High tunability and low loss tangent of ferroelectric thinfilms offer unique opportunity for the development of variousmicrowave devices. Silver tantalate niobate, which showsexcellent microwave properties, was selected for this study.Ag(Ta,Nb)O3(ATN) showed week dielectric dispersion in a widefrequency range from 1 kHz up to 100 GHz, negligible losses upto 30 GHz, and ease to tailor paraelectric state in a widetemperature range by Ta:Nb ratio. This thesis is mainly based on the synthesis andcharacterization of niobate ferroelectric ATN thin films. Thinfilms for various measurements were prepared by pulsed laserdeposition and rf-magnetron sputtering techniques. X-ray diffraction (XRD) pattern show that ATN/Pt80Ir20films have been found to be (001) preferentiallyoriented, while the epitaxial quality of ATN/LaAlO3heterostructures have been ascertained. Dielectricproperties were analyzed by measuring the relationship betweendielectric permittivity and frequency as well as dielectricpermittivity and temperature. Reliable tracing of theferroelectric hysteresis polarization versus electric loopsindicate the ferroelectric state in ATN films at temperaturebelow 125 K and yields the remanant polarization of 0.4µC/cm2@ 77 K. The fundamental current-voltage behavior in Ag(Ta,Nb)O3ferroelectric films was measured usingMe/Ag(Ta,Nb)O3/Pt80Ir20, Me = Pd, Au, Cr, and Al, vertical capacitivecell structures with different top electrodes. Various kinds ofconduction mechanisms such as Schottky emission, Poole-Frenkel,Fowler-Nordheim, and ionic conduction were classified. Finally, by fabricating interdigital capacitors on the oxidesubstrates, the characteristics and performances of Ag(Ta,Nb)O3varactors were examined. Au/Cr/ATN/LaAlO3interdigital capacitors exhibited loss tangent aslow as 0.0033 @ 1 MHz, weak frequency dispersion of 5.8 % in 1kHz to 1 MHz range, tunability as high as 16.4 %,K-factor (tunability/tanδ) higher than 48.

<p>High tunability and low loss tangent of ferroelectric thinfilms offer unique opportunity for the development of variousmicrowave devices. Silver tantalate niobate, which showsexcellent microwave properties, was selected for this study.Ag(Ta,Nb)O₃(ATN) showed week dielectric dispersion in a widefrequency range from 1 kHz up to 100 GHz, negligible losses upto 30 GHz, and ease to tailor paraelectric state in a widetemperature range by Ta:Nb ratio.</p><p>This thesis is mainly based on the synthesis andcharacterization of niobate ferroelectric ATN thin films. Thinfilms for various measurements were prepared by pulsed laserdeposition and rf-magnetron sputtering techniques.</p><p>X-ray diffraction (XRD) pattern show that ATN/Pt₈₀Ir₂₀films have been found to be (001) preferentiallyoriented, while the epitaxial quality of ATN/LaAlO₃heterostructures have been ascertained. Dielectricproperties were analyzed by measuring the relationship betweendielectric permittivity and frequency as well as dielectricpermittivity and temperature. Reliable tracing of theferroelectric hysteresis polarization versus electric loopsindicate the ferroelectric state in ATN films at temperaturebelow 125 K and yields the remanant polarization of 0.4µC/cm²@ 77 K.</p><p>The fundamental current-voltage behavior in Ag(Ta,Nb)O₃ferroelectric films was measured usingMe/Ag(Ta,Nb)O₃/Pt₈₀Ir₂₀, Me = Pd, Au, Cr, and Al, vertical capacitivecell structures with different top electrodes. Various kinds ofconduction mechanisms such as Schottky emission, Poole-Frenkel,Fowler-Nordheim, and ionic conduction were classified.</p><p>Finally, by fabricating interdigital capacitors on the oxidesubstrates, the characteristics and performances of Ag(Ta,Nb)O₃varactors were examined. Au/Cr/ATN/LaAlO₃interdigital capacitors exhibited loss tangent aslow as 0.0033 @ 1 MHz, weak frequency dispersion of 5.8 % in 1kHz to 1 MHz range, tunability as high as 16.4 %,<i>K</i>-factor (tunability/tanδ) higher than 48.</p>

In this dissertation, liquid crystal materials and devices are investigated in meeting the challenges for photonics and communications applications. The first part deals with polymer-stabilized liquid crystal (PSLC) materials and devices. Three polymer-stabilized liquid crystal systems are developed for optical communications. The second part reports the experimental investigation of a novel liquid-crystal-infiltrated photonic crystal fiber (PCF) and explores its applications in fiber-optic communications. The curing temperature is found to have significant effects on the PSLC performance. The electro-optic properties of nematic polymer network liquid crystal (PNLC) at different curing temperatures are investigated experimentally. At high curing temperature, a high contrast, low drive voltage, and small hysteresis PNLC is obtained as a result of the formed large LC micro-domains. With the help of curing temperature effect, it is able to develop PNLC based