Dominant monohydride bonding in hydrogenated amorphous silicon thin films formed by plasma enhanced chemical vapor deposition at room temperature

Hydrogenated amorphous silicon (a-Si:H) prepared by plasma enhanced chemical vapor deposition (PECVD) with silane, typically shows significant dihydride bonding and high defect density when the substrate temperature is less than 200 °C. Monohydride bonding is associated with low defect densities, and is usually observed only above 250 °C. Using rf (13.56 MHz) PECVD, we have deposited a-Si:H films at a substrate temperature of 35 °C using silane diluted with helium, and found that films with dominant monohydride bonding can be deposited without significant substrate heating. A specific ion enhanced reaction mechanism that is consistent with the results is proposed. As deposited, the films with predominant monohydride bonding show low dark conductivity (10^-9 S/cm) and low photoconductivity (10^-7 S/cm under 100 mW/cm² while light illumination). Annealing the films for 3–4 h at 150 °C, resulted in an improved photoconductivity with photo to dark conductivity ratio near 10⁵. These films may be valuable for the fabrication of thin film electronics on novel substrates compatible with only low temperature processes. © 1997 American Vacuum Society.