Showing papers by "Wolf Gero Schmidt published in 2004"
TL;DR: The comparison of the results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient‐corrected density‐functional theory (DFT‐GGA) calculations using ultrasoft pseudopotentials and a plane‐wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.
Abstract: We present equilibrium geometries, vibrational modes, dipole moments, ionization energies, electron affinities, and optical absorption spectra of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient-corrected density-functional theory (DFT-GGA) calculations using ultrasoft pseudopotentials and a plane-wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.
93 citations
TL;DR: A structure model, consisting of subsurface Ga-Ga dimers and surface Ga-As dimers, is proposed for the (4 x 6) surface and is found to be energetically favorable at the Ga-rich limit and agrees well with experimental data from STM and reflection high-energy electron diffraction.
Abstract: The Ga-rich reconstruction of the GaAs(001) surface has been studied. Using scanning tunneling microscopy (STM), we have found the existence of a well-ordered $(4\ifmmode\times\else\texttimes\fi{}6)$ reconstruction under extreme Ga-rich conditions. A structure model, consisting of subsurface Ga-Ga dimers and surface Ga-As dimers, is proposed for the $(4\ifmmode\times\else\texttimes\fi{}6)$ surface. This model is found to be energetically favorable at the Ga-rich limit and agrees well with our experimental data from STM and reflection high-energy electron diffraction.
51 citations
TL;DR: The results explain recent experimental studies reporting changes of surface reconstruction of Si and Ge(001) surfaces induced by the scanning tunneling microscope and the occurrence of p(2x2) reconstructions on (001) surface of n-doped Si.
Abstract: We perform density-functional calculations on the influence of external electric fields and electrons or holes injected into surface states on the relative stability of $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $p(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed Si(001) surfaces. It is shown that an electric field parallel to the $[001]$ direction or the insertion of electrons into surface states favors the formation of $p(2\ifmmode\times\else\texttimes\fi{}2)$ periodicities. Our results explain recent experimental studies reporting changes of surface reconstruction of Si and Ge(001) surfaces induced by the scanning tunneling microscope and the occurrence of $p(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructions on $(001)$ surfaces of $n$-doped Si.
49 citations
TL;DR: In this article, the first principles of the Bethe-salpeter equation are reviewed and a comparison of the results obtained within different treatments of the electron-hole screening is made.
Abstract: In this paper we review the first-principles theoretical framework of the Bethe–Salpeter equation which is nowadays the state-of-the-art approach for including self-energy, local fields and excitonic effects in the surface optical response. Two different approaches for calculating the dielectric screening will be described. In both cases a parallel and efficient iterative algorithm to find numerically the solution of the BSE equation has been implemented. In fact, if the surface states are not energetically separated from the bulk states and if one is interested in describing a large energy window, the traditional approach involving a full diagonalization of a very large excitonic Hamiltonian is prohibitive. The reflectance anisotropy spectrum of the monohydride Si(100) is considered and compared with experiments. A comparison of the results obtained within different treatments of the electron–hole screening is made. Convergence and numerical problems are discussed.
29 citations
TL;DR: In this article, the electronic properties of the uracil-covered Si (001) surface have been studied by density-functional calculations, and the results show that semiconductor surface electronic properties can be tuned within a very wide range by organic functionalization even with only one molecular species.
Abstract: The electronic properties of the uracil-covered Si (001) surface have been studied by density-functional calculations. Dative-bonded configurations are characterized by a high density of surface states in the energy region of the fundamental gap, whereas the surface is perfectly passivated when covalent bonds form between the molecule and the substrate. A remarkable influence of the adsorption configuration on the ionization energy is predicted. The results show that semiconductor surface electronic properties can be tuned within a very wide range by organic functionalization even with only one molecular species.
25 citations
Journal Article•
TL;DR: In this article, the first principles of the Bethe-salpeter equation were reviewed and a comparison of the results obtained within different treatments of the electron-hole screening was made.
Abstract: In this paper we review the first-principles theoretical framework of the Bethe-Salpeter equation which is nowadays the state-of-the-art approach for including self-energy, local fields and excitonic effects in the surface optical response. Two different approaches for calculating the dielectric screening will be described. In both cases a parallel and efficient iterative algorithm to find numerically the solution of the BSE equation has been implemented. In fact, if the surface states are not energetically separated from the bulk states and if one is interested in describing a large energy window, the traditional approach involving a full diagonalization of a very large excitonic Hamiltonian is prohibitive. The reflectance anisotropy spectrum of the monohydride Si( 100) is considered and compared with experiments. A comparison of the results obtained within different treatments of the electron-hole screening is made. Convergence and numerical problems are discussed.
20 citations
TL;DR: In this article, the adsorption of methyl chloride (CH3Cl) on Si(001) surfaces is studied using first-principles calculations based on gradient-corrected density functional theory (DFT-GGA) and ultrasoft pseudopote...
Abstract: The adsorption of methyl chloride (CH3Cl) on Si(001) surfaces is studied using first-principles calculations based on gradient-corrected density-functional theory (DFT-GGA) and ultrasoft pseudopote...
18 citations
TL;DR: Reflectance anisotropy spectra (RAS) for uracil covered Si(0, 0, 1) surfaces have been calculated within the independent particle approximation as discussed by the authors.
17 citations
TL;DR: In this article, the authors present ab initio calculations of the adsorption of methylchloride (CH3Cl) on Si(0, 0, 1) and find five thermodynamically favorable configurations.
14 citations
TL;DR: In this article, a short review on the simulation of reflectance anisotropy spectroscopy is presented, where the clean, hydrogenated and uracil-covered Si(001) surface is used to illustrate the microscopic origin of surface optical anisotropies and the present state-of-theart in computational modeling of optical spectra.
