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Open Access Publications from the University of California

Scholarly Works (20 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2020)

The field of material science and engineering aims to provide interdisciplinary communities with the physical understanding and control of various material structures and properties to allow for the development of advanced material systems and technology. Key to the developments of ubiquitous energy conversion, enhanced computer logic and memory performance, and artificial intelligence lies an increasing demand for highly-susceptible, next-generation materials which maintain unique functional ability under applied thermal, mechanical, and electrical stimuli. One such class of functional materials poised to address these needs are thin-film, complex-polar oxides which maintain a rich landscape of electromechanical, thermoelastic, and electrothermal properties. As such, although developments in the fields of dielectrics, piezoelectrics, and ferroelectrics have followed, the field of pyroelectrics (i.e., electrothermal response) has remained relatively understudied. Due to the complex material property intercoupling of pyroelectricity to dielectric, ferroelectric, and piezoelectric counterparts, identifying routes to understand and enhance the various contributions to pyroelectric response has continued to limit the field. As a result, pyroelectrics have remained primarily stagnant in the past 30 years, limited to applications only in IR sensor array technology. Without the proper understanding and control of the various material properties that contribute to the overall pyroelectric response needed for enhanced performance in next-generation applications (e.g., waste-heat energy conversion), researchers have traditionally defaulted to perching pyroelectric systems near material phase transitions. Specifically, proximity to the Curie transition temperature (T_C) typically allows for the overall enhancement of pyroelectric response, however, at the cost of small operating temperature windows and increased DC, background temperatures. In this Dissertation, I aim to elucidate the various contributions to pyroelectricity via comprehensive and direct characterization methodologies and provide the community with accessible routes for overall pyroelectric response enhancement . I aim to show that these coupled material properties do not simply follow design rules established in the fields of ferroelectric and piezoelectric materials, and thus require an unintuitive understanding of their interactions.In this work, I provide various insights into the complex pyroelectric property interplay in prototypical PbZrxTi1-xO3 and BaTiO3 thin-film ferroelectric oxides. With careful consideration of applied electric fields, temperature oscillations, and device geometry, I show that pyroelectric contributions may be individually suppressed and decoupled to provide quantitative understanding in overall pyroelectric response. With the engineering of domain architectures in PbZrxTi1-xO3 systems via underlying substrates, the evolution of ferroelectric, dielectric, and pyroelectric response is investigated to understand and quantify ferroelastic- domain contributions. Typically disregarded in the past, I demonstrate that these subtle extrinsic contributions may account for as much as 35% of the total measured pyroelectric response at room temperature for some films. Furthermore, through the use of applied, high-DC electric fields , I can systematically suppress these extrinsic contributions to understand the evolution of pyroelectric response across the morphotropic boundary of PbZrxTi1-xO3. Lastly, with the fabrication and synthesis of in-plane electrothermal test platforms and high-quality BaTiO3 thin-films on Si substrates, I systematically investigate the evolution of elastic boundary conditions on the extrinsic and secondary contributions to pyroelectricity. Through the quantification of the various pyroelectric contributions, I more clearly depict the underlying property complexities that have long hindered advancements in the field of pyroelectrics.

    Cover page: Quantifying Contributions to Pyroelectricity in Thin-Film Ferroelectric Oxides
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2018)
    The emergent ferroelectricity in HfO2-based systems has attracted significant attention as this simple binary high-k dielectric now offers the possibility of nonvolatile function. In this work we employ zero-field and field-dependent pyroelectricity to show that thin films of silicon-doped HfO2 do exhibit a broken-inversion symmetry and are indeed ferroelectric. In addition, the pyroelectric response is found to exhibit a wake-up behavior akin to the wake-up phenomenon observed in the ferroelectric polarization with electric-field cycling. Using polarization-electric field hysteresis measurements, this wake-up phenomenon is attributed to the presence of defect dipoles which explains the measured pyroelectric response. Finally, direct electrocaloric measurements are performed on these silicon-doped HfO2 thin films, revealing an electrocaloric coefficient ∼4 times larger in magnitude than that expected for the measured pyroelectric coefficient. This enhancement is explained using the plausible role played by defect dipoles that contribute to additional configurational or dipolar entropy.
      Cover page: Pyroelectric and electrocaloric effects in ferroelectric silicon-doped hafnium oxide thin films
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2021)
      Pyroelectrics are a material class that undergoes a change in polarization as the temperature of the system is varied. This effect can be utilized for applications ranging from thermal imaging and sensing to waste-heat energy conversion to thermally driven electron emission. Here, we review recent advances in the study and utilization of thin-film pyroelectrics. Leveraging advances in modeling, synthesis, and characterization has provided a pathway forward in one of the more poorly developed subfields of ferroelectricity. We introduce the complex physical phenomena of pyroelectricity, briefly explore the history of work in this space, and highlight not only new advances in the direct measurement of such effects but also how our ability to control thin-film materials is changing our understanding of this response. Finally, we discuss recent advances in thin-film pyroelectric devices and introduce a number of potentially new directions the field may follow in the coming years.
