Polarization comparison of InAlGaAsGaAs superlattice photocathodes having low conduction band offset

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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

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85-90
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Main depolarization mechanisms

• Interband absorption smearing d due to bandedge
  fluctuations
• Hole scattering between HH and LH states causes a
  LH broadening g
• Optical phonon scattering
• Non polarized electrons generated in the BBR
• Scattering of electrons in the BBR

100nm
electron
6nm
GaAs
D
E
c
BBR
Conduction Band
AlGaAs
Vacuum
Buffer
HH1
D
E
v
Valence Band
LH1
HH- LH separation determined by well/barrier
thickness and strain DEC/DEV determined by SL
structure
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Motivation for a flat Conduction Band structure

• Observed emission spectrum
• Emitted electron polarization

a optical absorption R reflection
coefficient t1 electron lifetime in BBR tem
time of electron emission in vacuum  P0 initial
electron polarization upon excitation with
circularly polarized light S0 surface
recombination velocity tS spin relaxation time
in SL tS1 spin relaxation time in BBR
A. Subashiev et. al. SLAC-PUB 10901
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Design of a flat conduction band structure
Strained Barrier AlyInxGa1-x-yAs/GaAs SL on GaAs
substrate
y determines the bandgap x lowers bandgap,
controls DEC and induces strain
DEC(x,y)QC1DEg1(x)QC2,def(DEg2(y)dEC,def)
Yu. A. Mamaev et al., PST2003
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Unstrained wells and compressively strained
barriers GaAs/AlyInxGa1-x-yAs SL

• dQW(Å) E hh2 E lh1
  E hh1 E e1
• 10 -0.2054E00 -0.1160E00
  -0.5120E-01 1.429
• 15 -0.1752E00 -0.1041E00
  -0.4413E-01 1.429
• 20 -0.1542E00 -0.9352E-01
  -0.3794E-01 1.428
• 25 -0.1389E00 -0.8409E-01
  -0.3267E-01 1.428
• 40 -0.9484E-01 -0.6199E-01
  -0.2145E-01 1.427

Yu. A. Mamaev et al., Mainz 2004
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Photocathodes
QW 1.5nm QB 4nm BBR 6nm SL doping 4e17cm-3
GaAsP/GaAs SL 89meV LH-HH splitting
DEC36meV, DEV19meV
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5506 In.17Al.18Ga.65As/GaAs SL
Green curve without BBR absorption Black curve
with BBR
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5501 In.20Al.21Ga.59As/GaAs SL
Green curve without BBR absorption Orange
curve with BBR
10
5503 In.23Al.25Ga.52As/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
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5777 In.20Al.23Ga.59As/GaAs SL
92
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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.

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6329 In.20Al.22Ga.58As/GaAs SL
St. Petersburg 78 CTS 76 77
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6410 In.28Al.35Ga.37As/GaAs SL
St. Petersburg 74 CTS 77
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(004) X-ray
5-777 is better
Superlattice thickness 5-777 5.01 nm
5501 5.10 nm
Indium fraction (assume 100 strain)
5-777 19.6 5501 20.7
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(004) simulation
Assume 100 strain
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Summary of Polarization Results
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How can we explain 90?

• Polarization calibration seems OK within 2-3.
• 5-777 had an As-cap, allowing a low temp
  heat-cleaning.
• When 5-777 was heated to gt540 C, polarization
  dropped to 84, which is consistent with
  SVT-5501.
• 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.

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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.

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More SVT results
SVT-5506
SVT-5503
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SVT-5501 (5-777 duplicate)
Peak polarization 84
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Dear Takashi, We have analyzed your recent
data on SL-5501-5506 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 401-405.).
We have reasonable agreement between the
theory and experiment for the overall behavior of
polarization spectra for SL 5501 and 5506. 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 5503 4 nm In0.23Al0.23Ga0.56A
s 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.
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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) 171911).
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 6-329 and
6-410. 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 5-777 together with our
calculations. We use the latest Ioffe PTI data on
its composition. According to this data SL 5-777
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.
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InAlGaAs-GaAs superlattice

• St. Petersburg group observed 90 polarization
  from InAlGaAs-GaAs superlattice (5-777).
• SVT grew three wafers
• 5506 4 nm In0.17Al0.18Ga0.59As 1.5 nm GaAs 18
  periods
• 5501 4 nm In0.20Al0.21Ga0.59As 1.5 nm GaAs 18
  periods (duplicate of 5-777)
• 5503 4 nm In0.23Al0.25Ga0.59As 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.
• 6-329 4 nm In0.2Al0.22Ga0.58As 1.5 nm GaAs 18.5
  periods
• 6-410 3.07 nm In0.28Al0.354Ga0.366As 0.56 nm
  GaAs 15 periods
• X-ray diffraction of Ioffe 5-777 and SVT-5501