Ferroelectric and antiferroelectric ultrathin films have attracted a lot of attention recently due to their remarkable properties and their potential to allow for device miniaturization in numerous applications. However, when the ferroelectric films are scaled down, it brings about an unavoidable depolarizing field. A partial surface charge compensation allows to control the residual depolarizing field and manipulate the properties of ultrathin ferroelectric films. In this dissertation we take advantage of atomistic first-principles-based simulations to expand our understanding of the role of the partial surface charge compensation in the properties of ferroelectric and antiferroelectric ultrathin films. The application of our computational methodology to study the effect of the partial surface charge compensation in ferroelectric ultrathin films led to the prediction that, depending on the quality of the surface charge compensation, ferroelectric thin films respond to an electric field in a qualitatively different manner. They can be tuned to behave like a linear dielectric, a ferroelectric or even an antiferroelectric. This effect was shown to exist in films with different mechanical boundary conditions and different crystal symmetries. There are a number of potential applications where such properties of ferroelectric thin films can be used. One of these potential applications is energy storage. We will show that, in the antiferroelectric regime, ferroelectric thin films exhibit drastic enhancement of energy storage density which is a desirable property. One of the most promising applications of ferroelectric ultrathin films that emerged only recently is the harvesting of the giant electrocaloric effect. Interestingly, despite numerous studies of the electrocaloric effect in ferroelectric thin films, it is presently unknown how a residual depolarizing field affects the electrocaloric properties of such films. Application of state-of-the-art computational methods to investigate the electrocaloric effect in ferroelectric films with partial surface charge compensation led to the prediction that the residual depolarizing field can perform a dual role in the electrocaloric effect in these films. When the depolarizing field creates competition between the monodomain and nanodomain states, we predict an enhancement of the electrocaloric effect due to the frustration that increases the entropy of the state and therefore the electrocaloric temperature change. On the other hand, when the depolarizing field leads to a formation of nanodomains, thin films either exhibit a small electrocaloric effect or lose their electrocaloric properties altogether to the irreversible nanodomain motion. When the residual depolarizing field is weak enough to permit the formation of monodomain phases, the electrocaloric effect is significantly reduced as compared to bulk. We believe that our findings could potentially reveal additional opportunities to optimize solid state cooling technology. While the electrocaloric effect has been a popular topic of interest in recent years [12], there still exists numerous gaps in the fundamental understanding of the effect. In particular, it is presently unknown whether the scaling laws, known to exist for magnetocaloric materials, can be applied to ferroelectric and antiferroelectric electrocalorics. We predict the existence of scaling laws for low-field electrocaloric temperature change in antiferroelectric and ferroelectric materials. With the help of first-principles-based simulations, we showed computationally that the scaling laws exist for antiferroelectric PbZrO3 along with ferroelectrics PbTiO3, BaTiO3 and KNbO3. Additional evidence of the scaling laws existence are provided using experimental data from the literature. Interestingly, our studies on ferroelectric films predicted the existence of antiferroelectric behavior in ultrathin films with partial surface charge compensation. One may wonder whether it is possible to stabilize the ferroelectric phase in antiferroelectric films and what role the surface charge screening would play in such a transition. Motivated to address these fundamental questions, we used computational experiments to study antiferroelectric ultrathin films with a residual depolarizing field. Our studies led to the following predictions. We found that PbZrO3 thin films exhibit the ferroelectric phase upon scaling down and under the condition of efficient surface charge compensation. We also found a strong competition between the antiferroelectric and ferroelectric phases for the thin films of the critical size associated with antiferroelectric-ferroelectric phase transition. This finding motivated us to study the electrocaloric effect in PbZrO3 thin films with antiferroelectric-ferroelectric phase competition. We found that high tunability of the phase transition by the electric field leads to a wide range of temperatures associated with a strong electrocaloric effect. In addition, we found that epitaxial strain provides further tunability to the electrocaloric properties. In summary, our studies led to a broader and deeper understanding of the abundantly many roles surface charge compensation plays in ultrathin ferroelectrics and antiferroelectrics.

