Microscopic origin of metal–insulator transition in two-dimensional Bi2O2Se unraveled by first-principles simulations

Abstract

It has been experimentally observed that Bi2O2Se—a semiconductor with nominally metallic conductivity—can undergo a metal–insulator transition (MIT), yet its microscopic origin remains unclear. Our hybrid density functional theory study uncovers the mechanism behind this transition. Under O- and Se-poor growth conditions, donor defects VO+ and VSe+ form at the highest concentrations, pushing the Fermi level above the conduction band minimum (CBM) and inducing metallic behavior in Bi2O2Se. As the chemical potentials of O and Se increase to moderate levels, the concentrations of VO+ and VSe+ drop, shifting the Fermi level down to the CBM and triggering the MIT. Further enrichment in O and Se yields only a weakly insulating phase, because the densities of VO2+ and VSe2+ rise unexpectedly under these rich conditions, preventing the emergence of a highly insulating phase. This counterintuitive trend is explained by defect-correlation mechanism.