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 (1.3×10 7 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.