Master of Science, The Ohio State University, 1970, Graduate School

Committee: Not Provided (Other) Subjects:

Doctor of Philosophy (PhD), Ohio University, 2016, Electrical Engineering & Computer Science (Engineering and Technology)

The technological advantages of III-nitride semiconductors (III-Ns) have been demonstrated among others in the area of light emitting applications. Due to fundamental reasons limiting growth of InGaN with high Indium content, rare earth (RE) doped III-Ns provide an alternative way to achieve monolithic red, green, blue (RGB) emitters on the same III-Ns host material. However, the excitation efficiency of RE3+ ions in III-Ns is still insufficient due to the complexity of energy transfer processes involved. In this work, we consider the current understanding of the excitation mechanisms of RE3+ ions doped III-Ns, specifically Yb3+ and Eu3+ ions, and theories toward the excitation mechanism involving RE induced defects. In particular, we demonstrate and emphasize that the RE induced structural isovalent (RESI) trap model can be applied to explain the excitation mechanism of III-Ns:RE3+. Specifically, we have investigated the Yb3+ ion doped into III-Ns hosts having different morphologies. The observed emission peaks of Yb3+ ion were analyzed and fitted with theoretical calculations. The study of Yb3+ ion doped InxGa1-xN nano-rod films with varied indium (In) concentration shown the improvement of luminescence quality from the nanorod due to the presence of Yb dopant. Then we report the optical spectroscopy and DLTS study toward an Eu and Si co-doped GaN and its control counterpart. The Laplace-DLTS and optical-DLTS system developed in this work improved spectrum resolution compared to the conventional DLTS. The high resolution L-DLTS revealed at least four closely spaced defect levels associated with the Trap B, identified with regular DLTS, with activation energy 0.259±0.032 eV (Trap B1), 0.253±0.020 eV (Trap B2), 0.257±0.017 eV (Trap B3), and 0.268±0.025 eV (Trap B4) below the conduction band edge, respectively. Most importantly, a shallow hole trap was observed at energy 30±20 meV above the valence band edge of the GaN:Si,Eu3+ which can be attributed to the RESI hole (open full item for complete abstract)

Committee: Wojciech Jadwisienczak (Advisor); Savas Kaya (Committee Member); Martin Kordesch (Committee Member); Eric Stinaff (Committee Member); Kodi Avinash (Committee Member); Harsha Chenji (Committee Member) Subjects: Electrical Engineering; Materials Science; Nanotechnology; Optics

Master of Science, The Ohio State University, 2011, Materials Science and Engineering

Environmental barrier coatings (EBCs) are used to prevent oxidation of underlying ceramic-matrix composite (CMC) structural components in aircraft gas-turbine engines. As operating temperatures increase, ingested airborne sand poses a serious threat to the stability of these coatings, because the sand adheres to the hot EBC surfaces and melts, forming calcium–magnesium–aluminosilicate (CMAS) glass. The reaction of EBCs with molten CMAS can lead to EBC delamination. Additionally, the interaction of water vapor with the CMAS–reacted EBCs can result in the formation of undesirable phases. Yb2Si2O7 has been identified as a promising EBC ceramic, based on its desirable properties: phase stability up to 1600°C, low thermal expansion coefficient mismatch with common CMCs, and potential resistance against degradation by CMAS and water vapor. As–sintered Yb2Si2O7 and CMAS–coated Yb2Si2O7 are tested in an air environment and in a water vapor environment, at a temperature of 1300°C in both cases. The behavior of these ceramics is compared to that of reference materials. Results from oxidation and chemical stability studies and analyses are presented.

Committee: Nitin P. Padture PhD (Advisor); Rudolph G. Buchheit PhD (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2006, Chemistry

New carbonyl complexes of divalent ytterbium and transition metals of groups 6, 7 of the periodic table have been prepared. The syntheses were carried out in systematic fashion with the aim of establishing general procedures suitable for preparation of a range of compounds of this type. The products obtained were characterized by means of IR spectroscopy and X-ray single crystal diffraction. Nineteen X-ray structures are reported herein, of which only one has been published before. The compounds studied can be divided into two major groups: the solvent-separated ion pairs, where the YbII cation is surrounded with solvent molecules acting as ligands, preventing interaction with the metal carbonylate anion; and complexes with the bridging carbonyl ligands (isocarbonyl ligands), where the cation and the anion are bound together through a –CO- link. New instances of condensation of the solvent-separated ion pairs into the isocarbonyl complexes have been discovered, and the mechanism for such transformation was proposed. The novel [Hg(W(CO)5)2]2- anion was discovered and characterized by X-ray single crystal diffraction. Its reactivity was briefly investigated.

Committee: Sheldon Shore (Advisor) Subjects: Chemistry, Inorganic

Master of Science (MS), Ohio University, 2009, Electrical Engineering (Engineering and Technology)

In this thesis we report on luminescence of ytterbium (Yb3+) ions-doped GaN and AlN semiconductors. Ytterbium is one of a few lanthanides which has not been extensively investigated as an optically active center in crystalline III-Nitride hosts. In general, observed luminescence spectra of Yb3+-doped GaN and AlN are complex. There are more emission lines than predicted by theory for Yb3+ ions occupying a C3ν symmetry site in these materials. The number of luminescence transition lines observed at low temperature between the spin-orbit levels 2F5/2-2F7/2 indicates that Yb3+ ions are involved in different optically active centers. The luminescence spectra of Yb3+-doped GaN and AlN were measured in 11 K - 300 K temperature range and weak luminescence thermal quenching was observed. The photoluminescence kinetics of Yb3+ ions was measured under different excitation conditions and revealed that Yb3+ ions occupy at least two different lattice sites in GaN and AlN. The nature of these centers and the excitation mechanism was investigated using photoluminescence excitation spectroscopy. The comparison between the emission spectra of Yb3+ ions in the GaN and AlN indicates the presence of some similarities between the lattice locations of Yb3+ ions in these hosts. Based on the obtained experimental data we propose that the luminescence spectra of Yb3+ ions in GaN and AlN result from the existence of a substitutional YbAl,(Ga) site and a VN-Yb complex defect in AlN (GaN) lattice. Furthermore, we have argued that the future research on binary doping of III-Nitrides with Yb3+ and other lanthanides resulting in upconversion processes can enhance energy transfer processes between hosts and RE3+ ion emitting centers and make emission from RE3+-doped III-Nitrides in the visible spectral range more efficient.

Committee: Wojciech M. Jadwisienczak PhD (Advisor); Martin Kordesch PhD (Committee Member); Ralph Whaley PhD (Committee Member); Savas Kaya PhD (Committee Member); David Matolak PhD (Committee Chair) Subjects: Electrical Engineering