EFFECTS OF DEPOSITION TEMPERATURE ON PROPERTIES OF RF GLOW-DISCHARGE AMORPHOUS-SILICON THIN-FILMS

Thin films of hydrogenated amorphous silicon (a-Si:H) were grown on different substrates by r.f. glow discharge decomposition of silane gas. Hydrogen content and structural properties of films, determined by IR spectroscopy, have been compared with optical properties and density of localized states determined by UV-visible ellipsometry and photothermal deflection measurements respectively. New evidence about the influence of the deposition temperature on the structure and properties of a-Si:H, through its action during the growth process, has been analysed. Some effects induced by deposition temperature on a-Si:H films are the linear decrease in both the hydrogen content and the gap energy as the deposition temperature rises to 400-degrees-C as well as the decrease in the density of localized states and the Urbach edge as the temperature increases to 350-degrees-C. Moreover, changes in deposition temperature induce changes in hydrogen incorporation, as has been evidenced through a microstructural parameter calculated from IR absorption peaks at 2000 and 2090 cm-1. These results suggest that the main mechanism operating in the growth process is thermal activation of the surface mobility which is limited by the hydrogen surface desorption. Above 350-degrees-C, a lower hydrogen coverage arises during the a-Si:H deposition process.