Elimination of local thickness modulation in GaAs/GaAsP strained superlattices for high spin-polarization photocathodes

We successfully developed a transmission-type photocathode, and a high spin-polarization (90%) with a super-high brightness (1.3×107 Acm−2sr−1) of electron beam was achieved. In this research, the elimination of thickness modulation of GaAs/GaAsP superlattice by introduction of a GaAs inter-layer on a GaP substrate is the key point to realize the high spin-polarization. The thickness modulation of the superlattice is related with the surface roughness of the buffer layer. The compressive strain introduced in the GaAsP buffer layer on the GaP substrate causes hillock formation, where several degree off-angle surfaces are formed. The GaAs inter-layer deposited on the GaP substrate introduced a tensile strain in the GaAsP buffer layer instead of the compressive strain and relatively smooth GaAsP buffer layer was achieved. The smooth GaAsP buffer layer was attributed to the periodic GaAs/GaAsP superlattice layer growth.