Chemical and structural properties of polymorphous silicon thin films grown from dichlorosilane

Abstract

We have examined the effects of hydrogen dilution (RH) and deposition pressure on the morphological, structural and chemical properties of polymorphous silicon thin films (pm-Si:H), using dichlorosilane as silicon precursor in the plasma enhanced chemical vapor deposition (PECVD) process. The use of silicon chlorinated precursors enhances the crystallization process in as grown pm-Si:H samples, obtaining crystalline fractions from Raman spectra in the range of 65–95%. Atomic Force Microscopy results show the morphological differences obtained when the chlorine chemistry dominates the growth process and when the plasma–surface interactions become more prominent. Augmenting RH causes a considerable reduction in both roughness and topography, demonstrating an enhancement of ion bombardment and attack of the growing surface. X-ray Photoelectron Spectroscopy results show that, after ambient exposure, there is low concentration of oxygen inside the films grown at low RH, present in the form of SiO, which can be considered as structural defects. Instead, oxidation increases with deposition pressure and dilution, along with film porosity, generating a secondary SiOx phase. For higher pressure and dilution, the amount of chlorine incorporated to the film decreases congruently with HCl chlorine extraction processes involving atomic hydrogen interactions with the surface. In all cases, weak silicon hydride (SiH) bonds were not detected by infrared spectroscopy, while bonding configurations associated to the silicon nanocrystal surface were clearly observed. Since these films are generally used in photovoltaic devices, analyzing their chemical and structural properties such as oxygen incorporation to the films, along with chlorine and hydrogen, is fundamental in order to understand and optimize their electrical and optical properties.