Influence of excitation frequency, temperature, and hydrogen dilution on the stability of plasma enhanced chemical vapor deposited a-Si:H

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The first comparative study of dc, rf, and very high frequency (VHF) excitation for the plasma enhanced chemical vapor deposition 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 and 300 °C and for various H2 dilution ratios. The optical band gap, 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 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 2. 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. We rule out a universal relation between hydrogen content of the a-Si:H films and their stability against light soaking. Enhanced stability may be obtained for films which incorporate either more or less hydrogen than standard a-Si:H.