UC Riverside

The intrinsic properties of ultra-wide-bandgap semiconductors are exceptional. Diamond alone combines a bandgap of 5.47 eV, a theoretical breakdown field exceeding 10 MV/cm, and a thermal conductivity unmatched by any other semiconductor material. Whether these properties translate into working devices is determined not only by the bulk crystal but also by the atomic and electronic structure of its surface. This dissertation establishes, through first-principles calculations, how atomic-scale bonding at surfaces and interfaces governs the electronic properties that cannot be understood from the bulk structure alone. The diamond (100) surface reconstructs without an activation barrier, driving a metallic-to-semiconducting transition. In contrast, the diamond (111) surface admits a secondary pathway through a previously unidentified metastable intermediate, with the final reconstructed surface remaining semi-metallic. These reconstructed surfaces serve as the structural and electronic references for all subsequent termination studies. Surface-termination chemistry controls the electron affinity and, therefore, the viability of transfer doping. The identity, geometry, and coverage of H- and O-terminating species each modulate the electron affinity. On the (100) surface, oxidized group-IV terminations shift the electron affinity toward positive values and recover bulk-like bandgaps, enabling normally-off device operation with thermal stability that hydrogen termination cannot provide. On the (111) surface, however, oxidized group-IV terminations yield thermally robust surfaces but do not achieve complete structural or electronic passivation. For TMD/GaN heterostructures, thermodynamic analysis informed by electron microscopy reveals covalent interfacial bonding rather than purely van der Waals interfaces. The MoS₂/GaN heterostructure exhibits a type-I band alignment, whereas the NbS₂/GaN interface remains metallic. For h-BN growth on Ni(111), substitutional oxygen incorporation impedes rather than promotes monolayer synthesis by severely restricting adatom diffusion. In every system examined, the surface is where the physics is decided.