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Plasmonic materials have optical cross sections that exceed by 10-fold their geometric sizes, making them uniquely suitable to convert light into electrical charges. Harvesting plasmon-generated hot carriers is of interest for the broad fields of photovoltaics and photocatalysis; however, their direct utilization is limited by their ultrafast thermalization in metals. To prolong the lifetime of hot carriers, one can place acceptor materials, such as semiconductors, in direct contact with the plasmonic system. Herein, we report the effect of operating temperature on hot electron generation and transfer to a suitable semiconductor. We found that an increase in the operation temperature improves hot electron harvesting in a plasmonic semiconductor hybrid system, contrasting what is observed on photodriven processes in nonplasmonic systems. The effect appears to be related to an enhancement in hot carrier generation due to phonon coupling. This discovery provides a new strategy for optimization of photodriven energy production and chemical synthesis.

The detailed structure of ZnO doped with 5 at.% (metal) of the large aliovalent Eu3+-ions was investigated using EXAFS to describe the local Eu and Zn coordination. The microstructure, crystalline phases, contents and ZnO unit-cell parameters for the ZnO:5%Eu sponges synthesised at 200 to 900 oC were obtained by XRD, SEM, and TEM analysis. XRD showed peaks solely of h-ZnO for the 600 oC sample, while heating at 700 oC and higher caused phase separation into h-ZnO:Eu and c-Eu2O3. XRD showed a close to zero increase in ZnO unit cell-volume of ca. 0.4 vol%, compared to un-doped ZnO for the non-phase separated, clean oxide made at 600 oC. The Zn EXAFS data showed an almost intact local ZnO structure. The Eu EXAFS showed an unusually low coordination number (CN) of ca. 5 for the 200-600 oC samples, while the CN increased for higher temperatures, in concert with the formation of c-Eu2O3. 23 DFT-generated theoretical ZnO structures containing Eu-clusters built from 1 to 4 Eu3+-Zn2+-vacancy- Eu3+ pairs were compared with the experimental data. The lowest formation energies and ZnO unit-cell volume increase versus un-doped ZnO (0.6-0.7 vol%), were obtained when combining two or four Eu3+-Zn2+-vacancy- Eu3+ pairs into Eu4 and Eu8 clusters showing an average Eu CN of ca. 5. These theoretically determined lowest energy structures were all in good agreement with the experimental results obtained by EXAFS and XRD. Photoluminescence excitation and emission spectra performed on the ZnO:5at%Eu sponges obtained at various temperatures, showed strong quenching of the characteristic Eu3+ transitions for samples obtained at 600 and 800 °C, most likely due to changes in the ZnO defect states, which are crucial for Eu3+ excitation, and due to self-quenching upon Eu clustering and c- Eu2O3 phase separation. Thus, the optical data further supported Eu clustering found by EXAFS, DFT and XRD techniques, corroborating structure-property relationships in these materials. Overall, as far as we can find, the findings reported herein point to a doping structure very different from those previously proposed in the literature. It demonstrates that the semiconductor ZnO can host molecular-sized clusters of metal-oxides, very dissimilar to ZnO.

Poly(ethylene oxide) (PEO) is the most widely used compound as a solid-state (solvent-free) polymer electrolyte for Li batteries, mainly due to its low glass transition temperature (Tg) and ability...

The effect of biaxial strain on the band structure of two-dimensional silicon nanosheets (Si NSs) with (111), (110), and (001) exposed surfaces was investigated by means of density functional theory calculations. For all the considered Si NSs, an indirect-to-direct band gap transition occurs as the lateral dimensions of Si NSs increase; that is, increasing lateral biaxial strain from compressive to tensile always enhances the direct band gap characteristics. Further analysis revealed the mechanism of the transition which is caused by preferential shifts of the conduction band edge at a specific k-point because of their bond characteristics. Our results explain a photoluminescence result of the (111) Si NSs [U. Kim et al., ACS Nano 2011, 5, 2176–2181] in terms of the plausible tensile strain imposed in the unoxidized inner layer by surface oxidation.

