INFLUENCE OF EXCITATION FREQUENCY, TEMPERATURE AND HYDROGEN DILUTION ON THE STABILITY OF PECVD-DEPOSITED a-Si:H

The first comparative study of DC, RF and VHF excitation for the plasma enhanced chemical vapor deposition (PECVD) of intrinsic layers of hydrogenated amorphous silicon (a-Si:H) is presented. The effects of hydrogen dilution on film stability are emphasized. Growth rates at comparable plasma power are presented for substrate temperatures between 100°C and 300°C and for various H2 dilution ratios. The optical bandgap, H content, and electronic transport properties in the light-soaked state were measured. H2 dilution strongly reduces the growth rate for all techniques. The growth rate for the highest H2 dilution ratio is higher for VHF (~4 Å/s) than for DC (~3 Å/s) or RF (0.5-1 Å/s) excitation. In all three cases increasing the substrate temperature reduces the optical gap and the H content CH. Raising the substrate temperature slightly enhances stability. H2 dilution increases the optical gap for all three techniques. The H content of RFand VHF-deposited samples increases with increasing H2 dilution ratio, while in DC deposition it produces an initial drop of the H content, followed by an increase.