Operational aspects of a gallium phosphide source of P2 vapor in molecular beam epitaxy

We report our operational experiences with a gallium phosphide source of P2 vapor in the MBE growth of GaP. The use of a P2 source results in much more favorable vacuum characteristics than the use of P4, as generated by an elemental phosphorus source, introducing no complications compared to the growth of arsenides. The phosphorus background pressure during growth is actually lower than the background pressure present during GaAs growth using an elemental arsenic As4 source, for comparable flux rates. The few P4 molecules that do form by association of P2 condense as white phosphorus on the cryobaffles and re-evaporate upon warmup. However, the pressure peaks at a value no higher than the background pressure during growth. Furthermore, the bakeout pressures are not significantly higher than those obtained using arsenic only. The P2 beam generated by the GaP source contains less Ga than one might expect from equilibrium vapor pressure considerations, thus greatly facilitating the independent control of Ga and P2 fluxes.