Atomic resolution interface structure and vertical current injection in highly uniform MoS2 heterojunctions with bulk GaN

The integration of two-dimensional MoS2 with GaN recently attracted significant interest for future electronic/optoelectronic applications. However, the reported studies have been mainly carried out using heteroepitaxial GaN templates on sapphire substrates, whereas the growth of MoS2 on low-dislocation-density bulk GaN can be strategic for the realization of “truly” vertical devices. In this paper, we report the growth of ultrathin MoS2 films, mostly composed by single-layers (1L), onto homoepitaxial n−-GaN on n+ bulk substrates by sulfurization of a pre-deposited MoOx film. Highly uniform and conformal coverage of the GaN surface was demonstrated by atomic force microscopy, while very low tensile strain (∼0.05%) and a significant p+-type doping (∼4.5 × 1012 cm−2) of 1L-MoS2 was evaluated by Raman mapping. Atomic resolution structural and compositional analyses by aberration-corrected electron microscopy revealed a nearly-ideal van der Waals interface between MoS2 and the Ga-terminated GaN crystal, where only the topmost Ga atoms are affected by oxidation. Furthermore, the relevant lattice parameters of the MoS2/GaN heterojunction, such as the van der Waals gap, were measured with high precision. Finally, the vertical current injection across this 2D/3D heterojunction has been investigated by nanoscale current-voltage analyses performed by conductive atomic force microscopy, showing a rectifying behavior with an average turn-on voltage Von = 1.7 V under forward bias, consistent with the expected band alignment at the interface between p+ doped 1L-MoS2 and n-GaN.