Overcoordinated Hydrogens in the Carbon Vacancy: Donor Centers of SiC

Epitaxial silicon carbide is likely to contain hydrogen and vacancies ( $V$); therefore, $V+n\mathrm{H}$ complexes are likely to influence its electronic properties. Using ab initio calculations we show that neutral and positive H atoms are trapped by carbon vacancies ( $V_{\mathrm{C}}$) in three-center bonds with two Si neighbors. The double positive charge state of $V_{\mathrm{C}}+\mathrm{H}$ is not stable in equilibrium and in the triply positive state H binds only to one of the Si neighbors. At most two H atoms can be accommodated by a single $V_{\mathrm{C}}$. The $V_{\mathrm{C}}+n\mathrm{H}$ complexes have donor character and exhibit rather atypical vibration modes for Si-H bonds. Occupation levels and spin distributions were calculated and compared for $V_{\mathrm{C}}+\mathrm{H}$ and $V_{\mathrm{C}}$.