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Long term operation of high quantum yield
GaAs-photocathodes at the electron target of the
Heidelberg TSR using multiple recleaning by
atomic hydrogen
D. A. Orlov1, A.S. Terekhov2, C. Krantz1, S.N.
Kosolobov2, A.S. Jaroshevich2, A. Wolf1
1 Max-Planck-Institut für Kernphysik, 69117,
Heidelberg, Germany2 Institute of semiconductor
Physics, 630090, Novosibirsk, Russia
0.2 ... 8 MeV/u

• Motivation Photocathode multiple recleaning
  technique. Reliable, closed cycle, QY recovering.
  
• TSR target. Photocathode performance.
• Atomic hydrogen cleaning.
• Capillary AH source at TSR target.
• Results UV-spectroscopy
• (H-treatment optimization).
• Results Multiple recleaning.
• Outlook

TSR
Detectors (ions and neutrals)
Photoelectron e-target
Collector
e-
e-source
Electron gun with magnetic expansion ?10...90
Interaction section 1.5m
Ion beam
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Photocathode performance at the electron target
(A)
Superconducting solenoid
Gun chamber
E-gun
Preparation chamber
Manipulator
Loading chamber
Hydrogen chamber
collector
Photoelectron target
Merging region

• Currents up to 1 mA (2 mA)
• Lifetime - 24 h at 1 mA (2mA)
• kT? 0.5-1.0 meV kT 0.02 meV

E-gun
3
Photocathode performance (B) Lifetime
H2O
O2
CO2
1. Dark lifetime (RT) gt weeks (UHV)
CO
CH4
H2O
O2
CO2
2. Dark lifetime (LT) hour-weeks(temperature)
Cryosorption! T gt 130 K
Cold
CO, CH4
3. Operating high-current lifetime
Ion deflection, barrier!
e
Ion back stream!
GaAs
E
(e-current, energy, pressure, geometry)
B
Beam profiles (D12 mm)
Start
Degraded
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Atomic hydrogen cleaning
RF coil
1. RF plasma discharge source.
H2
H
Energetic particles from the source!Risk of
photocathode damage!
GaAs
oven
2. Hot filament source.
H2
oven
Low efficiency!Cathode heating! High partial
pressure of W!
3. Hot capillary source
W-capillary
H2
GaAs
Just good -).
oven
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AH treatment at the TSR target. Hot capillary
source.

• Efficient
• Narrow angular distribution of H-atoms
• Low capillary temperature (no W-contamination)

