Transcript Activation Studies of Gallium Arsenide Photocathodes

Activation Studies of Gallium
Arsenide Photocathodes
BY MORGAN DIXON
CARLETON COLLEGE; NORTHFIELD, MN
MENTORS: DR. IVAN BAZAROV, DR. YULIN LI,
DR. XIANGHONG LIU
CORNELL UNIVERSITY, SUMMER 2011
Why Study
GaAs
Photocathodes?
-GaAs is a
semiconductor
-Activate to negative
electron affinity to create
electron emission
-Emitted electrons form
the electron beam in
applications such as
Energy Recovery Linacs
(ERL)
photocathode
Figure 1: Design of the current ERL
Injector at Cornell University
Quantum Efficiency (QE) and Lifetime
 QE: measure of the
number of emitted
electrons per incident
photon on the cathode
 Ip = photocurrent (aka
the current of emitted
electrons)
 Pl = laser power incident
upon the cathode
hc I p
QE  
100%
e Pl 
 Lifetime: span of time
over which a
photocathode produces a
satisfactory QE

Time for QE to reduce to
1/e of it’s original value
 Dark Lifetime vs.
Operational Lifetime
 Want to increase lifetime
– currently a maximum
of a few days
The Physics of Photocathodes
Figure 2: Energy band diagram of a GaAs crystal and a GaAs crystal activated to NEA.
QE Chamber
Figure 4: CAD drawing of the UHV chamber used for all experiments.
Cathode
Stock
1. Remove old cathode
by heating the base and
melting the Indium
2. Clean old Indium
from the Mo Base
3. Place clean Indium
on the base and attach
the new cathode
Figure 5: CAD drawing of the cathode stock
and the cathode heating elements.
LabVIEW program
Figure 6: Screenshots of the labVIEW program,
GUP.vi
Cathode Preparation Procedure
 In house diamond cutting process
 Acid etching to remove surface oxides and other
contaminants
 Attach the clean cathode to the Mo base and place in
the chamber
 Pump down chamber and bake out to achieve UHV
 Vacuum annealing to further remove oxide
contaminants on the surface of the cathode

Heat cathode to 650C and hold for two hours
Co-Deposition
Activation
1. Introduce Cs until
peak in QE, then
overdose.
2. Once QE drops 20%
– 30% of peak value,
introduce the oxidizing
agent to the system.
3. Cease Cs and
oxidizing agent
deposition once QE
peaks a second time.
Figure 7: Typical activation curve using the
co-deposition method.
Yo-Yo
Activation
-1. Introduce Cs until
photocurrent peaks,
then overdose.
Initial
Cs Peak
-2. Once photocurrent
drops to half of the peak
value, cease cesiaton
and introduce oxidizing
agent.
-3. When photocurrent
peaks again, shut leak
valve and introduce Cs
again.
First exposure to NF3
-4. Repeat “yo-yo’s”
until total gain in
photocurrent plateaus.
Figure 8: Typical activation curve using the Yo-Yo method.
Activation
with N2 : Before
Gas Purification
First two trials showed
same activation process
and same photocurrent.
Both activations were
very slow.
Figure 9: Activation of GaAs with N2, first and second trials.
Activation
with N2 : After
Gas Purification
Previous activations
due to contamination?
Purified the N2 used for
activation by pumping
out the gas manifold.
Reduced O2 content by
nearly a factor of ten.
Activation process
slowed by nearly a
factor of ten.
Conclusion: N2
DOES NOT
ACTIVATE
CATHODES
Figure 10: Activation using N2, before and after cleaning
out and recharging the gas manifold.
Activation with N2 : scaled time axes
Comparison to First N2
Activation
Comparison to Second N2
Activation
Figure 11: Activation of cathode with N2 before and after purification of gas.
Activation
with NF3
Wanted to study the
dark lifetime of
cathodes activated
using NF3 and compare
to activation with O2.
Accidently killed the
cathode so lifetime
studies could not be
conducted.
Figure 8: Typical activation curve using the Yo-Yo method.
Blank
Activation
with NF3
- “Activated” the
cathode with Cs and
NF3 even though no
photocurrent was
detected.
- No guide to see when
peaks, only have RGA
scans to show when NF3
or Cs are in the
chamber.
NF3
Cs peak
Figure 12: RGA scans from blank activation with NF3.
Thermal
Desorption
Spectroscopy
(TDS)
-Linearly increase
temperature of cathode
at a rate of 5.25 C/min.
-Adsorbates desorb
from the cathode
surface while RGA
records composition of
the desorbed materials.
-Temperature of
desorption gives
information on how
adsorbates are bonded
to the surface.
Figure 13: TDS graph of select masses from N2 activated cathode.
TDS of Cs on
activated GaAs
photocathode
-Proper activation,
Tdesorb = 394 C
-Blank activation,
Tdesorb = 354 C
-Not enough data to
make conclusions,
however, the sharp
peak suggests that the
Cs is strongly bonded
with the surface of the
cathode.
Figure 13: Cs desorbing from NF3 activated cathode and other
peaks that desorbed at the same temperature.
TDS to identify
unknown
species
-After the cathode died,
the dominant species in
the gas composition of
the chamber changed
and we had many
unknown species in the
chamber.
-TDS revealed possible
fragment peaks of an
unknown species that
we tried to use to
identify the unknown
composition of the
mass 85 peak.
-Never found a good fit
for the species.
Figure 14: TDS data of unknown species on NF3 activated cathode
with twin peaks at masses 84 & 85 and possible fragment peaks at
masses 49 & 64
Conclusions
 The equipment and computing resources are set up
to begin work with the new QE chamber.
 Nitrogen will NOT activate GaAs photocathodes

GaAs photocathodes are very sensitive to contaminants in
activating gasses, so gases used in activating processes should
be very pure to be sure which species are activating the
cathode.
 Cs appears to strongly bind with the GaAs surface
when the cathode is activated with NF3.
Acknowledgements
 Thank you very much to my mentors Ivan, Yulin and
Xianghong, and all other members of Cornell’s
photocathode project, for their wonderful support
throughout the summer.
 This project was funded by the NSF.