Polarized Photocathode R

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Polarized Photocathode RD Update
Victoria ALCW July 28-31 2004
Takashi Maruyama SLAC

• PPRC Collaboration
• A. Brachmann
• J. Clendenin
• E. Garwin
• T. Maruyama
• D. Luh
• R. Kirby
• C. Prescott
• R. Prepost (Wisconsin) LC Accelerator RD at
  Universities

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OUTLINE

• Polarized photoemission
• Strained superlattice RD
• E158 RUN III
• Atomic hydrogen cleaning
• Multi-bunch laser development
• Summary

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Polarized photoemission
Circularly polarized light excites electron
from valence band to conduction band
Electrons drift to surface L lt 100 nm to
avoid depolarization Electron emission to
vacuum from Negative-Electron-Affinity (NEA)
surface
NEA Surface Cathode Activation
Ultra-High-Vacuum lt 10-11 Torr Heat treatment
at 600 C Application of Cesium and NF3
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Strained-superlattice
Single strained GaAs (SLC)
Strained GaAs

• ? High gradient doping 5?1019/cm3 in the 5 nm
  surface layer and 5?1017/cm3
• in the rest ? No surface charge limit
• Each superlattice layer is thinner than the
  critical thickness
• ? GaAs layers are highly
  strained

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SBIR with SVT Associates
Advanced Strained-Superlattice Photocathodes for
Polarized Electron Sources

• July 2001 SBIR Phase I awarded
• Very first sample produced 85 polarization
• Sep. 2002 SBIR Phase II awarded (2 year project)
• Polarized photocathodes are commercially
  available
• (SLAC Spin Polarizer Wafers).
• Phosphorus containing wafers are usually grown by
  MOCVD.
• Gas-source MBE is used at SVT Associates.

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MBE- In Situ Growth Rate Feedback
Monitoring RHEED image intensity versus
time provides layer-by-layer growth rate feedback
Growth at monolayer precision not possible with
MOCVD
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Superlattice parameters
b
w
Parameters Barrier thickness 3 nm lt b lt 7 nm
Well thickness 3 nm lt w lt 7
nm Phosphorus x 0.3 lt x lt 0.4 No. of periods
l 70 200 nm
First systematic study of polarized photoemission
from strained-superlattice - to be published in
Applied Physics Letters
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Structural Analysis Using X-ray Diffraction

• Measure superlattice period.
• Measure well and barrier widths.
• Measure phosphorus fraction in GaAs1-xPx layers.
• Measure strain in GaAs layers.

Data
Simulation
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Structural Analysis using SIMS(Secondary Ion
Mass Spectroscopy)
Phosphorus concentration vs. depth

• Bombard with Cs ions and
• detect As, P, and Be.
• Measure superlattice period
• Measure Be doping profile

Superlattice structure does not degrade after 600
C heat-treatment.
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Strain effect Vary x in GaAs1-xPx

• Max. polarization 86
• QE 1 (x5 SLC Cathode QE)
• HH and LH transitions observed
• HH-LH splitting increases with x.

LH
HH
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Well Dependence

• Max. polarization 86
• QE 1
• Second peak is sensitive to HH2 ? opportunity
  to test superlattice model.

X 0.35
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Period dependence

• ? Strain relaxes steadily.
• Peak polarization is constant lt 15 periods,
• but decreases rapidly gt 20 periods.

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No Charge Limit
1?1012 e- in 60 ns ? 4.5?1012 e- in 270 ns (x3
NLC train charge)
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E158 RUN III

• Cathode installed in May 03.
• E158 ran successfully.
• ESA Moller measured 90.
• But it showed a charge limit 7?1011 e-/300 ns
• Could not make NLC train charge but OK for E158.
• What happened?
• The 600 C heat-cleaning is destroying the high
  gradient doping profile.

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Dopant diffusion
Be concentration vs. depth

• Be dopants diffuse out of the
• surface during 600 C heat-cleaning.

Need to lower the heat-cleaning temperature to lt
450 C without lowering QE.
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Atomic-Hydrogen Cleaning Appl. Phys. Lett. 82,
4184 (2003)
Bulk GaAs
Ga2O3 comes off at 600 C. Ga2O comes off at
450 C. Ga2O3 4H ?Ga2O 2H2O?
600 C heat-cleaning QE 11 AHC 450 C
heat-cleaning QE 15
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Multi-bunch laser development
Laser modulator
RF Amp.
RF Gen.
Currently RF amp. is limited to 200 MHz. Need 714
MHz for 1.4 ns.
119 MHz (8.4 ns)
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Summary

• High gradient doped strained-superlattice
  GaAs-GaAsP has been developed under SBIR and is
  commercially available.
• Peak polarization of 86 with 1 QE and no
  charge saturation up to 1?1012 e- in 60 ns.
• Used successfully in E158 Run 3, yielding 90
  polarization at ESA.
• High gradient doping is vulnerable to high
  temperature heat cleaning.
• To lower the heat-cleaning temperature, the
  atomic hydrogen cleaning technique has been
  developed.