Activation Layer Stabilization of High Polarization Photocathodes in Sub-Optimal RF Gun Environments

Gregory A. Mulhollan

关键词: GaAs;    cesium;    lithium;    activation;    negative electron affinity;    NEA;    chemical immunity;    background gas;    higher resilience;    carbon dioxide;    carbon monoxide;    photoemission;    spin-polarization;    electrons;    XPS;   

DOI  :  10.2172/992578
RP-ID  :  DOE/07ER84832
PID  :  OSTI ID: 992578

学科分类：核物理和高能物理

美国|英语

来源: SciTech Connect

【 摘 要 】

Specific activation recipes for bulk, 100 nm thick MBE grown and high polarization III-V photocathode material have been developed which mitigate the effects of exposure to background gasses. Lifetime data using four representative gasses were acquired for bulk GaAs, 100 nm unstrained GaAs and strained superlattice GaAs/GaAsP, all activated both with Cs and then Cs and Li (bi-alkali). Each photoemitter showed marked resilience improvement when activated using the bi-alkali recipe compared to the standard single alkali recipe. A dual alkali activation system at SLAC was constructed, baked and commissioned with the purpose of performing spin-polarization measurements on electrons emitted from the bi-alkali activated surfaces. An end station at SSRL was configured with the required sources for energy resolved photoemission measurements on the bi-alkali activated and CO2 dosed surfaces. The bi-alkali recipes were successfully implemented at SLAC/SSRL. Measurements at SLAC of the photoelectron spin-polarization from the modified activation surface showed no sign of a change in value compared to the standard activated material, i.e., no ill effects. Analysis of photoemission data indicates that the addition of Li to the activation layer results in a multi-layer structure. The presence of Li in the activation layer also acts as an inhibitor to CO2 absorption, hence better lifetimes in worse vacuum were achieved. The bi-alkali activation has been tested on O2 activated GaAs for comparison with NF3 activated surfaces. Comparable resilience to CO2 exposure was achieved for the O2 activated surface. An RF PECVD amorphous silicon growth system was modified to allow high temperature heat cleaning of GaAs substrates prior to film deposition. Growth versus thickness data were collected. Very thin amorphous silicon germanium layers were optimized to exhibit good behavior as an electron emitter. Growth of the amorphous silicon germanium films on the above substrates was fine tuned with respect to time and power to moderate plasma damage to the photo-generating layer. Auger electron spectroscopy was used to analyze the composition and thickness of the emitter layers. AFM studies showed conformal growth on the GaAs substrates. Measurements at SLAC on the photoemitted electrons from high polarization substrates coated with amorphous silicon germanium indicated an ~10% relative drop in spin-polarization at the wavelength corresponding to the maximum spin-polarization when compared to the uncoated material,