Intrinsic hydrogenated amorphous silicon ͑a-Si:H͒ has been deposited using a hot tungsten filament in pure silane to drive the deposition chemistry-the ''hot-wire'' deposition method. The electronic and infrared properties of the film have been measured as a function of deposition parameters, leading to three principal conclusions. First, to obtain a high quality material, the Si atoms evaporated from the filament ͑distance L from the substrate͒ must react with silane ͑density n s ͒ before reaching the substrate; this requires n s L greater than a critical value. Second, radical-radical reactions cause deterioration of film properties at high values of G(n s L), 3 where G is the film growth rate; this requires G(n s L) 3 less than a critical value. Finally, the film quality is a function of G, and as G is increased the substrate temperature must be correspondingly increased to obtain high film quality. By optimizing these parameters, we have produced films with excellent electronic properties ͑e.g., ambipolar diffusion length Ͼ200 nm͒ at Ͼ5 nm/s deposition rate. Based on these insights, formulas are also given for optimizing film properties in multiple-filament geometries and in diluted silane.

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The deposition parameter space has been extensively explored using the hot wire technique with 1% SiH4 in He as a source gas. To achieve reasonable deposition rates despite the high dilution, the filament was positioned at 1–2 cm from the substrate. This short distance introduced a large nonuniformity across the substrate in deposition rate as well as in film properties. These spatial variations were used to analyze which factors in the deposition determine film quality. Radiation from the filament as well as deposition rate cannot explain the large variation in film properties, leaving gas-phase reactions of Si and H from the hot filament as the primary cause. It is clear that radicals evaporated from the filament must undergo gas-phase reactions with SiH4 before deposition in order to produce high-quality material. Thus, conditions such as increasing the chamber pressure or going to a heavier carrier gas increase the fraction of radicals that can react before reaching the substrate and, therefore, improve the film quality. However, such conditions also enhance multiple radical reactions before such radicals reach the substrate and this can have a negative effect on film quality: this is attributed to gas-phase nucleation with incorporation of conglomerates. The gas-phase chemistry is quite different from that of plasma-enhanced decomposition in that no disilane or trisilane is formed in significant quantities. This, and the dependence on pressure, indicates that the pathway for formation of these heavier particles is radical–radical reactions.

Device quality a-Si:H has been deposited at 3 Å/s using the “hot wire” technique with 1% SiH4 in He as a source gas. To achieve this deposition rate despite the high dilution, the filament was positioned at 1–2 cm from the substrate. This short distance introduces a large non-uniformity across the substrate in the deposition rate as well as in the film properties. This experimental fact was used to analyze which factors in the deposition determine film quality. We find that radiation from the filament is not an important factor. Data taken from samples deposited at various distances, pressures and flows suggest that the film quality is influenced by radical reactions with SiH4. However, this assumption alone predicts too strong a pressure dependence. The influence of deposition rate as an additional factor can explain the results.