Thin film structures of water on the CeO2(111) surface for coverages between 0.5 and 2.0 water monolayers have been optimized and analyzed using density functional theory (optPBE-vdW functional). We present a new 1.0 ML structure that is both the lowest in energy published and features a hydrogen-bond network extending the surface in one-dimension, contrary to what has been found in the literature, and contrary to what has been expected due to the large bulk ceria cell dimension. The adsorption energies for the monolayer and multilayered water structures agree well with experimental temperature programmed desorption results from the literature, and we discuss the stability window of CeO2(111) surfaces covered with 0.5-2.0 ML of water.

The widespread use of ceria-based materials and the need to design suitable strategies to prepare eco-friendly CeO2 supports for effective catalytic screening induced us to extend our computational multiscale protocol to the modeling of the hybrid organic/oxide interface between prototypical fluorinated linear alkane chains (polyethylene-like oligomers) and low-index ceria surfaces. The combination of quantum chemistry calculations and classical reactive molecular dynamics simulations provides a comprehensive picture of the interface and discloses, at the atomic level, the main causes of typical adsorption modes. The data show that at room temperature a moderate percentage of fluorine atoms (around 25%) can enhance the interaction of the organic chains by anchoring strongly pivotal fluorines to the channels of the underneath ceria (100) surface, whereas an excessive content can remarkably reduce this interaction because of the repulsion between fluorine and the negatively charged oxygen of the surface.

We propose that crystalline ionic hydrates constitute a valuable resource for benchmarking theoretical methods for aqueous ionic systems. Many such structures are known from the experimental literature, and they contain a large variety of water–water and ion–water structural motifs. Here we have collected a data set (CRYSTALWATER50) of 50 structurally unique “in-crystal” water molecules, involved in close to 100 nonequivalent O–H···O hydrogen bonds. A dozen well-known DFT functionals were benchmarked with respect to their ability to describe these experimental structures and their OH vibrational frequencies. We find that the PBE, RPBE-D3, and optPBE-vdW methods give the best H-bond distances and that anharmonic OH frequencies generated from B3LYP//optPBE-vdW energy scans outperform the other methods, i.e., here we performed B3LYP energy scans along the OH stretching coordinate while the rest of the structure was kept fixed at the optPBE-vdW-optimized positions.

This paper concerns thin water films and their hydrogen-bond patterns on ionic surfaces. As far as we are aware, this is the first time H-bond correlations for surface water and hydroxide species are presented in the literature while hydrogen-bond relations in the solid state have been scrutinized for at least five decades. Our data set, which was derived using density functional theory, consists of 116 unique surface OH groups–intact water molecules as well as hydroxides–on MgO(001), CaO(001) and NaCl(001), covering the whole range from strong to weak to no H-bonds. The intact surface water molecules are found to always be redshifted with respect to the gas-phase water OH vibrational frequency, whereas the surface hydroxide groups are either redshifted (OsH) or blueshifted (OHf) compared to the gas-phase OH– frequency. The surface H-bond relations are compared with the traditional relations for bulk crystals. We find that the “ν(OH) vs R(H···O)” correlation curve for surface water does not coincide with ...

The role of hydrogen and other defects in the stabilization of polar oxide interfaces is a matter of significant fundamental and practical interest. Using experimental (scanning tunneling microscopy, x-ray photoelectron spectroscopy) and theoretical (density functional theory) surface science techniques, we find that the polar Zn-terminated ZnO(0001) surface becomes excessively Zn deficient during high-temperature annealing (780 K) in ultrahigh vacuum (UHV). The Zn vacancies align themselves into rows parallel to the $[10\overline{1}0]$ direction, and the remaining surface Zn ions alternately occupy wurtzite (hcp) and zinc-blende (fcc) lattice positions, giving a characteristic ``striped'' $c(\sqrt{12}\ifmmode\times\else\texttimes\fi{}\sqrt{12})\mathrm{R}30$\ifmmode^\circ\else\textdegree\fi{} surface morphology with three types of rows: wurtzite Zn, zinc-blende Zn, and Zn vacancies. Interstitial H plays a central role in such a reconstruction, as it helps to compensate the excessive Zn deficiency. We propose a model in which hydrogen occupies positions in half of the vacancy rows to form hydroxide ions that can participate in hydrogen bonds in the O subsurface layer as a result of the mixed wurtzite/zinc-blende stacking.

