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.
Victoria ALCW July 28-31 2004
R. Prepost (Wisconsin) – LC Accelerator R&D at Universities
• 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
GaAs(1-x)Px Graded Layer
Single strained GaAs (SLC)
→ GaAs layers are highly strained
“Advanced Strained-Superlattice Photocathodes for
Polarized Electron Sources”
Very first sample produced 85% polarization
(SLAC Spin Polarizer Wafers).
Monitoring RHEED image intensity versus time
provides layer-by-layer growth rate feedback
Growth at monolayer precision – not possible with MOCVD
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
Additional peaks from
Structural Analysis Using X-ray Diffraction
Phosphorus concentration vs. depth
Superlattice structure does not degrade after 600 C heat-treatment.
• Max. polarization ~86%
• QE ~ 1%
• Second peak is sensitive to HH2 → opportunity to test superlattice model.
X = 0.35
11012 e- in 60 ns → 4.51012 e- in 270 ns (x3 NLC train charge)
The 600° C heat-cleaning is destroying the high gradient doping profile.
Be concentration vs. depth
surface during 600 C heat-cleaning.
Need to lower the heat-cleaning temperature to < 450 C without lowering QE.
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%
Currently RF amp. is limited to 200 MHz.
Need 714 MHz for 1.4 ns.
119 MHz (8.4 ns)