Fred Walker

Origin of the magnetoelectric coupling effect in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} Multiferroic heterostructures.

The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in the absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, opening a new pathway to the electronic control of spin in complex oxide materials.

Crystalline oxides on silicon.

This review outlines developments in the growth of crystalline oxides on the ubiquitous silicon semiconductor platform. The overall goal of this endeavor is the integration of multifunctional complex oxides with advanced semiconductor technology. Oxide epitaxy in materials systems achieved through conventional deposition techniques is described first, followed by a description of the science and technology of using atomic layer-by-layer deposition with molecular beam epitaxy (MBE) to systematically construct the oxide-silicon interface. An interdisciplinary approach involving MBE, advanced real-space structural characterization, and first-principles theory has led to a detailed understanding of the process by which the interface between crystalline oxides and silicon forms, the resulting structure of the interface, and the link between structure and functionality. Potential applications in electronics and photonics are also discussed.

Ferroelectric Field Effect Transistors for Memory Applications

The non-volatile polarization of a ferroelectric is a promising candidate for digital memory applications. Ferroelectric capacitors have been successfully integrated with silicon electronics, where the polarization state is read out by a device based on a field effect transistor configuration. Coupling the ferroelectric polarization directly to the channel of a field effect transistor is a long-standing research topic that has been difficult to realize due to the properties of the ferroelectric and the nature of the interface between the ferroelectric and the conducting channel. Here, we report on the fabrication and characterization of two promising capacitor-less memory architectures.

Combinatorial development of bulk metallic glasses

The identification of multicomponent alloys out of a vast compositional space is a daunting task, especially for bulk metallic glasses composed of three or more elements. Despite an increasing theoretical understanding of glass formation, bulk metallic glasses are predominantly developed through a sequential and time-consuming trial-and-error approach. Even for binary systems, accurate quantum mechanical approaches are still many orders of magnitude away from being able to simulate the relatively slow kinetics of glass formation. Here, we present a high-throughput strategy where ∼3,000 alloy compositions are fabricated simultaneously and characterized for thermoplastic formability through parallel blow forming. Using this approach, we identified the composition with the highest thermoplastic formability in the glass-forming system Mg-Cu-Y. The method provides a versatile toolbox for unveiling complex correlations of material properties and glass formation, and should facilitate a drastic increase in the discovery rate of metallic glasses.

Active Silicon Integrated Nanophotonics: Ferroelectric BaTiO3 Devices

The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 ± 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications.

Temperature dependence of the magnetoelectric effect in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures

The magnetoelectric response of Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 (PZT/LSMO) artificial multiferroic heterostructures as a function of temperature, electric, and magnetic field, shows that the largest magnetoelectric coupling is attained at temperatures near the magnetic critical point of LSMO, at ∼180 K (−13.5 Oe cm kV−1). The magnetoelectric coupling displays a strong temperature dependence, changing sign at 150 K and saturating to positive values below ∼100 K (+6 Oe cm kV−1). The magnetoelectric curve switches hysteretically between two states in response to the ferroelectric switching. The peak in the magnetoelectric response coincides with the observation of on/off switching of magnetism in LSMO near the critical region, where the sensitivity to electric field is largest, making it a promising approach for device applications.

Interface-induced nonswitchable domains in ferroelectric thin films

Engineering domains in ferroelectric thin films is crucial for realizing technological applications including non-volatile data storage and solar energy harvesting. Size and shape of domains strongly depend on the electrical and mechanical boundary conditions. Here we report the origin of nonswitchable polarization under external bias that leads to energetically unfavourable head-to-head domain walls in as-grown epitaxial PbZr(0.2)Ti(0.8)O3 thin films. By mapping electrostatic potentials and electric fields using off-axis electron holography and electron-beam-induced current with in situ electrical biasing in a transmission electron microscope, we show that electronic band bending across film/substrate interfaces locks local polarization direction and further produces unidirectional biasing fields, inducing nonswitchable domains near the interface. Presence of oxygen vacancies near the film surface, as revealed by electron-energy loss spectroscopy, stabilizes the charged domain walls. The formation of charged domain walls and nonswitchable domains reported in this study can be an origin for imprint and retention loss in ferroelectric thin films.

Atomically Engineered Oxide Interfaces

New solid-state phenomena emerge when interfaces between different oxides are created with atomic-scale precision.

Epitaxial MgO as an alternative gate dielectric for SiC transistor applications

Power transistor applications require alternative gate dielectrics on SiC that can operate at high fields without breaking down, as well as provide a high quality interface in order to minimize mobility degradation due to interface roughness. We have grown epitaxial MgO (111) crystalline layers on 6H-SiC (0001) substrates and characterized their structural and electrical properties. Measurements of gate leakage, breakdown fields, and dielectric properties make epitaxial MgO a potential candidate gate dielectric for SiC-based transistors.

Electrical properties and interfacial structure of epitaxial LaAlO3 on Si (001)

The dielectric and structural properties of LaAlO3 make it an attractive epitaxial gate oxide for nanometer-scale field effect transistors. However, the growth of epitaxial LaAlO3 directly on Si has not been possible to date. In order to achieve LaAlO3 epitaxy, we use a SrTiO3 template layer whose thickness minimizes elastic strain and atomic-level buckling at the interface. We find that LaAlO3 grown on this template layer is crystalline and initially strained, but relaxes to its bulk lattice constant within 7 unit cells. Cross-sectional transmission electron microscopy and inelastic electron tunneling spectroscopy studies of the LaAlO3/SrTiO3/Si structure show no evidence of an amorphous SiO2 layer. Capacitance-voltage measurements on thin films of epitaxial LaAlO3/SrTiO3/Si with LaAlO3 thicknesses between 13 and 110 nm show a dielectric constant for the LaAlO3 layer of 24, the same value as for the bulk. After a post-deposition low temperature anneal, these oxide heterostructures show no Fermi level pin...