Thermal oxidation and encapsulation of silicon–carbon nanolayers

Abstract

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 14nm 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 40nm 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.