Perovskite ferroelectrics and antiferroelectrics have attracted a lot of attention owing to their potential for device applications including THz sensors, solid state cooling, ultra high density computer memory, and electromechanical actuators to name a few. The discovery of ferroelectricity at the nanoscale provides not only new and exciting possibilities for device miniaturization, but also a way to study the fundamental physics of nanoscale phenomena in these materials. Ferroelectric nanowires show a rich variety of physical characteristics which are advantageous to the design of nanoscale ferroelectric devices such as exotic dipole patterns, a strong dependence of the polarization and phonon frequencies on the electrical and mechanical boundary conditions, as well as a dependence of the transition temperatures on the diameter of the nanowire. Antiferroelectricity also exists at the nanoscale and, due to the proximity in energy of the ferroelectric and antiferroelectric phases, a phase transition from the ferroelectric to the antiferroelectric phase can be facilitated through the application of the appropriate mechanical and electrical boundary conditions. While much progress has been made over the past several decades to understand the nature of ferroelectricity/antiferroelectricity in nanowires, many questions remain unanswered. In particular, little is known about how the truncated dimensions affect the soft mode frequency dynamics or how various electrical and mechanical boundary conditions might change the nature of the phase transitions in these ferroelectric nanowires. Could nanowires offer a distinct advantage for solid state cooling applications? Few studies have been done to elucidate the fundamental physics of antiferroelectric nanowires. How the polarization in ferroelectric nanowires responds to a THz electric field remains relatively underexplored as well. In this work, the aim is to to develop and use computational tools that allow first-principles-based modeling of electric-field-induced phenomena in ferroelectric/antiferroelectric nanowires in order to address the aforementioned questions. The effective Hamiltonian approach is a well validated model which reliably reproduces many static and dynamic properties of perovskite ferroelectric and antiferroelectrics. We begin by developing an effective Hamiltonian for the prototypical ferroelectric potassium niobate, a lead-free material which undergoes multiple structural phase transitions. Density functional theory calculations within the LDA and GGA are used to determine the effective Hamiltonian parameters for KNbO3 . By simulating an annealing within an NPT ensemble, we find that the KNbO3 parameters found from first principles underestimate the experimental transition temperatures. We apply a universal scaling technique to all of the first-principles derived parameters and are thus able to more accurately reproduce the transition temperatures predicted by experiment as well as a number of other static and dynamic properties of potassium niobate. Having determined the parameters of the effective Hamiltonian for KNbO3 , we use this as well as previously determined effective Hamiltonian parameters for PbTiO3 and BaTiO3 to study the electrocaloric effect in nanowires made of these materials. We determined that, in general, the electrocaloric effect in ferroelectric nanowires is diminished due to the reduced correlation length resulting from the finite lateral dimensions. However, certain temperature ranges were identified near ambient temperature where the electrocaloric response is enhanced with respect to bulk. The effective Hamiltonian model was also employed to study the response of the spontaneous polarization and temperature to tailored electric fields. We identified a novel means of reversing the polarization in ferroelectric nanowires which could potentially be used in the design of nanoscale THz sensors of ultra high density ferroelectric memory devices. While the soft mode frequency dynamics of bulk ferroelectrics under various mechanical boundary conditions have been studied extensively, the effects of different mechanical boundary conditions on the soft mode dynamics in ferroelectric nanowires remains relatively under-explored. We conduct a comprehensive study on PbTiO3 nanowires which explores the effects of hydrostatic pressure, applied uniaxial stress, and biaxial strain on the structural properties, transition temperatures, and soft mode dynamics. We found that depending on the particular type of mechanical boundary condition, the nanowire can exhibit either monodomain or polydomain vortex phases, drastically different from what is found for PbTiO3 bulk and originates from the critical role of the depolarizing field. We found a rich variety of dipole patterns, particularly for the polydomain states with the dipoles arranged in single and double polarization vortices depending on the type and strength of the mechanical boundary conditions. The soft mode frequency dynamics are also strongly affected by the mechanical boundary conditions. In particular we find that the frequency of the E mode in the P4mm phase is significantly larger than the A 1 mode which is in contrast with bulk PbTiO3 . This striking finding is attributed to the presence of the depolarizing field along the truncated directions which leads to mode hardening. In the last chapter, we identify the emergence of a ferroelectric state in antiferroelectric PbZrO3 nanowires and describe possible ways to stabilize the ferroelectric phase. Finally, we explore how our findings could potentially be used to improve existing technologies such as energy storage devices and electromechanical actuators as well as future technologies like solid state cooling devices.

No description available.

