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As a real-space technique, atomic-resolution STEM imaging contains both amplitude and geometric phase information about structural order in materials, with the latter encoding important information about local variations and heterogeneities present in crystalline lattices. Such phase information can be extracted using geometric phase analysis (GPA), a method which has generally focused on spatially mapping elastic strain. Here we demonstrate an alternative phase demodulation technique and its application to reveal complex structural phenomena in correlated quantum materials. As with other methods of image phase analysis, the phase lock-in approach can be implemented to extract detailed information about structural order and disorder, including dislocations and compound defects in crystals. Extending the application of this phase analysis to Fourier components that encode periodic modulations of the crystalline lattice, such as superlattice or secondary frequency peaks, we extract the behavior of multiple distinct order parameters within the same image, yielding insights into not only the crystalline heterogeneity but also subtle emergent order parameters such as antipolar displacements. When applied to atomic-resolution images spanning large (~0.5 × 0.5 μm2) fields of view, this approach enables vivid visualizations of the spatial interplay between various structural orders in novel materials.
The diverse and fascinating properties of transition metal oxides stem from the strongly correlated electronic degrees of freedom; the scientific challenge and range of possible applications of these materials have caused fascination among physicists and materials scientists, thus capturing research efforts for nearly a century. Here, we focus on the binary VxOy and the ternary perovskite AVO3 and review the key aspects from the underlying physical framework and their basic properties, recent strides made in thin-film synthesis, to recent efforts to implement vanadium-based oxides for practical applications that augment existing technologies, which surpass limitations of conventional materials.
Thin films of NbO2 are synthesized by oxide molecular-beam epitaxy on (001) MgF2 substrates, which are isostructural (rutile structure) with NbO2. Two growth parameters are systematically varied in order to identify appropriate growth conditions: growth temperature and the partial pressure of O2 during film growth. θ-2θ X-ray diffraction measurements identify two dominant phases in this system at background oxygen pressures in the (0.2–6)×10–7 Torr range: rutile NbO2 is favored at higher growth temperature, while Nb2O5 forms at lower growth temperature. Electrical resistivity measurements were made between 350 K and 675 K on three epitaxial NbO2 films in a nitrogen ambient. These measurements show that NbO2 films grown in higher partial pressures of molecular oxygen have larger temperature-dependent changes in electrical resistivity and higher resistivity at room temperature.
We describe a hybrid pixel array detector (electron microscope pixel array detector, or EMPAD) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit. The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition system, and preliminary results from experiments with 80–200 keV electron beams.
Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.
This year marks a major materials milestone in the makeup of silicon-based field-effect transistors: in the microprocessors produced by leading manufacturers, the SiO2 gate dielectric is being replaced by a hafnium-based dielectric. The incredible electronic properties of the SiO2/silicon interface are the reason that silicon has dominated the semiconductor industry and helped it grow to over $250 billion in annual sales, as reported by the Semiconductor Industry Association (SIA), San Jose, CA. The shrinkage of transistor dimensions (Moore's law) has led to tremendous improvements in circuit speed and computer performance. At the same time, however, it has also led to exponential growth in the static power consumption of transistors due to quantum mechanical tunneling through an ever-thinner SiO2 gate dielectric. This has spurred an intensive effort to find an alternative to SiO2 with a higher dielectric constant (K) to temper this exploding power consumption. This article reviews the high-K materials revolution that is enabling Moore's law to continue beyond SiO2.
In this issue we have endeavored to answer the question, “Whither oxide electronics?” This issue provides a framework and perspective on the progress in the field of oxide electronics over the past several decades, as well as the challenges and opportunities in the years to come. Building on the foundations laid by the pioneers in the materials community and spurred by the discovery of high-temperature superconductivity, there has been both tremendous progress in understanding the complex science of oxide electronic materials and the discovery of other fascinating new phenomena, including colossal magnetoresistance, multiferrocity, and two-dimensional electron gases in correlated oxide systems. Thin-film heterostructures provide a pathway to create novel devices and combinations of physical phenomena. Indeed, the ability to synthesize and control oxide heterostructures using sophisticated deposition techniques has become a key enabler of the recent advances in this field. These oxides are beginning to enter mainstream products because of their higher performance, for example, ferroelectric memories and oxides with high dielectric constant for computers that run at higher speed and use less power.
The electrical properties of Al/LaAlO3/GaAs metal-oxide-semiconductor capacitors were investigated. A thick arsenic (As2) capping layer was used to protect the GaAs from oxidation and contamination during the air exposure that occurred between the deposition of the GaAs and LaAlO3 layers in different molecular-beam epitaxy systems. Amorphous LaAlO3 was deposited on c(4×4)- and (2×4)-reconstructed (100) GaAs surfaces. Post dielectric deposition annealing was found to improve the capacitance-voltage (C-V) characteristics by eliminating frequency dispersion in the depletion and weak inversion regimes and diminished the bi-directional C-V hysteresis to 210 mV. Reasonably low gate leakage current was maintained after annealing.
