Highly spin-polarized electron photocathode based on GaAs-GaAsP superlattice grown on mosaic-structured buffer layer

We successfully developed a transmission-type photocathode, and a high polarization (90%) with a super-high brightness (1.3×107 A cm−2 sr−1) of electron beam was achieved. In this research, we found that the polarization of electrons from the GaAs-GaAsP superlattice on the GaP substrate was lower than that from the same superlattice on the GaAs substrate, and that a GaAs inter-layer deposition on the GaP substrate recovered that from the GaAs-GaAsP superlattice. Mechanism of the polarization degradation was investigated from a viewpoint of strain-relaxation process in the GaAsP buffer layer that was adopted between the superlattice and the substrates. The buffer layer with in-plain tensile strain was relaxed by inducing cracks, but that with in-plain compressive strain was relaxed by inducing dislocations. The dislocations may disturb the spin-polarization of electrons more effectively than the cracks do.