Com base em resultados de DSC, análise estrutural por difração de raios X e observações de textura, analisamos os efeitos da mistura do solvente apolar hexadecano com cristais líquidos formados pelas moléculas banana 1,3-fenilenobis[4-(4-tetradecilfeniliminometil)benzoato] (MB14) e 4-cloro-1,3-fenilenobis[4-(4-tetradecilfeniliminometil) benzoato] (MB14Cl). Propusemos um modelo estrutural para explicar as modificações causadas no arranjo molecular da fase B2 pelo acréscimo gradual do solvente. Observamos a diminuição da temperatura de transição entre esta fase e a fase isotrópica, porém a transição entre a fase B2 e a fase a temperaturas mais baixas não sofre alteração significativa. Para concentrações em massa de hexadecano de 45 % no MB14 e de 55 % no MB14Cl, a fase B2 já não é mais observada. Para o MB14Cl, resultados de difração de raios X revelaram que as moléculas de hexadecano penetram entre as camadas esméticas, aumentando a distância intercamada em torno de 3 Å. Acima de 5 % de concentração do solvente, o aumento da distância intercamada satura e ocorre segregação de fases em escala nanométrica. O comportamento da fase B2 sob a ação de um campo elétrico variável também foi analisado para o MB14 puro. Apresentamos um modelo para a linha de base do sinal de corrente de polarização, que leva em conta a não linearidade da condutividade para valores altos de campo aplicado, devido à movimentação iônica na amostra. Para o cálculo da viscosidade, consideramos a não linearidade da constante dielétrica com o campo aplicado, e adaptamos outro modelo, inicialmente utilizado para cristais líquidos ferroelétricos sob a ação de um campo quadrado, para o caso de um cristal líquido antiferroelétrico sob um campo triangular. Quanto aos dois tipos de arranjo molecular da fase B2, o arranjo homoquiral se mostrou bem mais estável que o racêmico, mesmo sob aplicação de campo triangular, quando este último é inicialmente favorecido. O arranjo racêmico se apresentou mais viscoso que o arranjo homoquiral, contrariando nossas previsões. / Based on DSC results, structural analysis by X-ray diffraction and texture observations, we observed the effects of mixing the nonpolar solvent hexadecane with the banana molecules liquid crystals ,3-phenilenebis[4-(4-tetradecilpheniliminometil)benzoate] (MB14) and 4-chloro-1,3-phenilenebis[4-(4-tetradecilpheniliminometil) benzoate] (MB14Cl). We propose a structural model to explain the changes in the molecular ordering of the B2 phase caused by the gradual increase of the solvent. We observed a decreasing of the transition temperature between B2 and isotropic phases, however the transition between B2 and lower temperature phases did not change significantly. For hexadecane concentrations above 45 wt% in MB14 and 55 wt% in MB14Cl, the B2 phase is no longer present. In MB14Cl, X-ray diffraction results showed that the hexadecane molecules penetrate between the smectic layers, increasing the interlayer spacing by about 3 Å. Above 5 wt% of solvent concentration, the increasing of the interlayer spacing saturates, and a phase segregation in nanometric scale occurs. The behavior of the B2 phase under variable electric field was also analysed for the pure MB14. We present a model for the baseline of the polarization current signal, which considers the non-linearity of the conductivity for high values of applied field, due to the presence of ions in the sample. In order to calculate the viscosity, we considered the non-linearity of the dielectric constant with the applied field, and adapted another model, initially used in ferroelectric liquid crystals under rectangular field, for the case of an antiferroelectric liquid crystal under triangular field. Concerning the two kind of molecular ordering in the B2 phase, the homoquiral ordering proved to be far more stable than the racemic, even under triangular field, when the latest is favored. Our measurements resulted in a racemic ordering more viscous than the homoquiral, going against our predictions.

Metal oxides crystallized in perovskite structure are generally modified in two different ways. According to the general structural formula ABO3, the two ways are A-site modification and B-site modification. The primary significance of perovskite metal oxides rests on their importance in electronic devices. A particular class of perovskites, namely Lead Zirconate or modified Lead Zirconate has received a special attention because of their unique antiferroelectricity and various applications in devices. Among the other modifications, A-site modification of PbZrO3 by La is rare and not much explored. Chapter 1 describes various applications of antiferroelectric thin films along with the synthesis and characterization of pure and La modified PbZrO3, which are relevant to the work presented in this thesis. Sol-gel processing and spin coating technique to deposit solid oxide thin films are well known for their low cost of deposition as well as for their ability to achieve better stoichiometric chemical composition. Common crack formation problem of sol-gel grown films can be prevented by ‘drying control chemical adhesive’ like polyvinylpyrrolidone (PVP). Heat