Optical properties and structure of amorphous silicon films prepared by CVD

Abstract

Silicon films were deposited by pyrolytic decomposition of silane on substrates held at various temperatures, T s , in the range 550 to 800°C. The absorption coefficient, refractive index, anf X-ray diffraction pattern of these films were determined. The films deposited at temperatures, T s ≤660°C are amorphous, and their absorption profile resembles that reported in the literature for sputtered or evaporated amorphous films after long-time anneal. Films deposited on substrates at or above 670°C are partially crystallized, with particle size increasing gradually with substrate temperature. When the amorphous films are annealed, the resulting changes depend on length and temperature of the anneal. After a temperature-dependent induction period, the samples crystallize rapidly. The volume shrinks by ≈3% as determined from the decrease in film thickness. The onset of crystallization is indicated first by a red shift of the absorption edge, which after further anneal is overcompensated by a blue shift. The results demonstrate that the superior solar absorptance of amorphous silicon can be utilized in photothermal solar energy converters of sufficient stability without sacrificing the advantages of CVD fabrication.