COMPOSITION AND THERMAL-ANNEALING-INDUCED SHORT-RANGE ORDERING CHANGES IN AMORPHOUS HYDROGENATED SILICON-CARBIDE FILMS AS INVESTIGATED BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE AND INFRARED-ABSORPTION

We have studied the effect of the film composition and the post-annealing treatment on both the first-shell local structure around the Si atoms and the bonding states of amorphous hydrogenated silicon carbide films (a-SixC1-x:H), prepared by the plasma-enhanced chemical-vapor-deposition technique. The local structure was characterized by measuring the extended x-ray-absorption fine structure at the Si K edge, whereas the Si-H, C-H, and Si-C bond densities were determined by using Fourier-transform infrared spectroscopy. The Si/C atomic ratio and the total hydrogen content were measured by means of the elastic-recoil-detection nuclear method. We have found that the Si-C and Si-Si bond lengths in the first coordination shell are, respectively, 1.88 and 2.35, and are independent of both the film composition and the annealing temperature. Taking into account the presence of Si-H and C-H hydrogenated bonds, we have obtained both qualitative and quantitative analyses of the short-range order changes, as a function of (i) the composition of a-SixC1-x:H (0.26≤x≤0.91) alloys, and (ii) the annealing temperature (300°C≤T≤850°C) of a-SiC:H films (x=0.5). The type of local disorder in the films was determined by calculating their corresponding short-range-order coefficients (ηSi-C0), by means of a theoretical model. We were thus able to show that, depending on the film composition, the short-range order is characterized either by a chemical preference for Si-C nearest-neighbor pairs (for 0.26≤x≤0.55) or by chemical clustering that favors the formation of Si-Si bonds in the local Si environments (for x0.77). On the other hand, we show that thermal annealings of a-SiC:H films cause partial dissociation of hydrogenated bonds (Si-H and C-H), which results in evacuation of hydrogen atoms and additional Si-C bond formation. These microstructural rearrangements are enhanced as the annealing temperature is increased beyond 650°C, and occur with a strong local chemical ordering that favors the formation of Si-C bonds. Concomitantly the stress of a-SiC:H films varies from highly compressive (-1 GPa) to highly tensile (+1 GPa), as the annealing temperature is increased from 300 to 850°C. Finally, we show that this stress variation of a-SiC:H films correlates well with the variations of their partial coordination numbers, their bond densities, and their degree of structural disorder. © 1995 The American Physical Society.