First-principles study of neutral silicon interstitials in 3C-and 4H-SiC

The structures and stability of single silicon interstitials in their neutral state are investigated via first principles calculations in 3C- and 4H-SiC. By carefully checking the convergence with Brillouin zone (BZ) sampling and supercell size, we explain the disagreement between previous published results and show that the split interstitial along < 110 > direction and tetrahedrally carbon coordinated structure have competing formation energies in the cubic polytype. A new migration mechanism for the silicon interstitial in the neutral state is presented here, which could be important for the evolution of defect populations in SiC. For 4H-SiC, the most energetically favourable silicon interstitial is found to be the split interstitial configuration I-Sisp < 110 > but situated in the hexagonal layer. The defect formation energies in 4H-SiC are, in general, larger than those in 3C- SiC, implying that the insertion of silicon interstitial introduces a large lattice distortion to the local coordination environments and affects even the second- or third-nearest neighbours. We also present a comparison between well converged plane-waves calculations and calculations with three localised orbital basis sets; one of them, in spite of providing a reasonable description for bulk properties, is clearly not suitable to describe interstitial defects.