AB-INITIO STUDY OF HYDROGENATION EFFECTS IN AMORPHOUS-SILICON CARBIDE

We propose a structural model of hydrogenated amorphous silicon carbid e (a-SiC:K) at stoichiometric composition and low H content, based on the results of ab initio molecular-dynamics simulations. As found for a-SiC, the system has a complex structure which cannot be described as a distorted crystalline phase. Nevertheless, the degree of chemical o rder is larger in the hydrogenated network than in the pure network. H binds to both C and Si. H bound to C gives rise to sp(3) bonded monoh ydrated sites, thus favoring the formation of a tetrahedral network. H bound to Si forms diverse bonding configurations where weak and stron g bonds can be identified. These results are in agreement with recent experiments. Furthermore, our findings for the evolution of Si-H and C -H bonding as a function of temperature are consistent with recent mea surements of H desorption from Si-C alloys. Finally, we suggest an int erpretation of x-ray and neutron-diffraction data based on analyses of several total distribution functions.