The electronic and structural properties of the Ge/GeO2 interface are addressed through a density functional simulation scheme which includes the use of hybrid functionals for achieving accurate band gaps, band offsets and defect levels. The present work discusses the germanium dangling bond levels, the thermodynamics of GeOx, the stability of the oxygen vacancy across Ge/HfO2 interfaces, the atomic structure of GeOx, electron and hole trapping in GeOx, and the band alignment at the Ge/GeO2 interface. (C) 2011 Elsevier B.V. All rights reserved.

The electronic and structural properties at the Ge/GeO2 interface are addressed through a density-functional simulation scheme, which includes the use of hybrid functionals for achieving defect levels and band gaps comparable to experiment. The present work discusses the determination of the germanium dangling bond levels, the band alignment in a model structure of the Ge/GeO2 interface, the thermodynamics of GeOx, the stability of the oxygen vacancy across Ge/GeOx/HfO2 stacks, the structural and electronic properties of GeOx, and its ability to trap electrons.

We calculate defect levels of dangling bonds in germanium using hybrid density functionals. To validate our approach, we first consider the dangling bond in silicon finding two well-separated defect levels, in excellent correspondence with their experimental location. Application of our scheme to the dangling bond in germanium then yields two very close defect levels lying just above the valence band, consistent with the location of the experimentally determined charge neutrality level. The small correlation energy and the proximity to the valence band edge provide an explanation for the absence of clear defect signatures in electrical and electron spin resonance experiments. © 2008 Elsevier Ltd. All rights reserved.

Experimental characterization is facing severe difficulties in elucidating the nature of defects at interfaces between silicon and amorphous high-@k oxides. This motivates recourse to first principle simulation approaches based on density functional theory. A major issue for these approaches is the choice of the model structure. Indeed, the most trivial choice of parent crystalline structures may fail in describing the amorphous nature of the oxide or the specific bonding arrangements occurring at the interface. A second critical issue relates to the determination of energy levels with respect to band edges of the semiconductor and the oxide, which often represent the only contact with experimental data. An improved description of the electronic structure is required going beyond standard density functional approaches that severely underestimate electronic band gaps. Here, these issues are illustrated by a comparative study of the oxygen vacancy in monoclinic HfO"2, in amorphous HfO"2, and at the Si-HfO"2 interface.

An atomistic model of the Ge-GeO2 interface has been generated through first-principle methods based on density functional theory. The interface model consists of amorphous GeO2 connected to crystalline Ge through a substoichiometric oxide region showing regular structural parameters. Structural and electronic properties are compared to available experimental data and studied as they evolve across the Ge-GeO2 interface. (C) 2009 Elsevier B.V. All rights reserved.

We use theoretical simulations to study how oxidative and humid environments affect the chemical composition, shape and structure of ceria nanoparticles. Based on our calculations, we predict that small stoichiometric ceria nanoparticles will have a very limited stability range when exposed to these environments. Instead, we find that reduced ceria nanoparticles are stabilized without changing their inherent shape through the adsorption of oxygen molecules in the form of superoxo species and water in the form of hydroxo species. Based on our results, we propose a redox-cycle for meta-stable ceria nanoparticles without the formation of explicit oxygen vacancies, which is important for understanding the low-temperature oxygen chemistry of ceria at the nanoscale.

