No description available.

No description available.

The quadrupole coupling tensors at the B11 and Na23 sites in borax and tincalconite, two members of the hydrated sodium tetraborate family have been completely evaluated using broadline nuclear magnetic resonance (n.m.r.) techniques. The results have enabled deductions to be made concerning the crystal chemistry of boron, the coordinations of the sodium atoms and the crystallographic symmetries. Changes in the Li7 n.m.r. spectrum of ferroelectric lithium hydrazinium sulphate as a function of temperature were followed in detail. A previously undetected second order phase transition leading to a high temperature polymorph is complete at 164°C, The quadrupole coupling tensors at the Li7 sites have been completely evaluated in the two phases. The second moments or the proton n.m.r. spectrum from powdered lithium hydrazinium sulphate have been determined within the temperature range -183°C to 225°C, and single crystal data on the protons have been obtained at room temperature. It is shown that the nitrogen and hydrogen atoms exist in the structure as the hydrazinium ion NH2 and-NH3+. Detailed information is derived concerning the proton-proton vectors and the modes of reorientation of the NH2 and-NH3+ groups at various temperatures. Mechanisms for the ferro-electric switching at room temperature and for the transition to the high temperature polymorph are proposed on the basis of the experimental evidence. / Thesis / Doctor of Philosophy (PhD)

<p> The temperature dependences of the deuteron spin-lattice relaxation times, T1, in ferroelectric ammonium sulfate, (ND4)2SO4 , and ammonium fluoroberyllate, (ND4)2BeF4, have been studied by transient methods over the range 80 to 47S°K. The ability to resolve the individual correlations of the two independent ND4 groups in the deuteron experiment has thrown new light on the reorientation of the ammonium groups and their behaviour in the phase transition. It has been proposed that the mechanism of the phase transition in ammonium sulfate is a disordering, with respect to the a b plane in the paraelectric phase, of the ND4 dipoles which also make the dominant contribution to the spontaneous polarization. The present N.M.R. results show that in (ND4)2so4 ,T1 at both non-equivalent ND4 groups is affected at the phase transition, whereas in (ND4)2BeF4 , it is affected at only one ND4 group. This suggests that the phase transitions in these two materials are rather different and that in ammonium sulfate both types of ammonium ions are involved whereas in ammonium fluoroberyllate only one type is involved in the transition. Such a conclusion is supported by evidence from measurements of thermal and dielectric properties. An earlier study of the proton spectrum in (NH4)2so4 reported a value of 33 gauss^2 for the second moment at 20°K. This is considerably smaller than the expected rigid lattice value of 5O gauss^2 and it was concluded that while some of the NH4+ groups are effectively rigid at 20°K others are still reorienting. Our new results for the proton second moment confirm the low value down to 4.2°K but our results for T1 indicate that the proton line is not motionally narrowed. </p> / Thesis / Doctor of Philosophy (PhD)

No description available.

No description available.

No description available.

In recent years, the ferroelectric oxides belonging to the family of layered perovskite, e.g., SrBi₂(Ta₁₋_xNb_x)₂O₉ (or SBTN), were identified as promising candidates for nonvolatile memory applications. SrBi₂Ta₂O₉ (or SBT) thin films were found to exhibit no fatigue up to 10¹² switching cycles, very good retention properties and low leakage current densities on Pt electrodes. However, high temperature processing, ie. 750 - 800°C, is needed for SBT to exhibit ferroelectric properties. Moreover, the fundamental properties of SBTN have not been fully characterized. In this research, SBTN solid solutions were studied from two aspects: the technical aspect and scientific aspect. From the technical point of view, low temperature processing of SBTN ferroelectric thin films was developed. In this part of study, SBTN thin films were made by metalorganic decomposition method (MOD) and were deposited on Pt-electrodes. The structure development study by a non-destructive optical method, spectroscopic ellipsometry, was proposed to determine nucleation and grain growth temperatures. The information on structure development can be obtained by observing how the refractive indices and film thicknesses change as functions of annealing temperature. The results of structure development study for SBT thin films suggest that the ferroelectric properties are controlled by grain growth process rather than nucleation process. The critical factor for ferroelectric properties was to have grain size exceeding a critical value, i.e., 0.1 µm. Applying this concept, low temperature processing can be achieved by growing larger grains at lower temperature. The processing temperature of SBTN thin films was reduced by 50 - 100°C by adding excess Bi or increasing Nb/Ta ratio. The optimum excess Bi content in SBT was 30 - 50%; within this range, limited solid solution of Bi₂O₃ and SBT was formed. From the scientific aspect of view, optical properties and ionic transport phenomena of SBTN bulk ceramics were investigated for the first time. The reason of using bulk ceramics is to exclude the difficulties associated with thin film technology, e.g., grain size effect and electrode-ferroelectric interface effect. These bulk property studies provide fundamental understanding of SBTN materials and provide a guideline for process development in device applications. The optical dispersion functions of bulk SBTN were obtained by using various angle spectroscopic ellipsometry with a surface layer correction. The values of refractive indices were found to vary with composition, which are possibly associated with crystallographic orientation. Using the Lorentz Oscillator model, the approximate energy band gaps of SBTN solid solutions were estimated to be about 5 eV. The ionic transport phenomena of SBT and SrBi₂Nb₂O₉ (or SBN) were investigated by using impedance spectroscopy. This technique allows to separate the effect of ion transport in grain, grain boundary and electrode-ferroelectric interface. In this study, the fatigue model of bismuth layered oxides was discussed through ionic conductivity and interface absorption effect. One conducting species, oxygen vacancies with positive charges, was assumed in the model. High ionic conductivities of SBT and SBN (~ 10¯⁷ S/cm) comparing to Pb(Zr₁₋_xTi_x)O₃ (~ 10¯¹¹-10¯¹⁰ S/cm) suggests high defect concentration and high charge mobility in bismuth layered oxide materials. As a result, the most possible model to explain high resistance to fatigue of SBT/SBN was the easy recovery of oxygen vacancies from the entrapment at electrode-ferroelectric interfaces. / Ph. D.

Fatigue and electrical degradation including low voltage breakdown of ferroelectric lead zirconate titanate Pb(Zr_xTi₁)O₃ (i.e. PZT) thin films are the major limitations for commercial memory applications of these films. It is noted that the presence of oxygen vacancies and their entrapment at the electrode-ferroelectric interfaces are the sources of the degradation phenomena. Attempts were made in this study to solve these problems: 1) by minimizing oxygen vacancy entrapment at the interfaces by employing RuO₂ electrodes; 2) by lowering the oxygen vacancy concentration in PZT films using donor doping (e.g. La³⁺ at Pb²⁺ site and Nb⁵⁺ at Ti/Zr⁴⁺ site). For this study, PZT thin films were prepared by a sol-gel method and deposited on both Pt/Ti/SiO₂/Si and RuO₂/SiO₂/Si substrates. The microstructure and electrical properties, such as hysteresis properties, fatigue, leakage current, time-dependent dielectric breakdown (TDDB), and retention, were studied with regard to the Zr/Ti ratio, the excess lead, the annealing temperature, the electrode material, and the doping amount. Furthermore, the pyrochlore to perovskite phase transformation of PZT on RuO₂ electrodes was also investigated. It was shown that PZT films (Zr/Ti=50/50) with 10 at.% excess lead annealed at 650°C for 30 min possessed the best electrical properties for ferroelectric memory application. In confirmation with earlier theoretical and experimental results, no polarization loss was observed up to 10¹¹ switching cycles for the PZT films deposited on RuO₂ electrodes. However, the low Schottky barrier at the interfaces between RuO₂ and PZT films resulted in a higher leakage current at a high electric fields. Donor doping of PZT films decreased carrier concentrations in PZT films, and thus, decreased the leakage current to acceptable limits. In addition, it was also noted that the pyrochlore to perovskite phase transformation of PZT on RuO₂ was similar to that of PZT on Pt electrodes. It can be concluded that the combination of RuO₂ electrodes and donor doping produced PZT films with high fatigue endurance and low leakage currents which are suitable for memory applications. / Master of Science

This work targeted the growth of gadolinium (Gd)-doped gallium nitride (GaN) thin films (Ga₁₋ₓGdₓN) by metal organic chemical vapor deposition (MOCVD). Characterization and evaluation of these Ga₁₋ₓGdₓN thin films for application in spintronics/optoelectronics devices also formed part of this work. This work presents: (1) the first report of stable, reproducible n- and p-type Ga₁₋ₓGdₓN thin films by MOCVD; (2) the first Ga₁₋ₓGdₓN p-n diode structure; and (3) the first report of a room temperature spin-polarized LED using a Ga₁₋ₓGdₓN spin injection layer. The Ga₁₋ₓGdₓN thin films grown in this work were electrically conductive, and co-doping them with Silicon (Si) or Magnesium (Mg) resulted in n-type and p-type materials, respectively. All the materials and structures grown in this work, including the Ga₁₋ₓGdₓN-based p-n diode and spin polarized LED, were characterized for their structural, optical, electrical and magnetic properties. The spin-polarized LED gave spin polarization ratio of 22% and systematic variation of this ratio at room temperature with external magnetic field was observed.