treatment of sol-gel derived thin films is generally determined by TGA and DTA. Crystalline phase of deposited solid thin films is determined by XRD whereas effect of modification can be ascertained by XRD peak assignment and relative crystalline peak shifting. Sol-gel grown film thickness is measured by common cross sectional SEM whereas AFM can detail the surface morphology. Chapter 2 summarizes the deposition and characterization of pure and La modified PbZrO3 thin films. Any nonmetal, which is insulator, is dielectric material and show dielectric dispersion in a frequency domain of low field alternative current. Among the most common feature of dielectric dispersion, Maxwell – Wagner type dispersion is well known. Similar kind of dielectric dispersion, named Maxwell – Wagner like dispersion, can be observed while the equivalent circuit consists of parallel G – C along with a series R. Universal power law of ac conductivity is the deciding factor to distinguish the nature of dispersion. Structural phase transition can be determined by dielectric response and it is widely known as dielectric phase transition. Effect of La modification on dielectric phase transition of PbZrO3 thin films depends on stabilization or destabilization of antiferroelectricity. Maximum dielectric constants of pure and modified PbZrO3 thin films depend on the crystallographic orientations of the growth. Chapter 3 presents dielectric properties of pure and La modified PbZrO3 thin films and these properties are correlated to the stabilization or destabilization of antiferroelectricity, relative integrated intensity of (202)O film orientation and trapped electron charge due to oxygen vacancies. Charge storage property of a capacitor is determined by the polarization of the capacitor on application of electric field whereas field dependent integrated area of polarization on withdrawal of electric field determines the recoverable capacitive energy storage. Among the three kinds of capacitors like linear or paraelectric, ferroelectric and antiferroelectric capacitors, antiferroelectric capacitor is known to be best for their ability to store huge amount of recoverable energy. The recoverable energy in antiferroelectrics can be increased by increasing squareness of the P – E hysteresis loop, applicable electric field, polarization or by the all possible combinations of them. Chapter 4 describes the correlation of relative integrated intensity of (202)O [RI(202)O] with critical applied electric field of P – E saturation to provide enhanced squareness of the hysteresis loops. This chapter also describes the variation of charge and recoverable energy storage properties with respect to RI(202)O. Like magnetocaloric effect, electrocaloric effect is capable to alter the temperature of a system by adiabatic polarization or depolarization. From the Maxwell’s relation of thermodynamics, assuming, (∂p ) = (∂s )electrocaloric effect can be calculated from temperature dependent polarization value of a paraelectric, ferroelectric or an antiferroelectric. Chapter 5 presents the electrocaloric effect of pure and La modified PbZrO3 thin films. Summary of present study and discussion have been delineated in Chapter 6 along with the future work which can give more insight into the understanding of antiferroelectric PbZrO3 thin films with respect to Pb and Zr site modification and with respect to different electrodes. (For formulas pl see the pdf file of the thesis)

Com base em resultados de DSC, análise estrutural por difração de raios X e observações de textura, analisamos os efeitos da mistura do solvente apolar hexadecano com cristais líquidos formados pelas moléculas banana 1,3-fenilenobis[4-(4-tetradecilfeniliminometil)benzoato] (MB14) e 4-cloro-1,3-fenilenobis[4-(4-tetradecilfeniliminometil) benzoato] (MB14Cl). Propusemos um modelo estrutural para explicar as modificações causadas no arranjo molecular da fase B2 pelo acréscimo gradual do solvente. Observamos a diminuição da temperatura de transição entre esta fase e a fase isotrópica, porém a transição entre a fase B2 e a fase a temperaturas mais baixas não sofre alteração significativa. Para concentrações em massa de hexadecano de 45 % no MB14 e de 55 % no MB14Cl, a fase B2 já não é mais observada. Para o MB14Cl, resultados de difração de raios X revelaram que as moléculas de hexadecano penetram entre as camadas esméticas, aumentando a distância intercamada em torno de 3 Å. Acima de 5 % de concentração do solvente, o aumento da distância intercamada satura e ocorre segregação de fases em escala nanométrica. O comportamento da fase B2 sob a ação de um campo elétrico variável também foi analisado para o MB14 puro. Apresentamos um modelo para a linha de base do sinal de corrente de polarização, que leva em conta a não linearidade da condutividade para valores altos de campo aplicado, devido à movimentação iônica na amostra. Para o cálculo da viscosidade, consideramos a não linearidade da constante dielétrica com o campo aplicado, e adaptamos outro modelo, inicialmente utilizado para cristais líquidos ferroelétricos sob a ação de um campo quadrado, para o caso de um cristal líquido antiferroelétrico sob um