When semi-insulating GaAs samples are coated with pyrolytically deposited Si3N4 layers and subsequently irradiated with a Q-switched ruby laser with energy densities > 0.3 J/cm2, it is found that a thin layer (0.1–0.25 μm) of the underlying GaAs substrates becomes n-type. This phenomenon may be the reason why, in previous work on laser annealing of donor implanted GaAs samples, the percentage electrical activity increased when samples were coated with Si3N4.