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

摘要

High quality electrical contact to semiconducting transition metaldichalcogenides (TMDCs) such as $MoS_2$ is key to unlocking their uniqueelectronic and optoelectronic properties for fundamental research and deviceapplications. Despite extensive experimental and theoretical efforts reliableohmic contact to doped TMDCs remains elusive and would benefit from a betterunderstanding of the underlying physics of the metal-TMDC interface. Here wepresent measurements of the atomic-scale energy band diagram of junctionsbetween various metals and heavily doped monolayer $MoS_2$ using ultra-highvacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal thatthe electronic properties of these junctions are dominated by 2D metal inducedgap states (MIGS). These MIGS are characterized by a spatially growing measuredgap in the local density of states (L-DOS) of the $MoS_2$ within 2 nm of themetal-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 nearlyidentical for Au, Pd and graphite contacts, indicating that it is a universalproperty of the monolayer semiconductor. Our findings indicate that even inheavily doped semiconductors, the presence of MIGS sets the ultimate limit forelectrical contact.