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CRYSTALLIZATION PROCESSES IN AMORPHOUS HYDROGENATED SILICON BASED ALLOYS F.DEMICHELIS, C.F.PIRRI, E.TRESSO Dip. Fisica Politecnico - C.so Duca degli Abruzzi 24 10129 Torino (Italy). L.BATTEZZATI, E.GIAMELLO Dip. Chimica Inorganica, Chim. Fisica e Chim. Materiali Universita' di Torino - Via Pietro Giuria 7 10125 (Italy) P.MENNA ENEA, Centro Ricerche Fotovoltaiche - 80055 Portici (Italy) ABSTRACT Amorphous and microcrystalline films of silicon and silicon carbide have been deposited by means of PECVD at low substrate temperature (200°C), with reactive gases highly diluted in H2. Devices quality a-Si:H films have been annealed in vacuum at temperatures in the range 200- 1000 C. The transition from amorphous to crystalline structure was studied by X-ray, Raman and I.R. spectroscopies, optical analysis in UV-VIS-NIR region, Transmission Electron Microscopy, Differential Scanning Calorimetry and Electron Spin Resonance measurements. By comparing the results of the two methods to obtain microcrystalline films we have deduced information on the process of growth of Si and SiC microcrystals. INTRODUCTION In the last years amorphous silicon (a-Si:H) has been the subject of considerable research and development leading to a number of applications in photovoltaic solarcells, thin-film field-effect transistors for liquid crystals displays, logic circuits, electrophotography and so on [1]. Recently [2-5] amorphous and microcrystalline silicon carbon alloys (a-SiC:H and pic -SiC:H) have got interest for their applications, for instance the use of pc -SiC:H films as heterojunction window material. Nevertheless up today the processes of microcrystal growth and the phase transition from amorphous to microcrystalline state have not been completely studied. Using high H2 dilution, gtc -Si:H and tc -SiC:H can be grown at temperature as low as 200"C from SiH 4 and SiH 4+CH 4 by Plasma-Enhanced Chemical Vapor Deposition (PECVD). The microcrystallization process depends on the H2 dilution, on the power density and on the substrate temperature. On the other hand microcrystallization can be obtained by thermal treatments of amorphous films. We have realized amorphous and microcrystalline films of silicon and silicon carbide by PECVD. Furthermore amorphous films were annealed at high temperatures in order to obtain crystallization. The phase transition were studied by I.R. and Raman spectroscopies, Transmission Electron Microscopy (TEM), X-ray diffraction, Differential Scanning Calorimetry (DSC), Optical spectroscopy, Electron Spin Resonance (ESR) and D.C. conductivity. By comparing the results of the two methods for obtaining microcrystals we have deduced information on the transition from amorphous to crystalline structure. EXPERIMENTAL a-Si:H, pc -Si:H, a-SiC:H and ltc -SiC:H were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) of SiH 4 +H2 and SiH 4+CH 4+H 2 mixtures. The flow rates were for H, in the range 180-200 sccm, for SiH 4 3-13 sccm, and for CH4 1-4 sccm. The pressure during film growth was varied in the range 30-4