Superhigh brightness and highspinpolarization electron source based on a novel transmissiontype GaAs

1
Super-high brightness and high-spin-polarization
electron source based on a novel
transmission-type GaAs/GaAsP strained
superlattice- Defects and polarization -
PESP2008 Oct.1-3, 2008, _at_Jefferson Lab., VA USA

• Xiuguang Jin1, Toru Ujihara1,
• Masatoshi Tanioku1, Yuya Maeda1, Shingo Fuchi1,
  Yoshikazu Takeda1, Naoto Yamamoto1, Yasuhide
  Nakagawa1, Masahiro Yamamoto1,Shoji Okumi1,
  Tsutomu Nakanishi1, Takashi Saka2, Hiromichi
  Horinaka3, Toshihiro Kato4, Tsuneo Yasue5,
  Takanori Koshikawa5

1 Nagoya University, 2 Daido Institute of
Technology, 3 Osaka Prefecture University, 4
Daido Steel Co. Ltd., 5 Osaka Electro-Communicatio
n University
2
Contents

• Introduction of our group
• Outline of strained superlattice photocathode
• Development of a high brightness and high
  polarizarion transmission-type photocathode for
  SPLEEM
• Defects and polarization

3
Takeda group
Department of Crystalline Materials
Science, Graduate School of Engineering, Nagoya
University
4
Fields
Crystal growth gt Thin films and quantum
structures of III-V semiconductor materials
(1) Superlattice for Photocathode, (2) Quantum
dot diode for biological issue gt Bulk
growth of semiconductor crystal (SiC, AlN)
InAs-QD/InP
Y.Yasuno et al., Optics Express 12, 6184(2004).
Human eye
5
Photocathode group
Xiuguang JIN PhD course student
Yuya MAEDA Master course student
Masatoshi TANIOKU Master course student
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NEA-type strained superlattice photocathode
NEA surface
Conduction mini-band

