Van der Waals interfaces in epitaxial vertical metal/2D/3D semiconductor heterojunctions of monolayer MoS2 and GaN

A promising approach for high speed and high power electronics is to integrate two-dimensional (2D) materials with conventional electronic components such as bulk (3D) semiconductors and metals. In this study we explore a basic integration step of inserting a single monolayer ${\text{MoS}}_2$ (${\text{1L-MoS}}_2$) inside a $\text{Au}/p$-GaN junction and elucidate how it impacts the structural and electrical properties of the junction. Epitaxial ${\text{1L-MoS}}_2$ in the form of 1-2 microm triangle domains are grown by powder vaporization on a $p$-doped GaN substrate, and the Au capping layer is deposited by evaporation. Transmission electron microscopy (TEM) of the van der Waals interface indicates that ${\text{1L-MoS}}_2$ remained distinct and intact between the Au and GaN and that the Au is epitaxial to GaN only when the ${\text{1L-MoS}}_2$ is present. Quantitative TEM analyses of the van der Waals interfaces are performed and yielded the atomic plane spacings in the heterojunction. Electrical characterization of the all-epitaxial, vertical $\text{Au}/{\text{1L-MoS}}_2$/$p$-GaN heterojunctions enables the derivations of Schottky barrier heights (SBH) and drawing of the band alignment diagram. Notably, ${\text{1L-MoS}}_2$ appears to be electronically semi-transparent, and thus can be considered as a modifier to the Au contact rather than an independent semiconductor component forming a $\mathrm{pn}$-junction. The $I-V$ analysis and our first principles calculation indicated Fermi level pinning and substantial band bending in GaN at the interface. Lastly, we illustrate how the depletion regions are formed in a bipolar junction with an ultrathin monolayer component using the calculated distribution of the charge density across the $\text{Au}/{\text{1L-MoS}}_2$/GaN junction.