1. 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 Recent Polarized Photocathode R&D at SLAC

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

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

4. 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 NH 4 OH Transferred into loadlock immediately. Heat-cleaned at 600°C for one hour Activated by Cs/NF 3 co-deposition Heat-cleaned and activated twice High-Gradient-Doped Strained GaAsP GaAs(100) Substrate GaAs Buffer Layer Graded GaAs 1-x P x x = 0  0.34 GaAs 0.66 P 0.34 Active Layer GaAs 0.95 P 0.05 GaAs Surface Layer 5  10 18 5  10 17 5  10 19 Dopant Concentration (cm -3 ) 0.25  m 2.5  m 90nm 10nm

5. 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. Weakness of Current Cathode Growth and Preparation Techniques

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

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

8. Strained layers start relaxing beyond critical thickness (~10nm). Strained layers relax partially until reaching practical limit (~100nm). Strain relaxation  Lower polarization Strain Relaxation in Thick Strained Layers Active Layer Thickness (nm) Polarization (%) MO5-586890~82 MO5-6007170~70

9. 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 Possible Improvements on Cathode Growth and Preparation

10. 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. MBE vs. MOCVD

11. 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. Atomic-Hydrogen Cleaning

12. GaAs Reference Cathode: stripped its surface oxide by NH 4 OH, heat-cleaned, and activated GaAs Test Cathode: No NH 4 OH stripping. Cleaning procedures are indicated in the figure. Atomic-hydrogen cleaning shows promising results. Cleaning condition needs to be optimized. Preliminary Results from Atomic-Hydrogen Cleaning System

13. 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 Superlattice Photocathodes

14. Superlattice Band Structure Calculations 1 234N+1N+2 kp transfer matrix method (S. L. Chuang, Phys. Rev. B 43 9649 (1991)) D m : transmission and reflection at interfaces, P m : propagation and decay in layers Set A N+2 = 1, B N+2 = 0 ; Change incident electron energy, and look at 1/A 1 for transmittivity. Transmittivity maximum  Resonant tunneling  Energy level

15. Multiple Quantum Well Simulation

16. QE ~ Band Gap Polarization ~ HH-LH Splitting Effective Band Gap HH-LH Splitting widthBarrier = 50nm

17. 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 X-Ray Diffraction -- Theory  d

18. GaAs Bulk X-Ray Diffraction – Rocking Curves GaAs 0.64 P 0.36 Graded GaAs 1-x P x Strained GaAs Test cathode: strained GaAs (004) scan – distance between layers

19. Strained Superlattice GaAsP SVT-3682 and SVT-3984 Strained GaAs GaAsP Strained GaAs GaAsP Strained GaAs GaAsP 30 Å GaAs Substrate GaAs (1-x) P x Graded Layer GaAs 0.64 P 0.36 Buffer Active Region 25  m 1000 Å T. Nishitani et al, SPIN2000 Proceedings p.1021

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

21. Rocking Curve (004) scan from SVT-3682 GaAs Bulk GaAs 0.64 P 0.36 Graded GaAs 1-x P x Additional peaks from superlattice structure 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. 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. Superlattice Cathode Performance

23. 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 Conclusion To do 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.