First-principles total-energy pseudopotential calculations were used to study the so-called negative-U large lattice relaxation model for column-III and VII donors. The calculated binding energies of DX centers for Al, Ga, In, Cl, Br, and I donor impurities in CdTe were in good agreement with experimental data. Three distinct types of DX-like structure, characterized by either bond rupture or bond compression, were found for column-VII donors. The relative stabilities of these structures were impurity- and pressure-dependent. Only Ga gave rise to a stable DX center in CdTe. Three types of DX center were suggested for column-VII donors. The DX structure became stable at high pressures while, in the alloys, DX1 or DX3 structures were more stable than DX2. It was noted that a new type of very low-energy lattice instability had been found which was very effective in acceptor compensation in II-VI semiconductors. It was found that a similar type of lattice instability could give rise to donor passivation; especially for column-VII impurities in II-VI semiconductors. The instability involved the rupture of 2 host bonds and the formation of a cation-cation dimer bond. The defect was a negative-U center and, in the case of halogen dopants, it provided a very effective compensation mechanism.

C.H.Park, D.J.Chadi: Physical Review B, 1995, 52[16], 11884-90