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Polarized Photocathode R&D Update

Polarized Photocathode R&D Update. Victoria ALCW July 28-31 2004. PPRC Collaboration: A. Brachmann J. Clendenin E. Garwin T. Maruyama D. Luh R. Kirby C. Prescott R. Prepost (Wisconsin) – LC Accelerator R&D at Universities. Takashi Maruyama SLAC. OUTLINE.

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Polarized Photocathode R&D Update

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  1. Polarized Photocathode R&D Update Victoria ALCW July 28-31 2004 PPRC Collaboration: A. Brachmann J. Clendenin E. Garwin T. Maruyama D. Luh R. Kirby C. Prescott R. Prepost (Wisconsin) – LC Accelerator R&D at Universities Takashi Maruyama SLAC

  2. OUTLINE • Polarized photoemission • Strained superlattice R&D • E158 RUN III • Atomic hydrogen cleaning • Multi-bunch laser development • Summary

  3. Polarized photoemission • Circularly polarized light excites electron from valence band to conduction band • Electrons drift to surface L < 100 nm to avoid depolarization • Electron emission to vacuum from Negative-Electron-Affinity (NEA) surface NEA Surface – Cathode “Activation” • Ultra-High-Vacuum < 10-11 Torr • Heat treatment at 600° C • Application of Cesium and NF3

  4. GaAsP 3nm Strained GaAs 4nm Active Region 100nm GaAs0.64P0.36 Buffer GaAsP 2.5mm Strained GaAs GaAs(1-x)Px Graded Layer GaAsP 2.5mm Strained GaAs GaAs Substrate Strained-superlattice Single strained GaAs (SLC) Strained GaAs •  High gradient doping: 51019/cm3 in the 5 nm surface layer and 51017/cm3 • in the rest → No surface charge limit • Each superlattice layer is thinner than the critical thickness → GaAs layers are highly strained

  5. SBIR with SVT Associates “Advanced Strained-Superlattice Photocathodes for Polarized Electron Sources” • July 2001 SBIR Phase I awarded Very first sample produced 85% polarization • Sep. 2002 SBIR Phase II awarded (2 year project) • Polarized photocathodes are commercially available (SLAC Spin Polarizer Wafers). • Phosphorus containing wafers are usually grown by MOCVD. • Gas-source MBE is used at SVT Associates.

  6. MBE- In Situ Growth Rate Feedback Monitoring RHEED image intensity versus time provides layer-by-layer growth rate feedback Growth at monolayer precision – not possible with MOCVD

  7. Superlattice parameters b w Parameters: Barrier thickness : 3 nm < b < 7 nm Well thickness : 3 nm < w < 7 nm Phosphorus x : 0.3 < x < 0.4 No. of periods : l ~ 70 – 200 nm First systematic study of polarized photoemission from strained-superlattice - to be published in Applied Physics Letters

  8. Graded GaAs1-xPx GaAs Bulk Additional peaks from superlattice structure GaAs0.64P0.36 Structural Analysis Using X-ray Diffraction • Measure superlattice period. • Measure well and barrier widths. • Measure phosphorus fraction in GaAs1-xPx layers. • Measure strain in GaAs layers. Data Simulation

  9. Structural Analysis using SIMS(Secondary Ion Mass Spectroscopy) Phosphorus concentration vs. depth • Bombard with Cs ions and • detect As, P, and Be. • Measure superlattice period • Measure Be doping profile Superlattice structure does not degrade after 600 C heat-treatment.

  10. Strain effect: Vary x in GaAs1-xPx • Max. polarization ~86% • QE ~1% (x5 SLC Cathode QE) • HH and LH transitions observed • HH-LH splitting increases with x. LH HH

  11. Well Dependence • Max. polarization ~86% • QE ~ 1% • Second peak is sensitive to HH2 → opportunity to test superlattice model. X = 0.35

  12. Period dependence •  Strain relaxes steadily. • Peak polarization is constant < 15 periods, • but decreases rapidly > 20 periods.

  13. No Charge Limit 11012 e- in 60 ns → 4.51012 e- in 270 ns (x3 NLC train charge)

  14. E158 RUN III • Cathode installed in May 03. • E158 ran successfully. • ESA Moller measured 90%. • But it showed a charge limit ~71011 e-/300 ns • Could not make NLC train charge but OK for E158. • What happened? The 600° C heat-cleaning is destroying the high gradient doping profile.

  15. Dopant diffusion Be concentration vs. depth • Be dopants diffuse out of the surface during 600 C heat-cleaning. Need to lower the heat-cleaning temperature to < 450 C without lowering QE.

  16. Atomic-Hydrogen CleaningAppl. Phys. Lett. 82, 4184 (2003) Bulk GaAs Ga2O3 comes off at ~600° C. Ga2O comes off at ~450° C. Ga2O3 + 4H Ga2O + 2H2O 600° C heat-cleaning: QE ~ 11% AHC + 450 ° C heat-cleaning: QE ~15%

  17. Multi-bunch laser development Laser modulator RF Amp. RF Gen. Currently RF amp. is limited to 200 MHz. Need 714 MHz for 1.4 ns. 119 MHz (8.4 ns)

  18. Summary • High gradient doped strained-superlattice GaAs-GaAsP has been developed under SBIR and is commercially available. • Peak polarization of 86% with ~1% QE and no charge saturation up to 11012 e- in 60 ns. • Used successfully in E158 Run 3, yielding 90% polarization at ESA. • High gradient doping is vulnerable to high temperature heat cleaning. • To lower the heat-cleaning temperature, the atomic hydrogen cleaning technique has been developed.

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