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Thermal oxidation and encapsulation of silicon-carbon nanolayers
Silicon-carbon (Si-C) thin films play a key role in many technological applications such as hard coatings, high-power electronics, and photovoltaics. In photovoltaics in particular annealed Si-C thin films containing Si quantum dots are used to develop solar cells with improved efficiency. The oxidation of these films during the annealing step, which is unavoidable in the high-throughput processes required for photovoltaics, was explored using scanning electron microscopy, Fourier-transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. SiO2 surface layers 5 to 14 nm thick were observed even in nominally inert furnace atmospheres, while annealing with graphitic carriers leads to the formation of SiOxCy films a few nm thick. To avoid the formation of either compound and thereby reduce the impact of the particular furnace used on the Si-C film an encapsulation layer made of a-Si: H was developed. It is shown that 40 nm of this layer can protect an Si-C film from oxidation. The SiO2 and residual Si formed are removed using standard etchants with only minimal impact on the Si-C film. It was found that this process depends critically on the thickness of a-Si: H deposited but is fairly insensitive to the parameters of the etching process. The encapsulation process presented herein is a key step towards the fast large-scale annealing of Si-C films required for photovoltaic applications, and has the potential to greatly simplify the thermal treatment of a wide range of thin films.