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.

Abstract

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-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$:H), prepared by the plasma-enhanced chemical-vapor-deposition technique. The local structure was characterized by measuring the extended x-ray-absorption fine structure at the Si K edge, whereas the Si-H, C-H, and Si-C bond densities were determined by using Fourier-transform infrared spectroscopy. The Si/C atomic ratio and the total hydrogen content were measured by means of the elastic-recoil-detection nuclear method. We have found that the Si-C and Si-Si bond lengths in the first coordination shell are, respectively, 1.88 and 2.35 \AA{}, and are independent of both the film composition and the annealing temperature. Taking into account the presence of Si-H and C-H hydrogenated bonds, we have obtained both qualitative and quantitative analyses of the short-range order changes, as a function of (i) the composition of a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$:H (0.26\ensuremath{\le}x\ensuremath{\le}0.91) alloys, and (ii) the annealing temperature (300 \ifmmode^\circ\else\textdegree\fi{}C\ensuremath{\le}T\ensuremath{\le}850 \ifmmode^\circ\else\textdegree\fi{}C) of a-SiC:H films (x=0.5). The type of local disorder in the films was determined by calculating their corresponding short-range-order coefficients (${\mathrm{\ensuremath{\eta}}}_{\mathrm{Si}\mathrm{\ensuremath{-}}\mathrm{C}}^{0}$), 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\ensuremath{\le}x\ensuremath{\le}0.55) or by chemical clustering that favors the formation of Si-Si bonds in the local Si environments (for x\ensuremath{\ge}0.77). 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 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}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.