Transcript No Slide Title - Jefferson Lab

Atomic hydrogen exposure of
strained layer GaAs photocathodes
M. Baylac, JLab
[email protected]
P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J.
Hansknecht, M. Poelker, P. Rutt, C. Sinclair, M. Stutzman
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarized electrons
• Nuclear physics program requires an electron source with
High efficiency
& High polarization
• Photoemission out of strained layer GaAs semiconductors

e-
Quantum Efficiency:
GaAs
-100 kV
Polarization:
QE =
Ne
N
+
Ne - Ne ~ 75%
Pe = +
N + N-
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
~ 0.3%
e
e
Atomic hydrogen source
• Semiconductor samples
cleaned by exposure to
atomic hydrogen
wafer ~300C
• H2 dissociation via RF
inductive discharge
100 MHz
G
• Voltage can be applied on
wafer to enhance/reduce
effect of ions
20 W
Mc.Alpine & Schildknecht,
Proceeding of IRE, 1959Thomas
(2099)
H2,
Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
or
D2
~
Polarization vs Hydrogen dose
 Significant depolarization
 Wavelength dependent
 Effect seen with and w/o ions
Pe (dose) – Pe (bare) ~ -10%
at bandgap
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
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
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
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
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy