Role of the gas temperature and power to gas flow ratio on powder and voids formation in films grown by 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 plasma enhanced chemical vapour deposition technique, b y monitoring the plasma impedance behaviour under different process conditi ons. The aim is to determine in which conditions the particles formed do no t deteriorate the performances of the films grown or even can lead to an im provement of the properties of the films deposited. The results achieved sh ow the existence of two main boundary regions (beta- and theta-regions) sep arating the so-called alpha-regime (no powder formed) from the gamma-regime (powder formed). Those regions are reached either by heating the gas, chan ging the gas pressure or using high power to gas flow ratios. In the beta-r egion the probability of incorporating nanoparticles in the films is low an d the films exhibit properties similar to those of the ones grown in the al pha-regime, with a low density of voids incorporated. In the theta-region s mall nanoparticles can be incorporated leading to films with density of sta tes below 5 x 10(15) cm(-3), widened Urbach energies and photosensitivities about two orders of magnitude larger than that of conventional amorphous s ilicon grown in the alpha-regime. (C) 2000 Elsevier Science Ltd. All rights reserved.