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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

This dissertation presents a systematic study on idealized electrode/ferroelectric/electrode heterostructures, from film synthesis to capacitor fabrication. Single crystal BaTiO3 has shown a small switching field of ~ 1 kV cm-1 and small switching energy of ~ 0.1 J cm-3, making it an excellent candidate for next generation non-volatile ferroelectric applications. However, when BaTiO3 is incorporated into actual nanoscale devices, the ferroelectric properties get significantly worse. BaTiO3 thin films show either large coercive switching field, increasing the total power consumption, or diminished remanent polarization, putting non-volatility into question. In this dissertation, I demonstrate that by carefully controlling the O2 growth pressure in pulsed laser deposition process, we can synthesize SrRuO3/BaTiO3/SrRuO3 heterostructures with BaTiO3 ferroelectric properties comparable to bulk crystal. Thickness scaling is performed to explore the smallest coercive voltage, with BaTiO3 films with thicknesses ranging from 25-50 nm exhibiting best combinations of switching voltage (< 100 mV), switching energy (< 2 J cm−3) and remanent polarization (> 10 μC cm-2). Depolarization field is found to play an important role in suppressing the coercive field, leading to a deviation in coercive field scaling from the Janovec-Kay-Dunn law. Lateral scaling is performed to explore the fastest switching speed, with the projection of achieving sub-nanosecond switching time in capacitor sizes as small as ~ 10 μm2. Integration of SrRuO3/BaTiO3/SrRuO3 heterostructures onto silicon substrate is also shown, with similar switching behavior achieved, as well as good resistance to fatigue and retention. Besides heterostructure synthesis, different fabrication methods are also explored to bring out the best BaTiO3 capacitor performance in various ferroelectric functions. MgO hard mask process introduces surface defects at electrode/ferroelectric interface and results in a large imprint in the hysteresis loops. While the wet chemical etching method gives out idealized ferroelectric loops with almost no imprint, the high selectivity and isotropy makes it unsuitable for fabricating more complex device architectures with high precision. The ion mill methods, on the other hand, have better directionality and can be used to etch almost every type of material and fabricate advanced device structures. However, the Ar+ ion beam bombardment can result in horizontal imprints in the hysteresis loop. Transport studies and deep-level transient spectroscopies reveal the introduction of Ba-related defect types (???′′−??••, ???′′) during the milling process. Post-fabrication annealing in Ba-rich environment significantly lowers the defect levels and reduces the imprints. We also discuss what can be done to further optimize the ferroelectric properties (reducing coercive field, increasing remanent polarization, etc.) in ultrathin BaTiO3 films, for the ferroelectric material to function well for various application purposes.

    Cover page: Towards Intrinsic Switching in Ferroelectric Oxide - Materials Synthesis and Fabrication Process
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Mechanical displacement in commonly used piezoelectric materials is typically restricted to linear or biaxial in nature and to a few percent of the material dimensions. Here, we show that free-standing BaTiO3 membranes exhibit nonconventional electromechanical coupling. Under an external electric field, these superelastic membranes undergo controllable and reversible "sushi-rolling-like" 180° folding-unfolding cycles. This crease-free folding is mediated by charged ferroelectric domains, leading to giant >3.8 and 4.6 μm displacements for a 30 nm thick membrane at room temperature and 60 °C, respectively. Further increasing the electric field above the coercive value changes the fold curvature, hence augmenting the effective piezoresponse. Finally, it is found that the membranes fold with increasing temperature followed by complete immobility of the membrane above the Curie temperature, allowing us to model the ferroelectric domain origin of the effect.
