Ferroelectric random access memories (FRAM) are considered as future memories due to high speed, low cost, low power, excellent radiation hardness, nonvolatility, and good compatibility with the existing integrated circuit (IC) technology. The non-volatile FRAM devices are divided into two categories, based on reading technique: destructive readout (DRO) FRAM and non-destructive readout (NDRO) FRAM. Lead zirconate titanate (PZT) is recently considered as one of the most promising materials for DRO FRAM devices due to its excellent ferroelectric properties. There are remarkable advances in the applications of PZT thin films, but the direct integration into high density CMOS devices is restricted by high processing temperatures. Hence, it is desirable to lower processing temperature and develop novel high temperature electrode-barrier layers for achieving high density DRO FRAM devices. The NDRO FRAM devices have been developed mainly using metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure. This devices use the remanent polarization of ferroelectric films to control the surface conductivity of a silicon substrate. The problem of the NDRO FRAM is that the actual electric field applied to ferroelectric films is very small compared to the external electric field, because of the large depolarization field in the MFS structure and the high capacitance ratio of ferroelectric capacitor and SiO2 capacitor in series in the MFMIS structure. Since the typical ferroelectric films show very high dielectric constant over 400, it is desired to develop ferroelectric films with low dielectric constant and low coercive electric field. This research is primarily focused on developing low temperature processing and high temperature electrode-barrier layers for DRO FRAM application, and exploiting novel ferroelectric materials for NDRO FRAM application. The low temperature processing was achieved by a novel sol-gel processing, which takes advantage of in-situ electrode template layer, rapid heating-treatment without pyrolysis step, and molecularly modified precursors. The PZT films with various composition were also investigated as a function of Ti content. In order to study the integration issues for these PZT films, a substrate was constructed as Pt/TiN/TiSi₂/poly-Si, which represents a scheme of capacitor in high density DRO FRAM devices. The ferroelectric films were incorporated into the substrate, and their ferroelectric properties were investigated as a function of annealing temperature. Excellent ferroelectric properties were observed for the thin films processed at a low temperature of 500 °C as contacting between top Pt and bottom polysilicon. The other approach we have taken to overcome the integration problems in high density DRO FRAM devices is to develop high temperature electrode barrier layers. In this research, Pt/IrO2/Ir hybrid layers were prepared on poly-Si substrate as high temperature electrode-barriers. The PZT films fabricated on the Pt/IrO₂/Ir/poly-Si substrates exhibited good ferroelectric properties and outstanding fatigue properties after high temperature processing. It was observed from Auger electron spectroscopy (AES) profiles that the hybrid oxide electrode minimized fatigue problem by reducing the oxygen vacancies entrapment at the electrode/ferroelectric interfaces. This results indicated that Pt/IrO₂/Ir high temperature electrode-barrier layers promise to solve major problems of PZT integration into high density DRO memory devices. For the NDRO FRAM devices, Sr₂Nb₂O₇ and La₂Ti₂O₇ thin films were prepared on Pt-coated silicon, Si(100), and Pt/IrO₂/SiO₂/Si substrates by metalorganic deposition (MOD) technique. The Sr₂Nb₂O₇ and La₂Ti₂O₇ thin films showed the dielectric constant values of 48 and 46, respectively. However, no ferroelectricity was observed at room temperature, which might be attributed to extremely small grains. Extensive studies on preparation and properties of Sr₂(Ta_1-xNb_x)O₇ (STN) both in bulk and thin film form were carried out as a function of composition. The STN films exhibited small dielectric constant of around 46, irrespective of the composition. / Ph. D.

