Influence of gas phase chemistry on the properties of hydrogenated amorphous silicon and silicon–carbon alloys grown by HACVD

Abstract

We report on the properties of a-/ μc-Si:H and a-SiC:H grown by hydrogen assisted chemical vapour deposition (HACVD) of different gas mixtures depending on various deposition parameters. Silicon films were grown by using gas mixtures of H 2/SiH 4 and H 2/Si 2H 6, whereas silicon–carbon films were deposited using H 2/CH 4/SiH 4, H 2/C 2H 4/SiH 4, H 2/C 2H 4/Si 2H 6 and H 2/SiH 3CH 3. The composition and bulk properties of the deposited films are investigated by FT–IR and Raman, XPS, RBS, SEM, conductivity measurements and UV/VIS. As proved by Raman and conductivity measurements, μc-Si:H films are obtained using total pressures P tot smaller than 5.5 Torr. Above a certain total pressure P tot (6.5 Torr) a-Si:H films are obtained. As shown by SEM, the amorphous films exhibit a rather smooth surface morphology, whereas the μc films show a grain structure. In the pressure range from 5.5 to 6.5 Torr, a phase transition occurs. Carbon-rich a-SiC:H-films (up to X C=0.7) are obtained using C 2H 4 as carbon source, whereas the use of CH 4 leads to films with minor amounts of carbon. Films based on CH 3SiH 3 exhibit a nearly stoichiometric composition and a high degree of chemical order. The composition is influenced by several gas phase parameters, e.g., total pressure P tot, substrate temperature T s and the mole fraction of the reaction gases. A gas phase kinetic simulation has been carried out and the validation of the model is tested by comparison with experimental results of silicon film deposition. As a result, the pressure P calc, at which the concentration of disilene is a hundred fold higher than that of atomic hydrogen, and the characteristic pressure P * of the μc-/a-Si:H phase transition plotted as a function of Φ(SiH 4)/ Φ tot behave very similar. These results were used to extend the model to the deposition of silicon–carbon films in order to correlate and to discuss the material properties of the silicon–carbon system.