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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

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slide1

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

slide2

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
slide3

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
slide4

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
slide5

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.
slide6

Dopant Loss during Heat-Cleaning

  • High-gradient-doped cathode shows charge limit effect after three activations at 600C.
slide7

SIMS Analysis

  • SIMS (Secondary Ion Mass Spectroscopy) analysis confirms Zn dopant loss after repeated heat-cleaning at 600°C.
slide8

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
slide9

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
slide10

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.
slide11

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.
slide12

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.
slide13

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
slide14

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
slide16

Multiple Quantum Well Simulation

widthBarrier = 50nm

  • QE ~ Band Gap
  • Polarization ~ HH-LH Splitting

Effective Band Gap

HH-LH Splitting

slide17

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
slide18

X-Ray Diffraction – Rocking Curves

  • Test cathode: strained GaAs
  • (004) scan – distance between layers

GaAs Bulk

Graded GaAs1-xPx

GaAs0.64P0.36

Strained GaAs

slide19

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

slide20

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
slide21

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
slide22

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.
slide23

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