Showing papers by "Wolf Gero Schmidt published in 2003"

Efficient O(N 2 ) method to solve the Bethe-Salpeter equation

Abstract: We present a numerically efficient approach to solve the Bethe-Salpeter equation for the polarization function. Rather than from the usual eigenvalue representation, the macroscopic polarizability is obtained from the solution of an initial-value problem. This reduces the computational effort considerably and allows for calculating excitonic and local-field effects in optical spectra of complex systems consisting of many atoms. As an example we investigate the optical anisotropy of the monohydride $\mathrm{Si}(001)(2\ifmmode\times\else\texttimes\fi{}1)$ surface. While excitonic effects influence the surface optical properties considerably, the local-field effect induced changes are minimal.

InP(001)-( 2 × 1 ) Surface: A Hydrogen Stabilized Structure

Abstract: The InP(001)(2 x 1) surface has been reported to consist of a semiconducting monolayer of buckled phosphorus dimers. This apparent violation of the electron counting principle was explained by effects of strong electron correlation. Combining first-principles calculations with reflectance anisotropy spectroscopy and LEED experiments, we find that the (2 x 1) reconstruction is not at all a clean surface: it is induced by hydrogen adsorbed in an alternating sequence on the buckled P dimers. Thus, the microscopic structure of the InP growth plane relevant to standard gas phase epitaxy conditions is resolved and shown to obey the electron counting rule.

P-rich GaP ( 001 ) ( 2 × 1 ) / ( 2 × 2 ) surface: A hydrogen-adsorbate structure determined from first-principles calculations

Abstract: We perform first-principles total-energy and electronic structure calculations in order to determine the atomic structure of the P-rich $(2\ifmmode\times\else\texttimes\fi{}1)/(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction commonly observed for GaP(001) surfaces grown by gas-phase epitaxy. A monolayer of buckled phosphorus dimers stabilized by hydrogen adsorbed in an alternating sequence is found to be energetically stable and to be consistent with the experimental data.

Gallium-rich reconstructions on GaAs(001)

Abstract: Ga-rich reconstructions on GaAs(001) surfaces were prepared by annealing and Ga dosing of Molecular Beam Epitaxy grown samples and analyzed in-situ by Reflectance Anisotropy Spectroscopy and Reflection High-Energy Electron Diffraction. Annealing or dosing gallium above about 800 K invariably results in a (4 × 2)/c(8 × 2) reconstruction. Lowering the temperature or annealing below 800 K results in a (2 × 6)/(3 × 6) reconstruction. By dosing the (2 × 6)/(3 × 6) reconstruction with more than 0.2 monolayer of gallium, it transforms into a (4 × 6) reconstruction. The observed translational symmetries and measured RAS spectra are compared with results of first-principles calculations. None of the (2 × 6) structures proposed in the literature is energetically stable. The RAS spectrum calculated for the ζ(4 × 2) model resembles reasonably the data measured for the (4 × 2) surface. The RAS spectra calculated for (2 × 6) symmetries indicate that mixed Ga-As dimers likely are a structural element of the corresponding surface reconstructions. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cycloaddition reaction versus dimer cleavage at the Si(001):C 5 H 8 interface

Abstract: The interface formed between an ordered monolayer of cyclopentene and silicon is studied by first-principles density-functional calculations. Several different structural models of the interface are considered and their reflectance anisotropy spectra are calculated. The spectra turn out to be highly structure dependent and can therefore be used to monitor the interface formation. We also find that coadsorption of hydrogen, which leads to dimer cleavage, can stabilize the interface by saturating the dangling bonds and releasing the high strain energy.

Nanowire-induced optical anisotropy of the Si(111)-In surface

Abstract: Ab initio calculations of the reflectance anisotropy of Si(111)-In surfaces are presented. A very pronounced optical anisotropy around 2 eV is predicted for the structural model proposed by Bunk et al. [Phys. Rev. B 59, 12 228 (1999)] for the (4 × 1) reconstructed surface. The (4 X 2)/(8 × 2) reconstructed surface, induced by a slight distortion of the indium chains, is shown to result in a splitting of the 2 eV peak. The calculated results are in excellent agreement with recent polarized reflectance data acquired during the (4×1)→(4×2)/ (8×2) phase transition.

Uracil adsorbed on Si(001): Structure and energetics

Abstract: The adsorption of uracil on the Si(001) surface has been investigated by density-functional theory calculations using a plane-wave basis in conjunction with ultrasoft pseudopotentials. A large number of possible interface structures are studied. Electrostatic effects and the keto−enol tautomerism play an important role for the surface reaction. There exists a pronounced tendency for molecular fragmentation, leading to the dissociation of hydrogen from the molecules and possibly to oxygen insertion into Si dimers.