Leak valve
Leak valve
palladium tube
palladium tube
P1.0E-08 mbar
H2
manipulator
W-capillary
H
manipulator
filament
sample
oven
H2
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AH treatment at the TSR target. Hot capillary
source.
W-capillary
1900 K
H2
GaAs
oven
T450o C t5-10 min
Based on the data K.G. Tschersich, JAP 87, 2565
(2000)
Leak valve
palladium tube
P1.0E-08 mbar
When heat-cleaning does not help (after 3-5
times)
H2
W-capillary
H-treatment (typical) Tcathode4500 CH-flux
5E14 atoms/cm2/sExposure time 5-10
minExposure 50-200 L
manipulator
filament
sample
oven
In 5 min transfer the sample to Prep.
ChamberHeat-cleaning at 400-4500 C for 30 min.
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AH cleaning UV spectroscopy
Cs/O layer removing by H0 Clean -gt CsO -gt H
Cs/O layer removing by H0 H-dose optimization
different H0-exposures
10 L 200 L
1. After 4 CsO activations heat-cleaning
QY (electron/photon),
QY (electron/photon),
4. H-cleaning
3. Cs heat-cleaning
2. Clean (HCL ISO)
5.0
6.0
4.0
3.0
5.0
6.0
4.0
3.0
Photon energy, eV
Photon energy, eV
- Accumulation of Ga/As oxides after multiple
reactivations. - AH efficiently removes oxides. -
The small presence of Cs.
To remove Ga and As oxides the AH exposure of
about 100 L is enough.
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Atomic hydrogen multiple recleaning
25
40
1.5 year of operation! (21 AH treatment, gt 80
activation, 120 heat cleaning)
35
20
30
25
15
20
QY (electron/photon),
QY (electron/photon),
10
15
LPE grown transmission mode photcathode
MOCVD grown transmission mode photcathode
10
5
5
0
0
0
500
1000
1500
2000
2500
3000
3500
0
500
1000
1500
2000
2500
H0 dose, L
H0 dose, L
AH multiple cleaning works almost perfectly with
only slow QY decrease for MOCVD grown
photocathodes.
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QY degradation heat-induced?
1. Accumulation of oxygen? NO!
2. Arsenic vacancies defects? NO!
3. Heat-cleaning induced degradation of
transmission mode cathodes (mechanical strain)?
YES!
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QY degradation heat-induced dislocations?
1. Accumulation of oxygen? NO!
2. Arsenic defects (vacancies)? NO!
3. Heat-cleaning induced dislocations at the
substrate (sapphire)-heterostructure interface?
AFM-image of photocathode with smooth surface
AFM-image of photocathode after multiple
recleaning
Peaks height 30-50 nm
RMS 0.2 nm
Dislocation net
Outside of peaks RMS 0.5 nm
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Conclusions Outlook

• Multiple recleaning of high QY photocathodes
  it works!
• Slow QY degradation is probably due to
  heat-induced defects (dislocations at the
  sapphire-heterostrucrure interface).
• Still can be improved.

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TSR electron target section - overview
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Photocathode section - overview
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Closed cycle of operation with atomic hydrogen
treatment
QY (electron/photon),
QY (electron/photon),
The evolution of QY UV spectra for different
AH-exposures
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TSR photoelectron target
Detectors (ions and neutrals)
0.2 ... 8 MeV/u
Neutrals detector
Movable ion detector
e-target
Collector
Electron gun with magnetic expansion ?10...90
TSR dipole
e-source
Adiabatic acceleration
e-
Interaction section 1.5m
Ion beam
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Fig.3 The figure shows the history of the
N5-photocathode in the Heidelberg target (gt1
year). In total the sample experienced more than
100 heat-treatment. Each minimum correspond
Cs-activation which typically goes after
H-treatment, except of N85, where no Cs-cleaning
was used. Others intermediate points correspond
to 1, 2, 3 or 4-th activation. The values of
AH-exposure are also indicated on the figure.
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Atomic hydrogen cleaning UV spectroscopy
Fig.1 The spectra was measured after HCL or
H-treatment or after activation by Cs or Cs/O
with subsequent heating. The steps are described
in the picture and ordering goes from up to down
(the first step before HCL, the last on
Cs/O2 6.5 A for N5 and 7.0 A Cs 6.5 A
for N6). Find on the next page detailed
description of the steps.
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Cryogenic photocathode source
Cs/O layer removing by H0
Photocathode setup
different H0-exposures
low high
QY (electron/photon),
Quantum Yield vs UV photon energy
Photocathode at 100 K
5.0
6.0
4.0
3.0
Photon energy, eV
Atomic hydrogen cleaning
Vacuum conditions UHV (510-12 mbar) H2O, O2,
CO2 lt10-14 mbar
High requirements for surface preparation
Number of steps (H0 or heat-cleaning)
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Photocathode performance at the electron target
(A)
Photoelectron target
T-control (heat cleaning, operation)
Photoluminescence IR transmission
spectroscopes, photoelectron spectra Surface
cleaning quality UV QY spectroscopy
Emission properties 2D energy
distribution

• Currents up to 1 mA (2 mA)
• Lifetime - 24 h at 1 mA (2mA)
• kT? 0.5-1.0 meV kT 0.02 meV