Electrical transport and structural characterization of epitaxial monolayer MoS$_{2}$ /n- and p-doped GaN vertical lattice-matched heterojunctions

We investigate vertical semiconductor junctions consisting of monolayer MoS$_{2}$ that is epitaxially grown on n- and p-doped GaN crystals. Such a junction represents a building block for 2D/3D vertical semiconductor heterostructures. Epitaxial, lattice-matched growth of MoS$_{2}$ on GaN is important to ensure high quality interfaces that are crucial for the efficient vertical transport. The MoS$_{2}$/GaN junctions were characterized with cross-sectional and planar scanning transmission electron microscopy (STEM), scanning tunneling microscopy, and atomic force microscopy. The MoS$_{2}$/GaN lattice mismatch is measured to be near 1{\%} using STEM. The electrical transport in the out-of-plane direction across the MoS$_{2}$/GaN junctions was measured using conductive atomic force microscopy and mechanical nano-probes inside a scanning electron microscope. Nano-disc metal contacts to MoS$_{2}$ were fabricated by e-beam lithography and evaporation. The current-voltage curves of the vertical MoS$_{2}$/GaN junctions exhibit rectification with opposite polarities for n-doped and p-doped GaN. The metal contact determines the general features of the current-voltage curves, and the MoS$_{2}$ monolayer modifies the electrical transport across the contact/GaN interface.