The effect of the atomic relaxation around defects on the electronic structure and optical properties of beta-SiC

The electronic structure and the optical properties associated with antisit e defects in cubic SiC have been computed by means of the LMTO (linear muff in-tin orbital) method and the supercell approach. The orbital-dependent LD A + U potential (LDA = local density approximation) used in the present wor k gives rise to an improved description both of the electronic structure ne ar the energy gap and of the optical functions. Attention has been mainly f ocused on the effects caused by the local lattice relaxation around the def ects. For compositions that deviate from the stoichiometric SiC towards hig her content of carbon atoms, the small reduction of the energy gap which is observed experimentally can be explained only if the lattice relaxation is taken into account. The local electronic structure of antisite defects is characterized by s- and p-like resonance states in the valence band. Strong resonances occur also in the conduction band (especially for Csi). The Sic (Csi) antisite has more (fewer) valence electrons localized in the atomic sphere than the official Si (C) atom,but this difference is considerably re duced by the lattice relaxation. The results of the calculations show how t he presence of point defects modifies the shape of the optical functions of the perfect SiC crystal and how the lattice relaxation has a strong effect on the fine structure of the optical functions. Different kinds of defect lead to different shapes of the optical functions.