III-nitride mainly GaN semiconductors are the most important materials for a wide range of applications, in particular high-power devices, due to the tunable direct bandgap, their chemical, and thermal stability. However, their growth on suitable substrates is still problematic, and low UV GaN efficiency hinders the efforts aimed at improving the performance of emitting devices. This dissertation presents novel growth and device fabrication methods capable of overcoming these issues using different novel strategies. The work reported in this dissertation comprises five parts. The first two parts demonstrate a new low-cost pulsed laser deposition (PLD)-based strategy for large-scale applications. This was developed to grow high-quality dislocation-free GaN NWs epitaxially on any bulk, flexible, or two-dimensional (2D) substrates without a catalyst, irrespective of the lattice mismatch or type of the substrate. As part of the work reported here, Si, p-GaN, Ga2O3, sapphire, graphene, MXene, and transition-metal dichalcogenide (TMD) substrates were utilized. Also, the adopted growth mechanisms are discussed, along with the advanced structural and optical characterizations. Advanced structural and optical characterizations further confirm the growth mechanism and demonstrate the superior optical and structural quality of GaN NWs. In the third part, a novel multiple quantum wells (MQWs)-based structure grown on the NWs is described, indicating that these NWs can be used as a template to grow III-nitride-based devices. In the fourth part of the work, the significance of these GaN NWs is further demonstrated by reporting on the fabrication of a high-performance self-powered broadband photodetector incorporating these NWs hybridized by two perovskite types: organic/inorganic as well as all-inorganic perovskites (CH3NH3PbI3 and CsPbBr3), revealing two different self-powered photodetector characteristics with high photo-responsivity at 0V. In the last part of this work, the focus is given to a new environmentally friendly strategy to enhance the device UV emission efficiency by functionalizing GaN NWs with solution-processed p-MnO quantum dots (QDs) characterized by much wider bandgap energy than that of GaN. The energy transfer mechanism from QDs to NWs is also discussed using different structural and optical characterizations. This novel strategy is based on drop-casting QDs on NWs, which is simple, cost-effective, and applicable for large-scale applications.