14 citations
TL;DR: In this article, the authors study two potentially very interesting applications for RAS: the oxidation of Si(001) and the functionalization of the Si surface with organic molecules and confirm experimental indications that the polarity of the interface-induced optical anisotropy is reversed layer by layer with increasing oxide thickness.
Abstract: In the last couple of years there has been much methodological and computational progress in the modelling of optical properties from first principles .R ef lectance anisotropy spectra (RAS) can now be calculated with true predictive power and can thus be used to draw conclusions directly on the surface geometry. In the present work we study two potentially very interesting applications for RAS: the oxidation of Si(001) and the functionalization of the Si surface with organic molecules. Our calculations confirm experimental indications that the polarity of the interface-induced optical anisotropy is reversed layer by layer with increasing oxide thickness. The oscillation of the RAS amplitude should thus allow for the quantitative monitoring of the vertical progression of the oxidation. Our results for Si(001) surfaces modified by cyclopentene and 9,10-phenanthrenequinone adsorption show a strong sensitivity of the RAS signal with respect to the adsorption geometry. Comparison with experimental data shows that cyclopentene most probabl ya dsorbs vi aa cycloaddition reaction with the Si surface dimers, while phenanthrenequinone seems to adsorb across two Si dimers.
TL;DR: In this article, the surface selectivity of reflectance anisotropy spectroscopy (RAS) was used to analyze the electronic properties of the (4 × 2)/(8 × 2)-low-temperature phase of in nanowires on Si(1.1/1).
Journal Article•
TL;DR: In this paper, the authors used reflectance anisotropy spectra (RAS) with true predictive power and can thus be used to draw conclusions directly on the surface geometry.
Abstract: In the last couple of years there has been much methodological and computational progress in the modelling of optical properties from first principles. Reflectance anisotropy spectra (RAS) can now be calculated with true predictive power and can thus be used to draw conclusions directly on the surface geometry. In the present work we study two potentially very interesting applications for RAS: the oxidation of Si(001) and the functionalization of the Si surface with organic molecules. Our calculations confirm experimental indications that the polarity of the interface-induced optical anisotropy is reversed layer by layer with increasing oxide thickness. The oscillation of the RAS amplitude should thus allow for the quantitative monitoring of the vertical progression of the oxidation. Our results for Si(001) surfaces modified by cyclopentene and 9,10-phenanthrenequinone adsorption show a strong sensitivity of the RAS signal with respect to the adsorption geometry. Comparison with experimental data shows that cyclopentene most probably adsorbs via a cycloaddition reaction with the Si surface dimers, while phenanthrenequinone seems to adsorb across two Si dimers.
TL;DR: Reflectance anisotropy spectra for water-covered Si(0, 0, 1) surfaces have been calculated within the independent-particle approximation in this paper, showing that dissociative adsorption and oxygen insertion into the Si dimer give rise to different optical anisotropies.
TL;DR: In this article, the coupling of surface and bulk states at semiconductor surfaces through electron-phonon interaction is discussed and the governing equations are derived from a microscopic theory in the framework of the density matrix theory.
Abstract: The coupling of surface and bulk states at semiconductor surfaces through electron–phonon interaction is
discussed. The governing equations are derived from a microscopic theory in the framework of the density
matrix theory. To gain a first insight, model wave functions are used to simulate the dynamics of nonequilibrium
electron distributions in three- and two-dimensional states, coupled by Frohlich interaction. Typical
time scales for the coupling are found to be in the order of few hundreds of femtoseconds.
01 Jan 2004
TL;DR: In this paper, the coupling of surface and bulk states at semiconductor surfaces through electron-phonon interaction is discussed and the governing equations are derived from a microscopic theory in the framework of the density matrix theory.
Abstract: The coupling of surface and bulk states at semiconductor surfaces through electron-phonon interaction is discussed. The governing equations are derived from a microscopic theory in the framework of the density matrix theory. To gain a first insight, model wave functions are used to simulate the dynamics of nonequi- librium electron distributions in three- and two-dimensional states, coupled by Frohlich interaction. Typi- cal time scales for the coupling are found to be in the order of few hundreds of femtoseconds.
01 Jan 2004
TL;DR: In this paper, the authors performed ab-initio calculations of an adsorbate-surface system within the framework of gradient-corrected density functional theory (DFT-GGA), using a plane wave basis set and ultrasoft pseudopotentials to determine the structurally relaxed ground state.
Abstract: Within the last years extensive efforts have been devoted to functionalizing semiconductor surfaces by organic molecules. Ordered monolayers may serve as a starting point to alter surface optical and electronic properties in specific ways in order to eventually realize applications such as molecular electronic devices or organic sensors. Besides, there have been comprehensive experimental and theoretical studies on the optical properties of relevant clean semiconductur surfaces. Especially measuring the reflectance anisotropy spectra (RAS) turned out to provide useful informations on the individual surface reconstructions. In recent works [1, 2] the influence of adsorption of a π-conjugated molecule (9,10-phenanthrenequinone) on electronic properties and RAS signal of the Si(001) surface was investigated. We performed ab-initio calculations of this adsorbate-surface-system within the framework of gradient-corrected density functional theory (DFT-GGA), using a plane wave basis set and ultrasoft pseudopotentials to determine the structurally relaxed ground state. Optical properties are then calculated within the independent-particle approximation from all-electron wave functions obtained by the projector augmented wave method (PAW). We present the energetically most favored structural configurations and the respective RAS spectra for several degrees of coverage, discuss the character of the molecule-substrate bonds from a molecular orbital point of view and the origin of the distinct RAS features due to molecular and surface contributions.