Role of the gas temperature and power to gas flow ratio on powder formation and properties of films grown by the PECVD technique

This paper deals with the study of the role of gas temperature and of the ratio of r.f. power to gas flow on the particle’s formation in amorphous silicon films grown by the plasma enhanced chemical vapour deposition technique, by monitoring the plasma impedance behaviour under different process conditions. The results achieved show the existence of two main boundary regions separating the so-called α-regime (no powder formed) from the γ-regime (powder formed). Those regions are reached either by heating the gas, changing the gas pressure or using high power to gas flow ratios, corresponding to the establishment of a balance between the plasma resistance and the plasma reactance. In the β-region the probability to incorporate nanoparticles in the films is low and the films exhibit photosensitivity’s of about 105 with density of states determined by the constant photocurrent method below 6×1015 cm−3 with Urbach energies below 50 meV. In the θ-region small nanoparticles can be incorporated leading to films with density of states below 3×1015 cm−3, with Urbach energies above 50 meV and photosensitivity’s above 106, about two orders of magnitude larger than that of conventional amorphous silicon grown in the α-regime.