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Polarized Photocathode R&D Update. Victoria ALCW July 28-31 2004. PPRC Collaboration: A. Brachmann J. Clendenin E. Garwin T. Maruyama D. Luh R. Kirby C. Prescott R. Prepost (Wisconsin) – LC Accelerator R&D at Universities. Takashi Maruyama SLAC. OUTLINE.

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Polarized photocathode r d update
Polarized Photocathode R&D Update

Victoria ALCW July 28-31 2004

PPRC Collaboration:

A. Brachmann

J. Clendenin

E. Garwin

T. Maruyama

D. Luh

R. Kirby

C. Prescott

R. Prepost (Wisconsin) – LC Accelerator R&D at Universities

Takashi Maruyama

SLAC


Outline
OUTLINE

  • Polarized photoemission

  • Strained superlattice R&D

  • E158 RUN III

  • Atomic hydrogen cleaning

  • Multi-bunch laser development

  • Summary


Polarized photoemission
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


Strained superlattice

GaAsP

3nm

Strained GaAs

4nm

Active Region

100nm

GaAs0.64P0.36

Buffer

GaAsP

2.5mm

Strained GaAs

GaAs(1-x)Px Graded Layer

GaAsP

2.5mm

Strained GaAs

GaAs Substrate

Strained-superlattice

Single strained GaAs (SLC)

Strained GaAs

  •  High gradient doping: 51019/cm3 in the 5 nm surface layer and 51017/cm3

  • in the rest → No surface charge limit

  • Each superlattice layer is thinner than the critical thickness

    → GaAs layers are highly strained


Sbir with svt associates
SBIR with SVT Associates

“Advanced Strained-Superlattice Photocathodes for

Polarized Electron Sources”

  • July 2001 SBIR Phase I awarded

    Very first sample produced 85% polarization

  • Sep. 2002 SBIR Phase II awarded (2 year project)

  • Polarized photocathodes are commercially available

    (SLAC Spin Polarizer Wafers).

  • Phosphorus containing wafers are usually grown by MOCVD.

  • Gas-source MBE is used at SVT Associates.


MBE- In Situ Growth Rate Feedback

Monitoring RHEED image intensity versus time

provides layer-by-layer growth rate feedback

Growth at monolayer precision – not possible with MOCVD


Superlattice parameters
Superlattice parameters

b

w

Parameters:

Barrier thickness : 3 nm < b < 7 nm

Well thickness : 3 nm < w < 7 nm

Phosphorus x : 0.3 < x < 0.4

No. of periods : l ~ 70 – 200 nm

First systematic study of polarized photoemission from strained-superlattice - to be published in Applied Physics Letters


Graded GaAs1-xPx

GaAs Bulk

Additional peaks from

superlattice structure

GaAs0.64P0.36

Structural Analysis Using X-ray Diffraction

  • Measure superlattice period.

  • Measure well and barrier widths.

  • Measure phosphorus fraction in GaAs1-xPx layers.

  • Measure strain in GaAs layers.

Data

Simulation


Structural analysis using sims secondary ion mass spectroscopy
Structural Analysis using SIMS(Secondary Ion Mass Spectroscopy)

Phosphorus concentration vs. depth

  • Bombard with Cs ions and

  • detect As, P, and Be.

  • Measure superlattice period

  • Measure Be doping profile

Superlattice structure does not degrade after 600 C heat-treatment.


Strain effect vary x in gaas 1 x p x
Strain effect: Vary x in GaAs1-xPx

  • Max. polarization ~86%

  • QE ~1% (x5 SLC Cathode QE)

  • HH and LH transitions observed

  • HH-LH splitting increases with x.

LH

HH


Well dependence
Well Dependence

• Max. polarization ~86%

• QE ~ 1%

• Second peak is sensitive to HH2 → opportunity to test superlattice model.

X = 0.35


Period dependence
Period dependence

  •  Strain relaxes steadily.

  • Peak polarization is constant < 15 periods,

  • but decreases rapidly > 20 periods.


No charge limit
No Charge Limit

11012 e- in 60 ns → 4.51012 e- in 270 ns (x3 NLC train charge)


E158 run iii
E158 RUN III

  • Cathode installed in May 03.

  • E158 ran successfully.

  • ESA Moller measured 90%.

  • But it showed a charge limit ~71011 e-/300 ns

  • Could not make NLC train charge but OK for E158.

  • What happened?

    The 600° C heat-cleaning is destroying the high gradient doping profile.


Dopant diffusion
Dopant diffusion

Be concentration vs. depth

  • Be dopants diffuse out of the

    surface during 600 C heat-cleaning.

Need to lower the heat-cleaning temperature to < 450 C without lowering QE.


Atomic hydrogen cleaning appl phys lett 82 4184 2003
Atomic-Hydrogen CleaningAppl. Phys. Lett. 82, 4184 (2003)

Bulk GaAs

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%


Multi bunch laser development
Multi-bunch laser development

Laser modulator

RF Amp.

RF Gen.

Currently RF amp. is limited to 200 MHz.

Need 714 MHz for 1.4 ns.

119 MHz (8.4 ns)


Summary
Summary

  • High gradient doped strained-superlattice GaAs-GaAsP has been developed under SBIR and is commercially available.

  • Peak polarization of 86% with ~1% QE and no charge saturation up to 11012 e- in 60 ns.

  • Used successfully in E158 Run 3, yielding 90% polarization at ESA.

  • High gradient doping is vulnerable to high temperature heat cleaning.

  • To lower the heat-cleaning temperature, the atomic hydrogen cleaning technique has been developed.


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