2D materials such as Transition metal dichalcogenides (TMDs), MoS2 and WS2, possess unique properties, namely tunable bandgap, novel optical and electronic characteristics, generating substantial interest as promising materials for transistors and photodetectors. 2D materials incorporated with 3D material can enhance device performance by the interfacial and epitaxial relationship between 2D materials and 3D materials such as Gallium Nitride (III–V semiconductors), which is almost perfectly lattice matched with MoS2. Although several studies have explored an understanding of the interface and device performance of 2D/3D material integration, we offer in-depth insight into the optimization of the interface and a unique growth process for integrating TMDs and GaN under a high vacuum system (~10-7 torr). Therefore, the aim of this dissertation research is to discuss an epitaxial growth mechanism and characteristics of heterostructures. The first part of the dissertation research aimed to optimize the in-situ interface preparation of GaN by Neon sputtering and ammonia annealing treatment before growing MoS2 in a high vacuum system. We achieved epitaxial growth of MoS2 on nitrogen-terminated GaN surface, resulting in 2 × 2 reconstruction acquired by low-energy electron diffraction (LEED). The MoS2/GaN diode exhibited a high on-off ratio of ~105 at 1 V and a low turn-on voltage (~0.3 V), resembling a Schottky diode. The second part of the project attempted optimization of in situ epitaxial growth of GaN on MoS2 without MoS2 removed. GaN growth was controlled by substrate temperature, Gallium evaporation rate and high voltage for filaments to activate ammonia. The structural quality of the GaN/MoS2 films was evaluated through X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. In the third part, bright photoluminescence (PL) in TMD heterostacks, alternating layers of MoS2 and WS2 on GaN, was explored. Layer by layer and 3R (Rhombohedral) type stacking TMD films were confirmed by Cross-sectional transmission electron microscopy (TEM). In electrical transport characteristics, The TMDs heterostacks show a Schottky-diode behavior with a high on-off ratio of up to 106 and the breakdown voltage under -100V. Furthermore, the comparison of wafer scale WS2/MoS2 vs MoS2/WS2 is investigated.