Charge Transfer and Built-in Electric Fields between a Crystalline Oxide and Silicon.

We report charge transfer and built-in electric fields across the epitaxial SrNb_{x}Ti_{1-x}O_{3-δ}/Si(001) interface. Electrical transport measurements indicate the formation of a hole gas in the Si and the presence of built-in fields. Hard x-ray photoelectron measurements reveal pronounced asymmetries in core-level spectra that arise from these built-in fields. Theoretical analysis of core-level spectra enables built-in fields and the resulting band bending to be spatially mapped across the heterojunction. The demonstration of tunable charge transfer, built-in fields, and the spatial mapping of the latter, lays the groundwork for the development of electrically coupled, functional heterojunctions.

Band alignment, built-in potential, and the absence of conductivity at the LaCrO3/SrTiO3(001) heterojunction.

Core-level and valence-band x-ray photoemission spectra measured for molecular-beam-epitaxy-grown LaCrO(3)/SrTiO(3)(001) yield band offsets and potential gradients within the LaCrO(3) sufficient to trigger an electronic reconstruction to alleviate the polarity mismatch. Yet, the interface is insulating. Based on first principles calculations, we attribute this unexpected result to interfacial cation mixing combined with charge redistribution within CrO(2) layers, enabled by low-lying d states within LaCrO(3), which suppresses an electronic reconstruction.

Atomistic mesh generation for the simulation of nanoscale metal-oxide-semiconductor field-effect transistors.

We present a methodology for the finite-element discretization of nanoscaled semiconductor devices with atomic resolution. The meshing strategy is based on the use of patterns to decompose the unit cell of the underlying crystallographic structures producing unstructured tetrahedral meshes. The unit cells of the bulk semiconductors and, more importantly, of the interfaces between the substrate and the gate dielectric have been extracted from classical molecular dynamics and density functional theory simulations. A Monte Carlo approach has been then used to place the dopants in nodes of the crystal, replacing silicon atoms. The thus created "atomistic" meshes are used to simulate an ensemble of microscopically different double-gate Si metal-oxide-semiconductor field-effect transistors and the transition region at the Si/SiO_{2} interface. In addition, a methodology to approximate amorphous dielectrics is also presented.

Electronic and magnetic properties of epitaxial perovskite SrCrO3(0 0 1).

We have investigated the intrinsic properties of SrCrO3 epitaxial thin films synthesized by molecular beam epitaxy. We find compelling evidence that SrCrO3 is a correlated metal. X-ray photoemission valence band and O K-edge x-ray absorption spectra indicate a strongly hybridized Cr3d-O2p state crossing the Fermi level, leading to metallic behavior. Comparison between valence band spectra near the Fermi level and the densities of states calculated using density functional theory (DFT) suggests the presence of coherent and incoherent states and points to strong electron correlation effects. The magnetic susceptibility can be described by Pauli paramagnetism at temperatures above 100 K, but reveals antiferromagnetic behavior at lower temperatures, possibly resulting from orbital ordering.

Impact of lattice mismatch and stoichiometry on the structure and bandgap of (Fe,Cr)2O3 epitaxial thin films.

The structural properties of phase-pure epitaxial (Fe1-xCrx)2O3 thin films deposited on α-Al2O3(0 0 0 1) substrates by oxygen-plasma-assisted molecular beam epitaxy are investigated across the composition range using x-ray photoelectron spectroscopy, high-resolution x-ray diffraction, scanning transmission electron microscopy and electron energy loss spectroscopy, and non-Rutherford resonant elastic scattering measurements. The films possess a columnar grain structure with uniform mixing of cations on the nanometer scale. Fe-rich films are relaxed and appear to be slightly oxygen-rich, while Cr-rich films remain partially strained to the Al2O3 substrate and are found to be oxygen deficient. A model is proposed to explain the oxygen stoichiometry results based on the energetics of oxygen defect formation and rate of oxygen diffusion in the corundum lattice, and the dependence on the cation composition. Deliberately introducing residual compressive biaxial strain into (Fe1-xCrx)2O3 thin films (x = 0, 0.41, 0.52) by employing a Cr2O3 buffer layer is shown to narrow the optical bandgap, from 1.80(1) eV for relaxed (Fe0.47Cr0.53)2O3 to 1.77(1) eV for partially strained (Fe0.48Cr0.52)2O3. The relationships which are elucidated between epitaxial film structure and optical properties can be applied to bandgap optimization in the (Fe,Cr)2O3 system.

Reversible nano-structuring of SrCrO3-δ through oxidation and reduction at low temperature.

Oxygen vacancies are often present in complex oxides as point defects, and their effect on the electronic properties is typically uniform and isotropic. Exploiting oxygen deficiency in order to generate controllably novel structures and functional properties remains a challenging goal. Here we show that epitaxial strontium chromite films can be transformed, reversibly and at low temperature, from rhombohedral, semiconducting SrCrO(2.8) to cubic, metallic perovskite SrCrO(3-δ). Oxygen vacancies in SrCrO(2.8) aggregate and give rise to ordered arrays of {111}-oriented SrO(2) planes interleaved between layers of tetrahedrally coordinated Cr(4+) and separated by ~1 nm. First-principle calculations provide insight into the origin of the stability of such nanostructures and, consistent with the experimental data, predict that the barrier for O(2-) diffusion along these quasi-two-dimensional nanostructures is significantly lower than that in cubic SrCrO(3-δ). This property is of considerable relevance to solid oxide fuel cells in which fast O(2-) diffusion reduces the required operating temperature.

Thermodynamic instability at the stoichiometric LaAlO3/SrTiO3(001) interface.

Stoichiometric epitaxial LaAlO(3) grown on TiO(2)-terminated SrTiO(3)(001) by off-axis pulsed laser deposition is shown to exhibit strong cation intermixing. This result is corroborated by classical and quantum mechanical calculations of the relative stabilities of abrupt and intermixed interface configurations. The valence band offset was measured to be 0.16 ± 0.10 eV, and this value cannot be accounted for theoretically without including intermixing in the physical description of the interface.

Dynamics of low-coordinated surface atoms on gold nanocrystallites.

The authors highlight the importance of transient configurations of atoms on the surface of nanocrystallites, and present methodologies for their investigation. A Monte Carlo method has been developed and is used to simulate the thermodynamic equilibrium of nanometer sized Au nanocrystallites, both free and supported on a MgO(100) surface. The authors find that appreciable numbers of atoms transiently occupy adatom positions on Au(111) facets, even at room temperature. This type of dynamically appearing site is usually neglected in relation to catalysis but may have a significant activity (for CO oxidation, for example). They also observe a complex solid-solid roughening transition which involves a variety of transient local atom configurations on the surface of nanocrystallites.