Heavy hole mini-band
Light hole mini-band
It is important that electrons are excitedONLY
FROM HEAVY-HOLE MINI-BAND.
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Strained superlattice for band split
Strained GaAs
Strained superlattice
GaAs layer
Substrate
Superlattice
Mini-band fromation
Good strain, good superlattice and then good
polarization
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STM image of photocathode
Specimen image
Typical structure Well GaAs 4
nm Barrier GaAs0.6P0.4 4 nm Num. of layers
12 pairs
Buffer layer
12 mm
GaAs substrate
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High polarization
GaAs-GaAsP strained superlattice on GaAs substrate
Polarization 92 (2002)
Nakanishi (2002).
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Strain-compensated structure
Improvement of crystal quality for the further
increase of quantum efficiency.
Heavy doped GaAs
Heavy doped GaAs
GaAs/GaAs0.61P0.39 Superlattice 12 pairs
GaInAs/GaAsP Superlattice 12 pairs
GaAs0.80P0.20 buffer
No buffer layer
p-type GaAssubstrate
p-type GaAssubstrate
Presented at the poster of X. G. JIN.
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SPin Low Energy Electron Microscopy (SPLEEM)
Principle of SPLEEM
Polarized electron (polarization P)
Diffraction
Magnetization M
A Magnetic contrast
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SPLEEM images
q
Incident beam energy 0.7 eVScam rate 50
sec/image
Observation area f 30 ?m
f 10 ?m
f 6 ?m
f
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Performance requirements
To observe magnetic structure in spin-electronics
materials and magnetic storage devices, Higher
performance Spin Low Energy Electron
Microscope(SPLEEM) is expected!
High polarization ? High contrast High
brightness ? Fast imaging
Present(commercial) Requirement
Polarization 30 90 and over (low
contrast) (high contrast) Brightness 1x103
Acm-2sr-1 3x105 Acm-2sr-1 1-10
s/frame 0.03 s/frame (Video rate)
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Back illumination for high brightness
I
I Current of electron beam S Generation area
of electron beam ? Solid angle of electron beam
Brightness
S ? ?
Front-side illumination
Novel Back-side illumination
Diameter 1-2 ?m
Diameter 100 ?m
It is difficult highly to focus the laser light,
because it must be avoid that the optical system
obstructs electron beam.
Highly focusing lens can be used.
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Transmission-type Photocathode
Transmission-type
Heavy doped GaAs
GaAs/GaAs0.64P0.37 Superlattice 12 pairs
GaAs substrate
Band gap 1420 meV
GaAs0.64P0.37 buffer
GaAs substrate
GaP substrate
Pump laser l780 nm
x
GaP substrate
Band gap 2260 meV
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Growth method conditions
GaAs 5 nm Zn 6?1019 cm-3
MOCVD
GaAs 4 nm GaAs0.64P0.36 4 nm 12 pair Zn
1.5?1018 cm-3
GaAs0.64P0.36buffer 2?m Zn 1.5?1018 cm-3
GaP substrate
Growth temperature 660?C Feed materials TEG,
TBP,TBA V/III ratio 15
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Measurement system for transmit-type photocathode
Laser light
Photocathode
Brightness measurement chamber
Electron beam
Electron beam
Cross-section of electron gun
Mott scattering chamber for polarization
measurement
Poster session, Naoto Yamamoto
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Highly focused excitation light
Laser light
Laser spot image on the back-surface of the
photocathode
L531 mm
1.9 mm
Laser light intensity profile
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High brightness
I beam current r Generation area(radius) R
Beam size at the Faraday cup(radius) L Distance
from photocathode to Faraday cup
Measurements
Beam current I 3.2 ?A? Generation area (radius)
r 1.3 ?m Beam size at the Faraday cup R 1.9
mm Distance L 531 mm
Brightness 1.3?0.5?107 A?cm-2?sr-1 The largest
value in the category of polarized photocathode
is 10000 times larger than that of the commercial
production, 40 times larger than that of our
target.
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Polarization
Polarization5060 (Conventional structure on
GaAs substrate 8090 over)
Polarization was low, although the superlattice
structure was same as that on the GaAs of which
the polarization is 92.
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Discrepancy between front and back irradiations
Front irradiation Pol. 6065 Back
irradiation Pol. 5360
Front
Back
Front
Back
The back irradiation is not problem. The low
polarization is originated in the PHOTOCATHODE.
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Lattice mismatch between GaAsP buffer and
substrate
GaAs substrate
GaP substrate
Lattice constantGaAsPgtGaP
Lattice constantGaAsgtGaAsP
Compressive strain
Tensile strain
GaAsP buffer
GaAsP buffer
GaAs substrate
GaP substarete
SL on GaP sub.
SL on GaAs sub.
10 µm
10 µm
Strain relaxation process depends on substrate.
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Defects induced by strain relaxation process
GaAs substrate
GaP substrate
GaAsP 30 nm AFM surface image
GaAsP 15 nm AFM surface image
110
110
Crack
Ridge structure
1 µm
1 µm
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Control of strain relaxation process
Heavy doped GaAs
GaAs/GaAs0.6P0.4 Superlattice 12 pairs
Tensile strain is induced in the GaAsP buffer.
GaAsP buffer
GaAs interlayer
GaP substrate
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Effect of Interlayer
AFM image of the buffer layer(5 µm x 5 µm)
GaP substratewithout interlayer
GaP substratewith interlayer
The GaAs interlayer on GaP substrateinduced the
cracks as well as GaAs substrate.
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Interlayer improves polarization
Using the interlayer, the polarization increased
to 90 !! The transmission-type photocathode with
high polarization was successfully developed!
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Performance
LaB6 10 ?m 105106
Brightness10000 times Polarization 3 times
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How do defects affect polarization?

• Strain is reduced by introduction of defects.
  (The low polarization is caused by the excitation
  process.)
• Defects directly induce SPIN-FLIP. (The low
  polarization is caused by the diffusion process.)

