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

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 degrees C and for various H-2 dilution ratios. The optical band gap, H content, and electronic transport properties in the light-soaked state were measured. H-2 dilution strongly reduces the growth rate for all techniques. The growth rate for the highest H-2 dilution ratio is higher for VHF (similar to 4 Angstrom/s) than for dc (similar to 3 Angstrom/s) or rf (0.5-1 Angstrom/s) excitation. In all three cases, increasing the substrate temperature reduces the optical gap and the H content C-H. Raising the substrate temperature slightly enhances stability. H-2 dilution increases the optical gap for all three techniques. The H content of rf- and VHF-deposited samples increases with increasing H-2 dilution ratio, while in dc deposition it produces an initial drop of the H content, followed by an increase. In all three cases, H-2 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. (C) 1998 American Institute of Physics. [S0021-8979(98)06819-4].