Resonant and Bound States of Charged Defects in Two-Dimensional Semiconductors

TL;DR

This paper investigates charged acceptor impurities in monolayer WS$_2$, revealing localized defect states that influence electron transport, with implications for ultrathin electronic device development.

Contribution

It combines experimental spectroscopy and large-scale calculations to identify and analyze bound and resonant defect states in monolayer WS$_2$, highlighting their origin and impact.

Findings

Identification of localized defect states near the valence band edge.

Discovery of resonant states resulting from multi-valley band structure.

Resonant states can trap carriers for tens of picoseconds.

Abstract

A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS$_2$ using a combination of scanning tunnelling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multi-valley valence band structure of WS$_2$, whereby localized states originating from the secondary valence band maximum at $\Gamma$ hybridize with continuum states from the primary valence band maximum at K/K$^{\prime}$. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds.