Doctor of Philosophy, Case Western Reserve University, 2016, Physics

The Zn-IV-nitride family of semiconductors is an electronic and structural analogue of the III-nitride family. This new family of semiconductors could have similar applications in electronic and optoelectronic devices as the III-nitride family, but several challenges, such as control of crystal quality, doping, and cation ordering, need to be overcome first. Cation ordering in these ternary semiconductors affects the structure, band-gap, and vibrational properties, but it is not yet well understood. Polycrystalline ZnGeN2 was grown by exposing a Ge wafer to Zn and NH3 vapor at temperatures between 758°C and 914°C. The degree of cation lattice ordering was found to increase with the growth temperature, as evidenced by x-ray diffraction analysis. The degree of cation disorder was observed to correlate with the suppression of predicted Raman peaks and the emergence of phonon density-of-states features. Annealing experiments were performed on disordered ZnGeN2 grown at 758°C. After annealing for 3 hours at 850°C in the presence of Zn and NH3 vapor, the ZnGeN2 was found to be 90% ordered as evidenced by x-ray diffraction and Raman spectroscopy. ZnGeN2 was also grown from Ge diluted in liquid Sn, which resulted in platelet-shaped crystals. The platelet morphology allowed Raman modes with a2 and a1L symmetries to be measured for the first time. ZnGeN2 and ZnSnN2 have less of a lattice mismatch compared to GaN and InN, which could lead to alloys with less of the tendency for phase separation. Equilibrium thermodynamic modeling was used to predict outcomes of ZnGeN2-ZnSnN2 alloy growths from Sn-rich, Ge-poor liquids. The model predicted that the ZnSnN2 concentration in the alloy should be less than 0.1%. The measured ZnSnN2 concentration in the grown material was estimated to be 3-8% using Vegard's law analysis. The larger-than-predicted ZnSnN2 concentration was most likely a result of out-of-equilibrium growth.

Committee: Kathleen Kash (Advisor); Walter Lambrecht (Committee Member); Xuan Gao (Committee Member); Hongping Zhao (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, The Ohio State University, 2009, Chemistry

This work describes the synthesis and characterization of a number of new AA'BB'O6 perovskites which possess the unusual combination of rock salt ordering of B/B' and layered ordering of A/A'. These compounds have been structurally characterized by powder X-ray and neutron diffraction as well as transmission electron microscopy. Several of these compounds are found to adopt polar P21 space group symmetry as a result of the cation ordering in combination with a–a–c+ octahedral tilting. A number of other compounds are shown to have a compositional modulation of the A-site cations that is accompanied by a twinning of the octahedral tilt system. Using UV-Vis spectroscopy the band gaps of these compounds have been determined. This analysis shows that NaLnMgWO6 compounds are insulators while NaLnMnWO6 compounds are semiconductors. The dielectric constants are found to be in the range of approximately 20-35. The magnetic properties of the NaLnMnWO6 and NaLnMgWO6 compounds have been measured. All NaLnMnWO6 compounds are found to order antiferromagnetically at temperatures ranging from 6-15 K. The NaLnMgWO6 compounds do not show any indications of magnetic order. The magnetic structures of NaLaMnWO6, NaNdMnWO6, and NaTbMnWO6 have been determined from neutron powder diffraction. NaLaMnWO6 is found to order into a simple commensurate structure. NaNdMnWO6 orders incommensurately due to interactions between the two magnetic ions. NaTbMnWO6 is found to pass through two magnetic phase transitions. Just below its Neel temperature it has an incommensurate modulation of its structure which disappears as it is further cooled.

Committee: Patrick Woodward PhD (Advisor); Yiying Wu PhD (Committee Member); Malcolm Chisholm PhD (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2005, Chemistry

The syntheses, crystal structures, electrical properties, and optical absorbance spectra of perovskite oxynitrides, AMO2N (A = Ba, Sr, Ca; M = Ta, Nb), LaTaON2, LaTiO2N, LaMg1/3Ta2/3O2N, LaMg1/2Ta1/2O5/2N1/2, and BaSc0.03Ta0.97(O,N)3, have been investigated. The average crystal structure of BaTaO2N is a cubic perovskite, with a Ta–O/N distance of 2.056 A. SrTaO2N and CaTaO2N are distorted by octahedral tilting, showing noticeably smaller Ta–O/N distances of approximately 2.02 A. Electron diffraction studies of BaTaO2N are consistent with the simple cubic perovskite crystal structure determined using X-ray powder diffraction methods. Each of the niobium oxynitrides is isostructural with its tantalum analog, though the Nb–O/N distances are observed to be slightly longer. The Mg+2/Ta+5 and Sc+3/Ta+5 pairs have high preference for ordering in the oxides but in oxynitrides the mixed anion environment disturbs the long range order of cations. The optical band gaps are estimated from diffuse reflectance spectra to be as follows: BaTaO2N, 1.8 eV, SrTaO2N, 2.1 eV, CaTaO2N, 2.4 eV, BaNbO2N, 1.8 eV, SrNbO2N, 1.9 eV, CaNbO2N, 2.1 eV, LaTaON2, 1.9 eV, LaTiO2N, 1.9 eV, LaMg1/3Ta2/3O2N, 1.9 eV, LaMg1/2Ta1/2O5/2N1/2, 1.9 eV, and BaSc0.03Ta0.97(O,N)3, 1.8 eV. Impedance spectroscopy was carried out on sintered pellets of the ATaO2N, BaNbO2N, and BaSc0.03Ta0.97(O,N)3 to investigate the dielectric and electrical transport properties. The BaNbO2N sample shows metallic-type conductivity apparently from a slight reduction that occurs during sintering. In contrast, the tantalum compounds are semiconductors/insulators with conductivities of ~10-5 S/cm (A = Ba, Sr) and ~10-8 S/cm (A = Ca). Interpretation of the impedance data reveals that BaTaO2N and SrTaO2N have unexpectedly high bulk dielectric constants, κ ~ 4900 and 2900, respectively at room temperature. The dielectric constants of both compounds are frequency dependent and show a relatively weak, linear dependence upon temperature with (open full item for complete abstract)

Committee: Patrick Woodward (Advisor) Subjects: Chemistry, Inorganic