Transcript Strained Superlattice GaAs photocathodes at JLab M. Baylac Qweak collaboration meeting August 17, 2004 Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association.

Strained Superlattice GaAs
photocathodes at JLab
M. Baylac
Qweak collaboration meeting
August 17, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarized Electron Guns at JLab
photocathode
HV
insulator
anode
Laser
e
-100 kV
-
Photoemission from GaAs
semiconductor
Cs
NEG-coated
Beamline
NF
3
NEG
pump
s
Strained GaAs in Gun2 (“old”
material)
Strained-superlattice GaAs in
Gun3 (“new” material)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Strained layer GaAs photocathodes
• From 1998 through 2003, we have used strained layer
GaAs photocathodes at JLab (Bandwidth Semiconductor,
Inc.).
• Reliable, well understood material.
• Stained-layer GaAs provides;
• Good polarization: Pe ~ 75% at 840 nm
• Moderate quantum efficiency: QE ~ 0.2% at 840 nm
• Limitations that keep polarization < 80%:
• limited band splitting
• relaxation of the strain for thickness > critical thickness (~10 nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Strained GaAs/GaAsP superlattice
• Very thin quantum well layers alternating with latticemismatched barrier layers
• Each superlattice layer is < critical thickness
• Natural splitting of valence band adds to the strain-splitting
• Developed by SLAC with SVT Associates, Inc.
SLAC-PUB-10331 (2004), submitted to Appl.Phys.Lett
• First samples received at JLab October 2003,
characterized at the injector test cave
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Superlattice structure
Be doping (cm -3 )
5.10
GaAs (5 nm)
17
GaAsP (3 nm)
GaAs (4 nm)
18
GaAs 0.64 P0.36 (2.5 μm)
5.10
5.10
19
14 pairs
GaAs 1-x P x , 0<x<0.36 (2.5 μm)
p-type GaAs substrate
SVT associates, per SLAC specs.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Quantum Efficiency
QE ~ 1% versus 0.2%
from strained layer
material
we operate
here
Wavelength (nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Beam polarization
Highest polarization
ever measured at the
Test Cave
Wavelength for Good
QE and Polarization
Wavelength (nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Analyzing power (%)
Analyzing power (aka QE anisotropy)
Analyzing power
smaller by factor of
3 compared with
strained-layer GaAs:
4% versus 12%
This means smaller
inherent intensity &
position asymmetries
on beam.
Wavelength for good QE
and polarization
Wavelength (nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
QE (%)
QE vs hydrogen cleaning
Typical H-dose to clean
anodized samples
Drawback:
Delicate material
Can’t clean with
atomic hydrogen
Makes it tough to
anodize edge of
cathode
Thomas
Jefferson National
Hydrogen
exposure
timeAccelerator
(min) Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Superlattice Photocathodes at CEBAF
• Several failed attempts to load superlattice
photocathodes inside tunnel guns
• Successful installation of un-anodized superlattice
photocathode in Gun 3 (March, 2004)
• Activation gave QE ~ 0.4% at 780 nm (vs 1% in test cave)
• Used during HAPPEx-He and portion of HAPPEx-H (June,
2004)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Poor lifetime
• Frequent spot moves were
required to maintain 40 A
beam current at Hall A
every week at start of run,
every day as we approached
July 4 shutdown!
• HAPPEx-He OK. HAPPEx-H
not so good. Injector
conditions changing too often.
HC asymmetries were not
stable.
14 mm
QE profile after
3 weeks of running
• Poor gun lifetime atypical of
CEBAF photoinjector.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Polarimetry in hall A
• Compton (D. Lhuillier)
Pe ~ 85.2  3.2 %
photon
electron
• 5 MeV Mott (J. Grames)
Pe ~ 86  3 %
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Parity quality beam?
•
Short run + numerous spot moves
=> Jury is still out. Poor gun lifetime made it difficult to
assess performance of superlattice photocathode from a
parity violation experiment perspective.
•
HAPPEX reports;
•
•
From HAPPEx-H
Gun3 superlattice GaAs Gun2 strained layer GaAs
Charge asymmetry OK
for both photocathodes
Position asymmetries
were smaller using gun2
strained layer photocathode
(no active position feedback)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Surface Charge Limit
• QE drops as laser power increases: photoelectrons build up in
band bending region create opposing E field that reduces NEA
G.A. Mulhollan et al, Phys. Lett. A 282, 309 (2001)
QE is not constant
• Reduces maximum available beam current. Lose laser headroom.
Makes for shorter operating lifetime of gun.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Lasers
• Our new commercial Ti-Sapphire lasers
provide more laser power (~ 300 mW)
compared to our “old” diode lasers (~ 50
mW). http://www.tbwp.com
• They are wavelength tunable. Now we can
tune to peak polarization.
• Successful and reliable running since G0.
• Ti-Sapp laser + superlattice photocathode
a good match for high current Qweak
experiment. 300 mW laser power + QE of
1% can provide 1800 uA beam current.
• Max current only 360 uA with strained
layer cathode. Not as much headroom.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Summary
• Highest polarization ever measured at JLab: Pe = 86%
• Measurements of many samples at test stand indicates this is no fluke.
• 5 times higher QE than strained layer material.
• Smaller analyzing power should provide smaller inherent charge and
position asymmetry. (Recent HAPPEx results do not support this claim.)
• Delicate material, more difficult to handle. Cannot be H-cleaned.
Can’t recover QE from a dirty superlattice, unlike strained layer
• We suffered surface charge limit. QE drops with increasing laser
power. A concern for high current experiments like Qweak.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Outlook
• Poor lifetime due to supperlattice? Doubt it:
• Gun 3 has a bad lifetime in 2003 using strained layer
• Un-anodized wafer increases damage on the wafer
Reworked Gun 3 over the shutdown, hoping to boost lifetime
• QE lower in the tunnel than in test cave:
• Hopefully due to the gun itself, not the wafer
• Received arsenic capped samples: easier to handle and
anodize (to be tested in lab)
• Smaller inherent HC asymmetries? Surface charge
limit? Need more operating experience.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy