GaAsSb/AlGaAsP Superlattice Polarized Electron Source

The negative-electron-affinity (NEA) photocathodes which produce polarized electrons are a vital component of electron accelerators such as that at the Stanford Linear Accelerator Center (SLAC) and DoE Jefferson Lab. Future systems, such as the International Linear Collider (ILC), will require a polarized electron beam intensity at least 20 times greater than produced by strained GaAs, which is used in the current generation of photocathodes. Additionally, the degree of electron polarization needs to be increased beyond the 75% currently attainable and intrinsic material properties related to improving the surface charge limit must also be addressed, and the photocathodes should be more robust in an RF gun environment. The end result of the combined Phase IPhase II effort will be a new generation of robust photocathodes capable of yielding intense, highly polarized electron beams for use in advanced electron colliders. We have previously achieved & gt;85% polarization using a strained-compensated superlattice formed from alternating layers of GaAsSb and AlGaAsP approximately ten monolayers thick. For this program we will apply a strain-compensation concept utilizing antimony- and phosphorus-based material which should overcome material limitations of the GaAs/GaAsP alloys. In the Phase I we will design and fabricate a strain-compensated superlattice structure with GaAsSb/AlGaAsP material by molecular beam epitaxy. The first portion of the program will optimize the growth conditions to achieve the desired alloy composition and interface quality. Photocathode structures will then be fabricated, and their polarization and quantum efficiency will be measured. Commercial Applications and Other Benefits: A successful project will produce a highly efficient polarized electron source for use in experimental research at SLAC, Jefferson Lab, and other electron collider facilities. These devices have applications in other areas which include magnetic imaging research, surface analysis, Quantum computing and cryptography.