A comparative study of DC, RF and VHF excitation for the plasma enhanced chemical vapor deposition (PECVD) of intrinsic layers of a-Si:H is presented, with special emphasis on the effects of hydrogen dilution. Growth rates at comparable plasma power, for substrate temperatures between 100°C and 300°C and for various H2 dilution ratios are presented, along with optical bandgap, H content, and electronic transport properties in the light-soaked state.
H2 strongly reduces the growth rate for all techniques. The growth rate for the highest H2 dilution ratio is approximately four times higher for VHF than for DC or RF excitation. In all three cases increasing the substrate temperature reduces the optical gap and the H content CH. Raising the substrate temperature slightly enhances the stability of undiluted films. H2 dilution increases the optical gap for all three techniques. The H content of RF- and 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.
In all three cases, H2 dilution improves the electronic transport properties of the material by roughly a factor of two. The gain in stability is most pronounced for relatively small dilution; in the case of DC deposition, too strong a dilution even has an adverse effect on stability.