Theoretical investigation of random Si-C alloys

We have performed a first-principles investigation of the microscopic properties of random crystalline Si-C alloys. An ab initio tight-binding molecular-dynamics method is used to determine the microscopic atomic structure of the alloys. For small to moderate concentrations of C in Si, we find that the electronic structure shows a decrease of the band gap from that of pure Si. This result is unexpected since both ordered SiC and pure carbon (diamond) have much larger band gaps than Si. Plane-wave calculations were also done on ordered structures to further check this result and to determine the effeats of ordering. For the atomic structure, it is found that there are two different types of Si-A bonds. The first type is the Si-C bond near 1.7 Å as in bulk SiC, and the second type is a much shorter 1.65-Å bond for a carbon atom in a near-planar ${\mathit{sp}}^2$ configuration with its Si neighbors.