campo triangular. Quanto aos dois tipos de arranjo molecular da fase B2, o arranjo homoquiral se mostrou bem mais estável que o racêmico, mesmo sob aplicação de campo triangular, quando este último é inicialmente favorecido. O arranjo racêmico se apresentou mais viscoso que o arranjo homoquiral, contrariando nossas previsões. / Based on DSC results, structural analysis by X-ray diffraction and texture observations, we observed the effects of mixing the nonpolar solvent hexadecane with the banana molecules liquid crystals ,3-phenilenebis[4-(4-tetradecilpheniliminometil)benzoate] (MB14) and 4-chloro-1,3-phenilenebis[4-(4-tetradecilpheniliminometil) benzoate] (MB14Cl). We propose a structural model to explain the changes in the molecular ordering of the B2 phase caused by the gradual increase of the solvent. We observed a decreasing of the transition temperature between B2 and isotropic phases, however the transition between B2 and lower temperature phases did not change significantly. For hexadecane concentrations above 45 wt% in MB14 and 55 wt% in MB14Cl, the B2 phase is no longer present. In MB14Cl, X-ray diffraction results showed that the hexadecane molecules penetrate between the smectic layers, increasing the interlayer spacing by about 3 Å. Above 5 wt% of solvent concentration, the increasing of the interlayer spacing saturates, and a phase segregation in nanometric scale occurs. The behavior of the B2 phase under variable electric field was also analysed for the pure MB14. We present a model for the baseline of the polarization current signal, which considers the non-linearity of the conductivity for high values of applied field, due to the presence of ions in the sample. In order to calculate the viscosity, we considered the non-linearity of the dielectric constant with the applied field, and adapted another model, initially used in ferroelectric liquid crystals under rectangular field, for the case of an antiferroelectric liquid crystal under triangular field. Concerning the two kind of molecular ordering in the B2 phase, the homoquiral ordering proved to be far more stable than the racemic, even under triangular field, when the latest is favored. Our measurements resulted in a racemic ordering more viscous than the homoquiral, going against our predictions.

The ferroelectricity in fluorite structure based hafnium oxide (HfO2) material expanded the horizon for realizing nonvolatile ferroelectric memory concepts. Due to the excellent HfO2 ferroelectric film properties, CMOS compatibility, and scalability; the material is foreseen as a replacement of the lead based ferroelectric materials with a big game changing potential for the emerging ferroelectric memories. In this thesis, the development of novel memory concepts based on the ferroelectric or antiferroelectric HfO2 material is reported. The ferroelectric field effect transistor (FeFET) memory concept offers a low power, high-speed, nonvolatile, and one cell memory solution ideal for embedded memory realization. As an emerging concept based on a novel ferroelectric material, the FeFET is challenged with key performance aspects intrinsic to the underlying physics of the device. A central part of this thesis is the development of FeFET through material and gate stack engineering, in turn leading to innovative novel device concepts. The conceptual innovation, process development, and electrical assessment are explored for an ferroelectric or antiferroelectric HfO2 based nonvolatile memories with focus on the underlying device physics. The impact of the ferroelectric material on the FeFET physics is explored via the screening of different HfO2 based ferroelectric materials, thicknesses, and the film doping concentration. The impact of material interfaces and substrate doping conditions are explored on the stack engineering level to achieve a low power and reliable FeFET. The material optimization leads to the concept of ferroelectric lamination, i.e. a dielectric interlayer between multi ferroelectric ones, to achieve a novel multilevel data storage in FeFET at reduced device variability. Toward a low power FeFET, the stack structure tuning and dual ferroelectric layer integration are explored through an MFM and MFIS integration in a single novel FeFET stack. The charge trapping effect during the FeFET switching captures the dynamics of the hysteresis polarization switching inside the stack with direct impact on the interfacial layer field. Even though manifesting as a clear drawback in FeFET operation, it can be utilized in Flash, leading to a novel hybrid low power and high-speed antiferroelectric based charge trap concept. Furthermore, the FeFET reliability is studied covering the role of operating temperature and the ferroelectric wakeup phenomenon observed in the FeFET. The temperature modulated operation, role of the high-temperature pyroelectric effect, and the temperature induced endurance and retention reliability are studied.:Table of Contents Abstract Table of Contents 1. Introduction 2. Fundamentals 2.1. Basics of Ferroelectricity 2.2. The FeFET Operation Principle and Gate Stack Theory 2.3. Structure and Outline of the PhD Thesis 3. The