Browsing by Author "Moore, David J."
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- Degradation in lead zirconate titanate thin film capacitors for non-volatile memory applicationsBhattacharya, Mayukh (Virginia Tech, 1994-07-05)A study of the degradation of ferroelectric properties in Lead Zirconate Titanate (PZT) thin film capacitors is presented in this work. Metal- Ferroelectric - Metal capacitors were prepared by sputtering and metal organic decomposition (MOD) techniques. Samples with several different film thicknesses were considered in this study. Depolarization, leading to imprint has been studied at various temperatures. Changes in the dielectric properties of the capacitors as a function of the number of fatigue cycles is presented. Impedance and modulus spectroscopic techniques have been applied to study the effect of degradation on the ferroelectric thin film. It has been shown that with accurate low frequency impedance measurement equipment, new insight can be gained on the mechanisms of degradation in ferroelectric capacitors.
- Structure and properties interrelationships of SrBi₂(Ta1-xNbx)₂O₉Chen, Tze-Chiun (Virginia Tech, 1995-12-16)In recent years, the ferroelectric oxides belonging to the family of layered perovskite, e.g., SrBi₂(Ta₁₋xNbx)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₋xTix)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.
- Time domain device modeling of High Frequency Power MOSFETsHoagland, Richard W. (Virginia Tech, 1993-12-15)The development of the High Frequency Power MOSFET has brought about a need for accurate models. Now that the frequency range of these MOSFETs is in domains where typically scattering parameter measurements are used, a broad band device model can prove to be extremely useful. This thesis summarizes the research performed towards the development of a wideband Gate model for the Motorola MRF162 High Frequency Power Transistor. The device theory for typical MOSFETs will be explained. This theory will lead into the development of the Power MOSFET and its associated frequency limitations. The benefits of Time Domain Techniques will be explained and how a wideband model is achieved from this technique. The result from the analysis of the measurements and the device theory is a wideband Gate model developed for the frequency range from 100MHz to 400MHz. Verification is achieved by curve matching the measured Time Domain Reflected waveforms with the simulated waveforms generated using a proprietary program Modified Transient Analysis Program (MTCAP) and by comparison of expected and simulated parasitic values.