Unveiling the multi-level structure of midgap states in Sb-doped MoX$_2$ (X = S, Se, Te) monolayers

TL;DR

This study uses DFT calculations to analyze the electronic and structural effects of Sb doping in MoX2 monolayers, revealing impurity states, band gap modifications, and p-type behavior, with implications for optoelectronic applications.

Contribution

It provides a detailed theoretical analysis of Sb doping effects in MoX2 monolayers, highlighting the multi-level midgap states and their impact on electronic properties, which was not previously comprehensively studied.

Findings

Impurity states span the entire band gap.

Sb doping induces p-type behavior.

Band gap increases with doping.

Abstract

In this study, we use DFT calculations to investigate the electronic and structural properties of MoX$_2$ (X = S, Se, Te) monolayers doped with substitutional Sb atoms, with a central focus on the Sb(Mo) substitution. In MoS$_2$, we observe that this substitution is energetically favored under S rich conditions, where the S$_2$ gaseous phase is likely to be present. This result is compatible with a recent experimental observation in Sb-doped MoS$_2$ nanosheets grown by CVD. A similar behavior is found in MoSe$_2$, but in MoTe$_2$ the Sb(Mo) substitution is less likely to occur due to the possible absence of gaseous Te phases in experimental setups. In all cases, several impurity-induced states are found inside the band gap, with energies that span the entire gap. The Fermi energy is pinned a few tenths of eV above the top of the valence band, suggesting a predominant $p$-type behavior. The orbital nature of these states is investigated with projected and local density of states calculations, which reveal similarities to defect states induced by single Mo vacancies as well as their rehybridization with the $5s$ orbital from Sb. Additionally, we find that the band gap of the doped systems is increased in comparison with the pristine materials, in contrast with a previous calculation in Sb-doped MoS$_2$ that predicts a gap reduction with a different assignment of valence band and impurity levels. We discuss the similarities, discrepancies, and the limitations of both calculations. We also speculate possible reasons for the experimentally observed redshifts of the A and B excitons in the presence of the Sb dopants in MoS$_2$. We hope that these results spark future investigations on other aspects of the problem, particularly those concerning the effects of disorder and electron-hole interaction, and continue to reveal the potential of doped TMDCs for applications in optoelectronic devices.