The Ge core-level shift across the Ge/GeO2 interface is determined within semilocal and hybrid density functional schemes. We first assess the accuracy achieved within these theoretical frameworks by comparing calculated and measured core-level shifts for a set of Ge-based molecules. The comparison with experimental data results in rms deviations of 0.19 and 0.09 eV for core-level shifts calculated with semilocal and hybrid density functionals, respectively. We also compare calculated core-level shifts at the Ge(001)-c(4 x 2) surface with high-resolution x-ray photoemission spectra finding similar agreement. We then turn to the Ge/GeO2 interface, which we describe with atomistic superlattice models showing alternating layers of Ge and GeO2. The adopted models include a substoichiometric transition region in which all Ge atoms are fourfold coordinated and all O atoms are twofold coordinated, as inferred for Si/SiO2 interfaces. Since the calculation of core-level shifts involves charged systems subject to finite-size effects, we use two different methods to ascertain the core-level shift Delta E-XPS between the oxidation state Ge-0 and Ge+4 across the interface. In the first method, core-hole relaxations are first evaluated in bulk models of the interface components and then complemented by the initial-state shift calculated across the interface, while the second method consists of direct interface calculations corrected through classical electrostatics. Using the more accurate hybrid functional scheme, we obtain a shift Delta E-XPS of 2.7 +/- 0.1 eV. This value is significantly lower than experimental data, which typically fall around 3.3 eV or higher, but the underestimation is consistent with that found for the valence band offset of the same model. This leads to the conclusion that the adopted model structures yield an incorrect description of the interface dipole and emphasizes that Ge/GeO2 interfaces possess different structural properties than their silicon counterparts.

We calculate charge transition levels of various defects at the SiC/SiO2 interface within a scheme based on hybrid density functionals, which accurately reproduce the involved band gaps and band offsets. The defect levels are first evaluated in bulk models of the interface components and then aligned with respect to the interface band diagram through the use of a model interface showing good structural and electronic properties. Interface-specific polarization effects are evaluated separately through classical electrostatics. We considered carbon-related defects involving single atoms and dimers in both the crystalline SiC substrate and the amorphous SiO2 oxide. Our investigation also comprises oxygen-and hydrogen-related defects, including the Si-Si bond (O vacancy), the Si-2-C-O structure, the peroxy linkage, and the hydrogen bridge (Si-H-Si). Among the defects studied, the Si-2-C-O structure represents the best candidate for the high defect density measured near the conduction band of SiC.

We investigate carbon single-atom and pair defects at the SiC/SiO2 interface as candidate defects for the density of defect states in the SiC band gap. In order to accurately describe the electronic defect levels with respect to the SiC band edges, we use a hybrid density functional which reproduces the experimental band gap of SiC. The carbon pair defect consisting of two neighboring sp(2) hybridized carbon atoms is modeled in various configurations within a SiC/SiO2 model interface showing good structural parameters and an oxide density typical of amorphous SiO2. The carbon pair defect is found to contribute to the density of defect states not only in the lower and/or mid band gap of SiC, but also in the upper band gap, in contrast with previous studies. The carbonpair defect is also investigated via molecular models to achieve insight into the energy range spanned by its defect levels when the relative orientation of its sp(2) hybridization planes and the chemical nature of its neighbors are varied. Carbon single-atom defects on the oxide side of the interface are also modeled and found to contribute to the defect density in the band gap in a similar way as carbonpair defects. Comparison to the experimental defect density suggests that defects involving one or two carbon atoms cannot account for the high defect density observed in the vicinity of the SiC conduction band.

