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 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.@en

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 (¿Vbias >~ 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 ¿ to ¿' transition.@en

Abstract 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.@en

We applied pulse-shaped biasing (PSB) to the expanding thermal plasma deposition of intrinsic hydrogenated amorphous silicon layers at substrate temperatures of 200¿°C and growth rates of about 1¿nm/s. Fourier transform infrared spectroscopy of intrinsic films showed a densification with increasing deposited energy and a reduction in void content, whereas dual-beam photoconductivity measurements showed an increase in Urbach energy above 4.8¿eV/Si atom. From dark conductivity and photoconductivity measurements, we determined a maximum photoresponse of 2¿×¿106 at 3¿eV/Si atom, which decreased at higher deposited energies because of a higher dark conductivity as a result of a lower band gap. p¿i¿n solar cells with PSB applied during the intrinsic layer deposition showed initial energy conversion efficiencies of 7.4% at around 1¿eV/Si atom. Decreasing open-circuit voltage at >1¿eV/Si atom can be related to a lower band gap, whereas the short-circuit current drops at >4.8¿eV/Si atom, predominantly because of hole collection losses as determined from quantum efficiency measurements. The reduced fill factor for >1¿eV/Si atom was presumably related to a decrease in mobility-lifetime product because of an increase in defect density.@en

Hydrogenated amorphous silicon (a-Si:H) was deposited with the Expanding Thermal Plasma-CVD (ETP CVD) method utilizing pulse-shaped substrate biasing to induce controlled ion bombardment during film growth. The films are analyzed with in-situ real time spectroscopic ellispometry, FTIR spectroscopy, as well as reflection-transmission and Fourier transform photocurrent spectroscopy (FTPS) measurements. The aim of this work is to investigate the effect ion bombardment with well defined energy on the roughness evolution of the film and the material properties. We observe two separate energy regimes with material densification and relatively constant defect density below ~ 120-130 eV and a constant material density at increasing defect density > 120-130 eV substrate bias. We discuss our results in terms of possible ion ¿ surface atom interactions and relate our observations to reports in literature.@en

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.@en