Density of midgap states and Urbach edge in chemically vapor deposited hydrogenated amorphous silicon films

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Abstract
Intrinsic or lightly boron-doped amorphous silicon films were produced by chemical vapor deposition (CVD) from disilane at temperatures from 380 to 460 °C and at growth rates from 0.1 to 40 Å/s. The density of states (DOS) near the middle of the mobility gap was determined by space-charge-limited conduction on n-i-n structures and by steady-state capacitance-temperature spectroscopy on p-i-n structures. Very close agreement was found between the two techniques for intrinsic layers deposited under similar conditions. The DOS distribution is rather flat from 0.60 to 0.75 eV from the conduction-band edge and has a range of 1–6×1017 cm−3 eV−1 for intrinsic films, with the minimum occurring for depositions at 440 °C independent of growth rate or contaminants in the disilane. Diborane levels from 9 to 18 ppm in the gas phase reduces the DOS at 440 °C to 3–6×1016 cm−3 eV−1. The exponential absorption below the band edge in the range 1.4–1.6 eV was determined from primary photocurrent spectra of p-i-n structures. The Urbach edge parameter E0 ranged from 0.048 to 0.056 eV with the minimum also occurring for depositions at 440 °C, independent of growth rate and boron content. Comparison to glow-discharge a-Si:H films indicates that CVD a-Si:H films have a similar amount of structural disorder, determined from E0, but an inherently larger DOS deep in the gap. A model is proposed in which boron reduces the high midgap dangling bond density caused by insufficient weakly bonded hydrogen in the film as a consequence of the high deposition temperatures. The model is consistent with other measurements, such as hole collection width.