C. H. Ahn

Magnetoelectric Coupling Effects in Multiferroic Complex Oxide Composite Structures

The study of magnetoelectric materials has recently received renewed interest, in large part stimulated by breakthroughs in the controlled growth of complex materials and by the search for novel materials with functionalities suitable for next generation electronic devices. In this Progress Report, we present an overview of recent developments in the field, with emphasis on magnetoelectric coupling effects in complex oxide multiferroic composite materials.

Electric field effect in correlated oxide systems.

Semiconducting field-effect transistors are the workhorses of the modern electronics era. Recently, application of the field-effect approach to compounds other than semiconductors has created opportunities to electrostatically modulate types of correlated electron behaviour—including high-temperature superconductivity and colossal magnetoresistance—and potentially tune the phase transitions in such systems. Here we provide an overview of the achievements in this field and discuss the opportunities brought by the field-effect approach.

Ferroelectricity in thin perovskite films

We report on the investigation of ferroelectricity in thin tetragonal single-crystalline perovskite films of Pb(Zr0.2Ti0.8)O3 grown by off-axis rf magnetron sputtering. The local ferroelectric properties of atomically smooth films, with thicknesses ranging from a few unit cells to 800 A, were measured using a combination of electric force microscopy and piezoelectric microscopy. The time dependence of the measured signals reveals a stable ferroelectric polarization in films down to thicknesses of 40 A.

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.

High-Mobility Few-Layer Graphene Field Effect Transistors Fabricated on Epitaxial Ferroelectric Gate Oxides

The carrier mobility mu of few-layer graphene (FLG) field-effect transistors increases tenfold when the SiO2 substrate is replaced by single-crystal epitaxial Pb(Zr0.2Ti0.8)O3 (PZT). In the electron-only regime of the FLG, mu reaches 7x10(4) cm(2)/V s at 300 K for n=2.4x10(12)/cm(2), 70% of the intrinsic limit set by longitudinal acoustic (LA) phonons; it increases to 1.4x10(5) cm(2)/V s at low temperature. The temperature-dependent resistivity rho(T) reveals a clear signature of LA phonon scattering, yielding a deformation potential D=7.8+/-0.5 eV.

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.

Control and imaging of ferroelectric domains over large areas with nanometer resolution in atomically smooth epitaxial Pb(Zr0.2Ti0.8)O3 thin films

We have investigated the possibility afforded by epitaxial ferroelectric oxide thin films to control and image locally the polarization field of ferroelectrics over large areas with submicron resolution, using the metallic tip of an atomic force microscope as a mobile top electrode and local probe of the ferroelectric properties. Atomically smooth films of Pb(Zr0.2Ti0.8)O3, showing a root-mean-square roughness of typically a few angstroms, could be uniformly polarized and imaged over areas as large as 2500 μm2 without introducing any topographic disorder. Regular arrays of 100 nm wide lines and circular domains with a diameter less than 100 nm were written in arbitrary areas of the uniformly polarized regions.

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.

Unusual resistance hysteresis in n-layer graphene field effect transistors fabricated on ferroelectric Pb(Zr0.2Ti0.8)O3

We have fabricated n-layer graphene field effect transistors on epitaxial ferroelectric Pb(Zr_0.2Ti_0.8)O_3 (PZT) thin films. At low gate voltages, PZT behaves as a high-k dielectric with k up to 100. An unusual resistance hysteresis occurs in gate sweeps at high voltages, with its direction opposite to that expected from the polarization switching of PZT. The relaxation of the metastable state is thermally activated, with an activation barrier of 50-110 meV and a time constant of 6 hours at 300 K. We attribute its origin to the slow dissociation/recombination dynamics of water molecules adsorbed at the graphene-PZT interface. This robust hysteresis can potentially be used to construct graphene-ferroelectric hybrid memory devices.

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.