1,720,991 research outputs found
Nucleation and first stage growth process of extrinsic defects in GaAs triggered by self-interstitials
No full textNucleation and first-stage growth processes of extrinsic defects in GaAs triggered by self-interstitials
No full textThe growth process of small self-interstitial clusters In (n≤7) in crystalline GaAs has been addressed by semi-empirical tight-binding molecular-dynamics technique. The In ground-state structures have been found among many possible choices of topological properties and stoichiometric compositions. The stable structure have been fully characterised concerning the structural, electronic, energetic, and elastic properties; some remarkable findings emerged concerning, among the others, the stability scenario of the ground-state structures, the possible low-energy reaction paths involved in the growth process, the electrostatic and the elastic capture volumes and the Fermi-level pinning. It is demonstrated that compact geometries are no longer energetically favoured for n 5 and that the In growth proceeds via capture processes involving either isolated interstitials or di-interstitials. An extended pentainterstitial (I5) ground-state structure has been identified as the possible core-basic structure of extrinsic linear defects along the {111} direction of the GaAs lattice. © 2009 The American Physical Society
Stability of I3 complexes in III-V compound semiconductors by tight-binding molecular dynamics
No full textIntrinsic interstitials in GaAs are known to have a large formation energy that makes their concentration almost negligible in as-grown materials. However, interstitials must be explicitly considered in implanted GaAs where collision cascades, induced by the energetic ions, produce a large amount of Frenkel defects: these, indeed, can be considered as a source of interstitials that can migrate and evolve in larger aggregates. Due to the wide relaxation pattern induced by intrinsic interstitials, large supercells are necessary to avoid artifacts that, in practice, make first-principles calculations too computationally demanding, even for complexes involving just three interstitials. Following previous papers on di-interstitials, this paper reports on the
structural, stability, and electronic properties of I3 complexes, depending on the topology and the stoichiom-
etry, approached by semiempirical tight-binding molecular dynamics and damped dynamics. I3 complexes reveal a strong tendency to form these structures, and a stability hierarchy is determined between them. All the stable and metastable structures have been studied regarding the electronic structure, the charge distribution through the Mulliken analysis, the localization of the defect related wave function, and the local lattice strain field around the complex. The present study is aimed to define the selection rules that must be considered in order to build up larger In (with n > 4) self-interstitials complexes in GaAs
Hydrophobic coatings on hydrogenated (111) silico surface
No full textThe first stage functionalization of hydrogenated (111) Si surface with methyl-terminated monolayers to form
hydrophobic coatings has been studied by accurate ab initio density functional total energy calculations. The
adsorption have been
characterized from the geometrical and the energetic points of view; the ground-state adsorption configurations
together with the geometrical and the energetic parameters relevant to self-assembling processes have been
obtained, such as the polymers tilt angles, the rotation energy barriers, the binding energies, and the electrostatic
interactions. The above-mentioned quantities are related to the stability properties of self-assembled
monolayers and affect critically the stability and the uniformity of the hydrophobic film
Dielectric Properties of Self-Assembled Monolayer Coatings on a (111) Silicon Surface
No full textNovel nanomaterial systems as possible candidates for gate dielectric insulators play a key role in the fabrication of next-generation transistor devices in both metal-oxide-semiconductor (MOSFET) and organic thin-film transistors (OTFTs). We focus on one of these new alternative gate dielectric nanostructured materials: self-assembled monolayers (SAMs) of hydroxylated octadecyltrichlorosilane (OTS) chains deposited on a (111) Si substrate. Starting from the evaluation of the surface partial dipole of the SAM coating on the Si surface, we report a quantitative ab initio study of the static dielectric constant of the OTS thin-film coating at different coverage values of the hydrogenated (111) Si surface ranging from partial to full coverage. The main physical features of the OTS SAM films at different coverages have been studied with respect to their influence on the static dielectric constant, and a two-layer model is established. A linear dependence of the static dielectric constant versus the coverage is shown to hold, resulting from depolarization phenomena of the main contributors
Migration barriers of neutral As di-interstitials in GaAs