The (1-x)SrBi2Ta2O9-xBi3TaTiO9 thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi2Ta2O9: a leading candidate material for memory applications. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600 oC and grain size was found to be considerably increased for the (1-x)SrBi2Ta2O9-xBi3TaTiO9 compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the thin films was in the range 180-225 for films with 10-50 mol % of Bi3TaTiO9 content in the solid solution. Ferroelectric properties of (1-x)SrBi2Ta2O9-xBi3TaTiO9 thin films were significantly improved compared to SrBi2Ta2O9. For example, the observed 2Pr and Ec values for films with 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 composition, annealed at 650 oC, were 12.4 micro C/cm2 and 80 kV/cm, respectively. The solid solution thin films showed less than 5 % decay of the polarization charge after 10^10 switching cycles and good memory retention characteristics after about 10^6 s of memory retention. The size and temperature effect of 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 thin films were studied by determining how the ferroelectric properties vary with film thickness and temperature. It was found that the ferroelectric properties were determined by the grain size, and not by the thickness of the film in our studied thickness range of 80-350 nm. A 80 nm thick film showed good ferroelectric properties similar to the 350 nm thick film. Thermal stability of the 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 thin film was found to be much better compared to the SrBi2Ta2O9 and Pb(Zr,Ti)O9 thin films due to its higher Curie temperature and lower Schottky activation energy according to temperature changes. Also, 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 thin films has shown good ferroelectric properties on multilayer system such as PtRh/PtRhOx/poly-Si suggest their suitability for high density FRAM applications. / Ph. D.

The objective of this research is to implement monolithic analog phase shifters based on barium strontium titanate (BST) coated sapphire substrates for IEEE 802.11b wireless local area network (WLAN) applications. It has been known that several BST thin film properties such as high relative permittivity, electric field dependence, fast polarization response, relatively low loss, and high breakdown field, allow for miniaturization and high performance of analog phase shifters. Before attempting to implement BST phase shifters, coplanar waveguides (CPWs) and interdigital capacitors (IDCs) based on various BST compositions and thicknesses have been developed and characterized to capitalize on the electrical properties of BST thin films. Based on the characteristics of BST thin films, two design topologies have been studied to implement phase shifters. The first topology is a reflection-type structure. The reflection-type phase shifter composed of a 3-dB coupler and two identical reflective terminations has provided a large phase shift with a relatively low insertion loss. The second topology is an all-pass network structure. The all-pass network phase shifter consists of only lumped elements so that one can shrink in size of devices. The total chip area of the all-pass network phase shifter is only 2.6 mm * 2.2 mm with a loss figure-of-merit (FOM) of more than 69 deg/dB at 2.4 GHz. This is the smallest size and the best performance obtained to date for BST phase shifters in the 2.4 GHz band and comparable or even better than the state of the GaAs MMIC phase shifters. The nonlinear response of the all-pass network phase shifter also was investigated with two-tone intermodulation distortion (IMD) measurement. Furthermore, the all-pass network phase shifter was studied to ascertain a design to ensure minimum performance variation over a range of temperature and to determine which BST composition performed best in the face of temperature variations. Compact beamforming networks (BFNs) for WLAN systems using client-based smart antennas have been demonstrated to validate the feasibility of BST technology for WLAN applications. The two-element BFNs have been shown to increase throughput and network capacity by rejecting interference.

En relation avec le désordre de leur structure, les composés relaxeurs révèlent une dualité entre les échelles locale et globale. L'ordre polaire qui se développe localement ne se manifeste pas à l'échelle globale, la structure globale demeurant cubique. Bien que pertinente au vu de ses capacités à saisir la dualité d'échelle, une incorporation directe de la symétrie locale à l'appareil d'étude des relaxeurs reste absente. C'est autour de cette lacune que s'articule la thèse, en ce qu'elle constitue une implémentation explicite de la symétrie locale dans l'Hamiltonien effectif. Elle porte sur l'analyse du rôle de la symétrie locale à sous-tendre l'ordre locale dont les propriétés macroscopiques émergent. Une question sous-jacente s'adjoint à ces considérations et concerne la possible formulation du problème d'un point de vue topologique / Concomitantly with lattice disorder, there is a discrepancy between local and global scales in relaxor ferroelectrics, in that structural distortions occurring at the local scale are not reflected in the average global structure which remains cubic. There is an absence of direct implementation of the local symmetry in the modeling of relaxors, despite its considerable, but often unacknowledged, ability to encode local features. Central to the thesis is an explicit account for local gauge symmetry within the first-principles-derived effective Hamiltonian approach. The thesis thus aims to consider how an extended symmetry allowing independent transformations at different points in space can effectively bridge local features and macroscopical properties. An underlying question the thesis also seeks to answer is whether the disorder-induced non-trivial interplay between local and global scales can be described from a topological point of view

