Absence of a Band Gap at Interface of a Metal and Highly Doped Monolayer $MoS_2$

TL;DR

This study uses ultra-high vacuum scanning tunneling microscopy to reveal that metal-induced gap states dominate the electronic properties at the interface of heavily doped monolayer MoS2, setting fundamental limits for electrical contact quality.

Contribution

It provides the first atomic-scale measurements of the energy band diagram at metal-doped MoS2 interfaces, revealing universal metal-induced gap states that influence contact behavior.

Findings

MIGS dominate the electronic properties at the interface.

Measured gap in local density of states extends up to 2 nm from the interface.

Decay lengths of MIGS are similar across different metals, indicating universality.

Abstract

High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as $MoS_2$ is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer $MoS_2$ using ultra-high vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by 2D metal induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the $MoS_2$ within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ~0.55 nm near mid gap to as long as 2 nm near the band edges and are nearly identical for Au, Pd and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.