        Cover page: Pyroelectric thin films—Past, present, and future
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2015)
        Epitaxial strain has been widely used to tune crystal and domain structures in ferroelectric thin films. New avenues of strain engineering based on varying the composition at the nanometer scale have been shown to generate symmetry breaking and large strain gradients culminating in large built-in potentials. In this work, we develop routes to deterministically control these built-in potentials by exploiting the interplay between strain gradients, strain accommodation, and domain formation in compositionally graded PbZr1-xTixO3 heterostructures. We demonstrate that variations in the nature of the compositional gradient and heterostructure thickness can be used to control both the crystal and domain structures and give rise to nonintuitive evolution of the built-in potential, which does not scale directly with the magnitude of the strain gradient as would be expected. Instead, large built-in potentials are observed in compositionally-graded heterostructures that contain (1) compositional gradients that traverse chemistries associated with structural phase boundaries (such as the morphotropic phase boundary) and (2) ferroelastic domain structures. In turn, the built-in potential is observed to be dependent on a combination of flexoelectric effects (i.e., polarization-strain gradient coupling), chemical-gradient effects (i.e., polarization-chemical potential gradient coupling), and local inhomogeneities (in structure or chemistry) that enhance strain (and/or chemical potential) gradients such as areas with nonlinear lattice parameter variation with chemistry or near ferroelastic domain boundaries. Regardless of origin, large built-in potentials act to suppress the dielectric permittivity, while having minimal impact on the magnitude of the polarization, which is important for the optimization of these materials for a range of nanoapplications from vibrational energy harvesting to thermal energy conversion and beyond.
          Cover page: Complex Evolution of Built-in Potential in Compositionally-Graded PbZr1–x Ti x O3 Thin Films
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2019)
          There is growing interest in the study of thin-film pyroelectric materials because of their potential for high performance thermal-energy conversion, thermal sensing, and beyond. Electrothermal susceptibilities, such as pyroelectricity, are known to be enhanced in proximity to polar instabilities, and this is conventionally accomplished by positioning the material close to a temperature-driven ferroelectric-to-paraelectric phase transition. The high Curie temperature (TC) for many ferroelectrics, however, limits the utility of these materials at room-temperature. Here, the nature of pyroelectric response in thin films of the widely studied multiferroic Bi1-xLaxFeO3 (x = 0-0.45) is probed. While BiFeO3 itself has a high TC, lanthanum substitution results in a chemically induced lowering of the ferroelectric-to-paraelectric and structural-phase transition. The effect of isovalent lanthanum substitution on the structural, dielectric, ferroelectric, and pyroelectric response is investigated using reciprocal-space-mapping studies; field-, frequency-, and temperature-dependent electrical measurements; and phase-sensitive pyroelectric measurements, respectively. While BiFeO3 itself has a rather small pyroelectric coefficient at room temperature (∼-40 μC/m2 K), 15% lanthanum substitution results in an enhancement of the pyroelectric coefficient by 100% which is found to arise from a systematic lowering of TC
            Cover page: Enhanced pyroelectric properties of Bi1−xLaxFeO3 thin films
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2020)
            Ferroelectrics and related materials (e.g., non-traditional ferroelectrics such as relaxors) have long been used in a range of applications, but with the advent of new ways of modeling, synthesizing, and characterizing these materials, continued access to astonishing breakthroughs in our fundamental understanding come each year. While we still rely on these materials in a range of applications, we continue to re-write what is possible to be done with them. In turn, assumptions that have underpinned the use and design of certain materials are progressively being revisited. This perspective aims to provide an overview of the field of ferroelectric/relaxor/polar-oxide thin films in recent years, with an emphasis on emergent structure and function enabled by advanced synthesis, processing, and computational modeling.
              Cover page: Beyond Expectation: Advanced Materials Design, Synthesis, and Processing to Enable Novel Ferroelectric Properties and Applications
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2020)
              With the advent of increasingly elaborate experimental techniques in physics, chemistry and materials sciences, measured data are becoming bigger and more complex. The observables are typically a function of several stimuli resulting in multidimensional data sets spanning a range of experimental parameters. As an example, a common approach to study ferroelectric switching is to observe effects of applied electric field, but switching can also be enacted by pressure and is influenced by strain fields, material composition, temperature, time, etc. Moreover, the parameters are usually interdependent, so that their decoupling toward univariate measurements or analysis may not be straightforward. On the other hand, both explicit and hidden parameters provide an opportunity to gain deeper insight into the measured properties, provided there exists a well-defined path to capture and analyze such data. Here, we introduce a new, two-dimensional approach to represent hysteretic response of a material system to applied electric field. Utilizing ferroelectric polarization as a model hysteretic property, we demonstrate how explicit consideration of electromechanical response to two rather than one control voltages enables significantly more transparent and robust interpretation of observed hysteresis, such as differentiating between charge trapping and ferroelectricity. Furthermore, we demonstrate how the new data representation readily fits into a variety of machine-learning methodologies, from unsupervised classification of the origins of hysteretic response via linear clustering algorithms to neural-network-based inference of the sample temperature based on the specific morphology of hysteresis.
                Cover page: To switch or not to switch – a machine learning approach for ferroelectricity
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2022)
                Epitaxial PbHf1−xTixO3/SrTiO3(001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr1−xTixO3 system. End members, PbHfO3 and PbTiO3, were found to possess orthorhombic (Pbam) and tetragonal (P4mm) crystal structures and antiferroelectric and ferroelectric (∼87 μC/cm2) behavior, respectively. PbHf0.75Ti0.25O3 and PbHf0.25Ti0.75O3 solid solutions were both found to be ferroelectric with rhombohedral (R3c, ∼22 μC/cm2) and tetragonal (P4mm, ∼46 μC/ cm2) structures, respectively. For intermediate PbHf1−xTixO3 compositions (e.g., x = 0.4, 0.45, 0.5, and 0.55), a structural transition was observed from rhombohedral (hafnium-rich) to tetragonal (titanium-rich) phases. These intermediate compositions also exhibited mixed-phase structures including R3c, monoclinic (Cm), and P4mm symmetries and, in all cases, were ferroelectric with remanent (5−22 μC/cm2) and saturation (18.5−36 μC/cm2) polarization and coercive field (24−34.5 kV/cm) values increasing with x. While the dielectric constant was the largest for PbHf0.6Ti0.4O3, the MPB is thought to be near x = 0.5 after separation of the intrinsic and extrinsic contributions to the dielectric response. Furthermore, piezoelectric displacement−voltage hysteresis loops were obtained for all chemistries revealing displacement values as good as PbZr0.52Ti0.48O3 films in the same geometry. Thereby, the PbHf1−xTixO3 system is a viable alternative to the PbZr1−xTixO3 system offering comparable performance.
                  Cover page: Exploring the Morphotropic Phase Boundary in Epitaxial PbHf1–x Ti x O3 Thin FilmsCreative Commons 'BY-NC-ND' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2020)
                  Dielectric capacitors can store and release electric energy at ultrafast rates and are extensively studied for applications in electronics and electric power systems. Among various candidates, thin films based on relaxor ferroelectrics, a special kind of ferroelectric with nanometer-sized domains, have attracted special attention because of their high energy densities and efficiencies. We show that high-energy ion bombardment improves the energy storage performance of relaxor ferroelectric thin films. Intrinsic point defects created by ion bombardment reduce leakage, delay low-field polarization saturation, enhance high-field polarizability, and improve breakdown strength. We demonstrate energy storage densities as high as ~133 joules per cubic centimeter with efficiencies exceeding 75%. Deterministic control of defects by means of postsynthesis processing methods such as ion bombardment can be used to overcome the trade-off between high polarizability and breakdown strength.
                    Cover page: Ultrahigh capacitive energy density in ion-bombarded relaxor ferroelectric films
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2020)
                    The ability to produce atomically precise, artificial oxide heterostructures allows for the possibility of producing exotic phases and enhanced susceptibilities not found in parent materials. Typical ferroelectric materials either exhibit large saturation polarization away from a phase boundary or large dielectric susceptibility near a phase boundary. Both large ferroelectric polarization and dielectric permittivity are attained wherein fully epitaxial (PbZr0.8Ti0.2O3)n/(PbZr0.4Ti0.6O3)2n (n = 2, 4, 6, 8, 16 unit cells) superlattices are produced such that the overall film chemistry is at the morphotropic phase boundary, but constitutive layers are not. Long- (n ≥ 6) and short-period (n = 2) superlattices reveal large ferroelectric saturation polarization (Ps = 64 µC cm−2) and small dielectric permittivity (εr ≈ 400 at 10 kHz). Intermediate-period (n = 4) superlattices, however, exhibit both large ferroelectric saturation polarization (Ps = 64 µC cm−2) and dielectric permittivity (εr = 776 at 10 kHz). First-order reversal curve analysis reveals the presence of switching distributions for each parent layer and a third, interfacial layer wherein superlattice periodicity modulates the volume fraction of each switching distribution and thus the overall material response. This reveals that deterministic creation of artificial superlattices is an effective pathway for designing materials with enhanced responses to applied bias.
                      Cover page: Large Polarization and Susceptibilities in Artificial Morphotropic Phase Boundary PbZr1−xTixO3 Superlattices
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