      Cover page: Giant Superelastic Piezoelectricity in Flexible Ferroelectric BaTiO3 Membranes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2024)
      There is an increasing desire to utilize complex functional electronic materials such as ferroelectrics in next-generation microelectronics. As new materials are considered or introduced in this capacity, an understanding of how we can process these materials into those devices must be developed. Here, the effect of different fabrication processes on the ferroelectric and related properties of prototypical metal oxide (SrRuO3)/ferroelectric (BaTiO3)/metal oxide (SrRuO3) heterostructures is explored. Two different types of etching processes are studied, namely, wet etching of the top SrRuO3 using a NaIO4 solution and dry etching using an Ar+-ion beam (i.e., ion milling). Polarization-electric-field hysteresis loops for capacitors produced using both methods are compared. For the ion-milling process, it is found that the Ar+ beam can introduce defects into the SrRuO3/BaTiO3/SrRuO3 devices and that the milling depth strongly influences the defect level and can induce a voltage imprint on the function. Realizing that such processing approaches may be necessary, work is performed to ameliorate the imprint of the hysteresis loops via ex situ “healing” of the process-induced defects by annealing the ferroelectric material in a barium-and-oxygen-rich environment via a chemical-vapor-deposition-style process. This work provides a pathway for the nanoscale fabrication of these candidate materials for next-generation memory and logic applications.
        Cover page: Effect of fabrication processes on BaTiO3 capacitor propertiesCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2022)
        Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length scales. This review traces the evolution of ferroelectric thin-film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high-throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand-in-hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.
          Cover page: Thin‐Film Ferroelectrics
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO3 , production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm-1 and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.
            Cover page: Beyond Substrates: Strain Engineering of Ferroelectric Membranes
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2022)
            The hafnate perovskites PbHfO3 (antiferroelectric) and SrHfO3 ("potential" ferroelectric) are studied as epitaxial thin films on SrTiO3 (001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb1- x Srx HfO3 system. The resulting (240)-oriented PbHfO3 (Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm-2 at 1.6 MV cm-1 , weak-field dielectric constant ≈186 at 298 K, and an antiferroelectric-to-paraelectric phase transition at ≈518 K. (002)-oriented SrHfO3 films exhibited neither ferroelectric behavior nor evidence of a polar P4mm phase . Instead, the SrHfO3 films exhibited a weak-field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77-623 K) and electric field (-3 to 3 MV cm-1 ). While the lack of ferroelectric order in SrHfO3 removes the potential for MPB, structural and property evolution of the Pb1- x Srx HfO3 (0 ≤ x < 1) system is explored. Strontium alloying increased the electric-breakdown strength (EB ) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (Ur ) and efficiency (η). The composition, Pb0.5 Sr0.5 HfO3 produced the best combination of EB  = 5.12 ± 0.5 MV cm-1 , Ur  = 77 ± 5 J cm-3 , and η = 97 ± 2%, well out-performing PbHfO3 and other antiferroelectric oxides.
              Cover page: Exploring the Pb1−xSrxHfO3 System and Potential for High Capacitive Energy Storage Density and Efficiency
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2023)
              Complex-oxide superlattices provide a pathway to numerous emergent phenomena because of the juxtaposition of disparate properties and the strong interfacial interactions in these unit-cell-precise structures. This is particularly true in superlattices of ferroelectric and dielectric materials, wherein new forms of ferroelectricity, exotic dipolar textures, and distinctive domain structures can be produced. Here, relaxor-like behavior, typically associated with the chemical inhomogeneity and complexity of solid solutions, is observed in (BaTiO3 )n /(SrTiO3 )n (n = 4-20 unit cells) superlattices. Dielectric studies and subsequent Vogel-Fulcher analysis show significant frequency dispersion of the dielectric maximum across a range of periodicities, with enhanced dielectric constant and more robust relaxor behavior for smaller period n. Bond-valence molecular-dynamics simulations predict the relaxor-like behavior observed experimentally, and interpretations of the polar patterns via 2D discrete-wavelet transforms in shorter-period superlattices suggest that the relaxor behavior arises from shape variations of the dipolar configurations, in contrast to frozen antipolar stripe domains in longer-period superlattices (n = 16). Moreover, the size and shape of the dipolar configurations are tuned by superlattice periodicity, thus providing a definitive design strategy to use superlattice layering to create relaxor-like behavior which may expand the ability to control desired properties in these complex systems.
                Cover page: Engineering Relaxor Behavior in (BaTiO3)n/(SrTiO3)n SuperlatticesCreative Commons 'BY-NC-ND' version 4.0 license
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