Nanoscale GaN Epitaxy and Polytype Selection in Liquid-metal Mediated Environments and Writing-to-Learn in Materials Science and Engineering

Abstract

Most low dimensional materials possess properties different from their bulk counterparts and can be engineered to exhibit a wide range of physical properties for the fabrication of electronic, optoelectronic, and quantum devices. In the case of epitaxial nitride nanostructures, the role of liquid-metal-mediated environments on the polytype selection and layer dimensionality (i.e., films, NWs, and QDs) remains controversial. To elucidate the role of group-III metals (i.e., In and Ga) on nanostructure formation, we utilize advanced experimental and computational tools to monitor fundamental processes across growth conditions. In particular, droplet epitaxy of GaN and InN QDs, and the formation mechanism and crystal structure (polytype) selections of GaN nanowires on Si(001) were investigated in the thesis.

First, the formation mechanisms of GaN quantum dots (QDs) via annealing of Ga droplets in a nitrogen flux using a combined computational-experimental approach are presented. Temperature- and substrate-dependence of the size distributions of droplets and QDs, as well as the relative roles of Ga/N diffusivity and GaN nucleation rates on QD formation are considered. The relative roles of nucleation- and coarsening-dominant growth, as well as zincblende vs wurtzite polytype selection, on various substrates are discussed. These insights provide an opportunity for tailoring QD size distributions and polytype selection for a wide range of III-N semiconductor QDs.

Next, the formation of InN QDs during nitridation of In droplets was examined using ETEM and HRTEM. As the substrate temperature increases, the nanoparticle (NP) sizes decrease, while their density increases; meanwhile moiré pattern formation suggests the formation of coherently strained InN/In2O3 interfaces. At the lowest nitridation temperatures, the smallest NPs primarily exhibited 2D moiré fringes (MF) suggesting ZB InN formation. At the highest nitridation temperatures, larger NPs primarily exhibited 1D MF. We hypothesize that there is a higher probability of ZB nucleation from smaller particles and WZ nucleation from larger particles, with unintentional In2O3 formation on InN.

Furthermore, the influence of Ga surface saturation on GaN nanowire (NW) polytype selection is examined. The Ga surface saturation in the absence and presence of nitrogen determines the GaN polytype and morphology selection, respectively. The interplay between surface and step-edge diffusion barriers governing the NW-to-film-transition and the influence of SixNy interlayer formation on zinc blende (ZB) vs wurtzite (WZ) polytype selection of GaN is discussed. In addition, distinct exciton emissions associated with ZB and WZ GaN are observed, suggesting a type-I WZ/ZB GaN band-offset. This work provides a crucial step toward the realization of polarization-free GaN-based optoelectronics.

Finally, I discuss the impact of writing-to-learn (WTL) on promoting conceptual understanding of introductory materials science and engineering. Our WTL process asks students to write a response to a prompt, performing, and receiving content-focused peer review, and finally revising their initial response. MSE concept‐inventory‐style assessment is used to analyze students’ gain in conceptual knowledge. Across topics, the normalized gains that reflect conceptual knowledge gain are highest for students who completed WTL. These gains imply the presence of an additional source of learning beyond that of the traditional course components. Details of the findings and suggestions of strategies for future WTL design and implementation are presented.