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 pressure and alloy dependences 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. Theoretical results for the pressure and alloy dependences in (Cd,Zn)Te alloys were in good agreement with experimental data. 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. On the basis of the calculations, Al, Br and I donors were suggested to be the best shallow donors in (Cd,Zn)Te alloys with respect to compensation by DX centers. 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