Generation of vacancy-interstitial pairs as a possible origin of resistivity switching and ferroelectric properties in Cd1-xZnxTe

Formation of vacancy-interstitial Frenkel pairs, together with the properties of interstitials and vacancies in CdTe, ZnTe, and their alloys were investigated by first-principles calculations. Generation of Frenkel pairs on the cation sublattice strongly depends on the Fermi energy: the presence of excess free electrons reduces the energy barrier for the pair generation from 2.5 eV in intrinsic samples to 1.2 eV. Moreover, E-F determines both the stability of Frenkel pairs with respect to recombination and their binding energy, which varies from similar to 0.2 to similar to 1 eV. A strong dependence on the Fermi energy, i.e., on the charge state, is also found for stable sites and barriers for diffusion of isolated interstitials. In particular, neutral interstitials have two (meta)stable sites, corresponding to two local minima of energy, and diffuse by jumps between them. Positively charged interstitials have only one stable site and diffuse by twice longer and curvilinear jumps. For the relevant charge states, the barriers for diffusion range from 0.5 to 1 eV, which imply a high mobility of interstitials. Most of these properties are traced back to the defect-induced deep gap levels, their occupation, and their dependence on the defect's site. The important role of the ionicity of the host is pointed out. On the other hand, generation of Frenkel pairs on the anion sublattice requires energy of about 5 eV, and thus is nonefficient. The obtained results suggest that formation of Frenkel pairs is a microscopic origin of two effects recently observed in Schottky junctions based on CdZnTe and other II-VI alloys, namely, the reversible changes in conductivity by a few orders of magnitude, and the ferroelectric-like behavior of polarization.