Nonradiative Carrier Recombination Enhanced by Vacancy Defects in Ionic II-VI Semiconductors

Nonradiative-recombination-related defects are significant for optoelectronic semiconductor devices. Here, we analyze nonradiative-recombination processes in ionic semiconductors using first-principles density-functional theory. In ionic group II-VI semiconductors, we find that large lattice relaxations of anion vacancies caused by strong Coulomb interactions between different charged defect states can significantly enhance recombination processes through a two-level recombination mechanism. Specifically, we show that the defect level of the 2+ charged anion vacancy $\mathrm{(}{V}_{\mathrm{Se}}^{2+})$ in group II-VI $\mathrm{Zn}\mathrm{Se}$ is close to the conduction-band minimum and easily captures an electron to form a metastable 1+ charged state $\mathrm{(}{V}_{\mathrm{Se}}^{+})$; then, the large lattice relaxation, on account of the change in Coulomb interactions locally in the different charged states, rapidly changes this metastable state to a stable one and simultaneously move the defect level of ${V}_{\mathrm{Se}}^{+}$ closer to that valence-band maximum, and thus, increases the hole-capture rate. Compared with the Shockley-Read-Hall nonradiative-recombination theory based on a single defect level, this two-level recombination mechanism involving anion vacancies can greatly increase the nonradiative-recombination rate in ionic group II-VI semiconductors. This understanding is expected to be useful for the study of the nonradiative-recombination process in ionic semiconductors for applications in the field of optoelectronic devices.