of band gap with the lowering of temperature.

19. A comparison between PL and SPV

20. SPV spectrum at finite field Finite field results

21. Magnetic field dependence of SPV spectrum Magnetic field dependence of SPV

22. Evolution of energy levels with magnetic field Shift of peaks at higher energy with magnetic field SPV spectroscopy is suitable to detect inter band LL transition in single QW To characterized the transitions, QH experiment is necessary The width of join density of states can be measured J(h ) =  E g h (E)  g e (E + h ) dE

23. 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.