D.-A. Luh, A. Brachmann, J. E. Clendenin, T. Desikan, E. L. Garwin,

1. Recent Polarized Photocathode R&D at SLAC D.-A. Luh, A. Brachmann, J. E. Clendenin, T. Desikan, E. L. Garwin, S. Harvey, R. E. Kirby, T. Maruyama, and C. Y. Prescott Stanford Linear Accelerator Center, Stanford, CA 94025 R. Prepost Department of Physics, University of Wisconsin, Madison, WI 53706

2. Highlights • Current cathode in use (high-gradient-doped strained GaAsP) • Growth and preparation techniques for photocathodes and their weakness • Possible solutions/improvements and current progress

3. Laser pulse length : 100 ns Laser wavelength : 805 nm High-Gradient-Doped Strained GaAsP • Currently used in the accelerator • Peak polarization ~82% @805nm • QE ~0.4% @ 805nm • No charge limit effect with available laser energy

4. Dopant Concentration (cm-3) 10nm GaAs Surface Layer 51019 Active Layer GaAs0.95P0.05 90nm 51017 2.5m GaAs0.66P0.34 51018 Graded GaAs1-xPx x = 0 0.34 2.5m 51018 0.25m GaAs Buffer Layer 51018 GaAs(100) Substrate High-Gradient-Doped Strained GaAsP Cathode Growth • Grown by Bandwidth Semiconductor • Metal-Organic-Chemical-Vapor-Deposition (MOCVD) • Zn-doping Cathode preparation • Anodized at 2.5V to form a ~3 nm oxide layer • Waxed to a glass for cutting • Degreased in boiling Trichloroethane. • Stripped surface oxide layer by NH4OH • Transferred into loadlock immediately. • Heat-cleaned at 600°C for one hour • Activated by Cs/NF3 co-deposition • Heat-cleaned and activated twice

5. Weakness of Current Cathode Growth and Preparation Techniques • MOCVD • The base pressure of MOCVD growth chamber is in high-vacuum range, compared with ultra high-vacuum in other techniques. • MOCVD requires higher growth temperature. • MOCVD growth mechanism is complicated. • Zn-doping • The diffusion coefficient of Zn in GaAs is high at the heat-cleaning temperature we use. • The heat-cleaning capability of Zn-doped cathodes is limited. • Single strained layer • Strain relaxation in thick strained layers causes lower polarization.

6. Dopant Loss during Heat-Cleaning • High-gradient-doped cathode shows charge limit effect after three activations at 600C.

7. SIMS Analysis • SIMS (Secondary Ion Mass Spectroscopy) analysis confirms Zn dopant loss after repeated heat-cleaning at 600°C.

8. Strain Relaxation in Thick Strained Layers • Strained layers start relaxing beyond critical thickness (~10nm). • Strained layers relax partially until reaching practical limit (~100nm). • Strain relaxation  Lower polarization

9. Possible Improvements on Cathode Growth and Preparation • MBE (Molecular Beam Epitaxy) growth – High quality films • Ultra-high-vacuum environment • Lower growth temperature and simpler growth mechanism • More choices on dopants • Be/C doping – better heat-cleaning capability • Lower impurity diffusion coefficients in GaAs at high temperature • As-capped cathodes -- Lower heat-cleaning temperature • Atomic-hydrogen cleaning – Lower heat-cleaning temperature • Superlattice structure – Preserve strain in active layers  higher polarization

10. MBE vs. MOCVD • Both SVT-3982 and MO5-5868 are high-gradient-doped strained GaAsP. • SVT-3982 is MBE-grown Be-doped (SVT Associates). • MO5-5868 is MOCVD-grown Zn-doped (Bandwidth Semiconductor). • Preliminary result shows that MBE-grown cathode has better performance. • Heat-cleaning capability of Be-doped cathodes need to be determined.

11. Atomic-Hydrogen Cleaning • The goal: to achieve good QE with lower heat-cleaning temperature • Thanks to Matt Poelker of Jefferson Lab for many discussions and helps. • Cathodes are atomic-hydrogen cleaned, and then transferred into activation chamber through loadlock.

