The broad spectrum of electronic and optical properties exhibited by oxides offers tremendous opportunities for microelectronic devices, especially when a combination of properties in a single device is desired Here we describe the use of reactive molecular-beam epitaxy and pulsed-laser deposition to synthesize functional oxides, including ferroelectrics, ferromagnets, and materials that are both at the same time Owing to the dependence of properties on direction, it is often optimal to grow functional oxides in particular directions to maximize their properties for a specific application But these thin film techniques offer more than orientation control; customization of the film structure down to the atomic-layer level is possible Numerous examples of the controlled epitaxial growth of oxides with perovskite and perovskite-related structures, including superlattices and metastable phases, are shown In addition to integrating functional oxides with conventional semiconductors, standard semiconductor practices involving epitaxial strain, confined thickness, and modulation doping can also be applied to oxide thin films Results of fundamental scientific importance as well as results revealing the tremendous potential of utilizing functional oxide thin films to create devices with enhanced performance are described

/pdf/a-thin-film-approach-to-engineering-functionality-into-51t9902h5j.pdf

A Thin Film Approach to Engineering Functionality into Oxides

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.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769414000013

Elastic strain engineering of ferroic oxides

Metal oxide semiconductor field-effect transistors, formed using silicon dioxide and silicon, have undergone four decades of staggering technological advancement. With fundamental limits to this technology close at hand, alternatives to silicon dioxide are being pursued to enable new functionality and device architectures. We achieved ferroelectric functionality in intimate contact with silicon by growing coherently strained strontium titanate (SrTiO3) films via oxide molecular beam epitaxy in direct contact with silicon, with no interfacial silicon dioxide. We observed ferroelectricity in these ultrathin SrTiO3 layers by means of piezoresponse force microscopy. Stable ferroelectric nanodomains created in SrTiO3 were observed at temperatures as high as 400 kelvin.

/pdf/a-ferroelectric-oxide-made-directly-on-silicon-1f9oimse8z.pdf

A Ferroelectric Oxide Made Directly on Silicon

We present a hybrid approach for Bethe-Salpeter equation (BSE) calculations of core excitation spectra, including x-ray absorption (XAS), electron energy loss spectra (EELS), and nonresonant inelastic x-ray scattering (NRIXS). The method is based on ab initio wave functions from the plane-wave pseudopotential code abinit; atomic core-level states and projector augmented wave (PAW) transition matrix elements; the NIST core-level BSE solver; and a many-pole self-energy model to account for final-state broadening and self-energy shifts. Multiplet effects are also approximately accounted for. The approach is implemented using an interface dubbed OCEAN (Obtaining Core Excitations using abinit and NBSE). To demonstrate the utility of the code we present results for the K edges in LiF as probed by XAS and NRIXS, the K edges of KCl as probed by XAS, the Ti ${\mathrm{L}}_{2,3}$ edge in SrTiO${}_{3}$ as probed by XAS, and the Mg ${\mathrm{L}}_{2,3}$ edge in MgO as probed by XAS. These results are compared with experiment and with other theoretical approaches.

Bethe-Salpeter equation calculations of core excitation spectra.

We first present an extended introduction of the various methods used to extract electronic and structural information from the K pre-edge X-ray absorption spectra of 3d transition metal ions. The K pre-edge structure is then modelled for a selection of 3d transition metal compounds and analyzed using first-principles calculations based on the density functional theory (DFT) in the local density approximation (LDA). The selected compounds under study are presented in an ascending order of electronic structure complexity, starting with the Ti K-edge of rutile and anatase, and finishing with the Fe K-edge of the cyanomet-myoglobin. In most cases, the calculations are compared to polarized experimental spectra. It is shown that DFT-LDA methods enable us to reproduce satisfactorily the experimental features and to understand the nature of the electronic transitions involved in the pre-edge region. The limiting aspects of such methods in modelling the core-hole electron interaction and the 3d electron-electron repulsion are also pointed out.

/pdf/first-principles-calculations-of-x-ray-absorption-spectra-at-1l3i11fbob.pdf

First-principles calculations of X-ray absorption spectra at the K-edge of 3d transition metals: an electronic structure analysis of the pre-edge.

A phase-separation instability, resulting in the dewetting of thin ${\mathrm{SrTiO}}_{3}$ films grown on Si(100) is shown by scanning transmission electron microscopy. Plan-view imaging of 1-nm thick, buried ${\mathrm{SrTiO}}_{3}$ films was achieved by exploiting electron channeling through the substrate to focus the incident 0.2 nm beam down to a 0.04 nm diameter, revealing a nonuniform coverage by epitaxial ${\mathrm{SrTiO}}_{3}$ islands and $2\ifmmode\times\else\texttimes\fi{}1$ Sr-covered regions. Density-functional calculations predict the ground state is a coexistence of $2\ifmmode\times\else\texttimes\fi{}1$ Sr-reconstructed Si and Sr-deficient ${\mathrm{SrTiO}}_{3}$, in correspondence with the observed islanding.

Imaging the phase separation in atomically thin buried SrTiO(3) layers by electron channeling.

The local atomic geometries in ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{TiO}}_{3}$ thin films grown on MgO(001) substrates have been determined by Ti $1s$ near-edge x-ray-absorption fine-structure measurements and density-functional-theory calculations. The accuracy of the atomic geometries predicted by density-functional theory is demonstrated by simulations of the near-edge x-ray-absorption fine-structure spectra using a Bethe-Salpeter treatment of the Ti $1s$ core-hole interaction in the films. Our results show that films with either $cga$ or $cla$ tetragonal lattice distortions have their polarization vectors rotated toward or away from the [001] film-normal or $c$-axis direction, respectively. Both distortions result locally in the monoclinic $r$ phase of the strain-phase diagram of Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)], and the polarizations are rotated significantly larger than the geometry would suggest.

/pdf/local-structural-distortions-in-strained-ba-0-5-sr-0-5-tio-3-17o0uv9k3x.pdf

Local structural distortions in strained Ba 0.5 Sr 0.5 TiO 3 thin films

$\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}K$ and $\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}{L}_{2,3}$ x-ray absorption fine-structure near-edge spectra of ${\mathrm{SrTiO}}_{3}$ thin films grown coherently on $\mathrm{Si}(001)$ reveal the presence of a ferroelectric (FE) distortion at room temperature. This unique phase is a direct consequence of the compressive biaxial strain achieved by coherent epitaxial growth.

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=101778

Ferroelectric distortion in SrTiO 3 thin films on Si ( 001 ) by x-ray absorption fine structure spectroscopy: Experiment and first-principles calculations

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Imaging the phase separation in atomically thin buried SrTiO(3) layers by electron channeling.

Local structural distortions in strained Ba 0.5 Sr 0.5 TiO 3 thin films

Ferroelectric distortion in SrTiO 3 thin films on Si ( 001 ) by x-ray absorption fine structure spectroscopy: Experiment and first-principles calculations