OPTOELECTRONIC PROPERTIES OF HYDROGENATED AMORPHOUS-SILICON FILMS DEPOSITED UNDER NEGATIVE SUBSTRATE BIAS

We present a detailed study of the effects of ion bombardment on the optoelectronic properties of a-Si:H films. Two series of samples were deposited from a rf glow discharge at 30 and 100 mTorr of silane pressure, corresponding to two different deposition conditions. The energy of the ions impinging on the substrate was increased by applying a negative dc bias in steps of 25 V to the substrate holder. The increase of the substrate bias from 0 to - 100 V had no effect on the deposition rate of a-Si:H at 30 mTorr, whereas a factor of 2 decrease was observed for deposition at 100 mTorr. The density of states of the a-Si:H films, determined by photothermal deflection spectroscopy and by the constant-photocurrent method, decreased as the substrate bias was increased up to - 50 V, especially for the series deposited at 100 mTorr. At the same time the valence-band tail became sharper. These observations are consistent with the improvement of the electron drift-mobility deep-trapping-lifetime (mu-d-tau-d)e product, determined by time-of-flight, and of the hole-mobility recombination-lifetime (mu-tau-r)h product determined from the voltage dependence of the photocurrent collection efficiency. For both series of samples, the films with the best electronic properties were obtained at a negative substrate bias of 50 V. No correlation was found between the substrate bias and the light-induced degradation behavior of these films. The saturated density of light-induced defects shows a direct correlation with the optical gap.