The deep-acceptor nature of the chalcogen vacancies in 2D transition-metal dichalcogenides

TL;DR

This study uses advanced hybrid density functional calculations to show that chalcogen vacancies in monolayer TMDs are deep acceptors, clarifying their role in electronic properties and resolving previous conflicting interpretations.

Contribution

The paper provides the first accurate reference of defect charge transition levels to the fundamental band gap in 2D TMDs, demonstrating that vacancies are deep acceptors and not responsible for conductivity.

Findings

Chalcogen vacancies are deep acceptors in monolayer TMDs.

Vacancies do not induce n-type or p-type conductivity.

Transition levels occur within the band gap, leading to paramagnetic states.

Abstract

Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional (2D) TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/$-1$) and ($-$1/$-$2) transition levels occur in the gap, leading to paramagnetic charge states S=1/2 and S=1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.