Post Oxidation Effects of High Rate Microcrystalline Silicon Grown by PECVD for Solar Cell Applications

In this paper we investigate post oxidation phenomena in high rate microcrystalline silicon grown by means of PECVD for solar cell applications. Microcrystalline silicon thin films were deposited by PECVD with rates higher than 1.5 nm/s and up to 2.4 nm/s on silicon wafers and glass. The RF power of the PECVD process varied from 300 W to 1000 W in order to provide layers within the amorphous to microcrystalline transition. The resulting films exhibited a maximum crystallinity of 60 % when grown at high deposition rate of 2.4 nm/s. Fourier Transform Infrared (FTIR) spectroscopy show that the oxidation occurs in ambient conditions at the crystalline phase as well as in the amorphous phase depending on the deposition rate, thus in the possible existence of porosity within the bulk of the film. Moreover, the post-oxidation is time-dependent and affects the electrical properties of the film. More precisely, it was found that the photoconductivity to dark conductivity ratio as well as the activation energy are affected by the post-oxidation mechanisms.