Emerging Memory Optimization Cycle: From Conceptual Design to Fabrication 3.1. The FeFET Conceptual Design and Layout Implementation 3.2. Gate First FeFET Fabrication: Material and Gate Stack Optimization 3.3. Novel Gate First based Memory Concepts: Device Integration and Stack Optimization 3.4. Device Characterization: Electrical Testing Schemes 4. The Emerging FeFET Memory: Material and Gate Stack Optimization 4.1. Material Aspect of FeFET Optimization: Role of the FE Material Properties 4.2. The Stack Aspect of FeFET Optimization: Role of the Interface Layer Properties 4.3. The Stack Aspect of FeFET Optimization: Role of the Substrate Implant Doping 4.4. Summary 5. A Novel Multilevel Cell FeFET Memory: Laminated HSO and HZO Ferroelectrics 5.1. The Laminate MFM and Stack Characteristics 5.2. The Laminate based FeFET Memory Switching 5.3. The Laminate FeFET Multilevel Coding Operation (1 bit, 2 bit, 3 bit/cell) 5.4. The Maximum Laminate FeFET MW Dependence on FE Stack Thickness 5.5. The Role of Wakeup and Charge Trapping 5.6. The Laminate MLC FeFET Area Dependence 5.7. The Laminate MLC Retention and Endurance 5.8. Impact of Pass Voltage Disturb on Laminate based NAND Array Operation 5.9. The Laminate FeFET based Synaptic Device 5.10. Summary 6. A Novel Ferroelectric MFMFIS FeFET: Toward Low Power and High-Speed NVM 6.1. The MFMFIS FeFET P-E and FET Characteristics 6.2. The MFMFIS based Memory Characteristics 6.3. The Impact of MFMFIS Stack Structure Tuning 6.4. The Maximum MFMFIS FeFET Memory Window 6.5. The Role of Device Scalability and Variability 6.6. The MFMFIS Area Tuning for Low Power Operation 6.7. The MFMFIS based FeFET Reliability 6.8. The Synaptic MFMFIS based FeFET 6.9. Summary 7. A Novel Hybrid Low Power and High-Speed Antiferroelectric Boosted Charge Trap Memory 7.1. The Hybrid Charge Trap Memory Switching Characteristics 7.2. The Role of Polarization Switching on Optimal Write Conditions 7.3. The Impact of FE/AFE Properties on the Charge Trap Maximum Memory Window 7.4. The Hybrid AFE Charge Trap Multi-level Coding and Array Operation 7.5. The Global Variability and Area Dependence of the Charge Trap Memory Window 7.6. The AFE Charge Trap Reliability 7.7. The Hybrid AFE Charge Trap based Synapse 7.8. Summary 8. The Emerging FeFET Reliability: Role of Operating Temperature and Wakeup Effect 8.1. The FeFET Temperature Reliability: A Temperature Modulated Operation 8.2. The FeFET Temperature Reliability: Role of the Pyroelectric Effect 8.3. The FeFET Temperature Reliability: Endurance and Retention 8.4. The Impact of Ferroelectric Wakeup on the FeFET Memory Reliability 8.5. Summary 9. Closure: What this Thesis has Solved? 9.1. How material selection/development influence the FeFET? 9.2. Why the FeFET Still Operates at High Write Conditions? 9.3. Why the FeFET Endurance is still a Challenge? 9.4. Can the FeFET become Multi-bit Storage Memory? 9.5. How the Scalability Determine FeFET Chances? 10. Summary 11. Bibliography List of symbols and abbreviations List of Publications Acknowledgment Erklärung

The group of materials with ABO3 type perovskite structure are very important due to their attractive electrical and magnetic properties for technological applications and have been studied in the form of single crystals, bulk polycrystalline materials and thin films. Recently, efforts have been made to synthesize and understand the growth of ABO3 type perovskite nanostructures because of their distinctive physical properties and potential applications in the nanodevices. The primary aim of the present thesis is to synthesize the perovskites at nano-scale, with zero-dimension (0D), and one-dimension (1D) configurations. Basic work was carried in terms of synthesis – structure – composition correlation. Due to the small nature of the synthesized materials, few attempts were done to examine the physical properties, but to a limited extant. Efforts were also done to emphasize the structural behavior of nano perovskite in comparison with their bulk counterparts. Chapter 1 provides a brief introduction to perovskite materials and nanostructures, their technological applications and the fundamental physics involved. A brief review of the perovskite nanostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the experimental studies carried out in this thesis. It describes the methods used for the synthesis, experimental set up and the basic operation principles of various structural and physical characterizations such as X-ray diffraction (XRD), thermal analysis, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), compositional analysis (EDX), focused ion beam (FIB), electrical and magnetic studies of the materials prepared. Chapter 3 describes the fabrication of porous anodic aluminum oxide (AAO) templates with different pore size, basic steps for synthesis of nanotubes and the possible growth mechanism of nanotubes in the AAO template. In chapter 4, we report the synthesis of ferroelectric Ba1-xSrxTiO3 (x = 0.0, 0.3) nanoparticles (diameter range: 20-40nm) and Ba1-xSrxTiO3 (x = 0.0, 0.4) nanotubes with diameter about 200nm by the sol-gel method. The Ba1-xSrxTiO3 nanostructures so obtained were characterized by