12. Preliminary Results from Atomic-Hydrogen Cleaning System • GaAs Reference Cathode: stripped its surface oxide by NH4OH, heat-cleaned, and activated • GaAs Test Cathode: No NH4OH stripping. Cleaning procedures are indicated in the figure. • Atomic-hydrogen cleaning shows promising results. Cleaning condition needs to be optimized.

13. Superlattice Photocathodes • Critical thickness (~10nm) limits the size of strained active region. • Multiple quantum wells to preserve strain • Strained layers sandwiched between unstrained layers • The thickness of single strained layer is less than critical thickness. • Band structure calculation to determine cathode structure parameters (well width, barrier width, and phosphorus fraction, etc.) • X-ray diffraction to characterize cathode structure (layer thickness, composition, and strain, etc.) • Photoluminescence to check cathode band structure

14. 2 3 4 N+1 N+2 1 Superlattice Band Structure Calculations • k•p transfer matrix method (S. L. Chuang, Phys. Rev. B 43 9649 (1991)) • Dm: transmission and reflection at interfaces, • Pm: propagation and decay in layers • Set AN+2 = 1, BN+2 = 0; Change incident electron energy, and look at 1/A1 for transmittivity. • Transmittivity maximum  Resonant tunneling  Energy level

15. Multiple Quantum Well Simulation

16. Multiple Quantum Well Simulation widthBarrier = 50nm • QE ~ Band Gap • Polarization ~ HH-LH Splitting Effective Band Gap HH-LH Splitting

17.  d   X-Ray Diffraction -- Theory • Bragg’s Law: n  = 2 d sin • All lattice planes contribute to Bragg diffraction • Every layer contributes a Bragg peak • Repeating series of thin layers causes additional peaks

18. X-Ray Diffraction – Rocking Curves • Test cathode: strained GaAs • (004) scan – distance between layers GaAs Bulk Graded GaAs1-xPx GaAs0.64P0.36 Strained GaAs

19. GaAsP 30 Å Strained GaAs 30 Å Active Region 1000 Å GaAs0.64P0.36 Buffer GaAsP 25mm Strained GaAs GaAs(1-x)Px Graded Layer 25mm GaAsP Strained GaAs GaAs Substrate Strained Superlattice GaAsP SVT-3682 and SVT-3984 T. Nishitani et al, SPIN2000 Proceedings p.1021

20. Strained superlattice GaAsP SVT-3682 and SVT-3984 CB1 1.65 eV HH1 0.86 eV LH1 GaAsP GaAs GaAsP GaAs GaAsP • Photoluminescence confirms the simulation prediction

21. Graded GaAs1-xPx GaAs Bulk Additional peaks from superlattice structure GaAs0.64P0.36 Rocking Curve (004) scan from SVT-3682 • Both SVT-3682 and SVT-3984 are superlattice cathodes: • MBE grown Be-doped (SVT Associates). • Barrier width: 30Å • Well width: 30Å • Phosphorus fraction in GaAsP: 0.36 • Layer number: 16 • Highly-doped surface layer thickness: 50Å • XRD analysis on SVT-3682 • Well Width = Barrier Width = 32Å • Phosphorus fraction in GaAsP: 0.36

22. Superlattice Cathode Performance • Peak polarization > 85% • Good QE • SVT-3984 was tested in Gun Test Lab at SLAC, and there was no charge limit effect with available laser energy.

23. Conclusion • MBE-grown Be-doped cathodes show equal or better performance than MOCVD-grown Zn-doped cathodes. • Preliminary test of atomic-hydrogen cleaning shows promising result. • First strained superlattice cathodes show very good performance. To do • Study the heat-cleaning capability of Be-doped and C-doped cathodes. • Optimize the process of atomic-hydrogen cleaning. • Study As-capped cathodes. • Test superlattice cathodes with different structure parameters