Improved Superlattices for SpinPolarized Electron Sources

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Improved Superlattices for Spin-Polarized
Electron Sources

• Yu.A.Mamaev, L.G.Gerchikov, Yu.P.Yashin,
  V.Kuzmichev, D.Vasiliev
• St. Petersburg State Polytechnic University
• T. Maruyama, J.E. Clendenin
• Stanford Linear Accelerator Center
• V.M.Ustinov and A.E.Zhukov
• Ioffe Physico-Technical Institute
• mamaev_at_spes.phmf.spbstu.ru

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OUTLINE

• 1. Introduction
• 2. Strained-well InAlGaAs/AlGaAs SL structures
• with high valence band splitting
• 3. Strained-barrier InAlGaAs/GaAs SL structures
• with minimized conduction band offset
• 4. Summary Outlook

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Polarized beams enhance the luminosity of a
collider.

• The principal reason for the effectiveness of
  polarized electrons for energies gt MZ is that
  right-handed electrons have no weak interaction
  whereas left-handed electrons do.
• Consequently, above the Z0 mass, right-handed
  and left-handed electrons behave as distinctly
  different particles.
• Wherever cross sections have a strong
  dependence on polarization, about half the
  particles in an unpolarized beam are useless. By
  choosing only the desired particles for an
  interaction, the luminosity for a given beam
  intensity is effectively increased.

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Polarized Photocathode RD at St. Petersburg
Polytechnic University
Experimental setup
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Superlattice (SL) based photocathodewith
negative electron affinity

• Advantages
• Thick working layer without strain relaxation
• Large valence band splitting
• Band structure engineering

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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.

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Types of Strained Superlattices
I. Strained-well SLs
SL

• Feature
• Large valence band splitting due to combination
  of deformation and quantum confinement effects in
  QW

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MBE grown InAlGaAs/AlGaAs strained-well
superlattice
Eg1.543eV, Valence band splitting Ehh1 - Elh1
60 meV, Pmax92, QE0.6.
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SL In0.155Al 0.2Ga0.645As(5.1nm)/Al0.36Ga0.64As(2
.3nm), 4 pairs
Spectra of electron emission Polarization P and
Quantum Efficiency QE
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SL In0.155Al0.2Ga0.645As(5.1nm)/Al0.36Ga0.64As(2.
3nm), 12 pairs
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SL In0.155Al0.2Ga0.645As(5.1nm)/Al0.36Ga0.64As(2.
3nm)
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Types of Strained Superlattices
2. Strained-barrier SLs
GaAs BBR
SL

• Feature
• Deformation splitting in barrier layer
  increases energy spitting of hh1 - lh1 in QW
  layer

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MBE grown InAlGaAs/GaAs strained-barrier SLs
with minimal conduction band offsets
Eg1.471eV, Valence band splitting Ehh1 - Elh1
60 meV, Maximal polarization Pmax91, QE0.14.
Semiconductors 2006, Vol. 40, No. 11, p. 1326
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18.5 periods of SL In0.2Al0.23Ga0.67As (4nm)/Ga
As(1.5nm) Room temperature
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InxAlyGa
1-x-yAs/GaAs SLsBarrier heights for
electrons Uc Ec2 Ec1, heavy holes Uhh Evh2
Evh1 and light holes Ulh Evl2 Evl1. Negative
values of Uc imply that, for electrons, the GaAs
layer is a barrier and the InxAlyGa1-x-yAs layer
is a well. The splitting energy ?Ehh - lh Ehh1
E h1. The band gap of the superlattice Eg Ee1
Ehh1. B is the emission probability
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Sample 5501 (SLAC - SPTU)
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Summary Outlook

• Values of electron polarization of up to 92
  with corresponding Quantum efficiency of up to
  0.5 have been achieved.
• Till now the best structures are
• Strained InAlGaAs/AlGaAs and GaAs/GaAsP
  superlattices.
• The optimization of DBR superlattice structures
  is underway.

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Acknowledgments

• This work was supported by
• RFBR under grant 04-02-16038,
• NATO under grant PST.CLG.979966,
• the U.S. Department of Energy under contract
  DEAC02-76SF00515 and Swiss National Science
  Foundation under grant SNSF IB7420-111116.

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In expression for QE(?) we choose the emission
probability B so as to ensure the best agreement
with the experimental spectrum, assuming that the
reflection coefficient is R 0.3 and weakly
depends on the photon energy in the
frequency range considered. For sample 6-330, the
parameter B appeared to be equal to 0.089. The
parameter of the absorption edge tail d 30 meV
was found by comparing the behavior of QE(?) with
the decrease in the observed quantum yield below
the absorption threshold. The width of the hole
spectrum ? 30 meV by comparing P(?) with the
experimental data in the region of the maximum.
This value of ? corresponds to the width of the
photoluminescence peak of the SL measured at a
temperature of T 77 K. Rather large values of ?
and d are caused by the fluctuations of
heterolayer composition. For the sample
considered, we obtain 9 losses of the
initial photoelectron polarization, i.e., it is
almost twice as large as the 5 polarization
losses obtained at the emission from the BBR.