Oxide materials might form the basis for the next technological revolutions in fields such as electronics, photovoltaics or heterogeneous catalysis by enabling the production of inexpensive, high capacity and fast nonvolatile memories, neuromorphic computers and control systems, and efficient energy conversion devices. All these possible applications have in common that they promise to expand the limits of existing technologies by exploiting the inherent material-level complexity of oxides. However, the enhanced opportunities of this added complexity come at the price of significantly greater difficulties in understanding the materials' properties and their implications. Among many, the peculiar and variable mixture of ionic and covalent bonding behavior and the relative ease of changing the oxygen content of many compounds lead to fascinating effects. Furthermore, a deep level of understanding of the materials properties and their relation to synthesis is crucial for eventual applications. In recent years, an enormous amount of work has been devoted to the development of new experimental and theoretical tools, which have enabled much improved understanding of oxide behavior. Materials science is now becoming able to predict and engineer the behavior of oxide materials and explore brave new worlds of functionality. The recent advances in functional oxide science and technology prompted us to hold the CECAM workshop on “Functional Oxides for Emerging Technologies” at the Bremen Center for Computational Materials Science in October 2013, and it was during the preparation for this workshop that the Focus Issue on functional oxides was conceived. The three Review@RRL articles introduce the reader to innovative, low energy electron microscopy [1], controllable electric conduction at oxide interfaces [2], and ionic switching based RRAM [3]. The 20 original Letters present exciting new research on the topics of transport properties [4], super-reducible oxides [5], ferroelectricity [6], point defects [7], memory applications [8–10], bulk properties [11–14], interfaces [15–17], surfaces [18–20] and catalytic properties [21–23]. We sincerely hope that you will find our collection of articles from the forefront of functional oxide research to be informative and that reading our Focus Issue inspires you to come up with more and even better ideas and insights. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The defect levels of the nitrogen dangling bond at the Si/SiO2 interface are determined through a density-functional approach. The composition grading at the interface is modeled through crystalline and amorphous models of stoichiometric SiO2, nitrided SiO2, and substoichiometric silicon oxides. The relevant charge transition levels are first determined with respect to the band edges of the parent oxides within a semilocal density-functional scheme. Through the use of band edge shifts and band offsets previously obtained with hybrid functionals, the calculated defect levels are then positioned with respect to a band diagram of the Si/SiO2 interface which shows good agreement with the experimental one. We find that the 0/- charge transition level locates within the Si band gap. The level locates close to the Si valence band for nitrogen atoms in the stoichiometric oxide, but is found to rise across the silicon band gap as the environment of the nitrogen atoms becomes more silicon rich. The latter raise is accompanied by a stabilization of the incorporated nitrogen.

We determine energy levels of candidate defects for the high defect density observed just below the SiC conduction band at SiC/SiO2 interfaces. We address C interstitials and H bridge defects which both show defect levels in the upper part of the SiC band gap. However, these defects also give rise to defect levels in the lower part of the band gap, unlike the experimental defect density which lacks this correlation.

We use a scheme based on hybrid density functionals to calculate charge transition levels of defects in SiO2. We align the calculated levels to the band edges of Si at the Si‐SiO2 interface. We find a good agreement between calculated and measured defect energy levels.

Defect levels of dangling bonds in germanium are calculated through the use of hybrid density functionals. The theoretical approach is first validated through the consideration of the well‐characterized defect levels of the dangling bond in silicon, for which two well‐separated defect levels are found in excellent correspondence with their experimental location. For germanium, our scheme then yields two very close defect levels, located just above the valence band maximum. These findings are consistent with measurements of the charge neutrality level, lying within 0.1 eV from the valence band edge. The small correlation energy and the proximity to the valence band edge provide a rationale for the absence of clear defect signatures for germanium dangling bonds in electrical and electron‐spin‐resonance experiments.

We study the charge state of the diffusing O-2 molecule during silicon oxidation through hybrid functional calculations. We calculate charge-transition levels of O-2 in bulk SiO2 and use theoretical band offsets to align these levels with respect to the Si band edges. To overcome the band-gap problem of semilocal density functionals, we employ hybrid functionals with both predefined and empirically adjusted mixing coefficients. We find that the charge-transition level epsilon(0/-) in bulk SiO2 occurs at similar to 1.1 eV above the silicon conduction-band edge, implying that the O-2 molecule diffuses through the oxide in the neutral charge state. While interfacial effects concur to lower the charge-transition level, our estimates suggest that the neutral charge state persists until silicon oxidation.

