Semiconductor IISER-Pune

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1 Semiconductor IISER-Pune Past Ph.D Students: Kanika Bansal, Arthur Varghese, Padmashri Patil Current Phd/i-PhD students: Amit Bhunia, Saurav Kumar, Mohit Singh, Dipti D, Anweshi Dewan, Sumit Shrivastava, Gautam Sharma, Deepak Roy and Debesh Battacharjee Postdocs: Dr. Remya N, Dr. Satish Badadhe PI: Shouvik Datta, Dept. of Physics, IISER-Pune 1

2 Past & Current Research Things We have been Asking Are there any Electrical Signatures of Exciton Formation and Mott Transition to e-h plasma in Semiconductor Laser Diodes? How to probe the Condensed Matter Physics of Laser Diodes and Electronic Origin of Coherence? - by Kanika Bansal (Received her Ph.D, 2015) & Amit Bhunia. What is the Role of Fluid dynamics in Nano-fabrication: Do we need Fluid dynamical instability at Nanoscale? - by Arthur Varghese (Received his Ph.D, 2015). Semiconductor Quantum Dots: What is the - Role of photoexcitation intensity Fluorescence Blinking? Role of Quantum dot Heterostructure in Nano Photovoltaic Devices? - by Padmashri Patil (Received her PhD, 2015). How constant is Optical Dielectric Constant? - Meaning of Bohr Radius of Bound States like Excitons. - by Padmashri Patil (Received her Ph.D, 2015). 2

3 Ongoing & Future Research Things We Will be Asking How to connect all these following issues to probe the Electronic Origin of Coherence in Lasing and Excitonic BEC etc? 1) Amit Bhunia (Ph.D): Electrical and Optical Investigation of Excitonic and Exciton-Polaritonic BEC and BCS Transitions using GHz scale dielectric response & Energy-Momentum Spectroscopy of Excitons etc. Investigation of Coherent Electronic Transport in Semiconductor Laser Diodes as well as Plasmonic Materials. 2) Kumar Saurabh (Int.PhD): Study of Photon Statistics before and after Lasing in Semiconductor Laser Diodes. 3) Mohit Kumar Singh (Int.PhD): Study of Shot Noise before and after Lasing in Semiconductor Laser Diodes. 4) Dipti D. (Int.PhD): Fabrication & Investigation of Microcavity Polaritons. 3

4 VMEL Magnitude (a.u.) Inphase VMEL (a.u.) Electrons Dielectric Response of a Light Emitting Medium h - Probing the Condensed Matter Physics behind Lasing! Sample: AlGaInP based Single Crystalline Multi Quantum Well Laser Diodes; (Quantum Well width ~ 10nm) Voltage Modulated Electroluminescence (VMEL) Spectra Correlated Electronic and Optical behavior at low frequencies ( 100 khz) Negative Capacitance vs VMEL kHz 5kHz 10kHz 15kHz 25kHz C (nf) khz 30 khz 1 khz Light emission varies at low modulation frequencies Direct participation of defects in radiative emission process 1 Effective 1 radiative Holes Nonradiative Wavelength (nm) *Carriers Responding at khz-mhz will be absent from GHz Direct Modulation for Optical Communications *DST Grant ~ 54,00,000 for GHz Dielectric Measurements 4 Kanika Bansal and Shouvik Datta, Journal of Applied Physics, 110, (2011), Appl. Phys. Lett. 102, (2013) khz 30 khz 1 khz I dc (A)

5 EL Intensity(a.u) EL Intensity(a.u) (i) Electrical Investigation of Excitonic Electroluminescence and Mott Transition in AlGaInP Based Multi-Quantum Well Laser Diodes ~ 6.4 mev of Binding Energy of the intermediate bound state matches with that of III-V semiconductors (ii) Standard Optical Evidence of Exciton Mott Transition BGR Starts > 1.8V Proposed Electrical Evidence of Exciton Mott Transition 10 2 (iii) (iv) V 1.81V 1.82V 1.83V 1.84V 1.85V Energy(eV) Evolution of Exponential tails > 1.8V Amit Bhunia, Kanika Bansal & Shouvik Datta ; Manuscript Under Preparation V 1.86V 1.87V 1.88V 1.91V 1.93V 1.97V 2.00V 2.02V Energy(eV) Excitonic bound State signatures are enhanced in f(dc/df) response, whereas defect response are suppressed at Low Temperatures 20K 5

6 Towards Exciton-Polaritonic Condensation, Polaritonic Lasing Recent Review by Y. Yamamoto Nature Physics 10, (2014) Polariton Laser vs Photon Laser PNAS, 100, 15318, 2003; Nature 497, , 2013 by Y. Yamamoto et.al. Currently Probing here Reference: Phase Transitions in Electronhole Systems in Photoexcited Semiconductors By T. Suzuki, University of Tokyo,

