High Polarization and High Robustness Antimonide Based Superlattice Photocathodes for RF Gun Applications

Negative-electron-affinity photocathodes, which produce polarized electrons, are a vital component of electron accelerators such as that at the Stanford Linear Accelerator Center. Future systems, such as the International Linear Collider, 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 percent currently attainable, intrinsic material properties related to improving the surface charge limit must be addressed, and the photocathodes should be more robust in an RF gun environment. This project will develop a new generation of robust photocathodes capable of yielding intense, highly polarized electron beams for use in advanced electron colliders. Previously, polarization greater than 85 percent was achieved using a strained-superlattice, formed from alternating layers of GaAs and GaAsP approximately ten monolayers thick. In this project, a similar superlattice concept, which utilizes an antimony-based (Sb) material that can overcome the material limitations of GaAs/GaAsP alloys, will be used. Phase I will design the strained superlattice structure with an antimony-based material and fabricate it by molecular beam epitaxy. Growth conditions will be optimized 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 as described by the awardee: A highly efficient polarized electron source would be used in experimental research at SLAC and other electron collider facilities. These devices also would have applications in other areas, including magnetic imaging research, surface analysis, quantum computing, and cryptography.