| Density functional theory (DFT) investigations of defects in three materials systems have enabled an enhanced understanding of how the defects influence the final properties of materials. Point defects, surfaces, and the heterogeneous interface between dissimilar materials are all discussed. The formation energies of point defects in bulk AlN are used to show that the source of the unwanted deep ultraviolet absorption is carbon incorporation during growth. The carbon in these samples is also shown to form a donor-acceptor pair with nitrogen vacancies, leading to a 2.8 eV optical emission. The co-doping of these high-carbon AlN samples with Si and O donors is shown to render the samples optically transparent because the donor level acts as a trapping center for electrons and leads to a long-lived state. Finally, the challenges of n-type doping in AlN are discussed based on the activation energies of both the Si and O donors are discussed in terms of the DX-type transition between a shallow donor state and a deep acceptor state. To study the stable surfaces of polar rocksalt oxides, DFT calculations are extended to experimentally relevant temperatures and pressures using ab initio thermodynamics. This technique allowed the prediction of an experimental growth window where polar surfaces could be stabilized using a hydrogen surfactant which acted as a polarity compensator. The heterogeneous interface between wurtzite GaN and rocksalt MgCaO was examined based on the site preference during the initial stages of epitaxial oxide growth in order to explain the strong observed preference for MCO to have only one rotational configuration on GaN. |
