First Principles Calculations for Point Defects in 2H-, 3C- and 4H-SiC Polytypes.

摘要

The electronic and ionic structure of primary defects in silicon carbide (SiC) is studied using ab-initio plane-wave pseudopotential methods. The studied effects include the carbon and silicon vacancy, the silicon and carbon antisite, the antistructure-pair and the nearest-neighbor divacancy. The defects are studied in all their stable charge states in a variety of SiC crystals (in 3C-, 4H- and 2H-polytypes). The aim of this work is to numerically calculate the electronic structure of the named defect centers, to evaluate the defect formation energy and the relaxed ionic structure around the defect accurately using density-functional theory and efficient computational tools. The physical, electrical, structural and optical properties can be derived from the results of the electronic structure calculation. In this work, such results are the ionization levels, negative-U transitions, the symmetry point group of the relaxed atoms around the defect, the total spin, the positron lifetime and corresponding high momentum part of Doppler broadening of the annihilation radiation of the defect center. All these quantities are compared and discussed in context with experimental observations.