This is a copy of the slides presented at the meeting but not formally written up for the volume.
Description: Epitaxial thin films of the multiferroic BiFeO3 have been grown by molecular beam epitaxy in an adsorption-controlled growth regime where substrate temperature and bismuth oxide over pressure establish phase and stoichiometry control. 30 nm thick BiFeO3 films have been deposited directly on (001) SrTiO3 and SrRuO3/(110) DyScO3, and on (0001) GaN containing a 1 nm thick TiO2 overlayer to enable the epitaxial transition between (0001) GaN and (0001) BiFeO3. Films grown on (001) SrTiO3 possess rocking curves identical to that of the underlying substrate, e.g., a full width at half maximum (FWHM) in ù of 25 arc sec in the best case. This is over 40 times narrower than the best published result for epitaxial BiFeO3 films deposited by any technique. On all of the above substrates, the BiFeO3 films exhibit rhombohedral symmetry. The in-plane epitaxial alignment observed for BiFeO3/TiO2/(0001) GaN differs by a 30° in-plane rotation from that observed in previous work for BiFeO3/SrTiO3/TiO2/(0001) GaN. Specifically, our (0001) BiFeO3/TiO2/(0001) GaN is oriented in-plane with [10-10]BiFeO3 [11-20]GaN, accompanied by a 180° in-plane rotational twin variant. The results of this work will be discussed in the context of the interfacial and crystallographic orientation dependence on the ferroelectric, antiferromagnetic, and dielectric properties of this multiferroic.
Out-of-phase boundaries (OPBs), planar faults between regions of a crystal that are misaligned by a fraction of a unit cell dimension, occur frequently in materials of high structural anisotropy. Rarely observed in the bulk, OPBs frequently exist in epitaxial films of layered complex oxides, such as YBCO-type, Aurivillius, and Ruddlesden-Popper phases, and frequently propagate through the entire thickness of a film, due to their large offset and the improbability of opposite-sign OPB annihilation. OPBs have previously been demonstrated to have a significant impact upon properties, so it is important to understand their generation. These faults arise through the same few mechanisms in the various layered complex oxides.
An effort is made to unify the discussion of nucleation of these defects, common to layered oxide materials. OPBs can nucleate at the film-substrate interface (primary) via steric, chemical, or misfit mechanisms, or post-growth (secondary) through crystallographic shear during decomposition of volatile components. Examples of the mechanisms observed during high-resolution transmission electron microscopy (HRTEM) study of Aurivillius and Ruddlesden-Popper phases are presented. A method for estimating the relative OPB density in a film from correlation of x-ray diffraction (XRD) θ-2θ data with TEM information on OPBs is presented.
As a first step in the identification of suitable alternative gate dielectrics for metal oxide semiconductor field-effect transistors (MOSFETs), we have used tabulated thermodynamic data to comprehensively assess the thermodynamic stability of binary oxides and nitrides in contact with silicon at temperatures from 300 K to 1600 K. Sufficient data exist to conclude that the vast majority of binary oxides and nitrides are thermodynamically unstable in contact with silicon. The dielectrics that remain are candidate materials for alternative gate dielectrics. Of these remaining candidates, the oxides have a significantly higher dielectric constant (ĸ) than the nitrides. We then extend this thermodynamic approach to multicomponent oxides comprising the candidate binary oxides. The result is a relatively small number of silicon-compatible gate dielectric materials with ĸ values substantially greater than that of SiO2 and optical bandgaps ≥ eV.
Thin films of composition (Ba,Sr)yTiO2+y with 0.43 ≤ y ≤; 1.64, were deposited by metalorganic chemical vapor deposition on (100) MgO substrates at various growth conditions. X-ray diffraction and transmission electron microscopy studies showed that the films were composed of epitaxial Ba1–xSrxTiO3 (x ≈0.06) grains and an amorphous phase. The orientation of the tetragonal Ba1–xSrxTiO3 grains (pure a axis, pure c axis, or a mix of the two) was found to be strongly dependent upon film composition. This composition dependence is explained for the majority of the Ti-rich films by an analysis of average strains in the two-phase films, assuming a compressive strain of ≈1% in the amorphous phase.
Thin films of perovskite Pb–Ti–O3 families were heteroepitaxially grown by sputtering on (0001)sapphire and/or (001)SrTiO (ST). These epitaxial films contained microstructures, although X–ray diffraction analysis suggested formation of single crystal phase with three dimentional crystal orientation. Their microstructures were studied by the electron microscopy, atomic force microscopy, and spectroscopic ellipsometry so as to find factors which influence the formation of the microstructure. It was found that the orientation of the substrate surface and the chemical composition of adatoms during initial film growth strongly affected the formation of the microstructures. Sputtered PbTiO3 (PT) thin films under a stoichiometric condition on a miscut(001) ST(miscut 1.7 degree) realized the growth of continuous single crystal thin films of 10–100nm thick with extremely smooth surface with surface roughness less than 3nm. Deposition on a miscut substrate under a stoichiometric condition is essential to make continuous thin films of perovskite of single crystal phase.
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