Polarization comparison of InAlGaAs/GaAs superlattice photocathodes having low conduction band offset K. Ioakeimidi, T. Maruyama, J.E. Clendenin, A. Brachmann Stanford Linear Accelerator Center Yu.A.Mamaev, L.G.Gerchikov, Yu.P.Yashin, D. Vasilyev St. Petersburg State Polytechnic University V.M.Ustinov and A.E.Zhukov Ioffe Physico-Technical Institute R. Prepost University of Wisconsin

85-90%

Main depolarization mechanisms Interband absorption smearing  due to bandedge fluctuations Hole scattering between HH and LH states causes a LH broadening  Optical phonon scattering Non polarized electrons generated in the BBR Scattering of electrons in the BBR Vacuum electron 6nm GaAs 100nm  E c HH1 Valence Band LH1  E v AlGaAs Buffer Conduction Band BBR HH- LH separation determined by well/barrier thickness and strain  E C /  E V determined by SL structure

Motivation for a flat Conduction Band structure Observed emission spectrum Emitted electron polarization  : optical absorption R: reflection coefficient  1 : electron lifetime in BBR  em : time of electron emission in vacuum P 0 : initial electron polarization upon excitation with circularly polarized light S 0 : surface recombination velocity  S : spin relaxation time in SL  S1 : spin relaxation time in BBR A. Subashiev et. al. SLAC-PUB 10901

Vacuum electron 6nmGaAs 100nm AlGaAs Buffer Conduction Band HH1 BBR Valence Band LH1  E v  E c Design of a flat conduction band structure Strained Barrier Al y In x Ga 1-x-y As/GaAs SL on GaAs substrate y: determines the bandgap x: lowers bandgap, controls  E C and induces strain  E C (x,y)=Q C1 *  E g1 (x)+Q C2,def *(  E g2 (y)+  E C,def ) Yu. A. Mamaev et al., PST2003

Unstrained wells and compressively strained barriers GaAs/Al y In x Ga 1-x-y As SL Yu. A. Mamaev et al., Mainz 2004 d QW (Å) E hh2 E lh1 E hh1 E e E E E E E E E E E E E E E E E

Photocathodes Sample #In %Al % SL Bandgap eV BBR doping cm -3 E(meV) LH- HH  E C (meV)  meV)  meV) e e e e e e QW: 1.5nm QB: 4nm BBR: 6nm SL doping: 4e17cm -3 GaAsP/GaAs SL: 89meV LH-HH splitting  E C =36meV,  E V =19meV

Green curve – without BBR absorption; Black curve – with BBR #5506: In.17 Al.18 Ga.65 As/GaAs SL

Green curve – without BBR absorption; Orange curve – with BBR #5501: In.20 Al.21 Ga.59 As/GaAs SL

Green curve – In0.23Al0.25Ga0.54As – 1.5 nm GaAs without BBR absorption Blue curve –, In0.23Al0.23Ga0.54As – 1.5 nm GaAs without BBR absorption; Black curve – In0.23Al0.23Ga0.54As – 1.5 nm GaAs with BBR #5503: In.23 Al.25 Ga.52 As/GaAs SL

#5777: In.20 Al.23 Ga.59 As/GaAs SL 92%

Simulations vs data analysis Data indicates a blue shift for the polarization peak for samples 5501, 5503, 5506 In theory longer wavelengths give higher polarization because they obey better the selection rules In practice longer wavelengths photogenerate electrons primarily in the BBR where there is no polarization selectivity and also the electrons photogenerated in the SL structure in this case thermalize more in the BBR and they have lower tunneling probabilities due to tighter confinement and due to the CB peak at the interface between the SL and the BBR.

#6329: In.20 Al.22 Ga.58 As/GaAs SL St. Petersburg 78% CTS76 ~ 77%

#6410: In.28 Al.35 Ga.37 As/GaAs SL St. Petersburg74% CTS77%

(004) X-ray Indium fraction (assume 100% strain): % % Superlattice thickness: nm nm is better

(004) simulation Assume 100% strain

Summary of Polarization Results Sample #In %Al%Polarization %

How can we explain 90%? Polarization calibration seems OK within 2-3% had an As-cap, allowing a low temp heat-cleaning. When was heated to >540 C, polarization dropped to 84%, which is consistent with SVT Surface effect supported by the temperature of the heat cleaning dependence of the results X ray shows decreased barrier size that would imply higher strain Strained barrier structures do not preserve strain in the BBR; we think that 5777 might have reduced BBR thickness.

