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XIVth International Workshop on Polarized Sources, Targets & Polarimetry

Period Dependence of Time Response of Strained Semiconductor Superlattices. XIVth International Workshop on Polarized Sources, Targets & Polarimetry. Leonid G. Gerchikov Laboratory of Spin-Polarized Electron Spectroscopy Department of Experimental Physics State Polytechnic University

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XIVth International Workshop on Polarized Sources, Targets & Polarimetry

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  1. Period Dependence of Time Response of Strained Semiconductor Superlattices XIVth International Workshop on Polarized Sources, Targets & Polarimetry Leonid G. Gerchikov Laboratory of Spin-Polarized Electron Spectroscopy Department of Experimental Physics State Polytechnic University St. Petersburg, Russia

  2. SPESPSTP2011 Collaborators Department of Experimental Physics, St. Petersburg State Polytechnic University, St. Petersburg, Russia, Leonid G. Gerchikov, Yuri A. Mamaev, Yuri P.Yashin Institute of Nuclear Physics, Mainz University, Mainz, Germany,Kurt Aulenbacher, Eric J. Riehn

  3. SPESPSTP2011 Outline • Introduction • Goals and Motivation • Pulse response measurements • Experimental method and results • Partial electron localization • Theoretical approach • Kinetics of electron transport in SL • Role of electron localization. Pulse response and QE. • Analysis of the pulse response • Comparison of theory and experiment. Determination of localization times • Dependence of the response time on number of SL periods • Conclusions

  4. SPESPSTP2011 Best photocathodes

  5. SPESPSTP2011 SL In0.16Al0.2Ga0.64As(5.1nm)/Al0.36Ga0.64As(2.3nm)

  6. Strained-well SL GaAs BBR Unstrained barrier ab = a0 Strained QW aw > a0 Strained QW aw > a0 Unstrained barrier ab = a0 Buffer Layer a0 - latt. const GaAs Substrate SPESPSTP2011 SL Large valence band splitting due to combination of deformation and quantum confinement effects in QW

  7. SPESPSTP2011 MBE grown AlInGaAs/AlGaAs strained-well SL Eg = 1.536 eV, valence band splittingEhh1 - Elh1 = 87 meV, Maximal polarizationPmax= 92% at QE = 0.85%

  8. Experimental method Pulse response experiment: Time resolved measurements of electron emission excited by fs-laser pulse Photoexcitation

  9. Experimental method Pulse response experiment: Time resolved measurements of electron emission excited by fs-laser pulse Photoexcitation Beam deflection

  10. Experimental method Pulse response experiment: Time resolved measurements of electron emission excited by fs-laser pulse Photoexcitation Shift of transverse profile against slit Beam deflection

  11. Experimental method Pulse response experiment: Time resolved measurements of electron emission excited by fs-laser pulse Polarization measurements Photoexcitation Shift of transverse profile against slit Beam deflection

  12. SPESPSTP2011 Pulse response of SL Al0.2In0.16Ga0.64As(3.5nm)/ Al0.28Ga0.72As(4.0nm) 15 periods Time dependence of electron emission Evidence of partial electron localization Non-exponential decay 1< calc< 2 1 = 4 ps 2 = 12 ps calc= 6 ps

  13. Localized states h SPESPSTP2011 Electronic transport in SL Electron scattering BBR Buffer e1 Capture Detachment Photoexcitation Recombination Tunneling between QWs Tunneling to BBR Recombination hh1 lh1 Time of electron tunneling from last QW to BBR fexp(2b), f  200 fs Recombination time r 100 ps Time of resonant tunneling between neighbouring QWs QW= ħ/∆E exp(b), QW  20 fs Time of ballistic motion in SL SL= ħN/∆E Momentum relaxation time p 0.1 ps; Free pass N = QW/p 5 Capture time c 2-10 ps; Detachment time d 100 ps

  14. SPESPSTP2011 Electronic transport in SL Kinetic equation  – electronic density matrix H – effective Hamiltonian of SL in tight binding approximation, describes electron tunneling within SL I{} – collision term including: • collisions within each QW with phonons and impurities described in constant relaxation time , p, approximation • tunneling through the last SL barrier to BBR • optical pumping • electron capture by localized states and reverse detachment process

  15. SPESPSTP2011 Electronic diffusion inSL bulk GaAs N – number of SL periods V = E/4 = ħ/4QW– matrix element of interwell electron transition D = 40 cm2/s – diffusion coefficient S = 107 cm/s – surface recombination velocity For SL Al0.2In0.19Ga0.61As(5.4nm)/ Al0.4Ga0.6As(2.1nm) D = 12.6 cm2/s , S = 3.5*106 cm/s

  16. SPESPSTP2011 Role of partial localization: pulse response Electron localization Double exponential decay Fast decay rate 1-1= t-1 + c-1 Slow decay rate 2-1 = d-1( c/(t+ c)) 1 < t < 2 t- miniband transport time c- capture time d - detachment time No electron localization Single exponential decaywith decay time  = t

