Stability of hydrogenated amorphous silicon deposited by plasma-enhanced chemical vapour deposition from helium-diluted silane

The stability of undoped amorphous silicon deposited by r.f. glow discharge under standard conditions and from silane-helium mixtures has been studied. The optoelectronic properties have been obtained after thermal quenching and light soaking. Whereas standard hydrogenated amorphous silicon (a-Si:H) exhibits thermal metastability with changes in conductivity, photoconductivity and defect density induced by thermal quenching, a-Si: H films deposited from a mixture of diluted silane (40%) in helium at high deposition rates (up to 15 Ås^-1) behave in a strikingly different manner with no sign at all of thermal metastability up to 300°C. The saturation defect density measured after light soaking under high intensity (50 air mass 1) illumination is smaller by a factor of at least two than that in light-soaked standard a-Si: H. This result thus confirms the better stability of these ‘helium-diluted’ samples. The data are analysed in terms of recent equilibrium models in relation to other studies such as infrared spectroscopy and exodiffusion experiments which reveal completely different hydrogen bondings in this type of film.