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.

The etching of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon by hot tungsten filament generated atomic hydrogen has been investigated. Room-temperature etch rates of 27 Å s −1 for amorphous and 20 Å s −1 for microcrystalline silicon have been achieved. Boron doping decreases the etch rate, whereas phosphorus doping does not affect it. No surface roughening occurs, even for the highest a-Si:H etch rates. In the initial phase of the etch process, however, a bond structure modification arises close to the surface. An increase of microcrystalline silicon etch rates towards the substrate/film interface reflects the coalescence of the microcrystalline nuclei. Hot filament atomic hydrogen etching provides high etch rates of amorphous and polycrystalline silicon with a high selectivity against metals and thermal oxide. Due to its simple setup and control, this kind of hydrogen etching is very interesting for applications in semiconductor technology where F-or Cl-etchants are to be avoided.

The chemical reactions at the surface of transparent conductive oxides (SnO2, ITO and ZnO) have been studied in silane and hydrogen plasmas by in-situ ellipsometry and by SIMS as well as XPS depth profiling. SIMS and XPS of the interface reveal an increasing amount of metallic phases upon lowering a-Si:H growth rates (controlled by plasma power), indicating that the ion and radical impact is more than compensated by protecting the surface by a rapidly growing a-Si:H film. Hence, optical transmission of TCO films as well as the efficiency of solar cells can be improved if the first few nanometers of the p-layer are grown at higher rates. Comparing a-Si:H deposition on top of different TCOs, reduction effects on ITO and SnO2 have been detected whereas ZnO appeared to be chemically stable. Therefore an additional shielding of the SnO2 surface by a thin ZnO layer has been investigated in greater detail. Small amounts of H are detected close to the ZnO surface by SIMS after hydrogen plasma treatment, but no significant changes occur to the optical and electrical properties. In-situ ellipsometry indicates that a ZnO layer as thin as 20 nm completely protects SnO2 from being reduced to metallic phases. This provides for shielding of textured TCOs, and hence rising solar cell efficiencies, too. Regarding light trapping efficiency we additionally investigated the smoothing of initial TCO texture when growing a-Si:H on top by combining atomic force microscopy and spectroscopie ellipsometry.

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The growth of amorphous (a-Si:H) and microcrystalline (pc-Si) silicon by hot-wire chemical vapor deposition (HWCVD) has been studied by combining in-situ ellipsometry, atomic force microscopy (AFM), and Raman spectroscopy. Generally a dense nucleation layer is formed during a-Si:H HWCVD, containing nuclei about 0.8 nm high and 10 to 20 nm in diameter. The surface roughness gradually increases with film thickness and settles at a root mean square (RMS) value of 1.6 nm at about 200 nm thickness. For hydrogen dilution at gas flow ratios x=[H2]/[SiH4] of 15 to 120 microcrystalline material was obtained. The grain size and nucleation layer, however, are strongly dependent on x. Low H2 dilution enhances the formation of an amorphous-like interface layer from which the μc-Si:H growth eventually starts. Increasing x promotes the etching of amorphous regions and the surface diffusion of precursors, resulting in larger nuclei. X = 30 yields extended μc-Si nuclei (30 nm height, 90 nm diameter) and a pronounced increase in surface roughness for thicker films, but suppresses the formation of the amorphous-like nucleation layer. A further increase in x remarkably lowers the growth rate, but smoother surfaces at comparable film thickness and larger lateral dimensions of the grains occur. This is interpreted as incipient etching of the crystallites.

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We compare the electronic properties of nanocrystalline silicon from hot-wire chemical vapor deposition in a high-vacuum and an ultra-high-vacuum deposition system, employing W and Ta as filament material. From the constant photocurrent method we identify a band gap around 1.15 eV while, in contrast, a Tauc plot from optical transmission data guides to a wide band gap above 1.9 eV. The sudden change-over from nanocrystalline to amorphous structure in a hydrogen dilution series is also find in the dark and photoconductivity measurements. The samples show a metastability effect in the dark conductivity upon annealing in vacuum with an increase in the dark conductivity, with the large dark conductivity decreasing slowly after the annealing cycle when the cryostat is flushed with air. We identify larger values for the mobility-lifetime products, which corresponds to the smaller defect density shoulder in constant photocur- rent spectra, for the ultra-high-vacuum deposited material compared to the high-vacuun counterpart.

A hot tungsten wire effectively dissociates H2 into atomic hydrogen and thereby facilitates etching and hydrognation of silicon. Hot filament generated atomic hydrogen etches amorphous silicon (a-Si:H) at a rate of up to 27 Å/s and microcrystalline (μc) Si at rates up to 20 Å/s. A large laminar gas flow is the key to high etch rates. It provides for a fast transport of the etch products out of the reaction zone and thereby avoids redeposition. The etch rate increases with pressure and with H2 gas flow. Likewise, the etch rate rises with the filament temperature and saturates at a filament temperature of approximately 2150°C when approaching the maximum H2 dissociation probability. The decrease of the etch rate at higher substrate temperatures is attributed to the loss of the surface coverage by atomic hydrogen. The etch selectivity between a-Si:H and μc-Si drops at elevated substrate temperatures. Boron doping decreases the etch rates both for a-Si:H and μc-Si, whereas phosphorous doping does not significantly affect it. This etch selectivity is caused by a catalytic effect of BH3 on the surface hindering the formation of the main etch product silane. Even for highest etch rates no surface roughening of a-Si:H occurs, however, a bond structure modification of the near surfaces arises, an effect which results in the formation of a nanocrystalline surface layer. The increase of the μc-Si etch rate close to the film substrate interface characterizes the film thickness at which the coalescence of the microcrystalline nuclei starts.