Defect formation and carrier compensation in layered oxychalcogenide La2CdO2Se2: an insight from first principles

Abstract

Layered oxychalcogenide La₂CdO₂Se₂ is a candidate for an n-type transparent conductive material, but electrical insulating properties have hitherto been reported for both undoped and donor-doped La₂CdO₂Se₂. In this study, we investigate native defects and extrinsic dopants in La₂CdO₂Se₂ using first-principles calculations based on the Heyd–Scuseria–Ernzerhof hybrid functional approach. The calculated band structure and effective masses show two-dimensional characteristics, reflecting the layered structure. The energetics of the native defects indicates that negatively charged Cd vacancies and positively charged Cd interstitials are dominant at high and low Fermi level positions, respectively. The balance of these defects associated with Cd deficiency and excess leads the equilibrium Fermi level to a mid-gap position, which explains the experimentally reported nearly stoichiometric and electrical insulating behavior of undoped La₂CdO₂Se₂. Among various dopants selected from group-I to IV elements, Al at the Cd site forms a shallow donor level, and Sr at the La site forms a shallow acceptor level. However, the carriers generated by these dopants are severely compensated by the native defects charged with the opposite sign, consistent with the reported insulating properties of donor-doped La₂CdO₂Se₂. It is suggested that La₂CdO₂Se₂ is prone to form Cd vacancies and interstitials compared to zinc-blende CdSe, which has a similar local structure to the selenide layer in La₂CdO₂Se₂, partly because of a flexible structural relaxation related to its two-dimensional crystal structure. Such a detailed understanding of the defect properties in the non-Cu-based oxychalcogenide La₂CdO₂Se₂ is meaningful for further developing design principles for transparent conductive materials.