The quality of a rf glow discharge depositeda‐Si:H thin film is affected by the substrate material. As a result of ion bombardment, some of the reactive species from the glass substrate are introduced into the deposition chamber by desorption. These species are then adsorbed back into the surface with the decomposed gas and form impurities in the film. The presence of Na‐Si and K‐Si impurities in thea‐Si:H film lowers the electrical conductivity and decreases the film stability.