Abstract
We have performed a comprehensive theoretical study on the structural change of a tetrahedral (Td) four-site system, which is the most conceivable simple model for various point defects in covalent semiconductors, for all possible electron (hole) occupancies. The system is characterized by the electron transfer energy T, on-site Coulomb repulsion energy U and the lattice relaxation energy S. Possible stable structural configurations and their stabilities are determined exactly for the electronic ground and excited states. The symmetry of the stable structures are found to be Td, C3v, C2v, Cs or C depending on the electron number n (=0-- 8), the electron spin Sσ and the relative strengths of T, U and S. The minimum of the ground state energy of the lowest spin Sσ state is lower than those of higher spin states except for n=5 and 6. The obtained results are applied to discuss the structural change of point defects in group IV and III-V semiconductors. Because of the absence of the electron-hole symmetry, the off-center instability of a substitutional deep-impurity is more favorable for acceptor than for donor.
