In recent years, large investments into the research of semiconducting two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) have elucidated interesting device related physical phenomena such as valleytronics [1], 2D superconductivity [2], 2D excitonic effects [3] and vertical tunneling [4]. TMDs offer layer-dependent chemical tunability of electronic and optoelectronic properties governed by interlayer van der Waals (vdW) forces [5]. Because of their layered nature, these low-dimensional materials can be combined to form multifunctional heterostructure materials exhibiting entirely new physical systems offering new degrees of flexibility in designing electronics, optoelectronics and other novel devices [6], [7]. In the last couple of years, the focus in the 2D materials research have shifted from exploration of proof-of-concept devices using mechanically exfoliated materials to more advanced device processing using high-quality large-scale growth based on advanced scalable vdW-epitaxy techniques such as powder vapor deposition (PVD) and chemical vapor deposition (CVD). Most recently, an improvement in the device performance by integrating TMD materials with the conventional 3D semiconductors has been reported and remains an area of active research [8-10]. The first attempt to design a TMD/3D heterostructure was reported by Yamada and coworkers nearly two decades ago [11], [12]. Although, in that effort, the TMD was used as a substrate for GaN growth via molecular beam epitaxy (MBE), and having the specific purpose for comparing resulting layer quality to that of GaN grown on Al2O3 substrates. The high-quality epitaxial growth of GaN on MoS2 reported in this study opened up the possibility of defect and dislocation free growth of GaN/MoS2 heterojunction. Since this first attempt to combine 2D and 3D material systems, there has been very limited effort in utilizing scalability and multi-functionality features of 2D materials and combining them with the wide-bandgap materials such as GaN, SiC, etc., especially for electronic device applications. However, Lee and coworkers recently reported a 2D/3D device fabricated using transferred MoS2 onto a GaN substrate yielding excellent rectification and current-voltage (IV) characteristics [8]. In addition, our work on lattice-matched epitaxial growth of MoS2 on GaN resulting in high-quality and unstrained single layer MoS2 that is laterally registered with GaN was reported in Ref 9. In addition, a comprehensive experimental and theoretical follow-up study reveals interesting findings on the quality of interface, vdW-spacing between layers, and the role of MoS2 layer in the MoS2/GaN heterojunctions. We believe this effort will contribute significantly to the fundamental understanding of 2D/3D heterojunctions [10]. One of the key findings of this study was the observation that the single-layer MoS2 growth on GaN substrates with different charge carrier polarities, though structurally distinct, was unable to provide required rectification behavior and current densities. Our conclusion is that the single-layer MoS2 is electronically transparent. Figure 1 illustrates the uniformity of MoS2/GaN growth (1a), quality of the interface (1b), line-scan illustrating the interfacial layers (1c), and transport characteristics (1d). Experimentally, the vertical stacking of 2D on 3D semiconductors requires a clear understanding of the vdW gap and vdW-force between the layers, since these parameters determine the interlayer tunneling between the layers. The knowledge of interlayer coupling strength is also crucial for accurate material and device characterization for in-plane as well as out-of-plane carrier transport. In addition, detailed understanding of structural as well as electronic properties of metal/2D/3D interface during device operations is also fundamental for the successful implementation of 2D/3D heterojunctions into functional devices. Similarly, theoretical/simulation studies of 2D/3D and metal/2D/3D heterojunctions to support the experimental observations demand a balance between scale, complexity, and accuracy of methods/models. Figure. 2 highlights some of the underlying complexities and challenges associated in theoretical modeling and characterizations of 2D/3D and metal/2D/3D heterojunctions. In this talk, I will review ongoing experimental and theoretical efforts with regard to mitigating these issues and challenges. I will also highlight research findings from ARL's recent efforts on TMD/3D growth and metal/TMD/3D characterization for possible defense applications.
