Among a variety of ferroelectric materials, PbTiO₃ (PT) and Pb(Zr_xTi_1-x)O₃ (PZT) have been found to have unique properties for memory, sensor, actuation, and optical applications. The goal of this research is to employ MOCVD to fabricate epitaxial and polycrystalline PT and PZT thin films on various kinds of substrates. In particular, we are interested in the evolution of ferroelectric domain structure and the associated dielectric properties as a function of cycling in temperature or in applied electrical field. By way of a systematic study, it is expected that improved properties and more stable microstructure in the PT and PZT thin films can be achieved by controlling MOCVD processing conditions.

Tin oxide (SnO₂) thin films were deposited on Si(100) and Al₂O₃ substrates using metal-organic chemical vapor deposition (MOCVD) techniques at various temperatures. Microstructures were characterized using x-ray diffraction, SEM, TEM, and AES. Sensing properties associated with reducing gases such as H₂, alcohol, etc., were conducted using a modified 4-point probe with a constant current source. The microstructure in relation to the sensing properties is discerned and a model is developed to explain the temperature dependence of the sensing sensitivity.

Nonlinear oxides such as LiNbO₃ are widely used in single-crystal form for optical computing applications such as polarization control and signal modulation. The use of such materials allows subnanosecond data switching for photonic networks. Research is concerned with integration of nonlinear optical thin layers of LiNbO₃ on laser source materials such as (Al,Ga)As through Al₂O₃ and MgO buffer layers. A low-temperature solution deposition technique is used to deposit layers of high crystallographic perfection and low optical attenuation. Integration and device demon- stration are pursued for switching, amplification, and frequency mixing applications.

Solid-state nonlinear optic and electrooptic devices offer many advantages, including speed, compactness, simplicity, and reliability. Ferroelectric crystals show the strongest nonlinear properties and have high optical damage thresholds. The research is directed at the growth of high-quality single crystals of LiNbO₃, KNbO₃, and BaTiO₃. Mechanisms of crystal growth are determined. Methods to control the ferroelectric domains are developed. Electrical and optical properties are investigated using structure-property and processing-property relationships for nonlinear optical and electrooptic ceramics. Understanding of the role of dopants is pursued with regard to holographic storage, laser host, and ferroelectric switching applications.

The electrical and mechanical properties of polar materials are largely dependent on the direction of applied field in terms to crystallographic orientation. Unique and better properties are thus expected for ferroelectric and antiferroelectric thin layers with preferred orientation, heteroepitaxy, and homoepitaxy. The present research is concerned with the study of the heterogeneous nucleation and growth of sol-gel-derived complex oxides on substrate materials with modified surface chemistries. Emphasis is placed on identifying the dominant factors which control texture. The effects of solution chemistry and heat-treatment conditions as well as substrate materials are under investigation.

Lead zirconate titanate (PZT) perovskite materials are of considerable technological importance, particularly with regard to physical properties such as pyro- and piezoelectricity and electric field-induced antiferroelectric-to-ferroelectric phase switching. A systematic study of the effects of temperature, Sn- and La-substituents, and Zr/Ti ratio on the evolution of incommensurate phases in high Zr-content PZT is carried out by hot- and cold-stage TEM. Particular attention is paid to the possible development of 1/x[110] incommensurate modulations and to the resultant influence on the macroscopic properties. The purpose of this investigation is to obtain a fundamental understanding of microstructure-property relationships for electroceramics.

Polycrystalline ceramics exhibiting a large magnetore sistance effect are under investigation for sensor technology. Research is concerned with the effects of chemical substitution and the control of microstructure on properties. Ceramics are prepared by mixed-oxide routes and chemical methods and densified by hot-pressing. The role of grain boundaries on magnetoresistive characteristics is under investigation.

Sol-gel-derived PZT thin layers deposited on silicon were examined for their shrinkage behavior using in situ ellipsometry. Densification data were correlated with thermal analysis (DTA, TGA) to determine pyrolysis and crystallization effects on layer shrinkage. Experimental parameters such as water of hydrolysis, pH, heating rate, and spin speed were systematically varied and their effect on densification was evaluated. Stresses arising in the coating were measured as a function of heat treatment by a laser reflectance technique and related to associated densification phenomena and substrate/layer thermal expansion mismatch.

The high-frequency properties of Ba(Mg _1/3 Ta _2/3 )O₃ crystals, polycrystals, and thin layers are under investigation. Spectroscopic measurements are used to determine intrinsic lattice vibrations and dielectric loss mechanisms. The role of thermal processing and lattice ordering on high-Q dielectrics is under study.

