C. M. Bertoni

Theoretical investigation of hydrogen chemisorption on Ga‐containing III–V compounds

A fully self‐consistent pseudopotential calculation of the electronic properties of atomic hydrogen chemisorbed on GaAs and GaP(110) surfaces is reported. Different chemisorption geometries and substrate coverages are considered. The results are compared with the experimental information to select a structural model.

Image Plane for Surface Potential

A method of describing the nonlocality of exchange and correlation energy and potential, which gives the correct long-range imagelike behaviour, is adopted in a self-consistent calculation of a solid surface in the framework of the jellium model. The exchange-correlation potential Vxc(z), within the approximation proposed by Gunnarson and Jones, is written for a system with translational invariance in the plane parallel to the surface. The electron density n(z) and the total effective potential Veff(z) seen by an electron are self-consistently calculated. We show that within this scheme it is possible to calculate the position of the image plane, as a function of the bulk density.

Electronic structure and atomic configuration at the cleavage surface of zincblende compounds

The role of self-consistency and of the atomic distortions in determining the electronic structure of the cleavage surfaces of GaAs and ZnSe has been investigated using the layer method and an iterative tight-binding approach, which takes account of the difference between the surface and bulk electronic configurations. Several surface bands and resonances are identified near the gaps and empty lenses of the projected bulk band-structure and their sensitivity to the modifications of the surface potential are discussed. In the case of GaAs the inclusion of the atomic distortions at the surface is essential in order to obtain theoretical results in reasonable agreement with the experimental data. For ZnSe the available experimental information does not permit discrimination between the ideal and distorted configuration of the surface atoms, in spite of the sensitivity of the occupied surface states to the atomic displacements.

Third order perturbation theory and lattice dynamics of simple metals

The dynamical matrix of simple metals is set up by evaluating the total electron energy to third order in the electron ion pseudopotential. In this way three body unpaired non central forces arising from the off diagonal elements of the dielectric matrix are explicitly introduced. Since the usual on Fermi sphere approximation to a nonlocal pseudopotential gives a completely unreliable estimate of third order contributions and a full non local calculation is not feasible, a procedure is given to construct an average local potential, which disposes of most of the non locality and allows for a realistic calculation of the contributions of the unpaired forces to the lattice dynamics of simple metals. The phonon frequencies and the elastic constants of Li, Na, Al and Pb are evaluated. The results show that third order corrections are very important for Pb, while for the other metals they are about 10%. This indicates that three-body central forces are essential in order to describe the lattice dynamics of lead, while for Li, Na and Al a good description is obtained by considering only a central pairwise interaction between the ions.

Electronic Structure of Cubic GaN with Self-Energy Corrections

We present the results of a calculation for the bulk electronic structure of gallium nitride in the zincblende phase. We determine the equilibrium lattice constant, the cohesive energy and the bulk modulus in the Density Functional approach within the Local Density Approximation (DFT-LDA). The one-particle eigenvalues of the DFT Kohn-Sham equation do in principle not agree with the experimental band structure. Therefore, we calculate the quasi-particle energies by including self-energy corrections to the DFT-LDA exchange correlation potential, with the GW approximation for the electron self-energy. We use norm-conserving pseudopotentials and a large plane-wave basis set (100 Ry cut-off) for a converged calculation in the DFT-LDA. The LDA band gap turns out to be very sensitive to the crystal volume. We find that GW corrections to the LDA band gap are significant. A detailed comparison with other DFT-LDA results and approximate GW calculations and with existing experimental data is given.

The electronic structure of gallium nitride

The results of a density functional calculation on gallium nitride are given. We use norm-conserving pseudopotentials with sufficiently extended sets of plane waves to investigate the ground-state properties and the electronic band structure for the zincblende phase of GaN and compare them with the corresponding results for the wurtzite structure. A comparison with the outcomes of other calculations and with the existing experimental data is also given.

Optical properties of isolated and interacting silicon quantum wires

Abstract We have studied the effect of hydrogen passivation and inter-wire interaction on the electronic structure and optical properties of nanoscale Si wires through two first-principle techniques: linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA) and norm-conserving pseudopotential. We have considered free, partially and totally H-passivated [001] Si quantum wires with various rectangular cross-sections; moreover we have investigated the inter-wire interaction, by varying the wire density. The optical properties have been computed by evaluating the imaginary part of the dielectric function and the absorption coefficient. We find that wires with diameters as small as 10–25 A are active in the visible range. Inter-wire interaction leads to the presence of localized interface states which lower the bandgap energy. These results are important for the discussion about the dimensionality of confined Si quantum particles in porous Si and for the debate on quantum confinement models.

Structural models of reconstructed Si(110) surface phases

Abstract Structural models are proposed for the wide class of experimentally observed superstructures on the Si(110) surface: both for the case of Si(110) “16 × 2’ reconstruction and for the superstructures stabilized by Ni or Cu contamination, like (4 × 5), (2 × 1) and (5 × 1). These models are based on semi-empirical tight-binding total energy calculations for model subunits, on surface lattice dynamics experiments and also on scanning tunnelling microscope measurements. These models consist of a variety of building blocks, namely: new type of π-bonded dimers, adatoms, atoms with unsaturated dangling bonds, regular sequences of surface steps and missing rows. Surface stresses due to both the π-dimer or adatom formation and Ni contamination are considered to be the driving force of the Si(110) reconstructions.

Hydrogen-induced surface metallization of beta-SiC(100)-(3x2) revisited by density functional theory calculations.

Recent experiments on the silicon terminated (3 x 2)-SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of density-functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: in addition to saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, i.e., stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surface.

Surface vibrations at clean and hydrogenated GaAs(110) from ab-initio molecular dynamics

Abstract We present results of finite-temperature Car-Parrinello molecular dynamics simulations of the atomic geometry and the vibrations at clean and hydrogenated (1 and 0.25 ML coverages) GaAs(110) surfaces. Through an accurate analysis of the atomic motion based on signal processing techniques the phonon modes have been investigated. Some surface vibrational modes at high-symmetry k points, with layer-by-layer resolution of the eigenvectors, have been found. The results are compared with available experimental data and previous calculations.