ASU Electronic Theses and Dissertations
This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.
In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.
Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.
Filtering by
- All Subjects: Physics
- Creators: Ponce, Fernando A.
The first part of this work is focused on the evolution of microstructures of BAlN thin films. The films were grown by flow-modulated epitaxy at 1010 oC, with B/(B+Al) gas-flow ratios ranging from 0.06 to 0.18. The boron content obtained from X-ray diffraction (XRD) patterns ranges from x = 0.02 to 0.09, while Rutherford backscattering spectrometry (RBS) measures x = 0.06 to 0.16. Transmission electron microscopy indicates the sole presence of the wurtzite crystal structure in the BAlN films, and a tendency towards twin formation and finer microstructure for B/(B+Al) gas-flow ratios greater than 0.15. The RBS data suggest that the incorporation of B is highly efficient, while the XRD data indicate that the epitaxial growth may be limited by a solubility limit in the crystal phase at about 9%. Electron energy loss spectroscopy has been used to profile spatial variations in the composition of the films. It has also located point defects in the films with nanometer resolution. The defects are identified as B and Al interstitials and N vacancies by comparison of the observed energy thresholds with results of density functional theory calculations.
The second part of this work investigates dislocation clusters observed in thick InxGa1-xN films with 0.07 ≤ x ≤ 0.12. The clusters resemble baskets with a higher indium content at their interior. Threading dislocations at the basket boundaries are of the misfit edge type, and their separation is consistent with misfit strain relaxation due the difference in indium content between the baskets and the surrounding matrix. The base of the baskets exhibits no observable misfit dislocations connected to the threading dislocations, and often no net displacements like those due to stacking faults. It is argued that the origin of these threading dislocation arrays is associated with misfit dislocations at the basal plane that dissociate, forming stacking faults. When the stacking faults form simultaneously satisfying the crystal symmetry, the sum of their translation vectors does add up to zero, consistent with our experimental observations.
Part of this work is focused on InAs quantum dots (QDs) embedded in AlGaAs matrices. This QD system is important for the realization of intermediate-band solar cells, which has three light absorption paths for high efficiency photovoltaics. The suppression of plastic strain relaxation in the QDs shows a significant improvement of the optoelectronic properties. A partial capping followed by a thermal annealing step is used to achieve spool-shaped QDs with a uniform height following the thickness of the capping layer. This step keeps the height of the QDs below a critical value that is required for plastic relaxation. The spool-shaped QDs exhibit two photoluminescence peaks that are attributed to ground and excited state transitions. The luminescence peak width is associated with the QD diameter distribution. An InAs cover layer formed during annealing is found responsible for the loss of the confinement of the excited states in smaller QDs.
The second part of this work is focused on the investigation of the InxGa1-xN thin films having different bandgaps for double-junction solar cells. InxGa1-xN films with x ≤ 0.15 were grown by metal organic chemical vapor deposition. The defects in films with different indium contents have been studied. Their effect on the optical properties of the film have been investigated by cathodoluminescence. InxGa1-xN films with indium contents higher than 20% were grown by molecular beam epitaxy. The strain relaxation in the films has been measured from electron diffraction patterns taken in cross-sectional TEM specimens. Moiré fringes in some of the films reveal interfacial strain relaxation that is explained by a critical thickness model.