Conclusions The flat CB structures show promising polarization results with a record polarization of 92%. Further analysis needs to be done in order to understand depolarization effects primarily in the BBR. The flat CB photocathodes show sensitivity to heat cleaning temperature. The heat cleaning effect needs to be explored further with SIMS analysis.

More SVT results SVT-5503 SVT-5506

SVT-5501 (5-777 duplicate) Peak polarization ~84%

Dear Takashi, We have analyzed your recent data on SL and agree with your conclusion that the polarization losses could be due to the surface effect. To confirm this assumption we compare your experimental data (points) with our calculations (curves - see figures below). First we calculate the initial polarization of photoelectrons in the working layer using your data of layer composition and thickness. Then we reduce the resulting polarization of emitted electrons by 5% due to the polarization losses in the surface region (. “Energy resolved spin-polarized electron photoemission from strained GaAs/GaAsP heterostructure”, Yu.A.Mamaev, A.V.Subashiev, Yu.P.Yashin, H.-J.Drouhin and G.Lampel, Solid State Comm., Vol. 114, No 7, 2000, pp ). We have reasonable agreement between the theory and experiment for the overall behavior of polarization spectra for SL 5501 and For SL 5503 the agreement can be achieved if we assume that the actual Al concentration in barrier layer is 2% lower, i.e. SL nm In 0.23 Al 0.23 Ga 0.56 As – 1.5 nm GaAs. However there is a systematical discrepancy between the theory and experiment in the region of polarization maximum, namely experimental peak is blue shifted and smaller in amplitude. In our calculations the position of the polarization maximum is close to the photoabsorption edge. The QY spectrum behavior near the edge evidences that the position of photoabsorption edge is calculated correctly. Thus the observed blue shift of polarization maximum and large polarization losses might be caused by an additional physical reason.

We assume that this effect can arise from the photoabsorption in the surface (BBR) layer (see our last joint paperA. V. Subashiev, L. G. Gerchikov, Yu. A. Mamaev, Yu. P. Yashin, J. S. Roberts, D.- A. Luh, T. Maruyama, and J. E. Clendenin “Strain-Compensated AlInGaAs-GaAsP Superlattices for Highly-Polarized Electron Emission”, Appl. Phys. Lett. 86 (2005) ). In figures below we show the polarization spectra accounting for the BBR contribution to electron emission. Figures demonstrate that this contribution cuts the red side of the polarization maximum resulting in its sizable blue shift and decreasing of amplitude. The similar effect we also observe in SL and However it seems that in our best sample, SL 5-777, there is no visible BBR contribution. In the last figure we show the polarization spectrum of SL together with our calculations. We use the latest Ioffe PTI data on its composition. According to this data SL is 3.6 nm In0.20Al0.23Ga0.57As – 1.5 nm GaAs. The barrier thickness we reduce by 0.4nm according to X-ray results, though this change does not almost affect the polarization spectrum. Figure show that our calculations without BBR contribution (initial polarization – 5% surface losses) reproduce well the experimental spectrum. Unfortunately we do not understand at the moment why some samples are good and some samples with close or even the same design are bad.

Vacuum electron 6nmGaAs 100nm  E c HH1 Valence Band LH1  E v AlGaAs Buffer Conduction Band BBR

InAlGaAs-GaAs superlattice St. Petersburg group observed 90% polarization from InAlGaAs-GaAs superlattice (5-777). SVT grew three wafers –5506: 4 nm In 0.17 Al 0.18 Ga 0.59 As – 1.5 nm GaAs 18 periods –5501: 4 nm In 0.20 Al 0.21 Ga 0.59 As – 1.5 nm GaAs 18 periods (duplicate of ) –5503: 4 nm In 0.23 Al 0.25 Ga 0.59 As – 1.5 nm GaAs 18 periods Peak polarization of SVT-5501 was 84%. Three samples from St. Petersburg –5-777 Could not activate to high enough QE. No measurements. – nm In 0.2 Al 0.22 Ga 0.58 As – 1.5 nm GaAs 18.5 periods – nm In 0.28 Al Ga As – 0.56 nm GaAs 15 periods X-ray diffraction of Ioffe and SVT-5501