An investigation of the fast diffusion of Si in GaAs from deposited surface layers oxidized in an Ar/H2O ambient indicates excess As, formed by oxidation of Ga originating from the substrate, is responsible for the enhanced diffusion of Si into the substrate. The formation of the SiO2on the surface during oxidation prevents loss of the excess As, which accumulates in the remaining Si film. Higher H2O partial pressures during the oxidation produce higher As/Si ratios, resulting in an increase in the Si diffusion depth and concentration. However, then‐type carrier concentration decreases with higher As/Si ratios in the remaining Si layer. A second, nonoxidizing anneal on samples with the SiO2and Si layer removed produced different effects on the carrier concentration, depending on whether As was free to escape from the substrate. The results indicate that excess As related defects such as gallium vacancies are probably responsible for then‐type compensation in the fast diffused samples. ©1995 American Institute of Physics.