We have investigated the molecular-beam epitaxial growth and photoluminescence properties of GaAs on SiOx. It is seen that material with a grain size of 0.6-0.8 μm can be grown directly on the dielectric. The properties improve further when the layers are short-term annealed with a halogen lamp. Optimum grain sizes of 1.6 μm are obtained when the as-grown material is annealed at 950°C for 10 s, and very strong luminescence is observed in the same material. Photoconductive detectors made on the overgrown GaAs show large responsivities.