For materials of varying band gap, we compare energy levels of atomically localized defects calculated within a semilocal and a hybrid density-functional scheme. Since the latter scheme partially relieves the band gap problem, our study describes how calculated defect levels shift when the band gap approaches the experimental value. When suitably aligned, defect levels obtained from total-energy differences correspond closely, showing average shifts of at most 0.2 eV irrespective of band gap. Systematic deviations from ideal alignment increase with the extent of the defect wave function. A guideline for comparing calculated and experimental defect levels is provided., Comment: 4 pages, 3 figures

Using a density functional approach, we study structural and electronic properties of the 4H(0001)-SiC/SiO2 interface. Through the sequential use of classical and ab initio simulation methods, we generate an abrupt model structure which describes the transition between crystalline SiC and amorphous SiO2 without showing any coordination defect. The first step in our generation procedure consists in identifying suitable interfacial bonding patterns which account for the bond density reduction across the interface. In the second step, the connection to amorphous SiO2 is achieved through classical molecular dynamics. The atomic positions are then relaxed within a generalized gradient approximation of density functional theory. The final model structure shows good structural parameters and an oxide density typical of amorphous SiO2. We investigate the electronic structure of the generated model interface through the local density of states obtained with hybrid density functionals. In our atomically abrupt interface model, the full oxide band gap is recovered at a distance of similar to 5 A from the interface. This extent is in good agreement with estimates derived from internal photoemission measurements and provides support for the abrupt nature of the interface. We obtained band offsets through two different procedures: by evaluating the local density of states and by aligning the band extrema through the local electrostatic potential. Band offsets calculated with various functionals are compared to experimental values. The best agreement is achieved for a hybrid functional in which a screened Coulomb potential is used in the Hartree-Fock exchange term. For this case, the calculated band offsets underestimate the experimental values by similar to 25%, but agree with experiment within a few percent when expressed with respect to the oxide band gap.

We study structural and electronic properties of the oxygen vacancy in monoclinic HfO2 for five different charge states. We use a hybrid density functional to accurately reproduce the experimental band gap. To compare with measured defect levels, we determine total-energy differences appropriate to the considered experiments. Our results show that the oxygen vacancy can consistently account for the defect levels observed in optical absorption, direct electron injection, and trap-assisted conduction experiments.

The stability of oxygen vacancies across the Ge–HfO2 interface is studied through semilocal and hybrid density-functional calculations. On the semiconductor side, the formation energies are obtained for substoichiometric GeOx of varying x through the use of a bond-energy model. On the hafnium oxide side, the interface is modeled through bulk models with aligned band structures. Formation energies are compared for different charge states and Fermi energy levels. The oxygen vacancy is found to be most stable in the interfacial germanium oxide layer for both p-type and n-type doping. This favors the formation of substoichiometric GeOx, consistent with experimental observations.

The determination of band alignments and defect levels is demonstrated for the technologically relevant Si–SiO2–HfO2 gate stack. The proposed scheme, which combines first-principles molecular dynamics for model generation and hybrid density functionals for electronic-structure calculations, yields band offsets in close agreement with experiment. Charge transition and pinning levels associated with oxygen vacancies are aligned with respect to the silicon band edges. The vacancies are shown to preferentially reside in the amorphous transition layer, consistent with experimental observations of Fermi-level pinning.

Several studies have reported a dramatically increased oxygen storage capacity (OSC) for small ceria nanoparticles (∼5 nm). Both experiments and theory have correlated this effect with superoxide ion formation. In previous studies, density functional theory (DFT) calculations with the PBE+U density functional have been used, and the obtained results were only in qualitative agreement with the experimental observations. One severe problem is the underbinding of the O2 molecule upon superoxide ion formation, which suggests that such species should not exist above room temperature. In this work, we use hybrid DFT functional to resolve this problem. We find that the discrepancy between theory and experiment originates from an incorrect estimate of the energy associated with the localized f-electrons with respect to the oxygen p-levels. By using average O2 adsorption energies from hybrid DFT calculations, extrapolated to large nanoparticles (3−10 nm), in conjunction with first-order desorption kinetics, we fin...

