Recent Polarized Photocathode R&D at SLAC
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D.-A. Luh, A. Brachmann, J. E. Clendenin, T. Desikan, E. L. Garwin, - PowerPoint PPT Presentation


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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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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


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


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


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


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.


Dopant Loss during Heat-Cleaning Techniques

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


SIMS Analysis Techniques

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


Strain Relaxation in Thick Strained Layers Techniques

  • Strained layers start relaxing beyond critical thickness (~10nm).

  • Strained layers relax partially until reaching practical limit (~100nm).

  • Strain relaxation  Lower polarization


Possible Improvements on Cathode Growth and Preparation Techniques

  • 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


MBE vs. MOCVD Techniques

  • 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.


Atomic-Hydrogen Cleaning Techniques

  • 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.


Preliminary Results from Atomic-Hydrogen Cleaning System Techniques

  • 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.


Superlattice Photocathodes Techniques

  • 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


2 Techniques

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



Multiple Quantum Well Simulation Techniques

widthBarrier = 50nm

  • QE ~ Band Gap

  • Polarization ~ HH-LH Splitting

Effective Band Gap

HH-LH Splitting


Techniques

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


X-Ray Diffraction – Rocking Curves Techniques

  • Test cathode: strained GaAs

  • (004) scan – distance between layers

GaAs Bulk

Graded GaAs1-xPx

GaAs0.64P0.36

Strained GaAs


GaAsP Techniques

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


Strained superlattice GaAsP SVT-3682 and SVT-3984 Techniques

CB1

1.65 eV

HH1

0.86 eV

LH1

GaAsP

GaAs

GaAsP

GaAs

GaAsP

  • Photoluminescence confirms the simulation prediction


Graded GaAs Techniques1-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


Superlattice Cathode Performance Techniques

  • 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.


Conclusion Techniques

  • 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


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