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

High-Gradient-Doped Strained GaAsP 25 ) 20 15 10 5 0 0 50 Laser pulse length : 100 ns Laser wavelength : 805 nm 100 150 Laser Intensity (  J) 200 250 • • • • 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 • • • Cathode Growth Grown by Bandwidth Semiconductor Metal-Organic-Chemical-Vapor Deposition (MOCVD) Zn-doping 10nm 90nm • • • • • • • • 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 Transferred into loadlock immediately.

Heat-cleaned at 600 Activated by Cs/NF 3 ° C for one hour co-deposition Heat-cleaned and activated twice 4 OH 2.5

 m 2.5

 m 0.25

 m GaAs Surface Layer Active Layer GaAs 0.95

P 0.05

Dopant Concentration (cm -3 ) 5  10 19 5  10 17 GaAs 0.66

P 0.34

5  10 18 Graded GaAs 1-x P x x = 0  0.34

GaAs Buffer Layer GaAs(100) Substrate 5 5   10 10 18 18

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

100 Time (ns) 200 300 2.5x10

12 2.0

1.5

1.0

0.5

0.0

0 2 activations 3 activations 4 activations 5 activations 100 200 300 Laser Intensity (uJ) 400 500

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

10 20 6 5 4 3 2 10 19 6 5 4 3 2 10 18 6 5 4 3 2 0 20 unused heat-cleaned for 5 hours at 600°C 40 Depth (nm) 60 80

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 MO5-5868 MO5-6007 Active Layer Thickness (nm) 90 170 Polarization (%) ~82 ~70

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  polarization higher

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.

100 80 60 SVT-3982 M05-5868 40 20 0 650 700 750 800 Wavelength (nm) 850 10 1 0.1

0.01

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.

Preliminary Results from Atomic-Hydrogen Cleaning System • 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.

14 12 10 8 6 4 2 0 Atomic-hydrogen cleaned @ ~350°C, then heat-cleaned @450°C GaAs Reference Cathode GaAs Test Cathode Heat-cleaned @ 450°C 450 500 550 Heat-Cleaning Temperature (°C) 600

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

Superlattice Band Structure Calculations • • •

k •p

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 1 2 3 4 N+1 N+2  

A

1

B

1  

T

 

T

 

A N B N

 2  2  

D

1 .

P

2

D

2 .

P

3

D

3 ...

P N

 1

D N

 1 • • Set

A N+2 = 1

,

B N+2 = 0

; Change incident electron energy, and look at

1/A 1

transmittivity.

for Transmittivity maximum  Resonant tunneling  Energy level

Multiple Quantum Well Simulation

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

Effective Band Gap HH-LH Splitting

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   

d

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

P 0.36

Strained GaAs

Strained Superlattice GaAsP SVT-3682 and SVT-3984 GaAsP Strained GaAs 1000 Å 25  m 25  m Active Region GaAs 0.64

P 0.36

Buffer GaAs (1-x) P x Layer Graded GaAsP Strained GaAs GaAsP Strained GaAs GaAs Substrate 30 Å 30 Å T. Nishitani et al, SPIN2000 Proceedings p.1021

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

Rocking Curve (004) scan from SVT-3682 Graded GaAs 1-x P x GaAs Bulk • 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: 50 Å Additional peaks from superlattice structure GaAs 0.64

P 0.36

• XRD analysis on SVT-3682 – Well Width = Barrier Width = 32Å – Phosphorus fraction in GaAsP: 0.36

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.

100 80 60 40 SVT-3984 SVT-3682 20 0 640 660 680 700 720 Wavelength (nm) 740 760 780 1 0.1

0.01

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