number of techniques, including FE-SEM, XRD, DTA/TGA, FTIR spectroscopy, TEM, HRTEM as well as EDX and SAED. Formation of Y-junctions and multi-branches in Ba1-xSrxTiO3 nanotubes were also observed. The wall of the nanotubes were found to be made of randomly oriented nanoparticles which were confirmed from the HRTEM image. The average thickness of the wall of the nanotubes was found around 15(±5) nm and nanoparticles consisting the wall were found to be in the range of 5-10nm. Diffused phase transition (cubic to tetragonal), shifted to lower temperature side and leaky ferroelectric P–E loops were observed in Ba1-xSrxTiO3 (x = 0.0) ceramic prepared from nanoparticles. Curie temperature was observed at 120oC in the BT nanotube array as confirmed by the dielectric study. The P–E loops of as-prepared Ba1-xSrxTiO3 (x = 0.0) nanotube array were also measured and the hysteresis clearly demonstrates the room temperature ferroelectricity in the as prepared nanotubes, indicating these nanotube array is potential media as ferroelectric information storage. In chapter 5, we report the synthesis of single crystalline nanoparticles and polycrystalline nanotubes of Pb0.76Ca0.24TiO3 (PCT24) by sol-gel processing and characterized by various techniques. The crystallinity and phase purity of the PCT24 nanoparticles and nanotubes were confirmed by the XRD and SAED pattern. Compositional homogeneity and their crystalline structure confirms the formation of the tetragonal perovskite phase. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotubes was found around 20nm and nanoparticles consisting the wall were found to be in the range of 5-8nm. Formation of some single crystalline PCT24 nanorods was also observed as confirmed by SAED and HRTEM analysis. Formations of Y-junctions and multi-branches in this complex functional oxide were observed. Dielectric measurements shows the diffuse phase transition and frequency dependence of Tm (temperature at which real part of dielectric constant shows maxima) suggesting the relaxor type behavior in the PCT24 ceramic prepared from nanoparticles. Polarization study was carried out on PCT24 nanotube array, which shows the ferroelectric nature at room temperature. Chapter 6 reports the synthesis and studies of PbZrO3 (PZ) nanoparticles and PbZr1-xTixO3 for x = 0.0, 0.48 and 1.0 nanotubes. PZ nanoparticles were prepared by a novel sol-gel method based on diol-based solution. Initially, PZ was crystallized with some intermediate m-Z and t-Z phases at 400-550oC and start transforming to orthorhombic at around 600oC and then finally transformed into pure orthorhombic PZ phase at about 700oC. XRD and TEM confirmed the nanocrystalline nature of PZ particles. Curie temperature in the PZ ceramic prepared from PZ nanoparticles was observed around at 205oC, which is lower as compared to the bulk (233oC). P–E hysteresis loops of PZ ceramic prepared from nanoparticles were measured at different applied voltages and single ferroelectric loops of leaky nature were observed rather than antiferroelectrics. The lead zirconate nanoparticles produced may have potential applications as materials used in microelectronics and microelectromechanical systems. PbZr1-xTixO3 for x = 0.0 (PZ), 0.48 (PZT48) and 1 (PT) nanotubes were fabricated by sol-gel method within the closely packed porous alumina templates and characterized by various techniques. The crystallinity of the PZ, PZT48 and PT nanotubes were confirmed via XRD and SAED studies. EDX analysis demonstrated that stoichiometry was formed. Formation of Y-junctions in this complex functional oxide was also observed. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles, which were confirmed by the HRTEM studies and also by a typical SEM image. The average thickness of the wall of the nanotubes was found to be around 10-20nm and nanoparticles consisting the wall was found to be in the range of 3 – 8nm. The Curie temperature was observed at 220oC in the PZ nanotube array. For the first time, PLD has been employed for the synthesis of lead zirconate nanotubes using AAO template. Well-registered arrays of these nanotubes could function as three dimensional (3D) device elements in miniaturized ferroelectric random access memory (FRAM). In chapter 7, we report the synthesis of single crystalline 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT) nanoparticles. PMN-PT nanoparticles were developed by a novel sol-gel method based on diol route. After partial calcination at 450oC/1h, PMN-PT powder morphology started transforming from pyrochlore to perovskite phase. It is interesting to note that this partially crystallized PMN-PT powder was unstable under electron beam and generated freestanding lead nanoparticles after absorbing energy from a focused electron beam. PMN-PT powder annealed at 700°C was fully transformed to perovskite phase and was stable under electron beam. XRD calculations and TEM imaging confirmed the nanocrystalline nature of PMN-PT particles. Magnetic measurements on PMN-PT nanoparticles prepared at 650 and 750oC show room temperature ferromagnetic hysteresis, whereas the bulk or the agglomerated particles show diamagnetic behavior. With an increase of annealing temperature or the particle size the magnetic moment decreases. PMN-PT nanotubes with diameter about 200nm were fabricated successfully by the sol-gel method based on diol route within the closely packed porous nanochannel alumina templates. Phase purity and crystalline perovskite phase formation of PMN-PT nanotubes were confirmed by the XRD and SAED pattern. EDX analysis demonstrated that stoichiometry was formed within accepted limit. