Mahesh Neupane1 DeCarlos Taylor1 Dmitry Ruzmetov1 Robert Burke1 3 Anthony Birdwell1 Andrew Herzing2 Terrance O'Regan1 Edward Byrd1 Tony Ivanov1

1, U.S. Army Research Laboratory, Adelphi, Maryland, United States
3, General Technical Services LLC, Wall, New Jersey, United States
2, National Institute of Standards and Technology, Gaithersburg, Maryland, United States

Two-dimensional (2D) van-der-Waals (vdW) materials such as graphene, transition metal dichalcogenides (TMDCs), and hexagonal boron-nitride (h-BN) have received an increasing level of interest due to their unique properties. The electronic and optical properties of these materials depend on the number of stacking layers and substrate type and are mainly governed by vdW interactions between layers. An efficient integration of 2D functional materials with three dimensional (3D) substrates remains a challenge and our recent study demonstrates successful growth and device characterization of a single layer (SL) of molybdenum disulfide (MoS2) on a GaN substrate through vdW epitaxy for vertical transistor applications [1]. In that work, we observed that the SL-MoS2 acts as an interfacial barrier layer and modulates (to a small extent) the threshold voltage and on-current during device operation, suggesting a need for multilayer (ML) MoS2 growth on GaN [2].
Motivated by these observations, in this study, we perform a systematic first-principles study of layer-dependent structural and electronic properties of MoS2 on a GaN substrate. Material properties such as layer binding energies, formation energies, in-plane lattice constants, and vdW spacing between the layers have been computed and compared to experimental data. Further, we also attempt to establish a correlation between the calculated and observed electronic properties such as band gap, band alignment, work function, and Schottky barrier height (SBH). Finally, a qualitative understanding of the required critical 2D thickness for an appreciable on-current in a 2D/3D system will be presented.

1. Ruzmetov, D.; Zhang, K.; Stan, G.; Kalanyan, B.; Bhimanapati, G. R.; Eichfeld, S. M.; Burke, R. A.; Shah, P. B.; O’Regan, T. P.; Crowne, F. J.; Birdwell, A. G.; Robinson, J. A.; Davydov, A. V.; Ivanov, T. G. Vertical 2D/3D Semiconductor Heterostructures Based on Epitaxial Molybdenum Disulfide and Gallium Nitride. ACS Nano 2016, 10, 3580–3588.
2. O’Regan, T. P. Ruzmetov, D.; Neupane, M. R., Burke, R. A., Herzing, A. A.; Zhang, K.; Birdwell, A. G.; Taylor, D. E.; Byrd, E. F. C.; Walck, S. D.; Davydov, A. V.; Robinson, J. A.; Ivanov, T. G. Structural and Electrical Analysis of Epitaxial 2D/3D Vertical Heterojunctions of Monolayer MoS2 on GaN. Appl. Phys. Lett. 2017, 111, 051602.