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Atomic hydrogen exposure of strained layer GaAs photocathodes. M. Baylac, JLab baylac@jlab.org. P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J. Hansknecht, M. Poelker, P. Rutt, C. Sinclair, M. Stutzman. . N. GaAs. e. QE =. N. . -. -. +. +. -. N - N. N + N. e. e.

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Atomic hydrogen exposure of strained layer GaAs photocathodes

M. Baylac, JLab

baylac@jlab.org

P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J. Hansknecht, M. Poelker, P. Rutt, C. Sinclair, M. Stutzman

polarized electrons

N

GaAs

e

QE =

N

-

-

+

+

-

N - N

N + N

e

e

e

P =

Quantum Efficiency:

~ 0.3%

e

e

e

~ 75%

Polarization:

Polarized electrons
  • Nuclear physics program requires an electron source with

High efficiency

& High polarization

  • Photoemission out of strained layer GaAs semiconductors

-100 kV

atomic hydrogen source

~

G

Atomic hydrogen source

wafer

~300C

  • Semiconductor samples cleaned by exposure to atomic hydrogen
  • H2 dissociation via RF inductive discharge
  • Voltage can be applied on wafer to enhance/reduce effect of ions

15 cm

100 MHz

20 W

Mc.Alpine & Schildknecht,

Proceeding of IRE, 1959 (2099)

H2, or D2

polarization vs hydrogen dose

P (dose) – P (bare) ~ -10%

at bandgap

e

e

Polarization vs Hydrogen dose
  • Significant depolarization
  • Wavelength dependent
  • Effect seen with and w/o ions
why this depolarization
Why this depolarization?
  • Related to strain?

Tested and ruled out

  • Hydrogen trapped in material?

Increased anneal cycle (12 h instead of 2): no effect

  • Variation of angle of incidence of light onto wafer?

Roughness measurements with high resolution profilometry (Andy Wu @ Jlab ):

Bare surface: RMS ~ 155 A

Hydrogen exposed: RMS ~ 8500 A

conclusions
Conclusions
  • Atomic hydrogen exposure necessary to obtain high QE with anodized samples
  • Heavy dose depolarizes semiconductor significantly
  • Depolarization with/without H ions, unexplained enhancement
  • Excessive dose can reduce QE
  • Surface analysis shows roughened surface which can explain depolarization (underway)
  • Need prepare clean sample with minimal hydrogen exposure