The role of interfaces on barium titanate capacitors deposited on silicon by solution processing is under investigation. Microroughness and the chemistry of the silicon surface was found to be dependent on the cleaning, rinsing, and drying methods used. Microroughness and chemistry were characterized by a variety of analytical techniques and related to the reliability and performance characteristics of the integrated capacitors.

The preparation of homoepitaxial oxide thin layers from alkoxide solution precursors is under investigation. Materials of interest include LiNbO₃ and BaTiO₃. Crystallization behavior of the polar oxides is characterized in terms of interfacial and homogeneous nucleation mechanisms and grain growth. Detailed study of such model thin-layer sol-gel systems will lead to a better understanding of the evolution of microstructure in oxide thin layers crystallized on lattice-matched substrate materials.

Thin-layer ceramics have diverse applications as integrated capacitors, sensors, display components, and other devices. Solution processing allows deposition of such layers at greatly reduced temperatures on metal, oxide, and semiconducting substrates with control of stoichiometry, doping level, and crystalline order. Current research is directed at a new selective deposition technique made possible by functionalization of substrates with self-assembled layers. Derivitization of metal, oxide, and semi- conductor substrates with hydrophilic or hydrophobic assembled layers enables preferential wetting of surfaces by solution or vapor deposition techniques. Lithography-free selective deposition of ceramic thin layers has been demonstrated.

The complex piezoelectric properties of lead zirconate titanate based ceramics are being studied. Investigations are being performed using a modified Michelson-Morely interferometer, which can measure both the real and phase components of the response as a function of temperature and frequency. Investigations are being performed for various ac drive amplitudes and dc bias levels. It is anticipated that these studies will lead to a more comprehensive understanding of the fundamental electromechanical coupling mechanism in ferroelectrics.

The microstructure property relationships in various ferroelectric and piezoelectric ceramics are being studied by transmission electron microscopy. Investigations of the phase transformational characteristics are being performed in situ by hot- and cold-stage methods. Corresponding bulk property investigations are also being performed. Compositional systems currently being investigated include lead zirconate titanate (PZT), La-modified PZT, lead magnesium niobate, strontium barium niobate, and Sn-modified PZT. It is anticipated that these studies may lead to a more comprehensive understanding of the influence of impurities on ferroelectric and antiferroelectric domain structures and phase transformational characteristics.

The dielectric, piezoelectric, and electrically induced strain and polarization characteristics of antiferroelectric Sn-modified lead zirconate titanate ceramics are currently being investigated. Corresponding transmission electron microscopy studies are also being performed. These investigations are revealing the nature of the complex structure property relationships in these materials for the first time. An unusual incommensurately modulated polar structure has been observed. Changes in the modulated structure are being correlated with changes in the macroscopic response characteristics. Evidence is indicating that the nonlinear response is controlled by the modulation of the incommensurate structure by an applied electrical field.

The dependence of the dielectric response characteristics of ferroelectric, soft ferroelectric, and relaxor ferroelectric materials is being studied as a function of ac drive amplitude. Investigations are being performed as a function of temperature, frequency, and superimposed dc bias. These investigations have already revealed the presence of strong nonlinearities in critical compositional regions. The mechanisms underlying the anomalous nonlinearities are currently being studied and related to microstructural changes by transmission electron microscopy.

Studies of lead magnesium niobate lead titanate crystalline solutions are being studied for the purpose of achieving enhanced electrically induced strains and electromechanical coupling coefficients. Compositional modifications are being investigated that lead to improved performance coefficients with reduced hysteresis. Studies are being performed by dielectric spectroscopy, piezoelectric spectroscopy, and electrically induced strain and polarization methods. It is hoped that the investigations will lead to the development of the next generation of high-performance transducer/projector materials for underwater acoustical imaging applications in lateral waters.

Barium titanate is one of the most important ferroelectric materials. The interrelationships between the structure and properties in this material are being studied by changing the La and Sn contents. These investigations are being performed by dielectric spectroscopy and transmission electron microscopy, with particular attention being paid to the influence of impurities on the phase transformational characteristics and domain structure. The purpose of this study is to develop new lead-free relaxor-like ferroelectric materials for electrostrictive applications.

Minor amounts of CaO and SiO₂ are being added to MnFe₂O₄ to study their effects on the electrical transport properties of the ferrite. The additives are concentrated in the grain boundaries, which is believed to produce high-resistance grain boundaries, surrounding more conductive grains, thus reducing eddy current losses at high frequencies in the ferrite. It is also found that the concentration of dopant in the grain boundary prevents decomposition of the ferrite in oxidizing atmospheres, thus maintaining the stoichiometry of the high-resistance ferrite. Impedance spectroscopy, thermogravimetry, and electrochemical cell measurements are being used in the study.