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M. Baylac, JLab baylac@jlab

Effect of atomic hydrogen exposure on electron beam polarization from strained GaAs photocathodes. M. Baylac, JLab baylac@jlab.org. P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J. Hansknecht, M. Poelker, M. Stutzman. Plan. Motivations Experimental setup Test QE

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M. Baylac, JLab baylac@jlab

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  1. Effect of atomic hydrogen exposure on electron beam polarization from strained GaAs photocathodes M. Baylac, JLab baylac@jlab.org P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J. Hansknecht, M. Poelker, M. Stutzman

  2. Plan • Motivations • Experimental setup • Test • QE • Polarization measurements (2002 & 2003) • Analyzing power (2003) • Profilometry • Results, Conclusions

  3. 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

  4.  split degeneracy of P 0.1 μm x=0.29 3/2 250 μm 250 μm p-type GaAs substrate GaAs GaAs P P x x 1-x 1-x 600 μm Strained layer GaAs photocathode Bandwidth Semiconductor (formerly SPIRE) Strained GaAs • MOCVD-grown epitaxial spin-polarizer wafer • Lattice mismatch 0<x<0.29

  5. Photocathode preparation • 3” wafer cleaved (15.5 mm) • No wet chemical treatment, ie: no acid or base etching, no degreasing, no anodization • Sample on stalk w/ Indium Ta cup lucky

  6. 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

  7. ~ G Hydrogen source wafer ~300C • H2 dissociation via RF inductive discharge • Parameters adjusted then, wafer exposed • Dose measured with an “ion counter” at chamber bottom • Conditions kept identical for all cleanings 15 cm 100 MHz 20 W Mc.Alpine & Schildknecht, Proceeding of IRE, 1959 (2099) H2, or D2

  8. Hydrogen cleanerhttp://www.jlab.org/accel/inj_group/h2/portable_H2.html

  9. Test gun • 100 keV DC beam • Wavelength tunable Ti:Sapp • (750-850 nm) • 10 Hz helicity reversal • Mott polarimeter : P • Beam dump : QE lenses e correctors viewers

  10. Test gun (cont’d)

  11. 840 nm 2 3 y 5 4 1 x QE % ~ 13 mm Initial benchmark, untreated surface Syst. ~ 10% Stat. only M. Baylac et al, SPIN 2002, 15th International Spin Physics Symposium proceedings, 1073 (2003).

  12. 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

  13. 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

  14. 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

  15. Polarization with usual exposure P(H)-P(0)~-10% at bandgap 80 min dose

  16. Polarization with ion-reduced exposure P(H)-P(0)~-10% at bandgap 80 min dose

  17. Polarization with ion-enhanced exposure P(H)-P(0)~-20% at bandgap 80 min dose

  18. Vacuum chamber GaAs x x x Cs NF3 x x i Insertable dissociator Insertable powermeter l/2 Analyzing power • Measure A.P. across strained-layer GaAs • 7 consecutive H exposures up to 4 hours 860 nm diode laser

  19. Analyzing power (cont.) Analyzing power at 5 locations: ~ 5% and uniform

  20. Analyzing power • No variation with H exposure, or location => Effect unrelated to strain

  21. 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

  22. Profilometry after H exposure Bare surface • RMS ~ 155 A H cleaned • RMS ~ 8500 A

  23. 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)

  24. Polarization comparison

  25. Laser power systematic check

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