SURFACE ORDERING OF P-RICH InP(001): HYDROGEN STABILIZATION VS ELECTRON CORRELATION

Abstract: The influence of hydrogen on the reconstruction of InP(001) surfaces is studied by first-principles calculations. One-monolayer phosphorus forming oppositely buckled dimers with one hydrogen adsorbed per dimer is energetically favored for a wide range of surface preparation conditions. The electronic structure and STM image calculated for this adsorbate geometry agree well with the experimental findings obtained after annealing of MOVPE-grown InP samples. The Si(001)-like surface ordering as well as the surface band gap of more than 1 eV, supposedly arising from electron correlation effects [Li et al., Phys. Rev. Lett.82, 1879 (1999)], are naturally explained by the hydrogen-stabilized surface structure favored by the total-energy calculations.

Layer-by-layer analysis of surface reflectance anisotropy in semiconductors

Abstract: We propose a method to obtain the layer-resolved contributions to the reflectance anisotropy signal of semiconductor surfaces which are modeled using the slab approach. Following this method, a microscopic formulation based on the semiempirical tight-binding approach is used to calculate the reflectance anisotropy of two InP(001) surface structures possibly relevant to standard gas-phase epitaxy conditions. It is shown that (i) the elimination of the anisotropy signal from the bottom layers of the slab is essential to correctly compare with the experiment and (ii) the strong low-energy anisotropy characteristic for gas-phase-grown InP surfaces arises from the uppermost atomic layers.

Gas-Phase Epitaxy Grown InP(001) Surfaces From Real-Space Finite-Difference Calculations

Abstract: Density-functional calculations based on finite-difference discretization and multigrid acceleration are used to explore the atomic and spectroscopic properties of P-rich InP(001)(2x1) surfaces grown in gas-phase epitaxy. These surfaces have been reported to consist of a semiconducting monolayer of buckled phosphorus dimers. This apparent violation of the electron counting principle was explained by effects of strong electron correlation. Our calculations show that the (2x1) reconstruction is not at all a clean surface: it is induced by hydrogen adsorbed in an alternating sequence on the buckled P-dimers. Thus, the microscopic structure of the InP growth plane relevant to standard gas-phase epitaxy conditions is resolved and shown to obey the electron counting rule.

Excitonic and Local-Field Effects in Optical Spectra from Real-Space Time-Domain Calculations

Abstract: We present a novel approach to solve the Bethe-Salpeter equation for the polarisation function. Rather than from the usual eigenvalue representation, the macroscopic polarisability is obtained from the solution of an initial-value problem. This allows for the first time to calculate excitonic and local-field effects in optical spectra of large and complex systems such as surfaces. As an example we investigate the optical anisotropy of the hydrogen-passivated Si(ll0) surface. It is shown that the electron-hole attraction is largely responsible for the peculiar line shape of the surface optical spectrum.

Many-body and overlayer effects on surface optical properties

Abstract: We demonstrate the potential of recently developed total-energy and electronic-structure methods for the calculation of the optical properties of real surfaces. The many-body effects are fully taken into account by a solution of the combined Dyson and Bethe-Salpeter equations. We show that an initial-value formulation of the polarization function allows an efficient numerical calculation of the optical susceptibility for large slabs consisting of many atoms. As examples we investigate GaP(001) and Si(001) surfaces covered by hydrogen. In the case of P-rich GaP(001)2 x 2-H surfaces the low-energy region of the reflectance anisotropy (RA) is dominated by electron-hole pair excitations in surface states. Surface-induced modifications of bulk excitons near the E-1 and E-2 transitions are responsible for the RA of the monohydride Si(001)2 x 1-H surface. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Large-scale simulations of advanced materials and nanoscale devices

Abstract: Recent advances in theoretical methods and parallel supercomputing allow for reliable ab initio simulations of the properties of complex materials. We describe two current applications: pyro- and piezoelectric properties of BN nanotubes and optical signatures of organic molecules on Si(001) surface. BN nanotubes turn out to be excellent piezoelectrics, with response values significantly greater than those of piezoelectric polymers. However, their symmetry leads to exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes. Breaking of this symmetry induces spontaneous polarization comparable to those wurtzite semiconductors. Turning to organics on Si(100), we calculated the atomic structure and the optical signatures of a cyclopentene overlayer on Si(001). Cyclopentene can be used to attach a variety of organic molecules to Si devices, including DNA, and can therefore form a basis of a sensor structure. The spectra turn out to be highly structure-dependent and can therefore be used to monitor interface formation.

Pyrrole (C4H4NH) and Polypyrrole Functionalized Silicon Surfaces Calculated from First Principles

Abstract: The functionalization of the Si(001) surface by pyrrole and polypyrrole is investigated by means of first-principles pseudopotential calculations. We find dissociative reactions, leading to the partial fragmentation of the molecule, to be energetically most favored for pyrrole adsorption. The lowest-energy configuration for monolayer coverage is characterized by pyrrole rings bonded to the surface via Si–N linkage. In coexistence with adsorption geometries where both N and C are bonded to the surface, this structure accounts very well for the available experimental data. Chemisorption of pyrrole is found to effectively passivate the Si(001) surface, irrespective of the details of the adsorption geometry. The formation of well-ordered polypyrrole structures on Si(001) may require chemical modifications of the polypyrrole chains in order to account for the lattice mismatch.