Strained Superlattice GaAs photocathodes at JLab M. Baylac

1. Strained Superlattice GaAs photocathodes at JLabM. Baylac Qweak collaboration meeting August 17, 2004

2. photocathode anode Laser -100 kV - e Cs NF 3 Polarized Electron Guns at JLab HV insulator Photoemission from GaAs semiconductor NEG pumps NEG-coated Beamline Strained GaAs in Gun2 (“old” material) Strained-superlattice GaAs in Gun3 (“new” material)

3. Strained layer GaAs photocathodes • From 1998 through 2003, we have used strained layer GaAs photocathodes at JLab (Bandwidth Semiconductor, Inc.). • Reliable, well understood material. • Stained-layer GaAs provides; • Good polarization: P ~ 75% at 840 nm • Moderate quantum efficiency: QE ~ 0.2% at 840 nm • Limitations that keep polarization < 80%: • limited band splitting • relaxation of the strain for thickness > critical thickness (~10 nm) e

4. Strained GaAs/GaAsP superlattice • Very thin quantum welllayers alternating with lattice-mismatched barrier layers • Each superlattice layer is < critical thickness • Natural splitting of valence band adds to the strain-splitting • Developed by SLAC with SVT Associates, Inc. SLAC-PUB-10331 (2004), submitted to Appl.Phys.Lett • First samples received at JLab October 2003, characterized at the injector test cave

5. -3 Be doping (cm ) 19 18 17 5.10 5.10 5.10 GaAs (5 nm) GaAsP (3 nm) 14 pairs GaAs (4 nm) (2.5 μm) GaAs P 0.36 0.64 , 0<x<0.36 (2.5 μm) GaAs P 1-x x p-type GaAs substrate Superlattice structure SVT associates, per SLAC specs.

6. Quantum Efficiency QE (%) QE ~ 1% versus 0.2% from strained layer material we operate here Wavelength (nm)

7. Beam polarization Polarization (%) Highest polarization ever measured at the Test Cave Wavelength for Good QE and Polarization Wavelength (nm)

8. Analyzing power (aka QE anisotropy) Analyzing power smaller by factor of 3 compared with strained-layer GaAs: 4% versus 12% This means smaller inherent intensity & position asymmetries on beam. Analyzing power (%) Wavelength for good QE and polarization Wavelength (nm)

9. QE vs hydrogen cleaning Typical H-dose to clean anodized samples Drawback: Delicate material Can’t clean with atomic hydrogen Makes it tough to anodize edge of cathode QE (%) Hydrogen exposure time (min)

10. Superlattice Photocathodes at CEBAF • Several failed attempts to load superlattice photocathodes inside tunnel guns • Successful installation of un-anodized superlattice photocathode in Gun 3 (March, 2004) • Activation gave QE ~ 0.4% at 780 nm (vs 1% in test cave) • Used during HAPPEx-He and portion of HAPPEx-H (June, 2004)

11. 14 mm Poor lifetime • Frequent spot moves were required to maintain 40 A beam current at Hall A • every week at start of run, every day as we approached July 4 shutdown! • HAPPEx-He OK. HAPPEx-H not so good. Injector conditions changing too often. HC asymmetries were not stable. • Poor gun lifetime atypical of CEBAF photoinjector. QE profile after 3 weeks of running

12. Preliminary P ~ 85.2  3.2 % e photon electron Polarimetry in hall A • Compton (D. Lhuillier) • 5 MeV Mott (J. Grames) P ~ 86  3 % e

13. From HAPPEx-H Gun3 superlattice GaAs Gun2 strained layer GaAs Parity quality beam? • Short run + numerous spot moves => Jury is still out. Poor gun lifetime made it difficult to assess performance of superlattice photocathode from a parity violation experiment perspective. • HAPPEX reports; • Charge asymmetry OK for both photocathodes • Position asymmetries were smaller using gun2 strained layer photocathode (no active position feedback)

14. Surface Charge Limit • QE drops as laser power increases: photoelectrons build up in band bending region create opposing E field that reduces NEA G.A. Mulhollan et al, Phys. Lett. A 282, 309 (2001) • Reduces maximum available beam current. Lose laser headroom. Makes for shorter operating lifetime of gun. QE is not constant

15. Lasers • Our new commercial Ti-Sapphire lasers provide more laser power (~ 300 mW) compared to our “old” diode lasers (~ 50 mW). • They are wavelength tunable. Now we can tune to peak polarization. • Successful and reliable running since G0. • Ti-Sapp laser + superlattice photocathode a good match for high current Qweak experiment. 300 mW laser power + QE of 1% can provide 1800 uA beam current. • Max current only 360 uA with strained layer cathode. Not as much headroom. http://www.tbwp.com

16. Summary e • Highest polarization ever measured at JLab: P = 86% • Measurements of many samples at test stand indicates this is no fluke. • 5 times higher QE than strained layer material. • Smaller analyzing power should provide smaller inherent charge and position asymmetry. (Recent HAPPEx results do not support this claim.) • Delicate material, more difficult to handle. Cannot be H-cleaned. Can’t recover QE from a dirty superlattice, unlike strained layer • We suffered surface charge limit. QE drops with increasing laser power. A concern for high current experiments like Qweak.

17. Outlook • Poor lifetime due to supperlattice? Doubt it: • Gun 3 has a bad lifetime in 2003 using strained layer • Un-anodized wafer increases damage on the wafer Reworked Gun 3 over the shutdown, hoping to boost lifetime • QE lower in the tunnel than in test cave: • Hopefully due to the gun itself, not the wafer • Received arsenic capped samples: easier to handle and anodize (to be tested in lab) • Smaller inherent HC asymmetries? Surface charge limit? Need more operating experience.