Publications de Alfredo Pasquarello

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Using a model structure consisting of a network of corner sharing tetrahedra, we calculate the infrared spectrum of amorphous SiO2 within a first-principles approach and find good agreement with experiment both for the positions and the intensities of the main peaks. In addition to the vibrational properties, this required the dynamical charge tensors, which were obtained applying the recent quantum polarization theory. The relative intensities in the spectrum depend sensitively on the charge tensors and th

Using the variational density-functional perturbation theory, we investigate the dynamical and dielectric properties of tetragonal zirconia (t-ZrO2). We obtain the phonon frequencies at the center of the Brillouin zone, the Born effective charge tensors, and the dielectric permittivity tensors. For all these quantities, a comparison is made with the related values in the cubic phase. The Born effective charge tensors are found to be quite anisotropic. The calculated phonon frequencies present a better agree

We show that constant-Fermi-level ab initio molecular dynamics can be used as a computer-based tool to reveal and control relevant defects in semiconductor materials. In this scheme, the Fermi level can be set at any position within the band gap during the ...

Using density functional theory, we investigate the structural, vibrational, and dielectric properties of titanium oxides and silicates, which have attracted considerable attention in the framework of the quest for alternative high-K materials. For the oxides,. three crystalline phases of titanium dioxide are considered. The first two are hypothetical; they are obtained by similarity with the cubic and tetragonal structure of zirconia ZrO2 or hafnia HfO2. The third is the rutile, a crystal that occurs natur

The origin of the first sharp diffraction peak (FSDP) in the total structure factor of disordered network-forming glasses and liquids is frequently interpreted either as resulting from crystalline-like layers or from correlations between basic structural units (clusters) and interstitial voids. By applying appropriate criteria to configurations of liquid SiO2 and GeSe2 obtained by first-principles molecular dynamics, we find that neither interpretation holds. Despite the appearance of a FSDP in both liquids

After assessing the current status concerning the interpretation of Si 2p core-level shifts in Si-O systems, we model the atomic structure of the Si(001)-SiO2 interface using recent photoemission data obtained with synchrotron radiation. Our model structure reproduces the amount, the distribution, and the location of silicon atoms in intermediate states of oxidation. Our model also shows a SiO2 density in the neighborhood of the interface consistent with X-ray reflectivity measurements. (C) 2002 Elsevier Sc

Since the preliminary work of Anisimov and co-workers, the Hubbard corrected DFT+U functional has been used for predicting properties of correlated materials by applying on-site effective Coulomb interactions to specific orbitals. However, the determinatio ...

The close electronegativity of Ge and Se makes disordered GeSe2 systems ideally suited to investigate the relation between intermediate range order and bonding character. First-principles molecular dynamics simulations were carried out on liquid GeSe2 using two state-of-the-art functionals, which reproduce differently the ionic character in the bonds. Only the functional favouring more ionic bonding gave rise to a first sharp diffraction peak in the calculated neutron structure factor, in excellent agreemen

Using a density-functional approach, we study the dielectric permittivity across interfaces at the atomic scale. Focusing on the static and high-frequency permittivities of SiO2 films on silicon, for oxide thicknesses from 12 Angstrom down to the atomic scale, we find a departure from bulk values in accord with experiment. A classical three-layer model accounts for the calculated permittivities and is supported by the microscopic polarization profile across the interface. The local screening varies on lengt

An arsenic related defect at the In0.53Ga0.47As/oxide interface is studied through density functional theory. This defect corresponds to an As-As timer which transforms into two doubly occupied As dangling bonds upon capture of two electrons. The defect ch ...

We investigate the migration properties of hole polarons in rutile and anatase TiO2 using Hubbard U semilocal and hybrid functionals with parameters set to enforce the piecewise linearity condition. The polarons in anatase and in rutile are found to be sta ...

We calculate binding energies of shallow acceptors in GaAs/Ga1-xAlxAs quantum wells (QWs) for varying well widths. Variational calculations are performed in the framework of a multiband effective-mass theory, which accounts for the mixing between heavy and light holes. The Hamiltonian also takes into account the mismatch between the band parameters and the dielectric constants of well and barrier materials. The envelope function is expanded into a basis set consisting of products of two-dimensional hydrogen

We describe a scheme for calculating the infrared and Raman spectra of large model structures from first principles. Our scheme is based on the application of a finite electric field in density functional calculations with periodic boundary conditions. Coupling tensors for infrared and Raman spectra are obtained by numerically calculating first and second derivatives of the atomic forces with respect to the electric field. The method is illustrated through a study of the vibrational properties of vitreous s

