Light-induced degradation of hydrogenated amorphous silicon films deposited at different rf powers

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Abstract
Light-induced degradation has been investigated in glow-discharge deposited hydrogenated amorphous silicon (a-Si:H) films (≊1 μm), deposited at different rf powers by glow-discharge decomposition of pure silane. Changes in electrical, ellipsometric, and infrared parameters have been observed by exposing the films to white light of intensity 350 mW/cm2 for 4 h. The initial dangling bond density increases and the rate of change of the dangling bond density decreases in films successively deposited at higher powers up to 50 W. However, at still higher power (75 W), a reverse behavior has been observed. Spectroscopic ellipsometry shows maximum void fraction in films deposited at 50 W. Light soaking results in an increase in void fraction and hence, the dangling bond density. The unaltered position of ε2 max suggests that the tetrahedral coordination of the silicon network remains unchanged on light exposure. The IR transmittance difference curves indicate changes on light soaking in the stretching mode vibrations only in 50-W deposited film and in the rocking mode for all films except that deposited at 5 W. A greater change (maximum change for 50-W film) in the state of bonds has been observed for samples having a greater initial void fraction.