A method is described of forming a film of an amorphous material 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 therein. 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 said flow rate of the film precursor gas is altered during the course of deposition of material, so as to cause the bandgap to vary over the thickness of the deposited material.