We investigate the dielectric permittivity of SiO2 on Si(100) substrates using a first-principles approach. It is shown that both the static and high-frequency dielectric constants of the oxide overlayer increase when the oxide thickness is reduced. This behavior of the overlayer is found to be well reproduced by a classical two-layer model in which the pure oxide and the suboxide layer are treated as distinct dielectrics. Implications for interfaces between silicon and high-k gate oxides are discussed. (C)

The vibrational properties of amorphous SiO2 were studied within first-principles density functional theory. The calculated spectrum is in good agreement with neutron data, showing, in particular, a double peak in the high-frequency region. This doublet results from different local modes of the tetrahedral subunits and cannot be ascribed to a longitudinal-optic-transverse-optic (LO-TO) effect. This solves a long-standing controversy about the origin of the doublet in neutron spectra. A LO-TO splitting is re

We investigate the equivalent oxide thickness of a thin oxide interlayer in gate insulator stacks on silicon. Through the use of a first-principles approach, we map the profile of the local permittivity across two interface models showing different suboxide structures. These models incorporate the available atomic-scale experimental data and account for the amorphous nature of the oxide. The equivalent oxide thickness of the interfacial oxide layer is found to be smaller than the corresponding physical thic

Using a first-principles approach, we characterize dangling bond defects at Si-SiO2 interfaces by calculating hyperfine parameters for several relaxed structures; Interface models, in which defect Si atoms remain close to crystalline sites of the substrate upon relaxation, successfully describe P-b and P-b0 defects at (111) and (100) interfaces, respectively. On the basis of calculated hyperfine parameters, we discard models of the P-b1 defect containing a first neighbor shell with an O atom or a strained b

We treat homogeneous electric fields within density functional calculations with periodic boundary conditions. A nonlocal energy functional depending on the applied field is used within an ab initio molecular dynamics scheme. The reliability of the method is demonstrated in the case of bulk MgO for the Born effective charges, and the high- and low-frequency dielectric constants. We evaluate the static dielectric constant by performing a damped molecular dynamics in an electric field and avoiding the calcula

We present first-principles calculations of the electronic structure of BiVO4 at various levels of theory. In the calculations, we take into account a series of effects that affect the band gap, i.e., spin-orbit coupling, electron-hole interaction, nuclear ...

The structure of Liquid GeSe2 at T = 1373 K has been investigated by first-principles molecular dynamics. The calculated total neutron structure factor is in good agreement with recent experimental data. We found that the disappearance with increasing temperature of the first sharp diffraction peak (FSDP) in the total neutron structure factor is due to an increase of short-range chemical disorder. At T = 1373 K various bonding configurations coexist in close amounts, such as the Ge-GeSe3, Ge-GeSe2 and Se-Se

Alfredo Pasquarello

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Dynamical charge tensors and infrared spectrum of amorphous SiO2

First-principles study of dynamical and dielectric properties of tetragonal zirconia

Identification of Semiconductor Defects through Constant-Fermi-Level Ab Initio Molecular Dynamics: Application to GaAs

Titanium oxides and silicates as high-kappa dielectrics: A first-principles investigation

Origin of the first sharp diffraction peak in the structure factor of disordered network-forming systems: Layers or voids?

Atomic structure at the Si(001)-SiO2 interface: from the interpretation of Si 2p core-level shifts to a model structure

Hubbard U through polaronic defect states

Intermediate range order and bonding character in disordered network-forming systems

Dielectric discontinuity at interfaces in the atomic-scale limit: Permittivity of ultrathin oxide films on silicon

Arsenic related defect states resonant with the semiconductor conduction band at the In0.53Ga0.47As/oxide interface: A density functional study

Migration of hole polarons in anatase and rutile TiO2 through piecewise-linear functionals

Binding energies of ground and excited states of shallow acceptors in GaAs/Ga1-xAlxAs quantum wells

Infrared and Raman spectra of disordered materials from first principles

Dielectric effect of a thin SiO2 interlayer at the interface between silicon and high-k oxides

Origin of the high-frequency doublet in the vibrational spectrum of vitreous SiO2

Equivalent oxide thickness of a thin oxide interlayer in gate insulator stacks on silicon

Dangling bond defects at Si-SiO2 interfaces: Atomic structure of the P-b1 center

Ab initio molecular dynamics in a finite homogeneous electric field

Comprehensive modeling of the band gap and absorption spectrum of BiVO4

Breakdown of intermediate-range order in liquid GeSe2 at high temperatures