Study on defects related to local bonding of oxygen in hydrogenated silicon oxide films

In this study, intrinsic a-SiO:H films were prepared by a conventional radio frequency (13.56 MHz) plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of SiH₄, H₂ and CO₂. Changes in the optical, electrical and structural properties of the a-SiO:H films were investigated systematically by controlling the deposition parameters, mainly the gas ratio of CO₂ to SiH₄, and hydrogen dilution concentration. By introducing the CO₂ gas, three kinds of O-related IR features are found at 780, 980 and 2090 cm^-1. With increasing the CO₂/SiH₄ ratio, the absorption at 780 cm^-1 strongly coupled to the Si-H and Si-O-Si motions increases noticeably, indicating that the defect density in the films increases because the mode at 780 cm^-1 is unique signature of a particularly local geometry related to defects. In order to diminish the defect density level in the films, hydrogen dilution was performed. As the H dilution concentration increases, the defect density is reduced significantly and the photo-conductivity is improved to 10^-4 S/cm. Influences of oxygen incorporation into the Si network and H dilution on the performance of a-SiO:H solar cells are also examined in detail.