Comparison of VHF, RF and Dc Plasma Excitation for a-Si:H Deposition with Hydrogen Dilution

Abstract

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.