VHF glow discharges are employed for high-rate a-Si:H deposition, maintaining good optoelectronic properties. A more efficient radical generation, either due to higher electron densities or an enhanced high-energy electron tail, is generally assumed as the mechanism. A VHF a-Si:H depositing plasma was investigated between 40 and 250 MHz by optical emission spectroscopy, mass spectroscopy, ion energy measurements and electrical impedance analysis. The present study shows that the increase of deposition rate with frequency is essentially due to enhanced ion flux to the growth surface, such that models of deposition kinetics taking into account only neutral species and neglecting the role of ions impinging on the substrate can therefore not be applied to VHF plasma deposition.

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Amorphous hydrogenated silicon (a-Si:H) was deposited by SiH4 decomposition on a hot tungsten filament. The substrate temperature was held at 400 °C for all samples, maintaining conditions where material combining a low defect density and a low hydrogen content is obtained. A systematic study of the effects of gas pressure, substrate-to-filament distance, and filament temperature on film properties is presented, allowing insight into the growth condition required for this material as well as the significance of secondary gas phase reactions. Material of good optoelectronic quality is obtained at high growth rates. The stability with respect to light degradation was compared to typical plasma deposited films. Conditions for the transition from amorphous to microcrystalline films, observed under gas phase dilution with hydrogen, were investigated. By in situ ellipsometry and atomic force microscopy the nucleation and film morphology were shown to be significantly different from those for plasma-chemical vapor deposition material.

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The microstructure of amorphous Si1−xGex:H films with x<0.40 was studied using small-angle x-ray scattering (SAXS) and the results compared with those from opto-electronic and density measurements. The SAXS, the sub-band-gap absorption determined from photothermal deflection spectroscopy, and the photo/dark conductivity ratio all show relatively sharp changes above x=0.2. A corresponding sharp change in the anisotropic character of the SAXS is consistent with a transition to a columnar-like microstructure above x=0.2. The correlated results provide strong evidence for a direct link between degraded opto-electronic properties and the increased heterogeneity associated with the microstructural transition.