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Strain in superlattice
Pol. 92
Pol. 90
Pol. 60
e20.29
GaAs0.70P0.30
e10.72
GaAs0.98P0.02
GaAs0.70P0.30 buffer
GaAs interlayer
GaP substrate
93 meV
88 meV
68 meV
Calculated split width of hole mini-bands
Strain in superlattice does not affect the
polarization.
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Defect type
Pol. 60
Pol. 92
Pol. 90
Compressive
Tensile
Tensile
GaAsP buffer
GaAs interlayer
GaP substrate
110
110
110
Crack
Dislocation and/or Stacking fault
Crack
1 ?m
1 ?m
1 ?m
The defect type is strongly related with the
polarization. Moreover, the defect density is not
important.
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Effect of defect on polarization during diffusion
process
Crack
Dislocation or Stacking fault
GaP
They are along 111 plane.
Clacks are vertical to (001) substrate
The electrons go across the dislocations and
stacking faults. On the other hand, cracks do
not affect the electron diffusion.
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Conclusions

• We successfully developed the transmission-type
  high brightness and high polarization
  photocathode.
• The polarization strongly depends on the type of
  defect induced by the strain relaxation process.
  In our case, dislocation and stacking fault
  affect the polarization, while cracks are not
  bad.
• The introduction of the GaAs interlayer
  controlling the strain in buffer layer is
  effective on the improvement of polarization.

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Further Study A Transmission-type
strain-compensated structure
Improvement of crystal quality for the further
increase of quantum efficiency.
Heavy doped GaAs
Heavy doped GaAs
GaAs/GaAs0.61P0.39 Superlattice 12 pairs
GaInAs/GaAsP Superlattice 12 pairs
GaAs0.80P0.20 buffer
No buffer layer
p-type GaAssubstrate
p-type GaAssubstrate
Presented at the poster of X. G. JIN.
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Further studies

• In order to increase the quantum efficiency,
• interlayer material is changed,
• strain-compensate structure is used.

Poster presentation by X. G. JIN.
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Quantum efficiency spectra of up- and down-spin
electrons

• QE spectrum changes step-wise. Thus, the
  difference between both hole mini-bands is
  sufficient.
• Both spin-electrons simultaneously increase. This
  indicates that the SPIN-FLIP occurs during
  diffusion in superlattice.

Low polarization PES on GaP
Allover Pol.
QE
Down-spin Pol.
Up-spin Pol.
Both electrons simultaneously increase.
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XRD results
GaAs
GaAsP buf.
SL on GaAs substrate (Pol. 90)
XRD Intensity (a.u,)
GaP
Satellite peaks
SL on GaP substrate (Pol. 60)
GaP sub.
SL on GaP with GaAs interlayer (Pol. 90)
? (deg.)
The SL on GaP is the most PERIODIC. This result
is not consistent with the polarization.
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PL results
SL
GaAsP buf.
SL on GaAs substrate (Pol. 90)
PL Intensity (a.u,)
SL on GaP substrate (Pol. 60)
SL on GaP with GaAs interlayer (Pol. 90)
Wavelength (nm)
The PL intensities are almost same.
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Calculation of actual strain in superlattice
Strain in GaAs1-xPx barrier, e2
Strain in GaAs well, e1

• Calculation Procedure
• The average lattice constant of the superlattice
  is calculated from the XRD satellite peak
  position.
• The band gap of the superlattice is calculated
  from the PL peak position.
• The parameters are determined from the numerous
  calculations of the band gap based on the
  Kronig-Penny model and Model-solid theory.

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NEA-type strained superlattice photocathode
NEA surface
Ex) GaAsP/GaAs strained superlatice
Conduction mini-band

Strain-relaxed GaAsP buffer layer
Heavy hole mini-band
GaAs substrate
Light hole mini-band
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Band structure of GaAs/GaAsP superlattice
GaAs 5 nm Zn 6?1019 cm-3
136 meV
GaAs 4 nm GaAsP 4 nm 12 pair Zn 1.5?1018 cm-3
1857 meV
1572 meV
112 meV
233 meV
GaAs0.64P0.36buffer 2?m Zn 1.5?1018 cm-3
GaAs substrate Zn 1.3x1018 cm-3
GaAsP buffer
GaAs 4 nm
GaAsP 4 nm
The split between heavy hole band and light hole
band is 112 meV that is sufficient to high
polarization.
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Defects and substrate
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110
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R. M. Stroud et al., Phys. Rev. Lett., 89 166602
(2002)