Electrical contact properties between Yb and few-layer WS$_2$

TL;DR

This study investigates the contact properties of ytterbium (Yb) electrodes on WS$_2$, revealing low contact resistance, tunneling injection mechanisms, and weak temperature and thickness dependence, advancing understanding of charge injection in 2D electronics.

Contribution

It provides new insights into Yb/WS$_2$ contact mechanisms, highlighting tunneling injection and weak dependence on temperature and thickness, which were not well understood before.

Findings

Yb/WS$_2$ interfaces show strong energy level pinning and low contact resistance.

Contact resistance exhibits weak temperature dependence up to 77 K.

Significant tunneling injection occurs, invalidating pure thermionic emission models.

Abstract

Charge injection mechanism from contact electrodes into two-dimensional (2D) dichalcogenides is an essential topic for exploiting electronics based on 2D channels, but remains not well understood. Here, low-work-function metal ytterbium (Yb) was employed as contacts for tungsten disulfide (WS$_2$) to understand the realistic injection mechanism. The contact properties in WS$_2$ with variable temperature (T) and channel thickness (tch) were synergetically characterized. It is found that the Yb/WS$_2$ interfaces exhibit a strong pinning effect between energy levels and a low contact resistance ($R_\rm{C}$) value down to $5\,k\Omega\cdot\mu$m. Cryogenic electrical measurements reveal that $R_\rm{C}$ exhibits weakly positive dependence on T till 77 K, as well as a weakly negative correlation with tch. In contrast to the non-negligible $R_\rm{C}$ values extracted, an unexpectedly low effective thermal injection barrier of 36 meV is estimated, indicating the presence of significant tunneling injection in subthreshold regime and the inapplicability of the pure thermionic emission model to estimate the height of injection barrier.