High Polarization Photocathode R&d at Slac*

This paper describes recent progress on the development of high polarization photocathodes for polarized electron sources. A strained InGaAs cathode has achieved a maximum electron-spin polarization of 71% and has demonstrated the strain enhancement of polarization for the first time. Strained GaAs cathodes have yielded polarizations as high as 90% with much higher quantum efficiency. 1 c Introduction. Polarized electron sources based on negative-electron-affinity GaA-s were pioneered at SLAC. 1 The original PEGGY II source produced 2x10 11 e-per bunch at 120 Hz with an average spin polarization of 37%. The maximum polarization of this type of source was limited to 50% due to the valence-band degeneracy of the heavy-hole and light-hole bands of GaAs. Much effort has been devoted to achieving higher polarization by primarily focusing on the following three types of photocathodes: 1) ternary chalcopyrites which naturally have the appropriate band structure, 2) GaAs-AlGaAs superlattices, and 3) strained materials. These materials do not have a valence band degeneracy, and selective excitation of a single-transition is possible. Earlier efforts at SLAC were devoted to .-studies of the first two types of photocathodes When a thin layer " B " with a slightly of a crystal " A " is grown on a different crystal sm ati lattice constant, crystal " A " may grow " pseudomorphically, " introducing a biaxial compressive strain in the plane of the interface. The strain alters the band structure of crystal " A " such that the strain dependent energy difference of the heavy-hole and light-hole bands relative to the conduction band is given by: