Ferroelectric lead zirconate titanate thin films, Pb(ZrxTi 1-x)0 3 or PZT, have aroused considerable interest in recent years for the application in nonvolatile electronic memories because of their excellent ferroelectric properties. In this research, PZT thin films were studied from two aspects: the scientific aspect and the technical aspect. The optical properties of PZT solid solutions and the structure development in PZT films were extensively investigated in the scientific aspect. The PZT films used in this part of study were prepared by metalorganic decomposition (MOD) process. The envelope method, with consideration of light intensity loss from the back surface of the substrate, was demonstrated to be a simple and convenient tool for obtaining the optical properties of the PZT films in the medium and weak absorption regions. In the near optical band gap region, both the transmission and reflection spectra were used to successfully calculate the optical constants of the films. The film thickness derived from the envelope method was cross checked by a computer simulation method and was found to have an accuracy better than 2%. An effective, versatile, and nondestructive optical method was developed for the study of the structure development in MOD PZT films. Also, the models for the structure development were proposed and were verified by this optical method. Using this method, the characteristic temperatures (i.e., the initiation and completion temperatures) of each phase can be easily identified. In addition, the volume fraction of the perovskite phase in the pyrochlore-perovskite phase transformation region was obtained from this optical method. From the technical point of view, ferroelectric PZT films were successfully and reproducibly deposited for the first time by hot-wall metalorganic chemical vapor deposition (MOCVD). One of the problems associated with the MOCVD technique is the availability of the precursors. After intensive studies searching for the most suitable precursors for MOCVD PZT thin films, the safe and stable precursors, namely lead tetramethylheptadione [Pb(thd)2)], zirconium tetramethylheptadione [Zr(thd)4], and titanium ethoxide [Ti(OEt)4] were chosen. The films were deposited at temperatures as low as 55QOC and had pure perovskite phase in the as-deposited state. Also, the films were smooth, specular, crack-free, uniform, and adhered well on the substrates. The stoichiometry of the films can be easily controlled either by varying the individual precursor temperature and/or the flow rate of the carrier gas. Auger electron spectroscopic (AES) depth profile showed good uniformity through the thickness of the films. The AES spectra also showed no carbon contamination in the bulk of the films. As-deposited films were dense and showed uniform and fine grains. The film (Pb/Zr/Ti = 50/41/9) annealed at 6QQOC showed a spontaneous polarization of23.3 pC/cm² and a coercive field of 64.5 kV /em. / Ph. D.

Lead zirconate titanate (PZT) is a ferroelectric material which has many interesting properties. Recently, PZT thin films have been considered as one of the most promising materials for the application of nonvolatile electronic memories. In this study, a sol-gel process for PZT film preparation was adopted and greatly modified. PZT films with very desirable electrical properties have been successfully prepared by this modified sol-gel process. One of the problems of incorporating PZT films into the DRAM devices is the need of high post-deposition annealing temperatures which complicates their integration into the existing semiconductor manufacturing process. In this work, formation kinetics of PZT films were studied and the nucleation was found to be the rate-limiting step in the formation of the perovskite phase. Based on this finding, a seeding process was invented to encourage the nucleation of the perovskite phase. As a result of this seeding process, the transformation temperature has been lowered by as much as 100°C. The seeded PZT films also have good ferroelectric properties. The ferroelectric domain structures, and the metastable pyrochlore phase including its transformation to the perovskite phase have been investigated by transmission electron microscopy (TEM). The domain structures of the PZT films had the {lID} <110> orientation and most of them were 90° domains. The TEM study of the pyrochlore to perovskite transformation provides valuable insight on the formation of the perovskite phase. Among all the processing steps, the drying process of the sol-gel films created the highest growth stress. In addition, the thin film stress study was also used to determine the transformation stress and Curie temperature. The effects of composition, thermal processing conditions, and film thickness on electrical properties have been studied. Some of the notable results are as follow: (1) PZT films with a Zr/Ti ratio of 53/47, the morphotropic boundary (MPB) composition, have the highest remanent polarization and the lowest coercive field. (2) The optimum annealing temperatures for most of the PZT compositions are found to be about 50°C higher than the completion temperature of the perovskite formation (T_c^per) of the same composition. (3) PZT films with film thicknesses greater than or equal to 170 nm have electrical properties very close to those of the thicker films and are not susceptible to dielectric breakdown at an applied voltage of 5 V. / Ph. D.

Ferroelectric materials have been an area of keen interest for researchers due to their useful electro-mechanical coupling properties for a range of modern applications, such as sensing, precision actuation, or energy harvesting. The distribution of the piezoelectric coefficients, which corresponds to the piezoelectric properties, in traditional crystalline ferroelectric materials are determined by their inherent crystalline structure. This restriction limits the tunability of their piezoelectric properties. In the present work, ferroelectric lattice materials capable of a wide range of rationally designed piezoelectric coefficients are achieved through lattice micro-architecture design. The piezoelectric coefficients of several lattice designs are analyzed and predicted using an analytical volume-averaging approach. Finite element models were used to verify the analytical predictions and strong agreement between the two sets of results were found. Select lattice designs were additively manufactured using projection microstereolithography from a PZT-polymer composite and their piezoelectric coefficients experimentally verified and also found to be in agreement with the analytical and numerical predictions. The results show that the use of lattice micro-architecture successfully decouples the dependency of the piezoelectric properties on the material's crystalline structure, giving the user a means to tune the piezoelectric properties of the lattice materials. Real-world application of a ferroelectric lattice structure is demonstrated through application as a multi-directional stress sensor. / Master of Science / Ferroelectric materials have been an area of keen interest for researchers due to their useful electro-mechanical coupling properties for a range of modern applications, such as sensing, precision actuation, or energy harvesting. However, the piezoelectric properties of traditional materials are not easily augmented due to their dependency on material crystalline structure. In the present work, material architecture is investigated as a means for designing new piezoelectric materials with tunable sets of piezoelectric properties. Analytical predictions of the properties are first obtained and then verified using finite element models and experimental data from additively manufactured samples. The results indicate that the piezoelectric properties of a material can in fact be tuned by varying material architecture. Following this, real-world application of a ferroelectric lattice structure is demonstrated through application as a multi-directional stress sensor.

We developed a model to investigate the dielectric properties of the BaTiO₃ − Bi(Zn₁/₂Ti₁/₂)O₃ (BT-BZT) solid solution, which is a relaxor ferroelectric and exhibits long range disorder. The model uses ab initio methods to determine all polarization states for every atomic configuration of 2 x 2 x 2 supercells of BT-BZT. Each supercell is placed on a lattice with an Ising-like interaction between neighboring cell polarizations. This method allows us to consider long range disorder, which is not possible with ab initio methods alone, and is required to properly understand relaxor ferroelectric materials. We analyze the Monte Carlo data for a single lattice configuration using the multiple histogram method, and develop a modified histogram technique to combine data from multiple lattice configurations. Our calculated values of dielectric constant, specific heat, and polarization agree reasonably well with experiment. / Graduation date: 2012

Pulsed laser deposition has been used to grow Pb(Zr₀.₅₂Ti₀.₄₈)O₃ (PZT)/LaNiO₃ (LNO) heterostructures with restricted crystallographic orientations on bare Si(001) and SiO₂-coated Si(001) substrates, using TiN buffer layers. The effect of background gas pressure on orientation of the thin films was investigated in detail. XRD analyses showed that under optimized conditions, (001)-oriented PZT/LNO/TiN heterostructures could be grown on either Si(001) or SiO₂/Si substrates. The (001)-textured PZT films had remnant polarizations as high as 23µC/cm², and also had a low coercive field. Up to 10¹⁰ switching cycles have been achieved in these PZT films. / Singapore-MIT Alliance (SMA)

The primary focus of this research is to investigate the non-linear behavior of single crystal and polycrystalline relaxor ferroelectric PMN-xPT and PZN-xPT through experimentation and modeling. Characterization of single crystal and polycrystalline specimens with similar compositions was performed. These data give experimental insight into the differences that may arise in a polycrystal due to local interaction with inhomogeneities. Single crystal specimens were characterized with a novel experimental technique that reduced clamping effects at the boundary and gave repeatable results. The measured experimental data was used in conjunction with electromechanical characterizations of other compositions of single crystal specimens with the same crystallographic orientation to study the compositional effects on material properties and phase transition behavior. Experimental characterization provided the basis for the development of a model of the continuous phase transformation behavior seen in PMN-xPT single crystals. In the modeling it is assumed that a spatial chemical and structural heterogeneity is primarily responsible for the gradual phase transformation behavior observed in relaxor ferroelectric materials. The results are used to simulate the effects of combined electrical and mechanical loading. An improved rate-independent micromechanical constitutive model based on the experimental observations of single crystal and polycrystalline specimens under large field loading is also presented. This model accounts for the non-linear evolution of variant volume fractions. The micromechanical model was calibrated using single crystal data. Simulations of the electromechanical behavior of polycrystalline ferroelectric materials are presented. These results illustrate the effects of non-linear single crystal behavior on the macroscopic constitutive behavior of polycrystals.

<p>To sustain the upcoming paradigm shift in computations technology efficiently, innovative solutions at the lowest level of the computing hierarchy (the material and device level) are essential to delivering the required functionalities beyond what is available with current CMOS platforms. Motivated by this, in this dissertation, we explore ferroelectric-based devices for steep-slope logic and energy-efficient non-volatile-memory functionalities signifying the novel device attributes, possibilities for continual dimensional scaling with the much-needed enhancement in performance.</p> <p> </p> <p>Among various ferroelectric (FE) materials, Zr doped HfO₂ (HZO) has gained immense research attention in recent times by virtue of CMOS process compatibility and a considerable amount of ferroelectricity at room temperature. In this work, we investigate the Zr concentration-dependent crystal phase transition of Hf_1-xZ_xO₂ (HZO) and the corresponding evolution of dielectric, ferroelectric, and anti-ferroelectric characteristics. Providing the microscopic insights of strain-induced crystal phase transformations, we propose a physics-based model that shows good agreement with experimental results for 10 nm Hf_1-xZ_xO₂. Further, in a heterogeneous system, ferroelectric materials can exhibit negative capacitance (NC) behavior. Such NC effects may lead to differential amplification in local potential and can provide an enhanced charge and capacitance response for the whole system compared to their constituents. Such intriguing implications of NC phenomena have prompted the design and exploration of many ferroelectric-based electronic devices to not only achieve an improved performance but potentially also overcome some fundamental limits of standard transistors. However, the microscopic physical origin as well as the true nature of the NC effect, and direct experimental evidence remain elusive and debatable. To that end, in this work, we systematically investigate the underlying physical mechanism of the NC effect in the ferroelectric material. Based upon the fundamental physics of ferroelectric material, we investigate different assumptions, conditions, and distinct features of the quasi-static NC effect in the single-domain and multi-domain scenarios. While the quasi-static and hysteresis-free NC effect was initially propounded in the context of a single-domain scenario, we highlight that the similar effects can be observed in multi-domain FEs with soft domain-wall (DW) displacement. Furthermore, to obtain the soft-DW, the gradient energy coefficient of the FE material is required to be higher as well as the ferroelectric thickness is required to be lower than some critical values. Otherwise, the DW becomes hard, and their displacement would lead to hysteretic NC effects. In addition to the quasi-static NC, we discuss different mechanisms that can lead to the transient NC effects. Furthermore, we provide guidelines for new experiments that can potentially provide new insights on unveiling the real origin of NC phenomena.</p> <p> </p> <p>Utilizing such ferroelectric insulators at the gate stack of a transistor, ferroelectric-field-effect transistors (FeFETs) have been demonstrated to exhibit both non-volatile memory and steep-slope logic functionalities. To investigate such diverse attributes and to enable application drive optimization of FeFETs, we develop a phase-field simulation framework of FeFETs by self-consistently solving the time-dependent Ginzburg-Landau (TDGL) equation, Poisson’s equation, and non-equilibrium Green’s function (NEGF) based semiconductor charge-transport equation. Considering HZO as the FE layer, we first analyze the dependence of the multi-domain patterns on the HZO thickness (<i>T_FE</i>) and their critical role in dictating the steep-switching (both in the negative and positive capacitance regimes) and non-volatile characteristics of FeFETs. In particular, we analyze the <i>T_FE</i>-dependent formation of hard and soft domain-walls (DW). We show that, <i>T_FE</i> scaling first leads to an increase in the domain density in the hard DW-regime, followed by soft DW formation and finally polarization collapse. For hard-DWs, we describe the polarization switching mechanisms and how the domain density impacts key parameters such as coercive voltage, remanent polarization, effective permittivity and memory window. We also discuss the enhanced but positive permittivity effects in densely pattern multi-domain states in the absence of hard-DW displacement and its implication in non-hysteretic attributes of FeFETs. For soft-DWs, we present how DW-displacement can lead to effective negative capacitance in FeFETs, resulting in a steeper switching slope and superior scalability. In addition, we also develop a Preisach based circuit compatible model for FeFET (and antiferroelectric-FET) that captures the multi-domain polarization switching effects in the FE layer. </p> <p> </p> Unlike semiconductor insulators (e.g., HZO), there are ferroelectric materials that exhibit a considerably low bandgap (< 2eV) and hence, display semiconducting properties. In this regard, non-perovskite-based 2D ferroelectric -In₂Se₃ shows a bandgap of ~1.4eV and that suggests a combined ferroelectricity and semiconductivity in the same material system. As part of this work, we explore the modeling and operational principle of ferroelectric semiconductor metal junction (FeSMJ) based devices in the context of non-volatile memory (NVM) application. First, we analyze the semiconducting and ferroelectric properties of the α-In₂Se₃ van der Waals (vdW) stack via experimental characterization and first-principles simulations. Then, we develop a FeSMJ device simulation framework by self-consistently solving the Landau–Ginzburg–Devonshire equation, Poisson's equation, and charge-transport equations. Our simulation results show good agreement with the experimental characteristics of α-In₂Se₃-based FeSMJ suggesting that the FeS polarization-dependent modulation of Schottky barrier heights of FeSMJ plays a key role in providing the NVM functionality. Moreover, we show that the thickness scaling of FeS leads to a reduction in read/write voltage and an increase in distinguishability. Array-level analysis of FeSMJ NVM suggests a lower read-time and read-write energy with respect to the HfO₂-based ferroelectric insulator tunnel junction (FTJ) signifying its potential for energy-efficient and high-density NVM applications.

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. / Master of Science

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