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 degrees C. Monohydride bonding is associated with low defect densities, and is usually observed only above 250 degrees C. Using rf (13.56 MHz) PECVD, we have deposited a-Si:H films at a substrate temperature of 35 degrees 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/cm(2) while light illumination). Annealing the films for 3-4 h at 150 degrees C, resulted in an improved photoconductivity with photo to dark conductivity ratio near 10(5). These films may be valuable for the fabrication of thin film electronics on novel substrates compatible with only low temperature processes. (C) 1997 American Vacuum Society.