STABILITY OF HYDROGENATED AMORPHOUS-SILICON DEPOSITED BY PLASMA-ENHANCED CHEMICAL-VAPOR DEPOSITION FROM HELIUM-DILUTED SILANE

The stability of undoped amorphous silicon deposited by r.f. glow disc harge under standard conditions and from silane-helium mixtures has be en studied. The optoelectronic properties have been obtained after the rmal quenching and light soaking. Whereas standard hydrogenated amorph ous silicon (a-Si: H) exhibits thermal metastability with changes in c onductivity, photoconductivity and defect density induced by thermal q uenching, a-Si: H films deposited from a mixture of diluted silane (40 %) in helium at high deposition rates (up to 15 angstrom s-1) behave i n a strikingly different manner with no sip at all of thermal metastab ility up to 300-degrees-C. The saturation defect density measured afte r light soaking under high intensity (50 air mass 1) illumination is s maller 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 'heli um-diluted' samples. The data are analysed in terms of recent equilibr ium models in relation to other studies such as infrared spectroscopy and exodiffusion experiments which reveal completely different hydroge n bondings in this type of film.