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 r atio of r.f. power to gas flow on the particle's formation in amorphous sil icon films grown by the plasma enhanced chemical vapour deposition techniqu e, by monitoring the plasma impedance behaviour under different process con ditions. The results achieved show the existence of two main boundary regio ns separating the so-called alpha-regime (no powder formed) from the gamma- regime (powder formed). Those regions are reached either by heating the gas , changing the gas pressure or using high power to gas flow ratios, corresp onding to the establishment of a balance between the plasma resistance and the plasma reactance. Tn the beta-region the probability to incorporate nan oparticles in the films is low and the films exhibit photosensitivity's of about 10(5) with density of states determined by the constant photocurrent method below 6x10(15) cm(-3) with Urbach energies below 50 meV. In the thet a-region small nanoparticles can be incorporated leading to films with dens ity of states below 3 x 10(15) cm(-3), with Urbach energies above 50 meV an d photosensitivity's above 10(6), about two orders of magnitude larger than that of conventional amorphous silicon grown in the alpha-regime. (C) 2000 Elsevier Science S.A. All rights reserved.