C. Calandra

Transition metal silicides: aspects of the chemical bond and trends in the electronic structure

Presents a theoretical investigation of the electronic properties of bulk silicides of near-noble metals (NiSi2, NiSi, Ni2,Si, PdSi, Pd2Si, PtSi and Pt2Si). The theoretical approach is provided by the iterative extended Huckel method. The densities of states of various compounds, differing in silicon content and in the crystal structure, are presented and the main features of the chemical bond are discussed. A detailed comparison with the available experimental data and with other theoretical work is presented. The relevance of the present results for the interpretation of the silicide/silicon interfaces is also discussed.

Chemical Bonding at the Si-Metal Interface: Si-Ni and Si-Cr,

Chemical bonding at the interface of a near‐noble‐metal (Ni) and a transition metal (Cr) with Si is examined through synchrotron radiation photoelectron spectroscopy studies of in situ formed interfaces, of cleaved bulk silicides, and of disordered surfaces prepared by sputter etching of the silicides. Interpretation of these experimental results is guided by parallel linear combination of atomic orbitals (LCAO) (extended Huckel approximation) calculations of stoichiometric Ni and Cr silicides.

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.

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 nonpolar surfaces of II–VI compounds

The nature of surface states in cleaved surfaces of II–VI compounds with zincblende structure is studied. The tight‐binding method is used to calculate the electronic structure of a layer of twelve (110) planes. The results show various surface bands both in the gap and in empty lenses of the projected bulk band structure. The nature of these states is discussed and a comparison with the available experiments is presented. The results support the model of ionic surface states previously proposed for these surfaces.

Perturbation theory and three body forces in hcp metals

An analysis is presented of the role of the non pair forces in the lattice dynamics of sp bonded hcp metals: Be, Mg and Li. By expanding the total energy up to third order in the electron-ion pseudopotential, a dynamical matrix is derived which contains terms arising from central pairwise interactions between the ions as well as contributions due to sums of three body forces. The phonon dispersion relations are calculated using a local form of the model potential, which allows for a realistic evaluation of the third order contributions. The results show that the three body forces are very important in Be and are responsible for the peculiarities of the spectrum, as is most clearly seen from the analysis of the frequencies at the K point of the Brillouin zone.

Effects of chemical environment in the lineshape of silicon L2,3VV Auger spectra of nickel silicides

The Si L2,3VV Auger lineshape in nickel silicides of different compositions has been studied both experimentally and theoretically with the purpose of understanding the behaviour of silicon states in compounds where the Si atom has different local chemical environments. The experimental spectra provide evidence of significant modifications in the lineshapes, which are associated with changes in the distribution of the valence electrons. These modifications can be explained by theoretical calculations based on a single-particle description of the bulk electronic structure. Some discrepancies between theory and experiments can be attributed to approximations in the treatment of the final state of the Auger electron and to the neglect of surface effects in the electronic structure calculation.

Surface bands in relaxed cleavance surface of GaP

We have studied the dependence of the surface states upon relaxation in GaP (110) surface. Calculations were performed using a tight binding model with an approximate treatment of the self‐consistency. Different relaxation models, involving both rotation and stretching of the bonds were considered. The location and orbital composition of the surface bands their dispersion and the local density of states at the surface are presented for the various models. Unlike the other III–V compounds, we find that the relaxation does not remove the empty surface states from the band gap. Such a conclusion agrees with the experimental information on the pinning of the Fermi level in n‐doped samples of GaP. A comparison with partial‐yield photoemission data and a theoretical estimate of the surface excitonic binding energy are also given.

Self-consistent pseudopotential calculation of the electronic properties of the InP (110) surface

Self-consistent pseudopotential calculations for InP (110) surface have been carried out using the repeated slab method and assuming a relaxed surface geometry. Various surface states have been found and their character and symmetry properties have been investigated by analysing the corresponding pseudocharge density maps. Comparison between self-consistent and non-self-consistent calculations show significant differences both in the number and in the energy location of the surface states. The results are in agreement with the experimental findings obtained by different spectroscopical techniques.