We have investigated the dependence of the substrate temperature at which the transition from amorphous to polycrystalline state occurs, on the types of substrate, deposition conditions, and impurities. When crystallization is produced by annealing of an amorphous film already formed, the crystallization temperature is 600 °C when annealed under UHV, and higher when exposed to air prior to annealing. The increase is due to the necessity of out diffusion of trapped impurities introduced through the voids during exposure.