Abstract The work deals with the experimental study of ferroelectric (FE) performance scaling for nanometer-sized devices. In the emerging and advanced devices, it is desirable to couple FE performance with other functions. This requires integration of nanoscale FEs with other materials, which is especially promising in epitaxial heterostructures. Such heterostructures inevitably possess a large lattice mismatch, the effect of which on FE properties is unknown and is in the focus of the present work. In the study, heteroepitaxial thin and ultrathin films and superlattices of ABO3-type perovskite structure FEs were used, with A = Pb, Ba, Sr, K, and N, and B = Ti, Zr, Nb, and Ta. FE domains and local polarization switching were explored on the nanometer scale using piezoresponse force microscopy. The experiment was modified that allowed achieving images with high contrast and lateral resolution, and also allowed analysis of nanodomains in lateral capacitor configuration. Local properties were related to a macroscopic response. For this, the method of simultaneous on-wafer low-frequency impedance measurements was optimized allowing studies of thin and ultrathin (to 5 nm) films in a broad range of conditions and regimes. Experimental studies have reveled phenomena which cannot be explained in the frame of the existing theories. The observed new effects are important for applications such as multistate memory devices, storage capacitors, and FE tunnel junction devices.

In this thesis, we capitalize on the two-dimensional (2D) nature of MXenes by using them as precursors for the synthesis of 2D functional material. MXenes are easily intercalated with monovalent cations K, Na, Li due to their expanded d-spacing after etching. Based on these ideas, we have developed new synthesis processes of texture functional materials using MXenes as precursors. We have successfully synthesized two-dimensional Nb2C MXene based high aspect ratio ferroelectric potassium niobate (KNbO3) and well-oriented photoluminescent rare earth doped lithium niobate (LiNbO3:Pr3+) crystals, which have great potential in opto-electronics applications. In addition, this thesis demonstrates that poly(vinylidene fluoride) (PVDF)-based percolative composites using two-dimensional (2D) MXene nanosheets as fillers exhibit significantly enhanced dielectric permittivity. Furthermore, we fabricated MXene/in-plane aligned PVDF photo-thermo-mechanical solar tracking actuator for energy harvesting applications.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20699号 / 工博第4396号 / 新制||工||1683(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 西脇 眞二, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Investigations of polarization switching over broad time and electric field domains, in various modified Pb-based perovskite ferroelectrics, were systematically performed by ferroelectric switching current transient and bipolar drive P-E responses. Studies were performed from E«Ec to E»Ec, where Ec is the coercive field These investigations have shown the presence of broad relaxation time distributions for the switching process, which can extend over several decades in order of magnitude in time, and where the distribution is strongly dependent on the applied electric field. By performing the study of domain dynamics and polarization switching over extremely broad time domains (10⁻⁸ t < 10² sec), more complete information has been obtained that allows for development of a better mechanistic understanding. Prior polarization kinetics studies have focused on relatively narrow time ranges, and were fit to the Avarami equation, which contains a single relaxation time. However, our broad band width polarization dynamics and frequency relaxation studies have been fit to multiple stretched exponential functions extending over decades of order of magnitude in the time domain. Stretched exponential functions for domain nuclei formation, and for domain variant growth have been found. For example, [001]c, [110]c, and [111]c oriented PZN-4.5%PT crystals, nucleation was found to be a volume process (n=3) rather than just a domain wall restricted process. Consequently, nucleation is heterogeneous. And, growth of a domain variant with reversed polarization was found to be a boundary process (n=2), involving diffuse or rough domain walls. We have extended these studies to various types of ferroelectrics including hard, soft and relaxor types. / Master of Science

