Defect Tolerant Monolayer Transition Metal Dichalcogenides

TL;DR

This study systematically investigates defect tolerance in 29 monolayer transition metal dichalcogenides, identifying which are prone to deep gap states due to vacancies and proposing a descriptor for defect sensitivity.

Contribution

It introduces a first principles approach to classify TMDs based on defect tolerance and proposes a new descriptor correlating band orbital similarity with defect sensitivity.

Findings

Group VI and X TMDs form deep gap states with chalcogen vacancies.

Group IV TMDs are predicted to be defect tolerant with shallow defect levels.

Defect sensitive TMDs have valence and conduction bands with similar orbital composition.

Abstract

Localized electronic states formed inside the band gap of a semiconductor due to crystal defects can be detrimental to the material's optoelectronic properties. Semiconductors with lower tendency to form defect induced deep gap states are termed defect tolerant. Here we provide a systematic first principles investigation of defect tolerance in 29 monolayer transition metal dichalcogenides (TMDs) of interest for nanoscale optoelectronics. We find that the TMDs based on group VI and X metals form deep gap states upon creation of a chalcogen (S, Se, Te) vacancy while the TMDs based on group IV metals form only shallow defect levels and are thus predicted to be defect tolerant. Interestingly, all the defect sensitive TMDs have valence and conduction bands with very similar orbital composition. This indicates a bonding/anti-bonding nature of the gap which in turn suggests that dangling bonds will fall inside the gap. These ideas are made quantitative by introducing a descriptor that measures the degree of similarity of the conduction and valence band manifolds. Finally, the study is generalized to non-polar nanoribbons of the TMDs where we find that only the defect sensitive materials form edge states within the band gap.