| Gallium nitride is a semiconducting material that emits light in the blue/green region of the visible spectrum. The development of optoelectronic devices that utilize this part of the spectrum will revolutionize many aspects of the lighting industry by providing efficient, long-lasting sources of light. Furthermore, by alloying gallium nitride with other elements such as indium and aluminum and creating emission across the whole visible spectrum, it is even possible to create white light sources. Although devices utilizing the alloys of gallium nitride are already being fabricated, there are still major issues concerning the materials performance that have yet to be fully understood---issues that are limiting the full realization of the technology. In particular, perhaps the most intriguing issue from a fundamental materials physics perspective, is that devices are known to function despite a density of threading dislocations that is high enough to inhibit emission in other similarly structured semiconducting materials.; The aim of this Ph.D. dissertation is to explore the atomic scale origins of the intriguing properties of dislocations in gallium nitride. The analysis of the three types of threading dislocations present in the active layers of gallium nitride based devices, edge, screw and mixed, has been performed utilizing state-of-the-art imaging and analytical techniques in the scanning transmission electron microscope. This dissertation discusses the structural properties of all the cores determined using the incoherent Z-contrast imaging method and includes the first experimental determination of the core structure of mixed dislocations (including the case where it dissociates into partials). The composition of the cores and their electronic properties has been determined experimentally using electron energy loss spectroscopy, which has provided the first direct evidence that the electronic properties of dislocation cores can be controlled by the presence of oxygen impurities. The interpretation of the electronic structure changes observed at the cores has been supported by detailed simulations using both the multiple scattering methodology and ab-initio density functional theory. The correlation between these computational methods and the atomic scale experimental analysis is explored with reference to the development of new levels of resolution in aberration corrected and monochromated electron microscopes. |
