1. Atomic hydrogen exposure of strained layer GaAs photocathodes
M. Baylac, JLab
P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J. Hansknecht, M. Poelker, P. Rutt, C. Sinclair, M. Stutzman

2. Polarized electrons  + N - N P = N + N - e
Nuclear physics program requires an electron source with
High efficiency
& High polarization
Photoemission out of strained layer GaAs semiconductors 
GaAs e -
P =
N - N +
N + N N
QE =
Quantum Efficiency:
Polarization:
~ 0.3%
~ 75%
-100 kV

3. 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 ~ G
15 cm
100 MHz
20 W
Mc.Alpine & Schildknecht,
Proceeding of IRE, 1959 (2099)
H2, or D2

4. Polarization vs Hydrogen dose
Significant depolarization
Wavelength dependent
Effect seen with and w/o ions
P (dose) – P (bare) ~ -10%
at bandgap e

5. 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 Jlab ):
Bare surface: RMS ~ 155 A
Hydrogen exposed: RMS ~ 8500 A

6. 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