optical devices with highly desirable performances for optical communications. Such high performance is generally considered difficult to realize for a PNLC. In fact, the poor performance of PNLC, especially at long wavelengths, has hindered it from practical applications for optical communications for a long time. Therefore, the optimal curing temperature effect discovered in this thesis would enable PSLCs for practical industrial applications. Further more, high birefringence LCs play an important role for near infrared photonic devices. The isothiocyanato tolane liquid crystals exhibit a high birefringence and low viscosity. The high birefringence LC dramatically improves the PSLC contrast ratio while keeping a low drive voltage and fast response time. A free-space optical device by PNLC is experimentally demonstrated and its properties characterized. Most LC devices are polarization sensitive. To overcome this drawback, we have investigated the polymer-stabilized cholesteric LC (PSCLC). Combining the curing temperature effect and high birefringence LC, a polarization independent fiber-optical device is realized with over 30 dB attenuation, ~12 Vrms drive voltage and 11/28 milliseconds (rise/decay) response times. A polymer-stabilized twisted nematic LC (PS TNLC) is also proposed as a variable optical attenuator for optical communications. By using the polarization control system, the device is polarization independent. The polymer network in a PS TNLC not only results in a fast response time (0.9/9 milliseconds for rise/decay respectively), but also removes the backflow effect of TNLC which occurs in the high voltage regime.

Perovskite based artificial superlattices has recently been extensively investigated due to the immense promise in various device applications. The major applications include non-volatile random access memories, microwave devices, phase shifters voltage tunable capacitor applications etc. In this thesis we have taken up the investigation of two different types of symmetric superlattices, viz. BaZrO3/BaTiO3 and SrTiO3/BaZrO3, with possible applicability to voltage tunable devices. Chapter 1 deals with the introduction to the perovskite based functional oxides. Their various applications and the specific requirements for voltage tunable device applications has also been discussed in detail. The basic properties of BaTiO3 and SrTiO3, which are well documented in the literature, have been reviewed. The fundamental physics of interfacial interactions that influence the properties of superlattices is also discussed using existing models. The reason behind the choice of constructing artificial superlattices of BaZrO3/BaTiO3 and SrTiO3/BaZrO3 and the motivation behind this thesis is outlined. Chapter 2 gives a brief description of the basic characterization techniques that has been employed for studying the thin films. These include pulsed laser deposition of oxide thin films, structural characterization using X-Ray Diffraction and Atomic Force Microscope and electrical characterization of thin film metal-insulator-metal structures. The basic principle behind the techniques has also been included in various sections of this chapter. Chapter 3 introduces the reader to basic properties of the less studied perovskite material BaZrO3, one of the parent components of Ba(Zr,Ti)O3 based ceramics for high frequency applications. BaZrO3 is the common material in both the types of superlattices studied in this thesis. Initially the growth of polycrystalline BaZrO3 on (111)Pt/TiO2/SiO2/Si has been elaborated in this chapter. After characterizing the crystalline quality of the films and optimizing the growth conditions, epitaxial BaZrO3 films has been grown on (001) SrTiO3 substates. Dielectric properties of epitaxial BaZrO3 film have been measured as a function of temperature and frequencies. The electric field tunability of BaZrO3 films has been calculated from capacitance-voltage data for comparison with superlattice structures. Chapter 4 deals with the basic considerations involving growth of artificial superlattices and multilayers using pulsed laser ablation technique. The fundamental differences between formation of multilayers and superlattices have also been discussed, and the basic considerations for optimizing growth parameters are analyzed in this chapter. X-ray θ-2θ and φ-scans have been performed to investigate crystal quality of superlattices. The growth rates calculated from the satellite reflections in X-ray θ-2θ scans indicate fair degree of control over the growth and φ-scans confirms epitaxial cube-on cube growth of both types of superlattices. Atomic Force microscopy has been used to hcaracterize the film quality and surface morphology of superlattice structures and it has been found that the films have a very smooth surface with rms roughness of the order of few nanometres. Chapter5 