1. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility M. Baylac, JLab baylac@jlab.org P. Adderley, J. Brittian, J. Clark, A. Day, J. Grames, J. Hansknecht, M. Poelker, M. Stutzman Effect of atomic hydrogen exposure on electron beam polarization from strained GaAs photocathodes

2. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Plan Motivations Experimental setup Test QE Polarization measurements (2002 & 2003) Analyzing power (2003) Profilometry Results, Conclusions

3. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Strained layer GaAs photocathode Strained GaAs Bandwidth Semiconductor (formerly SPIRE)  split degeneracy of P 3/2 GaAs 1-x P x GaAs 1-x P x x=0.29 p-type GaAs substrate MOCVD-grown epitaxial spin-polarizer wafer Lattice mismatch 0<x<0.29 600 μm 250 μm 0.1 μm

5. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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/ lucky Indium Ta cup

6. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Hydrogen source 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 Mc.Alpine & Schildknecht, Proceeding of IRE, 1959 (2099) ~ G 100 MHz 20 W H2, or D2 ~300C 15 cm wafer

8. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Hydrogen cleaner http://www.jlab.org/accel/inj_group/h2/portable_H2.html

9. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Test gun 100 keV DC beam Wavelength tunable Ti:Sapp (750-850 nm) 10 Hz helicity reversal Mott polarimeter : P Beam dump : QE viewers lenses correctors e

10. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Test gun (cont’d)

11. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Initial benchmark, untreated surface ~ 13 mm 840 nm 1 2 3 5 4 QE % x y Stat. only Syst. ~ 10% M. Baylac et al, SPIN 2002, 15 th International Spin Physics Symposium proceedings, 1073 (2003).

12. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility QE at band-gap vs. Hydrogen exposure Significant drop in QE vs. H cleaning Confirmed by 2 similar tests in different chamber w/o breaking vacuum At 840 nm

13. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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  Reject 60+ min. data Depolarization due to low, high QE light

14. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Polarization with usual exposure P(H)-P(0)~-10% at bandgap 80 min dose

16. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Polarization with ion-reduced exposure P(H)-P(0)~-10% at bandgap 80 min dose

17. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Polarization with ion-enhanced exposure P(H)-P(0)~-20% at bandgap 80 min dose

18. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Analyzing power Measure A.P. across strained-layer GaAs 7 consecutive H exposures up to 4 hours 860 nm diode laser Vacuum chamber l /2 Insertable powermeter i GaAs Insertable dissociator CsCs NF3 x x x x x

19. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Analyzing power (cont.) Analyzing power at 5 locations: ~ 5% and uniform

20. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Analyzing power No variation with H exposure, or location => Effect unrelated to strain

21. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Profilometry after H exposure H cleaned Bare surface RMS ~ 155 A RMS ~ 8500 A

23. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility 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. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Polarization comparison

25. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Laser power systematic check