Improved Electron Yield and Spin-Polarization from III-V Photocathodes via Bias Enhanced Carrier Drift

72323- The exploration of certain regimes of high energy physics will require the development of photocathodes with emitted electron spin-polarizations above 90%. Emitted electron spin-polarizations approaching 90% have been achieved, but at the expense of quantum yield. Because natural diffusion spatial spread disfavors complete transport to the emitting surface, this project will develop technology for adding a drift component to the photoexcited electron diffusion velocity, thereby reducing electron residence time after excitation. This bias also should enhance overall emission probability by modifying the natural diffusion momentum with a drift component. In Phase I, a bias-capable, high-temperature, compliant cathode holder and contact mechanism were built and installed in the cathode test system at the Stanford Linear Accelerator Center (SLAC) to enable the electron emission characteristics from a front surface contacted GaAs photocathode to be tested. Bias-enhanced carrier drift was shown directly by the increase in photoemission with a forward bias resulting in a well modified quantum yield. Reverse bias gave the anticipated result as well, a decrease in the quantum yield with a likely accompanying increase in polarization. Phase II will design, build, and test a photocathode structure to accomplish optimal drift with minimal bias supply requirements. Photoyield and polarization measurements will be conducted in photocathode test facilities at SLAC. Commercial Applications and Other Benefits as described by awardee: The technology should be of use to the many laboratories utilizing GaAs based emitters. The final product also should be applicable for electron sources utilized in electron microscopes and other analytical tools for use in studying surface, atomic, and molecular physics. The enhanced yield of the forward biased cathode should translate into an effectively longer lifetime for those applications with less than ideal vacuum environments.