Published on October 9, 2007
Polarization Possibilities of Small Spin-Orbit Interaction in Strained-superllatice Photocathodes: Polarization Possibilities of Small Spin-Orbit Interaction in Strained-superllatice Photocathodes Collaborators: A. Brachmann, J. Clendenin, E. Garwin, K. Ioakeimidi, R. Kirby, T. Maruyama (SLAC), R. Prepost (Wisconsin), and A. Moy (SVT Associates) Takashi Maruyama SLAC SPIN2006 October 2-7 2006 Outline: Outline GaAs/GaAsP strained-superlattice and its limitation GaAs/InGaP strained-superlattice Preliminary results Possible explanation Summary GaAs/GaAsP Strained-superlattice: GaAs/GaAsP Strained-superlattice Strained GaAs Single strained GaAs HH – LH splitting from two sources: quantum size effect and strain → Higher polarization GaAs/GaAsP has a larger band gap → Higher QE High gradient doping: Be 51019/cm3 in the 5 nm surface layer and 51017/cm3 in the rest → No surface charge limit Each superlattice layer is thinner than the critical thickness → GaAs layers are highly strained Superlattice parameters: Superlattice parameters Parameters: Barrier thickness: 3 nm < b < 6 nm Well thickness : 3 nm < w < 6 nm Phosphorus x : 0.25 < x < 0.40 No. of periods : l ~ 70 – 200 nm b w Applied Physics Letters 85, 2640 (2004) Superlattice band calculations and polarization analysis are made using SpecCode and SpinSpec by Subashiev&Gerchikov GaAs/GaAsP Superlattice vs. Single strained GaAs: GaAs/GaAsP Superlattice vs. Single strained GaAs Peak polarization: 86% vs. 80% QE: 1% vs. 0.3% Clear HH and LH transitions. HH-LH splitting can be measured directly. The step-like behavior of 2D structure is observed. 2D 3D #3 HH LH Strain effect: Vary x in GaAs1-xPx: Strain effect: Vary x in GaAs1-xPx HH-LH splitting increases with x, but the polarization remains at ~85%. Strain relaxation Material specific depolarization: Material specific depolarization emit = 3-5 ps (Mainz) If s < 35 ps, the spin relaxation time has a significant effect on polarization. D’yakonov-Perel (DP) mechanism is dominant in low doped SL. DP mechanism comes from the spin-orbit interaction. Find materials with a smaller spin-orbit interaction. GaN GaP GaAs GaSb SO (eV) 0.01 0.08 0.34 0.76 Try GaAs/InGaP strained-superlattice P0: Initial polarization s : spin relaxation time emit : photoemission time PBBR: depolarization at BBR Three structures with 1.25% lattice mismatch: Three structures with 1.25% lattice mismatch Ec (eV) Ev (eV) Strained wells Strained wells Strained barriers HH LH Energy band comparison (Speccode): Energy band comparison (Speccode) Barrier = 4 nm Barrier = 4 nm GaAs/In0.32Ga0.68P GaAs/GaAs0.65P0.35 DEHL = 86 meV l = 782 nm DEHL = 109 meV l = 764 nm GaAs/In0.65Ga0.35P: GaAs/In0.65Ga0.35P Barrier = 4 nm Well = 4 nm GaAs/InGaP strained-superlattice structures: GaAs/InGaP strained-superlattice structures Strained wells 1) 5 nm GaAs cap Be: 11019 2) 4 nm In(0.32)Ga(0.68)P Be: 11017 3) 4 nm GaAs Be: 11017 repeat 2) and 3) 12 times 4) 2.5 um In(0.32)Ga(0.68)P Be: 51018 5) 2.5 um In(x)Ga(1-x)P x=0.48 -> 0.32 Be: 51018 6) In(0.48)Ga(0.52)P buffer lattice-matched to GaAs 7) GaAs substrate Strained Barriers-1 1) 5 nm GaAs cap Be: 11019 2) 4 nm In(0.65)Ga(0.35)P Be: 11017 3) 1.5 nm GaAs Be: 11017 repeat 2) and 3) 18 times 4) 1 um Al(0.3)Ga(0.7)As Be: 51018 5) GaAs buffer 6) GaAs substrate Five wafers have been grown at SVT Associates under DOE SBIR grant. Strained Barriers-2, same as 1 except 2) 1.5 nm In(0.65)Ga(0.35)P Be: 11017 3) 4 nm GaAs Be: 1 1017 QE and Polarization: QE and Polarization QE ~ 0.4% 1/2 ~ 1/3 of GaAs/GaAsP Peak polarization ~ 68% Strained Wells Superlattice structure affects dramatically: Superlattice structure affects dramatically 1.5 nm GaAs + 4 nm In0.65Ga0.35P 4 nm GaAs + 1.5 nm In0.65Ga0.35P QE ~ 0.002% Pol ~ 40% QE ~ 0.01% Pol ~ 68% Spin relaxation rate based on D’yakonov-Perel mechanism : Spin relaxation rate based on D’yakonov-Perel mechanism : spin-orbit-induced spin splitting coefficient E1e: confinement energy Narrower well has a larger confinement energy. Larger confinement energy Less vertical transport, thus lower QE More scattering, thus lower polarization. s ~ 10 ps s ~ 2 ps Why not measure s directly?: Why not measure s directly? If s > 50 ps, forget about the spin relaxation. If s depends on the structure and material, find the right material and structure. Faraday rotation system currently under development at SLAC. Time-resolved Faraday rotation spectroscopy IEEE J. Selected Topics in Quantum Electronics, 1, 1082 (1995) S. Crooker, D. Awschalom, and N. Samarth Summary: Summary GaAs/GaAsP strained-superlattice is currently the best structure. But the peak polarization seems saturated at 85% due to material specific depolarization. In an attempt to reduce depolarization, GaAsP is replaced with InGaP QE and polarization are strongly affected by SL parameters, requiring further optimization. Direct measurement of s using Faraday rotation may prove important.