A method is described for forming a film of amorphous silicon (a-Si:H) on a substrate by deposition from a plasma. The substrate is placed in an enclosure, a film precursor gas is introduced into the enclosure, and unreacted and dissociated gas is extracted from the enclosure so as to provide a low pressure in the enclosure. Microwave energy is introduced into the gas within the enclosure to produce a plasma therein by distributed electron cyclotron resonance (DECR) and cause material to be deposited from the plasma on the substrate. The substrate is held during deposition at a temperature in the range 200-600° C., preferably 225-350° C. and a bias voltage is applied to the substrate at a level to give rise to a sheath potential in the range −30 to −105V, preferably using a source of RF power in the range of 50-250 mW/cm2 of the area of the substrate holder.