A Polarized Electron Source Using Gallium Arsenide Strained by Differential Thermal Contraction

Since 1975, when efficient spin-polarized photoemission from a semiconductor was first achieved (Pierce et al., 1975a, 1975b; Pierce and Meier, 1976), there have been many semiconductor spin-polarized electron sources constructed in laboratories studying atomic, solid-state and high energy physics. The polarization of these sources, based on cubic III-V semiconductors (usually highly p-doped GaAs), has been intrinsically limited to a maximum of 50% by the band structure of the cubic crystals. In recent years, measured polarizations considerably exceeding 50% have been reported for a new generation of sources; these make use of III-V semiconductor based crystals in which the cubic symmetry has been lowered either by straining or by growing anisotropic heterostructures. In general, the yield of these high polarization sources (measured as the ratio of the emitted electron flux to the incident photon flux in the semiconductor) has been substantially below that obtained with conventional GaAs cathodes.