We have applied both sinusoidal and pulse-shaped rf substrate biasing techniques to the expanding thermal plasma deposition of hydrogenated amorphous silicon. Spectroscopic ellipsometry and Fourier transform infrared spectroscopy data demonstrate that both methods of substrate biasing can result in improved film properties at deposition rates ranging from 1.4 to 16 nm s −1 , at relatively low substrate temperatures (200 • C), as demonstrated by an increase in refractive index, and a decrease in the microstructure factor (R * ). An extra plasma forms in front of the substrate upon the application of both bias techniques, but optical emission spectroscopy data show that the emission intensities are significantly greater with the sinusoidal rf bias. Although the application of sinusoidal rf bias results in an initial improvement of the materials, the H content and R * values both increase at higher rf-induced substrate bias voltages (−V bias >∼ 70 V). The same bias conditions that result in increased H content and R * correspond to the conditions where the additional plasma in front of the substrate undergoes both a sharp increase in emission, and a decrease in measured ion current, suggesting an α to γ transition.

We have applied pulse-shaped biasing to the expanding thermal plasma deposition of hydrogenated amorphous silicon at substrate temperatures ∼200 °C and growth rates around 1 nm/s. Substrate voltage measurements and measurements with a retarding field energy analyzer demonstrate the achieved control over the ion energy distribution for deposition on conductive substrates and for deposition of conductive materials on nonconductive substrates. Presence of negative ions/particles in the Ar–H2–SiH4 plasma is deduced from a voltage offset during biasing. Densification of the material at low Urbach energies is observed at a deposited energy <4.8 eV/Si atom and attributed to an increase in surface mobility of mobile species as well as well as surface atom displacement. The subsequent increase in Urbach energy >4.8 eV/Si atom is attributed to bulk atom displacement in subsurface layers. We make the unique experimental abservation of a decreasing Tauc band gap at increasing total hydrogen concentration—this allows to directly relate the band gap of amorphous silicon to the presence of nanovoids in the material.

The surface roughness evolution of hydrogenated amorphous silicon (a-Si:H) films has been studied using in situ spectroscopic ellipsometry for a temperature range of 150–400 °C. The effect of external rf substrate biasing on the coalescence phase is discussed and a removal/densification of a hydrogen-rich layer is suggested to explain the observed roughness development in this phase. After coalescence we observe two distinct phases in the roughness evolution and highlight trends which are incompatible with the idea of dominant surface diffusion. Alternative, nonlocal mechanisms such as the re-emission effect are discussed, which can partly explain the observed incompatibilities.

Hydrogenated amorphous silicon (a‐Si:H) films are grown at different hydrogen dilutions. For high dilutions we observe a discrepancy in the surface roughness analysis between real‐time spectroscopic ellipsometry (RTSE) and atomic force microscopy (AFM) measurements. With RTSE a much higher roughness layer thickness is measured than with AFM. Additionally we observe what appears to be a strong roughening phase in the first 30‐50 nm of film growth for all conditions. A mechanism that involves the formations of a hydrogenrich overlayer and etching of higher hydrides in this overlayer is suggested.This mechanism explains the difference in RTSE roughness layer thickness evolution. At higher hydrogen dilutions, we obtain a thicker overlayer dominated by lower hydrides. RTSE interprets this overlayer as additional surface roughness and consequently overestimates the surface roughness for higher dilutions. The initial strong roughening phase is related to the built‐up of this over‐layer. In this phase, we obtain much denser films compared to the bulk film. The presence of columnar growth at higher dilutions is attributed to reduced surface mobility under these conditions (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)