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We investigate adatom self-diffusion on the rebonded step (RS) reconstructed Ge(105) surface. Activation energies for several paths are computed ab initio by applying the nudged elastic band method and used as input parameters for kinetic Monte Carlo simulations. We show that the RS reconstruction strongly influences adatom kinetics, which turns out to be fast at typical experimental temperatures, along both [010] and [(5) over bar 01] directions. The influence of strain on diffusion is also investigated. Our results are shown to be relevant for a better understanding of the growth modality of Ge pyramids on Si(001).

By comparing the scanning tunnel microscope images of the morphological evolution in Ge/Si(001) during Si overgrowth with the results of classical molecular-dynamic simulation and ab initio calculations for Si distribution in the wetting layer, we conclude that the disappearance of the (MxN) reconstruction is determined by Si-Ge intermixing in the third and fourth layers in the Ge wetting layer. The subsequent evolution of the (2xN) pattern, where N increases with further Si deposition, is shown to be produced by Si-Ge intermixing in the second layer. We suggest that this process corresponds to a crossover between thermodynamic behavior and kinetic behavior, which eventually leads to the Si capping.

Optimized crystal structure, electronic bands, and density of states near the band gap, and the dielectric function of beta-(Fe1-xCox)Si-2 with x equal to 0.0625 and 0.125 were obtained by means of total energy ultrasoft pseudopotential and full-potential linearized augmented plane-wave calculations. We address a preferential occupation of the Fe-1 and Fe-2 sites by the Co atoms at different concentrations indicating the Fe-2 site to be energetically favorable. We also discuss a variation in the energy gap and in the position of the donor level in beta-(Fe1-xCox)Si-2 with increasing Co content. Calculated imaginary and real parts of the dielectric function show a good agreement with results of ellipsometric measurements of samples grown by molecular-beam epitaxy.

Accurate first principles calculations reveal that a huge inward relaxation turns the originally stepped Ge(105) surface into a flat one, with surface energy equal to the Ge(001)p(2×2) at the bulk lattice parameter. Under in-plane compression (up to 4%) the surface energy of the (105) gets sizably lower than the (001), because of a combination of the elastic contribution provided by stretched dimers at the rebonded steps and the dangling-bond energy lowering generated by enhanced tilting of these dimers. © 2004 Elsevier B.V. All rights reserved.

Structural, electronic and optical properties of semiconducting rhenium silicide (ReSi_1.75) with various distributions of the silicon vacancies have been theoretically studied by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. We have found that the band dispersion is affected by vacancy positions, while the dielectric function and reflectivity display similar shapes for all considered variants, that can explain the rather scattered available experimental data on the gap value. Comparison between the calculated and ellipsometrically measured dielectric function and reflectivity on ReSi_1.8 polycrystals grown by the Czochralski technique shows a good agreement.

The morphological transition from pyramid to dome islands during the growth of Ge on Si(001)was investigated using high resolution scanning tunneling microscopy (STM). It was observed that the pyramids were grown from top to bottom and that from a critical size on incomplete facets were formed. It was also observed that the bunching of the steps delimiting the facets evolved into the steeper dome facets. It was found that a microscopic model was developed based on first principles and Tersoff-potential calculations for the onset of the morphological transition.

By using scanning tunneling microscopy we found that the surface reconstruction of Ge/Si(001) epilayers evolves from (MxN) to (2xN), and eventually to (2x1), during exposure to a Si flux. This sequence appears to be just the inverse of that observed during the growth of Ge or SiGe alloys on Si(001). However, molecular dynamics simulations supported by ab initio calculations allow us to interpret this morphological evolution in terms of Si migration through the epilayer and complex Si-Ge intermixing below the top Ge layer.

We report ab initio results for the ground-state properties, band diagrams, density of states, and dielectric functions of Ca(2)Z (Z = Si, Ge, Sn, and Pb) both in the orthorhombic and cubic structures. The calculations are performed by means of total energy ultrasoft pseudopotential and full potential linearized augmented plane wave methods within local density and generalized gradient approximations. The estimated difference in the cohesion energy shows the orthorhombic structure to be the stable phase. We also demonstrate that these materials are semiconductors independently of the phase. A direct energy gap and sizable anisotropy in optical spectra characterize orthorhombic Ca(2)Z, whereas a competitive direct-indirect character of the gap and a high value of oscillator strength of the first direct transition are predicted for cubic Ca(2)Z

