Gate-modulated conductance of few-layer WSe_2 field-effect transistors in the subgap regime: Schottky barrier transistor and subgap impurity states

TL;DR

This study investigates the gate-dependent conductance of few-layer WSe2 transistors, revealing Schottky barrier behavior and impurity states that influence contact and transport properties, providing insights for improving TMDC device quality.

Contribution

It establishes a connection between impurity states and contact behavior in WSe2 transistors, highlighting the role of lattice defects and thermionic field emission in subgap conduction.

Findings

Thermionic field emission dominates contact transmission at room temperature.

Impurity states density is approximately 1-2x10^13 /cm^2/eV.

Lattice defects are likely the primary source of impurity states.

Abstract

Two key subjects stand out in the pursuit of semiconductor research: material quality and contact technology. The fledging field of atomically thin transition metal dichalcogenides (TMDCs) faces a number of challenges in both efforts. This work attempts to establish a connection between the two by examining the gate-dependent conductance of few-layer (1-5L) WSe2 field effect devices. Measurements and modeling of the subgap regime reveal Schottky barrier transistor behavior. We show that transmission through the contact barrier is dominated by thermionic field emission (TFE) at room temperature, despite the lack of intentional doping. The TFE process arises due to a large number of subgap impurity states, the presence of which also leads to high mobility edge carrier densities. The density of states of such impurity states is self-consistently determined to be approximately 1-2x10^13 /cm^2/eV in our devices. We demonstrate that substrate is unlikely to be a major source of the impurity states and suspect that lattice defects within the material itself are primarily responsible. Our experiments provide key information to advance the quality and understanding of TMDC materials and electrical devices.