Ohmic contacts to 2D semiconductors through van der Waals bonding

TL;DR

This paper demonstrates that van der Waals-bonded buffer layers, including metallic monolayers like NbS2, can create low-resistance, barrierless contacts to 2D MX2 semiconductors by removing interface states and tuning Schottky barriers.

Contribution

It introduces a novel approach using van der Waals-bonded buffer layers to achieve ohmic contacts with 2D semiconductors, overcoming previous interface state issues.

Findings

Van der Waals buffer layers remove interface states.

High work function metallic monolayers yield barrierless contacts.

Possible buffer layers include graphene, h-BN, MoO3, and NbS2.

Abstract

High contact resistances have blocked the progress of devices based on MX2 (M = Mo,W; X = S,Se,Te) 2D semiconductors. Interface states formed at MX2/metal contacts pin the Fermi level, leading to sizable Schottky barriers for p-type contacts in particular. We show that (i) one can remove the interface states by covering the metal by a 2D layer, which is van der Waals-bonded to the MX2 layer, and (ii) one can choose the buffer layer such, that it yields a p-type contact with a zero Schottky barrier height. We identify possible buffer layers such as graphene, a monolayer of h-BN, or an oxide layer with a high electron affinity, such as MoO3. The most elegant solution is a metallic M'X'2 layer with a high work function. A NbS2 monolayer adsorbed on a metal yields a high work function contact, irrespective of the metal, which gives a barrierless contact to all MX2 layers.