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotube was found around 20 nm and nanoparticles consisting the wall were found to be in the range of 10-20 nm. Since electroceramic materials are following a similar trend to miniaturization as conventional semiconductors, the synthesis of nanosized oxidic building blocks is moving into the focus of scientific and technological interest. Ferroelectrics are promising class of materials for the fabrication of electronic devices, as they are already an integral part of modern nanotechnological operations. Chapter 8 deals with the synthesis and properties of BiFeO3 (BFO) nanoparticles and nanotubes. Single crystalline BFO nanoparticles of different size and polycrystalline BFO nanotubes were prepared by sol-gel method. As prepared nanostructures were characterized by various techniques such as XRD, TGA-DTA, FTIR, scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM and energy-dispersive X-ray spectroscopy (EDX). The crystallinity and phase purity of the BFO nanoparticles and nanotubes were confirmed by the XRD, SAED pattern and HRTEM analysis. Compositional homogeneity and their crystalline structure confirms the formation of the rhombohedrally distorted perovskite phase. EDX analysis demonstrated that stoichiometric BiFeO3 was formed within accepted limit. The HRTEM analysis confirmed that wall of the BFO nanotubes was made of nanoparticles, which were randomly oriented in the wall. The average thickness of the wall of the nanotubes was found to be around 15 nm and nanoparticles consisting the wall were found to be in the range of 3-6nm. Formation of Y-junctions in this complex functional oxide was observed. Magnetic measurements show clearly the enhancement of ferromagnetism in BFO nanotubes and ferroelectric loops were also observed in these nanotubes, that indicates the multiferroic nature of these nanotubes. BFO nanostructures at a large scale might be important for many applications such as memory elements in nanoscale devices in future. Chapter 9 reports the synthesis of a series of crystalline La1-xCaxMnO3 (x = 0, 0.3, 0.5, 0.7) nanoparticles with average diameter about 20 nm by an improved sol-gel method. The crystallinity and phase formation of as prepared nanoparticles was confirmed via XRD, SAED and HRTEM studies. EDX analysis demonstrated that desired stoichiometric was formed. Magnetic characterization reveals that the PM-FM transitions (Tc) occurs around at 205, 235, 235 and 230 K for x = 0, 0.3, 0.5, 0.7, respectively. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves, a cusplike peak in ZFC curve and unusual shape of M versus H loop at T = 5 K gives strong support for surface spin glass behavior. The highly stable charge ordering state in bulk manganites is suppressed, while the ferromagnetism is enhanced in these nanoparticles (x = 0.5 and 0.7). La0.7Ca0.3MnO3 were fabricated by sol-gel method within the closely packed porous alumina templates. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles (8-12nm) as confirmed by HRTEM studies. The strong irreversibility between ZFC and FC magnetization curves as well as a cusplike peak in ZFC curve gives strong support for surface spin glass behavior. Magnetization value as obtained from M-H loop was about 28.5% of expected value, suggesting the existence of a magnetic dead layer, which avoids the propagation of exchange interaction between magnetic grains. The PM-FM transition was observed at 235 K. Chapter 10 gives the summary and conclusions of the present study and also discusses the possible future work that could after more insights into the understanding of the perovskite nanostructures. Highlight of the present work (i) Successful growth of nanostructures in both particles and tube forms, and study of their structure – composition correlations. (ii) Present work could optimize the necessary chemistry to successfully grow nanoparticles and nanotubes of various perovskite compositions. (iii) Successful studies of physical properties of nanoparticles and nanotubes, ofcourse, to a limited extent. However the properties observed in the present nanostructures have a strong indication of nonlinear phenomena similar to their bulk counterparts. (iv) It was reported in the literature, the observation of ferromagnetic behavior in several nonmagnetic compositions at nano-scale. Surprisingly, similar ferroelectric behavior was noticed even in our perovskite complex oxides such as relaxors (PMN-PT). A clear interaction of magnetic spin and an electric dipole was evident in these oxides such as relaxors and also multiferroics at nano-scale (~10-20 nm). (v) In ferromagnetic compositions such as LCMO, a very interesting spin-glass type behavior was observed.