7 Radiative Transitions compete with excitonic BEC - Tuning Excitonic Coupling 1) Amit Bhunia (Ph.D): Electrical and Optical Investigation of Excitonic and Exciton-Polaritonic BEC and BCS Transitions using GHz scale dielectric response & Energy-Momentum Spectroscopy of Excitons etc. Investigation of Coherent Electronic Transport in Semiconductor Laser Diodes as well as Plasmonic Materials. 2) Kumar Saurabh (Int.PhD): Study of Photon Statistics before and after Lasing in Semiconductor Laser Diodes. Reference: Phase Transitions in Electron-hole Systems in Photoexcited Semiconductors By T. Suzuki, University of Tokyo, ) Mohit Kumar Singh (Int.PhD): Study of Shot Noise before and after Lasing in Semiconductor Laser Diodes. 4) Dipti D. (Int.PhD): Fabrication & Investigation of Microcavity Polaritons. Excitonic and Excitonic-Polaritonic BEC ***MBE Grown Quantum Well Laser Diodes with Single, Double and Triple Barrier Resonant 7 Tunneling structures procured from Prof. Henini, Nottingham University.

8 Directionally Asymmetric Self-Assembly of Cadmium Sulfide Nanotubes using porous alumina Nano-Reactors: **Do we Need Chemo-Hydrodynamic Instability at Nanoscale? Experimental Setup Arthur Varghese and Shouvik Datta, Physical Review E 85, , 2012 CdCl 2 Na 2 S Porous alumina before synthesis. After few hours: CdS nanotubes grow on porous alumina facing Na 2 S Solution only. CdS Nanotube Growth Nanotubular Growth of CdS on the edges of Porous Alumina Porous Alumina Hollow CdS Nanotube Theoretical Modeling by Dr. Apratim Chatterji et.al in J. Chem. Phys., Volume 138, p024905, (2013) & Phys. Rev. E, Volume 89, , (2014). 8

9 Cadmium Vacancy Minority Defects as Luminescence Centers in Size and Strain Dependent Photoluminescence Shifts in CdS Nanotubes Journal of Physical Chemistry C 118, pp (2014) Directionally Asymmetric Self-Assembly of Cadmium Sulfide Nanotubes using porous alumina Nano-Reactors: Do we Need Chemo-Hydrodynamic Instability at Nanoscale? Arthur Varghese and Shouvik Datta, Physical Review E 85, , 2012 Ongoing Work Fabrication of Nano/Micro Scale Electrical Contacts to Single CdS Nanotubes for Electronic Transport and Electroluminescence 9

10 Normalised Intensity (a.u) Length (m) Length(m) Length (m) Growth Rate (m/min) Nanotube Length (m) What is the precise role of fluid dynamics in wet chemical nano fabrication? Arthur Varghese and Shouvik Datta, Growth dynamics of CdS Nanotubes Role of Fluid Dynamics, Manuscript under preparation Diamter of these CdS nanotubes are ~100nm 0.1M CdCl 2 and 0.1M Na 2 S 100 Growth Duration (min) Growth Duration (min) Arthur Varghese, Journal of Nanoscience and Nanotechnology, 14, 4495 (2014). Is this Runway Growth point Towards Some Fluid Dynamical Nanoscale? variation of CdCl 2 0.1M Na 2 S Growth Duration 1hr Concentration of CdCl2 (M) Variation of Na2S 0.1M CdCl2 Growth Duration 4hr Concertration of Na2S(M) CdS nanotubes grown in different diameters of porous alumina Concentration 0.025M for 2 hr Nanotube Diamter(nm) More is Not Always Most Effective in PL? Not-so-Abundant Defects can be most Prolific!! Photoluminescence (PL) Spectra min 1hr 2hr 3hr 4hr Arthur Varghese, Prasenjit Ghosh & Shouvik Datta, Effects on Strain in Luminescence of CdS Nanotubes, Journal of Physical Chemistry C 118, pp (2014) Energy(ev) 10

11 Quantum Yield (%) A.V. R. Murthy, Padmashri Patil, Shouvik Datta, Shivaprasad Patil Journal of Physical Chemistry C, 117 (25), pp (2013). FCS: Autocorrelation time ( D ) and the hydrodynamic sizes of NCs are found to be function of light intensity Question : How Time Correlated Spectroscopies depend on the wavelength and intensity of the photo excitation? We can estimate the fraction of photo-darkened CdTe QDs and estimate photo-darkening probability upon photon absorption. Photodarkening probability depends on CdTe QD size and not so much on temperature. Blinking occurs due to charge transfer from core to surface states most likely through tunneling. CdTe/CdSe Core/Shell & AgInS 2 NanoCrystals Intensity Dependence of the Blinking Dynamics in CdTe Nanocrystals (NCs) using FCS in microsecond times scales Current Density (ma/cm 2 ) 0-2 Not Sintered Sintered at 200 O C Sintered at 400 O C Highly Fluorescent Core-Shell CdTe/CdSe Nano -4 Crystals synthesized without purification are as 40 good as the purified ones ~300% increase in nanophotovoltaic efficiency 0 after annealing CdTe CdTe/CdSe; CdTe unpurified CdTe/CdSe; CdTe purified Voltage (V) Peak Abs.Wavelength(nm) Journal of Alloys and Compounds 607, 230 (2014). Investigating the competition between Role of Strain & 11 Cryst. Eng. Comm 16, 3605, (2014). Type II transition in Light Emission for these Composite Heterostructure Nanoparticles