We introduce a scheme for the calculation of band offsets and defect energy levels at semiconductor-oxide interfaces. Our scheme is based on the use of realistic atomistic models of the interface structure and of hybrid functionals for the evaluation of the electronic structure. This scheme is herein applied to the technologically relevant Si–SiO2–HfO2 stack. Calculated band offsets show a very good agreement with experimental values. In particular, we focus on the energy levels of the oxygen vacancy defect and the interstitial hydrogen impurity. The defect levels are aligned with respect to the interface band structure and determined for varying location in the dielectric stack. The most stable charge states are identified as the Fermi level sweeps through the silicon band gap.

Through hybrid density functional calculations, we compare the Ge-Ge bond energy with the formation energy of a valence alternation pair as the O concentration varies across the Ge/GeO2 interface. First, hole trapping energies are calculated for three atomistic models with different O concentrations: bulk Ge with isolated O atoms, amorphous GeO, and amorphous GeO2 with an O vacancy. The reaction is then broken down in three steps involving the breaking of a Ge-Ge bond, charge transfer processes involving dangling bonds, and the formation of a threefold coordinated O atom. The energy of each elemental reaction is estimated through suitable model calculations. The charge transition levels resulting from this analysis agree with those obtained for the atomistic models. Our estimates indicate that hole trapping at low O concentrations occurs at no energy cost for p-type germanium owing to the formation of threefold-coordinated O atoms. Applied to n-type Ge, our analysis indicates that electron trapping in dangling bonds obtained from the breaking of Ge-Ge bonds is unfavorable. The formation energy of a valence alternation pair is evaluated and discussed in relation to previous results. (C) 2011 Elsevier B.V. All rights reserved.

Band offsets at semiconductor-oxide interfaces are determined through a scheme based on hybrid density functionals, which incorporate a fraction $\alpha$ of Hartree-Fock exchange. For each bulk component, the fraction $\alpha$ is tuned to reproduce the experimental band gap, and the conduction and valence band edges are then located with respect to a reference level. The lineup of the bulk reference levels is determined through an interface calculation, and shown to be almost independent of the fraction $\alpha$. Application of this scheme to the Si-SiO$_2$, SiC-SiO$_2$, and Si-HfO$_2$ interfaces yields excellent agreement with experiment., Comment: 4 pages, 4 figures

Electronic band gaps and dielectric constants are obtained for amorphous hafnium silicates using first-principles methods. Models of amorphous (HfO2)(x)(SiO2)(1-x) for varying x are generated by ab initio molecular dynamics. The calculations show that the presence of Hf gives rise to low-lying conduction states which explain the experimentally observed nonlinear dependence of the band gap on hafnium content. Static dielectric constants are found to depend linearly on x, supporting recent experimental data. (c) 2007 American Institute of Physics.

Defect levels of dangling bonds in silicon and germanium are determined within their respective band gaps through the use of hybrid density functionals. To validate our approach, we first considered the dangling bond in silicon finding two well-separated defect levels in excellent correspondence with their experimental location. Application to the dangling bond in germanium then yields two very close defect levels lying just above the valence band, which is consistent with the experimental location of the charge neutrality level. The occurrence of negative-U behavior leads to a reduced fraction of neutral dangling bonds, thereby suppressing the electron-spin-resonance activity.

Oxygen vacancy migration is studied in monoclinic HfO2 and across its interface with SiO2 through density functional calculations. In HfO2, long-range diffusion shows activation barriers of 2.4 and 0.7 eV for the neutral and doubly positively charged vacancy, respectively. In the latter case, the migration preferentially occurs along one-dimensional pathways. A HfO2/SiO2 interface model is constructed to address O vacancy migration across high-kappa gate stacks. The vacancy is shown to stabilize in its neutral charge state upon entering the SiO2 layer.