No full textThe recent discovery of intrinsic di-interstitial stability against the isolated self-interstitial point defects in GaAs has evidenced the importance of such complexes in, for instance, irradiated GaAs. In this paper, we illustrate and discuss diffusion of such complexes in comparison with isolated self-interstitials. In particular, the diffusion barriers of neutral di-interstitials have been calculated in the framework of density functional theory, showing that, in addition to their being stable, di-interstitials can also diffuse rapidly through the lattice, similarly to isolated self-interstitials
Atomistic Modeling of Gas Adsorption in Nanocarbons
Full text linkCarbon nanostructures are currently under investigation as possible ideal media for gas storage and mesoporous materials for gas sensors. The recent scientific literature concerning gas adsorption in nanocarbons, however, is affected by a significant variation in the experimental data, mainly due to the different characteristics of the investigated samples arising from the variety of the synthesis techniques used and their reproducibility. Atomistic simulations have turned out to be sometimes crucial to study the properties of these systems in order to support the experiments, to indicate the physical limits inherent in the investigated structures, and to suggest possible new routes for application purposes. In consideration of the extent of the theme, we have chosen to treat in this paper the results obtained within some of the most popular atomistic theoretical frameworks without any purpose of completeness. A significant part of this paper is dedicated to the hydrogen adsorption on C-based nanostructures for its obvious importance and the exceptional efforts devoted to it by the scientific community
Calculation of di-interstitial diffusion barriers in GaAs by density functional theory
No full textIntrinsic di-interstitial are stable against the
isolated self-interstitial point defects in GaAs making such complexes important in, for instance, irradiated GaAs. We illustrate and discuss diffusion of such complexes in comparison with isolated self-interstitials studied by first priciples tota energy calculations.
It is shown that di-interstitials can diffuse rapidly through the lattice, similarly to isolated self-interstitials
Properties of charged intrinsic di-interstitials in GaAs
No full textStable intrinsic di- interstitials configurations in GaAs with different stoichiometric compositions are studied by first principles total- energy calculations. For each composition, fully relaxed stable and metastable structures have been obtained at different charge states to study the stability properties in different doping conditions and calculate the thermodynamic transition states, showing that the studied structures are stable against the isolated interstitials with large binding energies. The structural and electronic properties of the different configurations are also discussed in order to determine the relevant observables and to plan for possible future experiments aimed to detect them. In this context, we have focused particularly on different Ga di- interstitials configurations showing that the most stable one can be detected by means of state of the art deep level detection techniques, whereas more sophisticated experiments are required, involving symmetry and thermal activation processes, to detect As di- interstitials. Combining the different techniques in an sort of " differential diagnosis," at least two of the three possible, stoichiometry dependent, di- interstitial configurations can be identified
Augmented methane adsorption at Ca decorated carbon nanotubes-a DFT study
No full textMethane adsorption at Ca decorated single wall carbon nanotubes has been studied by ab initio total energy calculations based on the density functional theory. The adsorption configurations have been studied by using various exchange-correlation energy functionals also including two possible long-range interaction correction schemes. Our calculations show that methane adsorption at Ca decorated carbon nanotubes is markedly enhanced when impurity atoms are considered as individual adsorption sites. We demonstrate that up to six CH4 molecules can bind at a single Ca impurity at room temperature. The phenomenon responsible for the measured adsorption energy is recognized as a Kubas-type interaction that involves the orbital overlap between the Ca d state and the methane sigma molecular orbitals. The adsorption values obtained with the different energy functionals used are discussed showing that local density approximation, often employed in the recent literature for similar adsorption problems, is affected by severe limitations when orbital overlap and physisorption co-exist. Lastly the well-known problem of Ca clustering is studied and discussed showing that, different from the hydrogen case, it does not induce any molecular dissociation
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