12 Mean free path (nm) How to probe the Nature of Dielectric Response for above band edge photo-excitations? Do We Need to Revisit Bohr Exciton Radius? 4 a B 2 e 2 Is Does this classic definition hold good for optical transitions much above the fundamental band gap? Is it still precise enough to be useful to describe transitions involving hot carriers much above the fundamental band gap? How does it modifies in presence of strong dispersions of the dielectric response? or k,? ~ ( L) Padmashri Patil & Shouvik Datta, arxiv: Approximating the dielectric response under optical excitation with an energy independent limiting value such as e (0) ~ is only valid for E g where E g is the band gap and T is the frequency of transverse optical phonon. However, in case of this above band gap transitions or hot excitons Bulk g E Bulk E ~ 17 ~ Diameter (nm) Reference: Yu, P. & Cardona, M. Fundamentals of 12 Semiconductors Ch. 6 (2nd edition, Springer, 1999) T Radiation field may not appear to be static for charge carriers 2 ME Ex 24 kbt a * Ex From Phys. Rev. B 13, 1692 (1976) Therefore, we suggest an optimum size window for these QDs to exploit such carrier multiplications events to improve the power conversion efficiency in the next generation nano-photovoltaic cells.

13 Past, Present and Future Towards BEC of Exciton-Polaritons Is there any Electrical Signatures of Exciton Formation and Mott Transition to e-h plasma in Semiconductor Laser Diodes? How to probe the Condensed Matter Physics of Laser Diodes and Electronic Origin of Coherence? - by Kanika Bansal (Submitted Ph.D Thesis) & Amit Bhunia. What is the Role of Fluid dynamics in Nano-fabrication: Do we need Fluid dynamical instability at Nanoscale? - by Arthur Varghese (Submitted Ph.D Thesis). Semiconductor Quantum Dots: What is the Role of photoexcitation intensity? Role of Quantum dot Heterostructure in Nano Photovoltaic Devices. - by Padmashri Patil (Submitted Ph.D Thesis). Time of Flight Photoconductivity for Mobility Measurements. How constant is Optical Dielectric Constant? - Meaning of Bohr Radius of Bound MBE grown Quantum Well Samples States like Excitons. - by Padmashri Patil (Submitted Ph.D Thesis). Electron and Hole wavefunctions are localized on different sides of the single & double barrier QW samples From Prof. M. Henini, Dept of Physics, Nottingham University, UK 1) Amit Bhunia (Ph.D): Electrical and Optical Investigation of Excitonic and Exciton-Polaritonic BEC and BCS Transitions using GHz scale dielectric response & Energy-Momentum Spectroscopy of Excitons etc. Investigation of Coherent Electronic Transport in Semiconductor Laser Diodes as well as Plasmonic Materials. 2) Kumar Saurabh (Int.PhD): Study of Photon Statistics before and after Lasing in Semiconductor Laser Diodes. 3) Mohit Kumar Singh (Int.PhD): Study of Shot Noise before and after Lasing in Semiconductor Laser Diodes. 4) Dipti D (Int.PhD): Fabrication & Investigation of Microcavity Polaritons. shouvik@iiserpune.ac.in 13

14 Excitons within Semiconductor Quantum Structure in Room Temperature Claus Klingshirn, in Semiconductor Optics & PHYSICAL REVIEW B 75,

15 Planning Additional Experiments: Excitonic BEC Theoretical Proposal Using Tunneling Current & Shot Noise 15

16 New Research Directions : MBE Grown p-i-n Quantum Structures procured from Prof. Henini, Nottingham University, UK Spatially delocalized electron and hole Wavefunctions embedded within Laser Cavity to reduce and tune radiative recombination which competes with BEC Barrier Width d = 6, 8 & 12 nm Many Body Excitonic Physics by tuning the excitonic interaction 16

17 Planning Additional Experiments: Effects on Shot Noise Figure Reference: Quantum Optics, Mark Fox 17

18 Planning Additional Experiments: Balanced Detection Classical Thermal Noise can be Nullified Figure Reference: Quantum Optics, Mark Fox 18

19 Planning Additional Experiments: Studying Correlation Functions APDs -High QE% (40-80%) - Up to GHz Modulation -Low Gain (~100) -Small Area PMTs -Low QE% (< 40%) -ns Time Domain -Very High Gain (~10 7 ) -Large Area Figure Reference: Quantum Optics, Mark Fox 19

20 Planning Additional Experiments: Interferometers Nature Materials 13, (2014) Figure Reference: Quantum Optics, Mark Fox 20

21 Planning Additional Experiments: Energy-Momentum Spectroscopy Nature Materials 13, (2014) 21

22 New Research Directions Have Emerged from Our Project Work ***In general, people investigate the many body excitonic physics using optical means alone. However, we also want to use frequency (GHz) and time domain (ns) electronic response studies currently being developed in this project to supplement these standard optical investigations of excitonic physics. 22