Photoinduced cation interstitial diffusion in II–VI semiconductors

The electronic structure, stability, and chemical trend of cation interstitials in II--VI semiconductors are studied through ab initio methods. We find that interstitials in the neutral charge state are more stable in the tetrahedral interstitial site near the cation (TC), whereas in the $(2+)$ charge state, they are more stable near the anion (TA). This bistability is explained through the Coulomb coupling and energy-level repulsion in the two different interstitial sites. We show that the diffusion energy barrier changes when the defect charge state changes. We suggest that if electrons or holes are taken from the defect level by light, changing its charge state, the interstitial atom will be able to diffuse almost spontaneously due to a reduced diffusion barrier. These results can help unravel the nature of the recently observed photoinduced migration of interstitials in II--VI semiconductors.