Nature of carrier injection in metal/2D semiconductor interface and its implications to the limits of contact resistance

TL;DR

This paper investigates the carrier injection mechanisms at metal/2D TMDC interfaces, revealing a multi-barrier process, deriving a modified Richardson equation, and establishing a theoretical limit for contact resistance, with experimental validation.

Contribution

It introduces a new understanding of carrier injection involving multiple barriers, proposes a novel SBH extraction method, and derives the Landauer limit for contact resistance in 2D devices.

Findings

Carrier injection occurs through two potential barriers influenced by metal-TMDC interaction.

The conventional Richardson equation is invalid for these multi-dimensional barriers.

A new transfer length method (TLM) accurately extracts Schottky barrier height.

Abstract

Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (Rc), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers - the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multi-dimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.