The following investigations have been carried out in this thesis 1)Cordierite is already well known for its low thermal expansion behaviour. Chemical substitutions at various octahedral and tetrahedral sites have been done and their thermal expansion characteristics have been studied Synthesis of cordierite in more reactive environment provided by AlF3 used as sintering aid has been attempted 2) Diffuse ferroelectric phase transition of lead based perovskite materials leads to low expansion region. Solid solutions of lead iron niobate with lead titanate is investigated to increase the structural distortion and see it this low expansion region can be extended to wider temperature Preparation of materials with higher tetragonal distortion In PbTi03- BlFeO3 system is undertaken to study the thermal expansion anisotropy. 3) Composites between lead iron niobate(+(x) and lead titanate (-(x below Tc) has been undertaken to prepare low expansion hulk over a wide temperature range 4) Acoustic emission has been employed as a tool to detect the microcracking in solid solutions between PFN1-x, PTx, and PT1-x, ,BFx, It is hoped to understand relation between magnitude of lattice distortion transition temperature and microcracking in ceramics of the class of materials.

<p>Ferroelectrics are a group of advanced electronic materialswith a wide variety of properties useful in applications suchas memory devices, resonators and filters, infrared sensors,microelectromechanical systems, and optical waveguides andmodulators. Among the oxide perovskite-structured ferroelectricthin film materials sodium potassium niobate or Na0.5K0.5NbO3(NKN) has recently emerged as one of the most promisingmaterials in microwave applications due to high dielectrictunability and low dielectric loss. This licentiate thesispresents results on growth and structural, optical, andelectrical characterization of Na0.5K0.5NbO3 thin films. Thefilms were deposited by rf-magnetron sputtering of astoichiometric, high density, ceramic Na0.5K0.5NbO3 target ontosingle crystal LaAlO3 and Al2O3, and polycrystalline Pt80Ir20substrates. By x-ray diffractometry, NKN films on c-axisoriented LaAlO3 substrates were found to grow epitaxially,whereas films on hexagonal sapphire and polycrystallinePt80Ir20 substrates were found to be preferentially (00l)oriented. Optical and waveguiding properties of theNa0.5K0.5NbO3/Al2O3 heterostructure were characterized using aprism-coupling technique. Sharp and distinguishable transversemagnetic (TM) and electric (TE) propagation modes wereobserved. The extraordinary and ordinary refractive indiceswere calculated to ne = 2.216±0.003 and no =2.247±0.002 for a 2.0 μm thick film at λ = 632.8nm. This implies a birefringence Δn = ne - no =-0.031±0.003 in the film. The ferroelectric state inNKN/Pt80Ir20 films at room temperature was indicated by apolarization loop with polarization as high as 33.4 μC/cm2at 700 kV/cm, remnant polarization of 9.9 μC/cm2 andcoercive field of 91 kV/cm. Current-voltage characteristics ofvertical Au/NKN/Pt80Ir20 capacitive cells and planar Au/NKN/LaAlO3 interdigital capacitors (IDCs) showed very goodinsulating properties, with the leakage current density for anNKN IDC on the order of 30 nA/cm2 at 400 kV/cm. Rf dielectricspectroscopy demonstrated low loss, low frequency dispersion,and high voltage tunability. At 1 MHz NKN/LaAlO3 showed adissipation factor tan δ of 0.010 and a tunability of 16.5% at 200 kV/cm. For the same structure the frequencydispersion, Δεr, between 1 kHz and 1 MHz was 8.5%.</p><p><b>Key words:</b>ferroelectrics, sodium potassium niobates,thin films, rf-magnetron sputtering, waveguiding, refractiveindex, prism coupling, dielectric tunability</p>