deals with the detailed electrical characterization of both types of superlattices structures. Dielectric response showed nearly temperature invariance for both types of superlattices. Polarization measurements show that the heterostructures are in paraelectric state. Even for paraelectric/ferroelectric BaZrO3/BaTiO3 superlattices, stress induced stabilization of the paraelectric state is exhibited in low period superlattices. Paraelectric/paraelectric-SrTiO3/BaZrO3 superlattices exhibited a tunability of ~20% at intermediate modulation periods and an extremely stable dissipation factor with respect to temperature which is very attractive for device application point of view. A maximum tunability of ~40% has been observed for lowest period BaZrO3/BaTiO3 superlattice. Relatively high Quality Factors has been observed for both type of superlattices and their dependence on the modulation periods has been analyzed. Dielectric relaxation data showed that Maxwell-Wanger type of behaviour is exhibited but the presence of a conductance component G had to be realized in the equivalent circuit representation, which originates from the observation of a square law dependence of the alternating current on the frequency. Finally DC electrical characteristics were investigated as a function of temperature to determine the type of conduction mechanism that is involoved. The data has been analyzed using existing theories of high field conduction in thin dielectric films and it has been found that at different temperature ranges, the conduction mechanism varied from bulk limited Poole-Frenkel to Space Charge limited conduction. The activation energy calculation indicate that the physical processes responsible for dielectric relaxation and dc conduction are identical.

L'intégration d'oxydes ferroélectriques permet la réduction des dimensions de dispositifs électroniques pour des applications en télécommunications, tout en leur apportant la reconfigurabilité. Parmi ces matériaux multifonctionnels, KxNa1-xNbO3 (KNN) se présente comme un candidat oxyde sans plomb prometteur pour un grand nombre d'applications. L'objectif de cette thèse est l'élaboration de couches minces de KNN et l'étude de leurs propriétés diélectriques en hyperfréquences en vue de leur intégration dans des antennes miniatures et reconfigurables. La permittivité εr, les pertes tanδ et l'agilité seront caractérisées à partir de couches minces de KNN déposées par la technique d'ablation laser. De plus, les avancements sur les dépôts par pulvérisation cathodique seront présentés. Après une étude sur la composition du matériau, l'influence des propriétés structurales sur les propriété diélectriques à travers l'utilisation de deux types de substrats a permis l'obtention d'une agilité de 20% sous un champ Ebias de 90 kV/cm. Une caractérisation diélectrique en température aura permis d'observer, à 10 GHz, une augmentation de la permittivité de 360 à 20°C jusqu'à 1000 à 240°C au niveau de la transition de phase polymorphique. Lors de ces travaux, la phase bronze de tungstène tétragonale (TTB), encore très peu étudiée dans le système K-Na-Nb-O a été préparée en couches minces fortement orientées et une permittivité élevée a été obtenue à basses (~200 à 10 kHz) et hautes fréquences (~130 à 10 GHz). Enfin la conception, la réalisation et la mesure d'antennes miniatures intégrant du KNN ont été effectuées. / Ferroelectric materials are a solution for reducing the size of electronic devices for telecommunication applications while also enabling reconfigurability. Among the multifunctional materials, KxNa1-xNbO3 (KNN) is a promising lead-free oxide for a large number of applications. The main goal of this work is the elaboration of KNN thin films and their dielectric characterisations in order to integrate the thin film to obtain miniature reconfigurable antennas. The permittivity εr, the loss tanδ and the tunability, at microwave frequencies, of the KNN were retrieved from thin films prepared by pulsed laser deposition. Also, the progress on the deposition of KNN thin films by RF magnetron sputtering will be presented. After the investigation of the effect of the composition and the structural properties of the KNN thin films on their dielectric properties, the tunability has been increased up to 20% under a 90 kV/cm electric field for x = 0.5. A dielectric characterisation of depending on the temperature, at 10 GHz, has shown an increase of the permittivity value from 360 at 20°C up to 1000 at 240°C, indicating the polymorphic phase transition. The tetragonal tungsten bronze phase (TTB), barely studied in the K-Na-Nb-O system, has been prepared in thin film and exhibiting high values of permittivity at both low and microwave frequencies (~200 à 10 kHz and ~130 à 10 GHz). Finally the design, realisation and measurements of miniature antennas integrating KNN has been done.