Atomic Hydrogen Cleaning of Super Lattice photo cathodes

1
Atomic Hydrogen Cleaningof Super Lattice photo
cathodes
2

• Introduction
• Long term storage problem of photo cathodes
• Hydrogen Cleaning
• QE/Polarization investigations
• Conclusion

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MAMI overview
MAMI C

• beam parameters
• 1508.4MeV
• max. 100mA
• eh10 nm rad

MAMI B

• beam parameters
• 855MeV
• max. 100mA cw current
• eh8 nm rad
• ca. 6000h 7000h operation / year

4
Our standard way from cathode package to work
position in source

• 1. sample from wafer is unpacked under nitrogen
  atmosphere, inserted in cathode holder and placed
  in transport vessel.
• 2. Transport vessel is connected to load-lock
  chamber and pumped to below 10-7 torr.
• 3. Cathode holder is transferred through valve
  from load-lock chamber to preparation chamber.
• 4. Preparation chamber at a few 10-11 torr.
  Cathode is heat cleaned and NEA-activated.
• 5. Activated photo cathode be placed into source
  (at probably even lower pressure).

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MAMI standard photocathodes

• For test used bulk GaAs (Wafer Tech. LTD.,
  England, U.K.)
• For the beam production strained layer (up to
  2004)
• Since then Super lattice cathodes (Sankt
  Petersburg State Technical University, Russia)
• Example S-45 piece of wafer SL5-998

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Preparation.Q.E. Trend History of the super
lattice cathode S-45
l680nm.
final state worse than it looks.
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New DBR and non-DBR photocathodes
Non-DBR-type 7-395
DBR type 7-396
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non-DBR-wafer SL 7-395
Data measured directly after wafer production at
SPSTU
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First results of new SLs at our installation
Disappointment QE is much too small, unpleasant
nonlinearity.
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Oxide problem?

• Probably due to insufficient As-passivation of
  surface
• Deterioration seems to appear after typical
  storage times of months
• Possible Reason oxide transfer As?Ga. Maybe
  not avoidable even if sample is stored under
  vacuum.
• Investigated ,e.g., by D. A. Allwood et al. for
  GaAs epi-ready surfaces. (Thin solid films, 412
  (2002) 76-83)
• Allwood suggests slowing down oxide transfer by
  cooling to -20C too late for our stock.
• Oxides not removed by conventional heating.
• Attempted solution Atomic Hydrogen cleaning

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Atomic Hydrogen Sources

• 1. Radio frequency source. Atomic hydrogen
  cleaning of polarized GaAs photocathodes was
  successfully applied to strained GaAs cathodes
  used for producing highly polarized electrons.
  (see for example T. Maruyama et al. APL, 82,23
  (2003) 4184)
• 2. Thermal cracking atomic beam sources are used
  successfully to remove native oxidation from GaAs
  and provide extremly good surface quality. See
  for example V. Andreev et al Proc. Spin 2000,
  p.901.
• Open question Polarisation after super lattice
  treatment?
• Note 6 nm thin functional structure in SL
  top layer

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Hydrogen Cleaning HABS
Hydrogen Atomic Beam Source (commercial system
by Dr. Eberl MBE-components GmbH)

• H2 dissociation typically 80-98 depending on
  operational conditions
• Atomic hydrogen flux density up to 11016/(cm2 s)
  
• No high-energy neutrals or ions
• Low power consumption (P lt 200 W)
• Integrated water cooling, low thermal load on
  other experimental equipment

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Atomic Hydrogen Cleaning Installation
Preparation Chamber
UHV transport vessel
Atomic Hydrogen Source
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Preparation with / without AHC
l680nm
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Cathode transport from HABS to test source PKAT
Preparation Chamber
UHV transport vessel
Photo E. Riehn
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Results SL 7-395
Improves 5 at low intensitiesabsence of
saturation! (50 improvement for high
intensities at MAMI)
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Structure SL 7-395
395H second activation 17.7 at 802nm?
difference insignificant. (other 395 H sample
achieves 853 of Polarisation at MAMI)
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DBR Structure SL 7-396H
Highest Q.E values ever measured at high
polarization in our lab (1.2) Stands 5 times
more incident power than conventional GaAs
cathode (preliminary!)
Extended P, QE datasets measured directly after
production by Y. Yashin, SPSTU
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Conclusion

• GaAlAs / InAlGaAs Super lattice photo cathodes
  are the standard type at our facility.
• Storage problem present in some (not all)
  wafers
• Atomic hydrogen cleaning by thermal cracker
  results in dramatically improved surface
  condition
• no significant polarization loss.
• Typical quantum efficiency 3-6 µA/mW at working
  point of high polarization (P85), Operation at
  accelerator started, now observing long term
  behaviour
• Promising first results from hydrogen cleaned DBR
  super lattice (7-12 µA/mW at max P).

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The End
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Appendix
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Cathode lifetime under different operating
conditions.
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Muster Title

• Based on GaAs strained layer muster text
• Quantum Efficiency
• Wave length
• And go on.

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Story of MAMI

• 1972 - 1975 Project studies about Microtrons
• 1975 Proposal of a Race Track Microtron
• 1979 First stage 14 MeV beam of MAinz MIcrotron
  MAMI
• 1983 Second stage at 183 MeV energy, maximal beam
  current 30µA
• 1990 Third stage 855 MeV beam of MAMI B
• 1991 Beam from distant upstairs polarized
  electron source
• 1992 First acceleration of polarised electrons to
  full energy
• 1999 Approval of the 1.5GeV Harmonic Double Sided
  Microtron (HDSM) as a fourth stage of MAMI
• Dec. 19, 2006, Beam through HDSM ! 1508MeV
  reached !
• Feb. 23, 2007 until Mar. 05., 2007 Start the
  first production beam time with 10µA polarized
  beam polarization 84 at 1.508GeV
• Feb.27, 2007 performed a high current test and
  with reasonable radiation level in the HDSM halls
  50µA beam current (75.4kW beam power)
• Oct. 5, 2007 Inauguration ceremony of MAMI C.

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PKA2
PKAT
PKA1
EKAN
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Vacuum chamber handling

• All vacuum components from 304 stainless steel,
• Vacuum and beam line downstream to differential
  stage bake able to 250 C
• Using continuously bake-out procedure.
• Heating elements taps and special ordered
  jackets
• Heating 200 C during one week.
• One of test source (PKA2) is coved now by NEG,
  under investigations.
• For example of chamber handling in CEBAF
  Stutzmann et al. NIM A 574 (2007) 213-220.

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Preparation.Example Cathode S-45
1. Thermal heating 30 min, P100 W, T550-600C
2. Cooling 45 min
3. Switch on Cs
4. Waiting 10 min., before photocurrent
5. Let in O2 pressure 2x10-9 torr
6. Control maximum rise velocity of current
7. Stop after 45 min
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PKA1 Main Source of MAMI
Chamber
High voltage insulator
NEG Pump
Cathode Position
Load Lock Chamber
Ion Pump
Preparation Chamber
Manipulator
Alpha Magnet
Ion Pump
Spin Rotator
Differential Stage
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MAMI overview.Polarised electron source
2005 6140h operation, 68 with polarised
beam 2006 5950h operation, 65 with polarised
beam 2007 7100h operation, 50 with polarised
beam 2008 yet more then 50 with polarised beam
Polarised beam produced by photo cathodes based
on A3B5 semiconductors by illuminating by
circular polarised laser light. Activation by
CsO Layers.

• Polarised beam means
• High quantum efficiency.
• High degree of the polarisation