Identification and tackling of a parasitic surface compound in SiC and Si-rich carbide films

Silicon carbide and silicon rich carbide (SiC and SRC) thin films were prepared by PECVD and annealed at 1100 degrees C. Such a treatment, when applied to SiC/SRC multilayers, aimed at the formation of silicon nanocrystals, that have attracted considerable attention as tunable band-gap materials for photovoltaic applications. Optical and structural techniques (X-ray photoelectron spectroscopy, Reflectance and Transmittance, Fourier Transformed Infrared Spectroscopy) were used to evidence the formation, during the annealing treatment in nominally inert atmosphere, of a parasitic ternary SiOxCy surface compound, that consumed part of the originally deposited material and behaved as a preferential conductive path with respect to the nanocrystal layer in horizontal electrical conductivity measurements. The SiOxCy compound was HF-resistant, with composition dependent on the underlying matrix. It gave rise to a Si-O related vibration in FTIR analysis, that may be misinterpreted as due to silicon oxide. The compound, if neglected, can affect the structural and electrical characterization of the material. To overcome this problem, a procedure is analyzed, based on the deposition of a sacrificial capping a-Si:H layer that partially oxidizes, and is removed by tetra methyl ammonium hydroxide (TMAH) after annealing. XPS analysis revealed that the resulting surface is mainly made up of SiC regardless of the composition of the underlying SRC layer. Subsequent SF6:O-2 dry etching results in a porous SiC-rich surface layer. The proposed method is effective in controlling the SRC surface configuration, and allows the performance of reliable optical and electrical characterization.