FORMATION ENERGIES, ELECTRONIC-STRUCTURE, AND HYPERFINE FIELDS OF CHALCOGEN POINT-DEFECTS AND DEFECT PAIRS IN SILICON

The electronic structure of substitutional and interstitial S, Se, and Te point defects and of defect pairs is investigated by ab initio total-energy calculations. The results show that under normal conditions the chalcogen point defects are preferentially built in as substitutional atoms. However, interstitial S point defects cannot be excluded for n-type material. According to our calculations, the formation energy (without Jahn-Teller distortion) is 1 eV higher than for the substitutional S point defect. Stable pairs are formed in n-type material for nearest-neighbor substitutional S and Se atoms. For Te pairs the formation energy is always larger than that for two isolated point defects. Lattice relaxation (which is ignored in our calculation) could lower this value in order to allow for stable Te pairs. Pairs of interstitial chalcogen defects and also mixed pairs of interstitial and substitutional chalcogen would also be stable if there was a sufficient concentration of interstitials. We find that more distant pairs can also be stable for S and Se whereas for Te pairs they are unlikely. We calculate the matrix elements of the hyperfine interaction with the impurity nuclei and also the ligand hyperfine matrix elements with several Si ligands and for several different paramagnetic configurations of the point defects and the pairs. We find good agreement between the experimental data and our results for those configurations that have been found to be stable according to total-energy calculations. We also find that next-nearest-neighbor pairs can be stable and speculate if such pairs can be identified with the chalcogen X center.