Surface States Engineering of Metal/MoS2 Contacts Using Sulfur Treatment for Reduced Contact Resistance and Variability

Abstract

Variability and lack of control in the nature of contacts between metal/MoS2 interface is a major bottleneck in the realisation of high-performance devices based on layered materials for several applications. In this letter, we report on the reduction in Schottky barrier height at metal/MoS2 interface by engineering the surface states through sulphur treatment. Electrical characteristics for back-gated MoS2 field effect transistor structures were investigated for two high work-function metal contacts Ni and Pd. Contacts on MoS2 treated with sulphur exhibited significant improvements in Ohmic nature with concomitant reduction in variability compared to those on untreated MoS2 films leading to a 2x increase in extracted mobility. X-ray Photoelectron Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold voltages indicated absence of additional doping or structural changes due to sulphur treatment. The Schottky barrier heights were extracted from temperature-dependent transfer characteristics based on the thermionic current model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2 and Pd/MoS2 contacts respectively is hence attributed to the increase in surface states (or stronger Fermi level pinning) due to sulphur treatment. The corresponding charge neutrality levels at metal/MoS2 interface, were extracted to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur treatment. This first report of surface states engineering in MoS2 leading to superior contacts is expected to significantly benefit the entire class of devices based on layered 2D materials.