Presentation on theme: "Record Values of Electron Beam Polarization and Quantum Efficiency for Semiconductor Photocathodes Yu.A.Mamaev St. Petersburg State Polytechnic University."— Presentation transcript:
Record Values of Electron Beam Polarization and Quantum Efficiency for Semiconductor Photocathodes Yu.A.Mamaev St. Petersburg State Polytechnic University firstname.lastname@example.org
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 7- 307 4. 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 10 18 cm -3 Be Al 0.36 Ga 0.64 As SL 23 A 3 10 17 cm -3 Be In 0.155 Al 0.2 Ga 0.645 As 51 A Al 0.4 Ga 0.6 AsBuffer 0.3 m6 10 18 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 0.155 Al 0.2 Ga 0.645 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 0.155 Al 0.2 Ga 0.645 As(5.1nm)/Al 0.36 Ga 0.64 As(2.3nm), 12 pairs
SL In 0.155 Al 0.2 Ga 0.645 As(5.1nm)/Al 0.36 Ga 0.64 As(2.3nm)
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. 126.96.36.1995 in the frames of a program “Development of the High School scientific potential” - the U.S. Department of Energy under contract DE—AC02-76SF00515 - Swiss National Science Foundation under grant SNSF IB7420-111116