Structure and properties of amorphous hydrogenated silicon carbide

Abstract

We have studied a compositionally varying series of amorphous hydrogenated silicon carbide (a-SiC:H) thin films deposited from silane-methane-hydrogen plasmas. The carbon-bonding environments were examined by $^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) while the hydrogen micro- structure was characterized by multiple-quantum nuclear magnetic resonance as well as conventional single-quantum solid-state NMR techniques. In addition, Fourier-transform infrared absorption, electron-spin resonance, Rutherford backscattering, and optical-absorption spectra were obtained. The $^{13}\mathrm{C}$ nuclear magnetic resonance spectra showed that all films contain both ${\mathrm{sp}}^{3}$ (tetrahedral) and ${\mathrm{sp}}^{2}$ (planar) carbon-bonding environments. The ${\mathrm{sp}}^{3}$ carbon is predominantly hydrogenated while the ${\mathrm{sp}}^{2}$ carbon has no bound hydrogen. Two distinct types of microstructure were found within the compositional series. At low carbon contents (20 at. %), this alloy system forms ``amorphous-silicon-like'' lattices, containing clusters of 6\ifmmode\pm\else\textpm\fi{}1 hydrogen atoms and regions devoid of hydrogen, due to inclusions of nonhydrogenated ${\mathrm{sp}}^{2}$ carbon. Higher-carbon-content samples also consisted of hydrogenated and nonhydrogenated regions, but did not have the well-defined hydrogen microstructure of the lower-carbon-content films. Also, there is a greater fraction of ${\mathrm{sp}}^{3}$ carbon in the higher-carbon-content films. The transition between the two types of hydrogen microstructure was clearly reflected in the deposition rates, defect density, disorder, and optical band gaps of the alloy. Thus, the heterogeneous nature of the carbon-atom-bonding configurations and hydrogen microstructure affect the optoelectronic properties of a-SiC:H.