Record Values of Electron Beam Polarization and Quantum Efficiency for Semiconductor Photocathodes Yu.A.Mamaev St. Petersburg State Polytechnic University

Collaborators Yu. P. Yashin, L.G.Gerchikov, V.V. Kuz’michev, D.A.Vasiliev Department of Experimental Physics, St.Petersburg State Polytechnic University, Russia James E. Clendenin, Takashi Maruyama Stanford Linear Accelerator Center, Stanford, CA, USA V.M. Ustinov, A.E. Zhukov, V.S. Mikhrin, A.P. Vasiliev A.F. Ioffe Physicotechnical Institute RAS, Russia

1. Introduction 2.Strained-well InAlGaAs/AlGaAs SL structures with high valence band splitting 3.Sample Summary OUTLINE

Superlattice (SL) based photocathode with negative electron affinity Advantages: Thick working layer without strain relaxation Large valence band splitting Band structure engineering

To get the best P & QE Valence band splitting > 60 meV Sharp heterointerfaces Uniformity of composition & the SL layers thicknesses (low fluctuations) Low depolarization in the course of transport High NEA value Heavy doped BBR region High E g value

 Working layer Polarization Losses 1. Photoabsorbtion stage(5-10%): Mixture of hh and lh states due to smearing of band edge and broadening of hole spectrum caused by doping and fluctuations of layer composition. Photoabsorption in BBR. 2. Transport stage (1%): Spin relaxation due to DP and BAP mechanisms. 3. Emission stage (5%): Spin relaxation in BBR due to DP mechanism.

Experimental setup Polarized Photocathode R&D at St. Petersburg Polytechnic University

T Unstrained barrier a b = a 0 GaAs Substrate Buffer Layer a 0 - latt. const GaAs BBR Strained QW a w > a 0 Strained QW a w > a 0 Unstrained barrier a b = a 0 SL Strained-well SLs Feature: Large valence band splitting due to combination of deformation and quantum confinement effects in QW

CompositionThicknessDoping As cap GaAs QW60 A 7  cm -3 Be Al 0.36 Ga 0.64 As SL 23 A 3  cm -3 Be In Al 0.2 Ga As 51 A Al 0.4 Ga 0.6 AsBuffer 0.3  m6  cm -3 Be p-GaAs substrate MBE grown InAlGaAs/AlGaAs strained-well superlattice E g =1.543eV, Valence band splitting E hh1 - E lh1 = 60 meV, P max =92%, QE=0.6%.

SL In Al 0.2 Ga As(5.1nm)/Al 0.36 Ga 0.64 As(2.3nm), 4 pairs Spectra of electron emission: Polarization P and Quantum Efficiency QE

SL In Al 0.2 Ga As(5.1nm)/Al 0.36 Ga 0.64 As(2.3nm), 12 pairs

SL In Al 0.2 Ga As(5.1nm)/Al 0.36 Ga 0.64 As(2.3nm)

X-ray SLAC

X-ray diffraction Ioffe PTI)

Comparison of SPb & SLAC data for 7-308

Repeatability

Summary P = 91.5% and QE = λ = 830 nm

Best photocathodes SampleCompositionP max QE(  max ) Team SLSP16GaAs(3.2nm)/ GaAs 0.68 P 0.34 (3.2nm) 92%0.5%Nagoya University, 2005 SL5-777GaAs(1.5nm)/ In 0.2 Al 0.23 Ga 0.57 As(3.6nm) 91%0.14%SPbSPU, 2005 SL7-307Al 0.4 Ga 0.6 As(2.1nm)/ In 0.19 Al 0.2 Ga 0.57 As(5.4nm) 92%0.85%SPbSPU, 2007

Acknowledgments This work was supported by - Russian Ministry of Education and Science under grant N.P in the frames of a program “Development of the High School scientific potential” - the U.S. Department of Energy under contract DE—AC02-76SF Swiss National Science Foundation under grant SNSF IB