The effects of strain on tin diffusion in GaAs were studied by using laser Raman and photoluminescence spectroscopies. It was found that an increase of compressive strain on GaAs causes an increase of the carrier concentration while a decrease of compressive strain or an increase of tensile strain causes a decrease of the carrier concentration at the surface of GaAs. The results are due to the decrease of the diffusion coefficient of tin in GaAs with compressive strain and the increase of it with tensile strain. Photoluminescence data showed that the peak due to Ga antisite defects increased with increase of compressive stress. This indicates a decrease of Ga vacancy concentration from the equilibrium concentration in an unstressed sample. On the other hand, photoluminescence data from the tensile stressed sample showed an increase of Ga vacancy concentration from the equilibrium concentration in an unstressed sample. Thus, the change of the diffusion coefficient with strain seems to be related to Ga vacancy. It was also found that the diffusion coefficient decreased exponentially with compressive strain and increased exponentially with tensile strain with the result that the activation energy of tin diffusion in GaAs varied linearly with strain. ©1997 American Institute of Physics.