6,087 research outputs found
Effects of strain and local charge on the formation of deep defects in III-V ternary alloys
No full textThe effects of external and internal strains and of defect charges on the formation of gallium vacancies and arsenic antisites in GaAs and In0.5Ga0.5As have been investigated by ab initio density functional methods, Present results show that a proper understanding of strain and defect charge permits the development of a defect engineering of semiconductors. Specifically, they predict that arsenic antisites in InGaAs ternary alloys can form, upon p-type doping in the presence of an arsenic overpressure, even in the case of high-temperature epitaxial growths
Defect engineering in III-V ternary alloys: Effects of strain and local charge on the formation of native deep defects
No full textThe effects of external and internal strains, and of defect charges on the formation of vacancies and antisites in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that a proper use of strain and defect charges permits the development of a defect engineering of III-V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while the formation of vacancies may be favored only by extreme conditions of compressive strain. (C) 2002 Elsevier Science B.V. All rights reserved
Defect engineering in III-V ternary alloys: effects of strain and local charge on the formation of substitutional and interstitial native defects
No full textThe effects of external and internal strains and of defect charges on the formation of vacancies, antisites and interstitials in GaAs and In0.5Ga0.5As have been investigated by first principles density functional methods. Present results show that strain and doping permit a defect engineering of III-V semiconductors. Specifically, they predict that doping may have major effects on the formation of antisites while vacancies may be favored only by extreme conditions of compressive strain. Interstitials may be moderately favored by doping and tensile strain. (C) 2001 Elsevier Science B.V. All rights reserved
Nitrogen passivation by atomic hydrogen in GaAsyN1-y and InxGa1-xAsyN1-y alloys
No full textPrevious theoretical studies on N-H complexes in GaAsN have been extended here to new di-hydrogen complex configurations and to N-H complexes in the In0.25Ga0.75As0.97N0.03 alloy. Moreover, a deeper analysis has been performed on the structure, formation energies, chemical bonding and electronic properties of old and new N-H complexes in the above alloys. On the ground of the achieved results, the existence of a novel di-hydrogen complex is predicted that is characterized by a C-2v symmetry and peculiar vibrational properties. Complexes with this symmetry are not stable in N-free GaAs. Further, we propose a sound model for the N passivation founded on the characteristics of the electronic states and the local atomic relaxations induced by the N-H complexes. This model explains why the N passivation is not achieved in the case of monohydrogen complexes and realized through the formation of the N-H-2(*) dihydrogen complexes. Finally, it is suggested that different N-H complexes (and different vibrational spectra) should be observed in hydrogenated p-type and n-type N-containing alloys
SILICON-HYDROGEN-ACCEPTOR COMPLEXES IN CRYSTALLINE SILICON
No full textAn extensive investigation of the equilibrium sites of H in p-type silicon has been performed in order to clarify the influence of the specific impurity on the geometry of the silicon-hydrogen-acceptor complexes. Previous studies focused onto the B and Al cases have been extended to the isovalent acceptors Ga and In, making clear the relevance of the impurity atomic size. The on-axis BC site is shown to be a marginal equilibrium position, which evolves toward an off-axis position as soon as the acceptor size exceeds that of B. A novel H metastable site has been estimated, only in Si:In, at the AB-In site, thus suggesting a dependence of H equilibrium sites on the full chemistry of the impurity. These results account well for far-infrared measurements in Si:Al and Si:Ga, as well as for perturbed gamma-gamma-angular correlation results in Si:In
Structure, kinetics, and passivation of hydrogen-acceptor complexes in gallium arsenide: A theoretical study
No full textThe structure, reorientation kinetics, and passivation mechanism of complexes formed by a H atom and As-site or Ga-site accepters, e.g., the Si(As)-H-Ga and the Zn(Ga)-H-As complexes in GaAs, have been investigated by first-principles local-density-functional methods. In both complexes, the stable configuration is found for the H atom located at a bond-centered site. The H atom is bound to the Si acceptor in the Si(As)-H-Ga complex and to the As atom in the Zn(Ga)-H-As complex. In spite of the different H bonds, similar vibrational properties and reorientation kinetics have been theoretically found for the two complexes. The present calculations well reproduce the experimental values of the vibrational frequencies and of the complex dissociation energies, as well as account for the acceptor passivation. A good agreement is also found with the reorientation energy of the Si(As)-H-Ga complex, while the unusually high relaxation rates of the Zn(Ga)-H-As complex measured by anelastic-relaxation investigations remains unexplained. In fact, neither the energy barrier estimated for the complex reorientation nor two different tunneling models account for those high relaxation rates
STRUCTURAL AND VIBRATIONAL PROPERTIES OF THE SI-H-AL COMPLEX IN CRYSTALLINE SILICON
No full textThe atomic complex formed by silicon, hydrogen, and aluminum atoms has been investigated by use of the pseudopotential-density-functional method in a supercell approach. The stable equilibrium geometry found for this complex is an off-axis configuration, with the hydrogen atom located near a Si-acceptor pair, noticeably different from the on-axis configuration obtained in the boron case. This result, together with a H local mode at quite low frequency, explains the fine structure reported for the vibrational spectra in hydrogenated Si:Al
VIBRATIONAL FREQUENCIES OF SI-P-H COMPLEXES IN CRYSTALLINE SILICON - A THEORETICAL-STUDY
No full textHydrogen-induced states near the GaAs band edges
No full textIn the present study the link between the inclusion of H species in GaAs and the appearance in the band gap of near-band-edge energy levels has been investigated. The equilibrium geometries and the electronic structures relative to different locations of H atoms, ions, and dimers in the GaAs lattice have been evaluated by first-principle local-density-functional methods. Atomic arrangements in GaAs identical to those found for different equilibrium geometries of the H species, but with the H species removed, have also been investigated in order to separate the effects of the lattice deformations due to the H inclusion from those produced by the formation of H-host-atom bonds. Discrete near-edge levels appear in the band gap, which are induced by the H interactions with the GaAs lattice as well as by charge effects in the case of H ions. Finally, radiative transitions from the conduction-band to the valence-band near-edge states are shown to account for the main features of the emission bands observed in the photoluminescence spectra of hydrogenated III-V compounds
Short hydrogen bonds at the water/TiO2 (anatase) interface
No full textThe nature of peculiar, short H bonds formed by water molecules in contact with the (101) anatase surface and their effects on the structural and vibrational properties of the first water layers adsorbed on the same surface have been investigated by performing density functional theory (DFT) total energy calculations and ab initio molecular dynamics (AIMD) simulations at different temperatures. Present results show that these short H bonds originate from a water/anatase interface effect related to an electronic charge transfer from surface Ti atoms to surface O atoms, mediated by water molecules. Further, AIMD simulations per-formed at low temperature indicate that such short H bonds are at the ground of both the atomic arrangements of the water layers and the peculiar features appearing in the corresponding vibrational spectra. The same interface effect significantly influences also the atomic arrangements and the vibrational properties of intermediates of the O-2 photoreduction reaction, which turn out to be involved in similar charge transfer processes as well as in the formation of short H bonds. AIMD simulations show that these short H bonds are still present at room temperature and give estimates of the vibrational frequencies of the same intermediates, which are in a quite good agreement with the experimental findings. Such an agreement supports the unifying theoretical picture proposed here for water molecules and O-2 photoreduction intermediates in contact with the anatase surface
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