LOCAL VIBRATIONAL-MODES AND ANHARMONIC FORCES OF MG2- IN ZNTE AND CDTE CRYSTALS( AND S2)

We present a simple model to describe local vibrational modes at impur ities in semiconductors using a two-particle anharmonic potential. We calculated the breathing-type distortion at the impurity in the tight- binding approximation, the frequencies of the local vibrational modes associated with the Mg2+ and S2- impurities in ZnTe and CdTe, their is otope shifts, and the anharmonic forces without any parameters adjuste d to defect properties. Our results for the energies of the local vibr ational modes and their isotope shifts at Mg and S impurities in ZnTe and CdTe are in good agreement with experiments. We found an inward di stortion of the ligands of Mg2+ in CdTe, an outward distortion in ZnTe , and an inward distortion at S2- in both crystals.