Optoelectronic properties of hydrogenated amorphous silicon films deposited under negative substrate bias

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Abstract
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 (μdτd)e product, determined by time‐of‐flight, and of the hole‐mobility recombination‐lifetime (μτr)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.