Transcript Effect of atomic hydrogen exposure on electron beam polarization from strained GaAs photocathodes M.

Effect of atomic hydrogen exposure
on electron beam polarization from
strained GaAs photocathodes
M. Baylac, JLab
[email protected]
P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames,
J. Hansknecht, M. Poelker, M. Stutzman
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Plan
• Motivations
• Experimental setup
• Test
• QE
• Polarization measurements (2002 & 2003)
• Analyzing power (2003)
• Profilometry
• Results, Conclusions
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Motivations
• Early days at JLab, wet chemical etching provided unreliable results:
sometimes good, sometimes bad QE
•
Since 1995, atomic hydrogen cleaning provides high QE and
reproducible results at JLab; other labs have adopted this method
since then (MAMI, Nagoya, Bates, SLAC)
• Polarization varied from wafer to wafer originating from the same
manufacturer and across one single wafer (12 mm diameter)
• What was JLab doing differently from SLAC?
 Study effect of hydrogen
cleaning on wafer properties
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Strained layer GaAs photocathode
Bandwidth Semiconductor (formerly SPIRE)
0.1 μm
Strained GaAs
• MOCVD-grown epitaxial
250 μm GaAs1-x Px
spin-polarizer wafer
• Lattice mismatch
 split
degeneracy of P3/2
250 μm
GaAs1-x Px
0<x<0.29
600 μm
p-type GaAs
substrate
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
x=0.29
Photocathode preparation
• 3” wafer cleaved (15.5 mm)
• Sample on stalk w/
• No wet chemical treatment,
ie: no acid or base etching, no
degreasing, no anodization
Indium
Ta cup
lucky
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Test Plan
• Stalk installed in gun vacuum chamber,
chamber evacuated & bake (250 C)
•
Wafer heated 2 hours at ~ 570 C (estimated wafer temperature)
• NEA activation (Cs+NF3) in gun chamber, QE scan of wafer
• 100 keV beam :
QE, polarization vs. wavelength and vs. wafer location
• Break vacuum, remove wafer from gun
• Load in portable hydrogen cleaning chamber, pump down
• Expose wafer to atomic hydrogen
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hydrogen source
• H2 dissociation via RF
inductive discharge
wafer ~300C
• Parameters adjusted then,
wafer exposed
100 MHz
• Dose measured with an “ion
counter” at chamber bottom
G
20 W
• Conditions kept identical for
all cleanings
Mc.Alpine & Schildknecht,
H2, or
Proceeding of IRE, Thomas
1959Jefferson
(2099)
National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
D2
~
Hydrogen cleaner
http://www.jlab.org/accel/inj_group/h2/portable_H2.html
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Test gun
• 100 keV DC beam
• Wavelength tunable Ti:Sapp
(750-850 nm)
• 10 Hz helicity reversal
• Mott polarimeter : Pe
lenses
• Beam dump : QE
correctors
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
viewers
Test gun (cont’d)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Initial benchmark, untreated surface
840 nm
2
y
3
5
4
1
x
Stat. only
Syst. ~ 10%
QE %
~ 13 mm
M. Baylac et al, SPIN 2002, 15th International Spin Physics Symposium proceedings,
1073 (2003).
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
QE at band-gap vs. Hydrogen exposure
At 840
nm
Significant drop in QE vs.
H cleaning
Confirmed by 2 similar
tests in different chamber
w/o breaking vacuum
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarization vs. cumulative H dose
(central location)
• Depolarization as H dose
is increased
• -dependent, strongest
at band-gap
• Found systematic effect:
High H dose, QE low:
ND filter removal
 increase non-IR light
Depolarization due to low , high QE light
 Reject 60+ min. data
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Depolarization tests
• Confirm/infirm previous depolarization results
• Test H+ ions effect
• 3 independent tests, 3 photocathodes, only one change:
• Usual exposure (no bias on photocathode during cleaning)
• Ion-enhanced exposure (negative bias)
• Ion-reduced exposure (positive bias)
• Same method as before with one single exposure of 80
minutes for each test
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarization with usual exposure
P(H)-P(0)~-10%
at bandgap
80 min dose
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarization with ion-reduced exposure
P(H)-P(0)~-10%
at bandgap
80 min dose
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarization with ion-enhanced exposure
P(H)-P(0)~-20%
at bandgap
80 min dose
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Analyzing power
Vacuum chamber
GaAs
x
x
i
x
x
C
s
NF3
x
Insertable
dissociator
• Measure A.P. across
strained-layer GaAs
• 7 consecutive H
exposures up to 4 hours
l/2
Insertable
powermeter
860 nm
diode laser
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Analyzing power (cont.)
Analyzing power at 5 locations:
~ 5% and uniform
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Analyzing power
• No variation with H exposure, or location
=> Effect unrelated
to strain
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Profilometry after H exposure
•A roughened surface could explain depolarization as
effective angle of incidence of light onto wafer varies
•High resolution 3d profilometry @ Jlab (Andy Wu)
area = 80 mm X 20 mm
vertical resolution = 7.4 Angstrom
• Measurement of RMS roughness
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Profilometry after H exposure
Bare surface
RMS ~ 155 A
H cleaned
RMS ~ 8500 A
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Hydrogen exposure : results
• Depolarization at band-gap induced by H exposure:
confirmed at ~ 10% level
systematics explains stronger effect seen on older data
• QE seems to be only lowered by heavy H dose
• Depolarization with/without H ions, unexplained enhancement
• Increased anneal cycle (12 h instead of 2) does not restore Pe
• Analyzing power independent of H
• Surface analysis shows roughened surface which can explain
depolarization (underway)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarization comparison
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Laser power systematic check
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy