The mechanism of silicon diffusion in GaAs, Al0.3Ga0.7As, and the silicon diffusion-induced layer disordering of multiquantum wells have been studied by photoluminescence, secondary-ion-mass spectroscopy, and transmission electron microscopy across a corner of a wedge-shaped sample. The diffusion source was a grown in highly Si-doped layer. The main photoluminescence properties of point defects in GaAs and Al0.3Ga0.7As are reviewed to interpret the experimental data. The depth profile of the photoluminescence allows the spatial correlation between the luminescence spectra and the Si concentration profile obtained from secondary-ion-mass-spectroscopy measurements. On the basis of the photoluminescence results, the physical processes occurring during the Si diffusion are discussed. Frenkel defects (pairs of element-III vacancies and interstitials) are generated in the highly Si-doped region. The element-III interstitials rapidly diffuse towards the surface where they react with the element-III vacancies generated at the surface when annealing is performed in an external As pressure. This induces a supersaturation of element-III vacancies in the Si-doped region which drives the Si diffusion. Annealing in vacuum reduces the oversaturation of element-III vacancies and, hence, reduces the Si diffusion. A domination of the Si donor-element-III vacancy complex emission band was found in the spectra taken in the Si-diffused region. This gives evidence for the vacancy-assisted mechanism in the Si diffusion and in the impurity-induced disordering.