# Presentation: 546456

## Polarized Photocathode ResearchCollaboration  PPRCR. Prepost – University of WisconsinCornell ALCW  July 13-16  2003

- A. Brachmann
- J. Clendenin
- E. Garwin
- T. Maruyama
- D. Luh
- S. Harvey
- R. Kirby
- C. Prescott
- R. Prepost

## Some Considerations

- Technique is Bandgap Engineering of Strained GaAs.
- Polarization will be < 100% - But 90% possible.
- Active layer must be < 10% of photon absorption length to preserve strain and polarization.
- Uniform Strain over larger thickness in principle possible with Superlattice structures
- Strained GaAs used at SLAC since 1986 with ~85% Polarization and ~.2% QE.
- R & D has been continuous since 1985.

## Outline

- Polarized photoemission
- Standard SLC photocathode
- Surface charge limit
- Charge limit vs. doping
- Polarization vs. doping
- High gradient doped strained GaAsP
- High gradient doped strained superlattice
- Atomic-hydrogen cleaning
- Summary

## Polarized photoemission

- •  Circularly polarized light excites electron
-     from valence band to conduction band
- •  Electrons drift to surface
-         L < 100 nm to avoid depolarization
- •  Electron emission to vacuum from
-     Negative-Electron-Affinity (NEA) surface

- NEA Surface –
-      Cathode “Activation”
- • Ultra-High-Vacuum < 10-11 Torr
- • Heat treatment at 600° C
- • Application of Cesium and NF3

## Facilities

- • QE and Polarization at 20 kV

- • QE and Polarization at 120 kV
-    under accelerator condition

## Standard SLC Strained GaAs

- 100 nm GaAs grown on GaAsP
- Uniformly doped at 51018 cm-3
- Peak polarization ~80%
- QE ~0.2 – 0.3%
- Max. charge ~7 1011 e-/270ns

- GaAs substrate

- GaAs1-xPx x=0.3

- GaAs1-xPx x=0 0.3

- 100 nm GaAs

- Charge saturation

## Beam structure

## Surface Charge Limit

- Photon absorption excites electrons to conduction band
- Electrons can be trapped near the surface; electron escape prob.  20%
- Electrostatic potential from trapped electrons raises affinity
- Affinity recovers after electron recombination
- Increasing photon flux counterproductive at extremes

- TESLA does not have a charge limit problem.

## Charge limit (cont.)

- pump

- probe

- Two short pulses

- Probe signal

- Long pulse

## Higher doping solves charge limit problem.

- Four samples with
- different doping level:
-      51018 cm-3
-      11019 cm-3
-      21019 cm-3
-      51019 cm-3

- Phys. Lett. A282, 309 (2001)

## But higher doping depolarizes spin.

## High-gradient doped strained GaAsP

- NIM A492, 199 (2002)

## 80% Polarization and No charge limit

- E158 cathode

## But polarization is still 80%.

- Actual strain is 80% of design

- Strain relaxation

## Strained-superlattice

**Notes:**
Nominal Structure

## SBIR with SVT Associates

- July 2001 SBIR Phase I awarded
-     Very first sample produced 85% polarization
- Sep. 2002 SBIR Phase II awarded
- MBE growth		MOCVD growth
- Be doped			Zn doped

- “Advanced Strained-Superlattice Photocathodes for
- Polarized Electron Souces”

- SLC photocathode

## MBE- In Situ Growth Rate Feedback 

**MBE- In Situ Growth Rate Feedback **

- Monitoring RHEED image intensity versus time
- provides layer-by-layer growth rate feedback

- Growth at monolayer precision

## b

- GaAs1-xPx

- GaAs

- GaAs1-xPx

- GaAs

- GaAs1-xPx

- CB1

- HH1

- LH1

- 1.65 eV

- 86 meV

- 1.42 eV

- w

- 	Parameters:
- 	barrier layer thickness, 30 Å < b < 100 Å
- 	well layer thickness    , 30 Å < w < 100 Å
- 	phosphorus fraction   ,  0.3 < x < 0.4
- 	No. of periods           , active layer ~ 1000 Å

## Multiple Quantum Well Simulation

## Multiple Quantum Well Simulation

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

**Effective Band Gap**

**HH-LH Splitting**

- Barrier = 5 nm

- x

## Polarization and QE

- • Peak polarization 85%
- • QE ~ 0.8 – 1%
- • Wavelength dependence is consistent with the simulation.

## 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

## No Charge Limit

- 11012 e- in 60 ns

## QE Anisotropy

- E

- Strain relaxation

- Strained GaAs

- Strained superlattice

- 10%

- 1.5%

## E158 again

- Cathode installed in May.
- But it shows a charge limit ~71011 e-/300 ns
- Cannot make NLC train charge but OK for E158.
- What happened?
- The 600° C heat-cleaning is destroying the high gradient doping profile.

## Atomic-Hydrogen Cleaning        

- Ga2O3 comes off at ~600° C.
- Ga2O  comes off at ~450° C.
- Ga2O3 + 4H Ga2O + 2H2O

- 600° C heat-cleaning:	     QE ~ 11%
- AHC + 450 ° C heat-cleaning: QE ~15%

- Bulk GaAs