  17. SPESPSTP2011 Role of partial localization: QE n –total electron concentration nm – concentration of miniband electrons nl – concentration of localized electrons nm< n = nm + nl Electron diffusion in SL Stationary pumping Decrease of diffusion length Bulk GaAsLD 1m Perfect SL 6-905 LD = 0.4m Real SL 6-905 LD = 0.08m Maximal QE, infinite working layer

  18. SPESPSTP2011 Role of partial localization: QE QE as a function of working layer thickness

  19. SPESPSTP2011 Pulse response of SL 5-998Al0.2In0.16Ga0.64As (3.5nm)/Al0.28Ga0.72As(4.0nm) 15 periods Time dependence of electron emission Parameters t = 5.8 ps – miniband transport time, calculated parameter c = 4.5 ps – capture time, fitting parameter d= 6.0 ps – detachment time, fitting parameter = 12 ps – total extraction time r*= 44 ps – effective recombination time LD = 0.27 m – diffusion length BSL = 0.88 - extraction probability

  20. SPESPSTP2011 Pulse response of SL 7-396Al0.2In0.19Ga0.61As (5.4nm)/Al0.4Ga0.6As(2.1nm) 12 periods Time dependence of electron emission Parameters t = 4.5 ps – miniband transport time, calculated parameter c = 9.0 ps – capture time, fitting parameter d= 110 ps – detachment time, fitting parameter = 23 ps – total extraction time r*= 15 ps – effective recombination time LD = 0.14 m – diffusion length BSL = 0.77 - extraction probability

  21. SPESPSTP2011 Pulse response of SL 6-905Al0.2In0.16Ga0.64As (5.1nm)/Al0.36Ga0.64As(2.3nm) 10 periods Time dependence of electron emission Parameters t = 2.5 ps – miniband transport time, calculated parameter c = 2.1 ps – capture time, fitting parameter d= 130 ps – detachment time, fitting parameter = 40 ps – total extraction time r*= 3.6 ps – effective recombination time LD = 0.077 m – diffusion length BSL = 0.59 - extraction probability

  22. SPESPSTP2011 Pulse response of SL 6-908Al0.2In0.16Ga0.64As (5.1nm)/Al0.36Ga0.64As(2.3nm) 6 periods Time dependence of electron emission Parameters t = 1.2 ps – miniband transport time, calculated parameter c = 4.5 ps – capture time, fitting parameter d= 50 ps – detachment time, fitting parameter = 9.4 ps – total extraction time r*= 12 ps – effective recombination time LD = 0.14 m – diffusion length BSL = 0.91 - extraction probability

  23. SPESPSTP2011 Results

  24. SPESPSTP2011 Calculated response time dependence on the length of SL 6 - 905-908

  25. SPESPSTP2011 Summary • Partial electron localization leads to non-exponential decay of pulse response. • Analysis of pulse response allows to determine the characteristic times of capture and detachment processes. • Partial electron localization decreases considerably the diffusion length in SL • Partial electron localization limits QE for thick working layer. • For practical application one should employ SL photocathodes with no more than 10 – 12 periods.

  26. SPESPSTP2011 Outlook • study spin polarized electron transport for various excitation energies, doping levels and SL parameters. • clarify the nature of localized states. • figure out how localization can be reduced in order to increase QE.

  27. SPESPSTP2011 • This work was supported by • Russian Ministry of Education and Science under grant 2.1.1/2240 • DFG through SFB 443 Thanks for your attention!

  28. Ballistic transport Tunneling resonances En = E0− ∆E/2Cos(qnd) qn = πn/d(N+1) ∆E – width of e1 miniband N – number of QW in SL Time of resonant tunneling SL= ħN/∆E  N·exp(b) Transport time = ħ/Γ N·exp(2b) Γ << ∆E ,  >>  SL b b b p >> SL= ħN/∆E, p = 10-13 s, ∆E = 40 meV, ∆Ep /ħ = 6 Optimal choice: bf = b/2

  29. Pulse response of SL Al0.2In0.19Ga0.61As (5.4nm) / Al0.4Ga0.6As(2.1nm) 12 periods Time dependence of electron emission: intensity and polarization Gradual depolarization with s = 81ps Long tail of emission current - - emission from localized states

  30. Pulse response of SL 6-908 (6 periods)at different wavelength Time dependence of electron emission Emission spectra P, QE

  31. Transport below conduction band edge

  32. Calculations of SL’s energyspectrum and photoabsorption within 8-band Kane model Miniband spectrum: Photoabsorption coefficient: Polarization:

  33. Photocathode with DBR Goal: considerable increase of QE at the main polarization maximum and decrease of cathode heatingMethod: Resonance enhancement of photoabsorption in SL integrated into optical resonance cavity Photoabsorption in the working layer: L << 1,  - photoabsorbtion coefficient, L - thickness of SL Resonant enhancement by factor 2/(1-(RDBRRGaAs) 1/2)2 Heating is reduced by factor L

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