Ab initio survey of the electronic structure of tetrahedrally bonded phases of silicon

We present an ab initio study of the electronic structure of a number of high-pressure metastable phases of silicon with tetrahedral bonding. The phases studied include all experimentally determined phases that result from decompression from the metallic beta-Sn phase, namely, the BC8 (Si III), hexagonal diamond (Si IV), and R8 (Si XII) phases. In addition to these we have also studied the hypothetical ST12 structure that is found upon decompression from beta-Sn in germanium. We find that the local-density approximation incorrectly predicts the R8 phase to be semimetallic and that the quasiparticle spectrum exhibits a band gap. The effective masses found in R8 suggest that R8 may be useful for high-mobility applications. In addition, the ST12 phase is found to have a large density of electronic states at the band edge which could lead to interesting superconducting behavior.