1,720,988 research outputs found
Energetics and diffusivity of atomic boron in silicon by density-functional based tight-binding simulations
No full textWe have applied a density-functional derived tight-binding method (DF-TBMD) to the study of the energetics and the dynamics of boron defects in silicon. This study is motivated by a number of interstitial-driven phenomena observed in experiments, as the transient enhanced diffusion of B atoms in implanted silicon samples together with the formation of immobile B precipitates. We discuss first the DF-TBMD results for equilibrium structures and formation energies of different defect configurations containing a single boron atom and a silicon self-interstitial. Moreover, DF-TBMD molecular dynamics simulations at finite temperature allow us to investigate boron diffusivity in a temperature range between 900 and 1500 K. We provide for the first time a dynamical picture of B diffusion in silicon characterized by a migration energy of 0.7 eV
Neutral boron-interstitial clusters in crystalline silicon
No full textThe formation of B clusters inside ultrashallow junctions is one of limiting factors in the miniaturization process of electronic devices. The assembling of these clusters corresponds to a reduction of the electrical activity of the doping process. Exploiting hierarchically different simulation techniques, we investigate struc- tural and electronic properties of small B clusters inside a crystalline Si matrix. Density-functional–theory- tight-binding molecular dynamics simulations are carried out as scouts selecting the candidates to be analyzed in depth via ab initio calculations. The latter provide insights into the electronic properties of the B clusters, identifying the fingerprints of interstitialcy and chemical composition in their densities of states
Impurity-vacancy complexes and ferromagnetism in doped sesquioxides
No full textBased on hybrid density-functional calculations, we propose that ferromagnetism in the prototypical bixbyite sesquioxide In2O3 doped with Cr is due to Cr-oxygen vacancy complexes, while isolated Cr cannot support carrier-mediated magnetic coupling. Our proposal is consistent with experimental facts such as the onset of ferromagnetism in O-lean conditions only, the low or vanishing net moment in unintentionally doped material, and its increase upon intentional doping. © 2014 American Physical Society
Atomistic study of boron clustering in silicon
No full textWe present the results of our theoretical investigation on the energetics of boron-interstitial clusters in silicon. In particular, we discuss the results obtained within a Density Functional derived Tight Binding (DFTB) scheme and further analyzed by Density Functional Theory calculations performed in the Local Density Approximation (DFT-LDA). We show the lowest-energy geometries, analyzing symmetry properties and atomic coordinations. The comparison of binding energy values allows us to discuss the competition between the formation of mixed and monoatomic clusters, in the search for a growth pattern in either high or low Si self-interstitial content. An upper limit for B-clusters dimensions (with n,m similar or equal to 4) is proposed. We focus then on the characterization of the electronic structures of the B-Si clusters. A preliminary analysis of the calculated electronic properties of selected cluster structures attributes features appearing in the fundamental gap to specific bonds between B and interstitial Si atoms
Magnetism and unusual Cu valency in quadruple perovskites
No full textWe study a selection of Cu-containing magnetic quadruple perovskites (CaCu3Ti4O12 LaCu3Fe4O12, and YCu3Co4O12) by ab initio calculations, and show that Cu is in an effective divalent Cu(II)-like state or a trivalent Cu(III) state depending on the choice of octahedral cation. Based on the electronic structure, we also discuss the role of Mott and Zhang-Rice physics in this materials class
Electronic Structure of Bulk and Defected CaCu3Ti4O12
No full textThe calcium copper titanate CaCu3Ti4O12 is a distortedperovskite exhibiting a giant dielectric constant withnon-ferroelectric temperature behavior. Recent investigationstend to attribute the origin of the anomalous behavior ofCaCu3Ti4O12 dielectric properties to the presence ofheterogeneous microstructure and grain boundary layers,excluding that the giant dielectric constant is present inperfectly stoichiometric, defect-free crystals. The intrinsicorigin is also ruled out by the normal dielectric behaviorobtained in ab initio Density Functional Theory (DFT)calculations. In this work, we present ab initio calculationsperformed to estimate the energies associated with the creationof defects in the CaCu3Ti4O12 lattice, including oxygen andcopper vacancies, and investigate their electronic properties.Furthermore, with a view at ascertaining whether correlationeffects may modify the electronic structure and dielectricity ofCaCu3Ti4O12, we present an application to CaCu3Ti4O12 of aDFT-based pseudo-selfinteraction-correction (pSIC) method already applied successfully to Mott insulators
Multigap absorption in CaCu3Ti4O12 and the prediction capability of ab initio methods
No full textWe report the electronic properties of the quadruple perovskite CaCu3Ti4O12 as obtained via several density- functional based methods and propose a new interpretation of optical experiments to the effect that four distinct transitions (centered around 0.7, 1.5, 2.5, and 3.5 eV) contribute to the spectrum. The comparison with experiment is satisfactory, especially after we account for the effects of spin disorder, which does not close the fundamental gap but suppresses the transition intensity. We find that some of the methods we employ tend to overestimate considerably the gaps for standard values of the respective adjustable parameters
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