A novel method for preparing photoconductive Si thin films termed “Spontaneous Chemical Deposition”, is proposed, in which silane is decomposed spontaneously by gas phase reactions with fluorine at reduced pressure. With the external parameters in the gas phase reaction such as a gas flow ratio of SiH4 to F2 and the reaction pressure and temperature, the Si-network structure of the films can be controlled intentionally, resulting in a reduction of the hydrogen content, CH and a variety of the films from “amorphous” to “microcrystalline”.