Thermally deposited silicon films doped with oxygen atoms and used as passivation films on silicon devices have been studied with transmission electron microscopy, x‐ray diffraction, and ESCA. The films contain at least two phases, silicon microcrystals and silicon oxide. The size of the microcrystals in as‐deposited films was dependent both on the deposition temperature and on the oxygen concentration. Heat treatment caused crystal growth which depended mainly on the annealing temperature and weakly on the annealing time. The lattice constant of the microcrystals was directly related to their size. The silicon oxide phase in as‐deposited films was found to be SiO1.4. The results suggest ’’mosaic’’ model of the films which are amorphous when the diameter of the silicon microcrystals was less than 10 A˚.