Low-temperature optical properties of hydrogenated amorphous silicon

We report experimental studies of the optical properties, in the energy range 0.5-3.0 eV, of magnetron sputtered hydrogenated amorphous silicon films in the temperature range 4-295 K. Raman measurements made at room temperature confirm the amorphous nature of the samples. For samples containing high hydrogen concentration, a slight change in the general behavior of the optical properties is observed as temperature approaches 30 K, indicating possible contributions from phase transitions associated with hydrogen to the optical absorption processes. The well-known Tauc rule and the other alternate relations being proposed in recent years are then employed to evaluate the optical gaps. At least at low temperatures, Cody's formulation of the energy gap is seen to yield values of the gap close to those obtained from the Tauc rule. The refractive index variations with energy are examined from the point of view of application of spectroscopic models like those of Wemple-Didomenico. All these studies have been carried out near the band-gap region of amorphous silicon.