The goal of this paper is to investigate the morphology and the structure of ion-beam synthesized beta-FeSi2 precipitates and the defects in the Si matrix by using transmission electron microscopy (TEM) and, in a few cases, conversion electron Mossbauer spectroscopy or Rutherford backscattering spectrometry. We shall point out how the different process parameters affect the resulting structure and the optimum process window for light emitting application of beta-FeSi2. In particular, we will also show that within the optimum process window at least two different types of precipitates are obtained in separate regions: small ball-shaped precipitates in the surface region and large disc-shaped precipitates deeper in the sample. The latter are shown to display a very good interface to the Si matrix, as obtained by TEM analysis and dedicated molecular dynamics simulation. In the second part of this paper [Martinelli , Phys. Rev. B 66, 085320 (2002)], the nature and the origin of the 1.54-mum photoluminescence signal obtained from our best samples will be analyzed in detail.

We have performed a comparative study of structural, electronic and optical properties of Ru2Si3, Ru2Ge3, Os2Si3 and Os2Ge3 by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. The estimated difference in the cohesion energy between the low-temperature orthorhombic phase and the high temperature tetragonal one for all these compounds indicates that the former phase is lower in energy with respect to the latter one. All materials in the orthorhombic structure are found to be direct band-gap semiconductors, still some of them in the tetragonal structure display an indirect nature (Os2Si3) or a competitive direct-indirect character (Ru2Ge3) of the gap. Optical properties are discussed by analyzing the imaginary part of the dielectric function and the dipole matrix elements corresponding to different interband transitions indicating for osmium silicide and germanide the presence of low-energy transitions with an appreciable value of the oscillator strength.

We show by molecular dynamics simulations on a scale comparable to experimental dimensions that a peculiar surface reconstruction of the {105} facets is responsible for the stability of Ge pyramids on Si(001). This finding is confirmed by ab initio total energy calculations for competing surface reconstructions and a very satisfactory comparison of the corresponding charge density maps to scanning tunneling microscopy measurements of the facets, both for filled and empty states

We have performed a comparative study of optical properties of isostructural beta -FeSi2, OsSi2, and two ternary configurations of (FeOs)Si-2 by first-principle calculations of band structure and the imaginary part of the dielectric Function. The latter for beta -FeSi2 and OsSi2 were compared to those deduced from ellipsometric measurements: indicating an excellent agreement. From theoretical calculations both binaries are found to be indirect gap semiconductors, whereas one ternary is characterised by a direct transition with high oscillator strength. (C) 2001 Elsevier Science B.V. All rights reserved.

We discuss the rather scattered predictions on the orthorhombic lattice parameters for the C49 TiSi2 phase, which are reported in the literature. New and accurate x-ray diffraction measurements and ab initio calculations are presented. Both agree in indicating that the density is higher than the one commonly assumed and it is close to the stable C54 phase. An unexpected tetragonalization of the orthorhombic cell has been observed and the theoretical analysis shows that this phenomenon could be originated by the massive presence of stacking faults. (C) 2001 American Institute of Physics

In this letter, we discuss the interesting changes in the band structure of two ternary (FeOs)Si-2 configurations with respect to the isostructural binary beta -FeSi2 and OsSi2 compounds. Our first-principle calculations also demonstrate that one of the ternaries is a direct-bandgap semiconductor with an appreciable value of the oscillator strength of the first direct transition at 0.78 eV. (C) 2001 American Institute of Physics

The effect of a thin Ta layer at the Si/Ti interface on the intermediate phase formation has been studied in detail by in situ sheet resistance, x-ray diffraction, transmission electron microscopy and Rutherford backscattering spectroscopy of partially reacted samples. When a Ta layer is deposited at the Si/Ti interface, a new intermediate phase has been detected, i.e. the hexagonal TiSi2 C40. This phase grows on the C40-TaSi2 that is formed at the interface with silicon. The lattice parameters of the C40-TiSi2 obtained by ab initio calculations agree quite well with the experimental ones. (C) 2001 American Institute of Physics.

We present both theoretical and experimental investigations of electronic and optical properties of isostructural beta -FeSi2 and OsSi2 by means of a full-potential linear augmented plane wave method and optical measurements. We report also ellipsometric and reflectance measurements on samples of polycrystalline osmium disilicide prepared by mechanical alloying. From ab initio calculations these compounds are found to be indirect band-gap semiconductors with the fundamental gap of OsSi2 larger some 0.3-0.4 eV than the one of beta -FeSi2. In addition to that, a low value of the oscillator strength is predicted for the first direct transitions in both cases. Computed optical functions for these materials were compared to the ones deduced from optical measurements, indicating very good agreement and the presence of some anisotropic effects