Showing papers by "Wolf Gero Schmidt published in 2001"

Bulk excitonic effects in surface optical spectra

Abstract: We calculate the surface optical properties of the passivated Si(110) surface using a real-space multigrid technique and ab initio pseudopotentials. Rather than from the usual eigenvalue representation, the macroscopic polarizability is obtained from the solution of an initial-value problem, which allows inclusion of excitonic and local-field effects in addition to the electronic self-energy in the surface calculations. It is shown that the electron-hole attraction is largely responsible for the peculiar line shape of the surface reflectance anisotropy.

Terrace and step contributions to the optical anisotropy of Si(001) surfaces

Abstract: The contributions of atomically flat terraces as well as of SA , SB , andDB steps to the optical anisotropy of Si~001! surfaces have been calculated using a real-space multigrid method together with ab initio pseudopotentials. Our results for ideal (1 32), p(232), andc(234) reconstructed surfaces show a distinct influence of the dimer arrangement on the optical spectra. The calculated spectrum for the Si(001) c(234 surface agrees best with the signal measured for atomically smooth terraces. The significant optical anisotropy around 3 eV observed for vicinal surfaces, however, is induced by surface steps. Both electronic transitions directly at the surface as well as in deeper layers contribute to the optical anisotropy. We identify two mechanisms causing anisotropy signals from layers beneath the surface: the influence of the anisotropic surface potential on the bulk wave functions as well as minor contributions from atomic relaxations caused by surface-induced stress.

GaAs(001): Surface Structure and Optical Properties

Abstract: The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.

X-ray diffraction analysis of the gallium-rich surface of GaAs(001)

Abstract: Laboratoire de Physique de la Matie `re Condense ́ , Ecole Polytechnique, 91128 Palaiseau Cedex, France LURE, CNRS-MR-CEA, Ba ˆtiment 209d, Centre Universitaire Paris-sud, Boı ˆte Postale 34, F91898, Orsay, France Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica dell’ Universita’ di Roma ‘Tor Vergata,’ Via della Ricerca Scientifica, I-00133 Roma, Italy Institut für Festkörpertheorie und Theoretische Optik, Friedrich-Schiller-Universita ̈t, Max-Wien-Platz 1, 07743 Jena, Germany ~Received 10 January 2001; revised manuscript received 22 May 2001; published 3 October 2001 !

Atomic structure and optical anisotropy of III–V(001) surfaces

Abstract: The optical anisotropy of materials with isotropic bulk crystal structure depends to a large extent on the surface atomic structure. For instance, data obtained by reflectance anisotropy spectroscopy (RAS) on (001) surfaces of zinc blende semiconductors such as InP and GaAs, have a fingerprint character for the various surface reconstructions. Here we present RAS spectra for GaAs(001) and InP(001) recorded at room temperature and at low temperature. We show that by comparison with a theoretical analysis based on ab initio density functional theory in local-density approximation calculations the origin of characteristic spectral features can be identified and thus RAS spectra utilized to discriminate between different competing structural models. We identify contributions related to electronic transitions between surface states as well as features arising from surface perturbed bulk wave functions. We explain the high sensitivity of RAS to the surface structure and chemistry as due to the surface state related features.

Structure and Energetics of P‐rich GaP(001) Surfaces

Abstract: The microscopic structure of the GaP(001) surface is not well understood, in marked contrast to other III–V compounds, in particular GaAs(001) (see Ref. [1] for a recent review). It has been suggested that ion bombardment and annealing of GaP(001) results in a (4 2) reconstructed Ga-rich surface [2, 3] in analogy to GaAs. On the other hand, based on ion scattering spectroscopy and low-energy electron diffraction (LEED) experiments, a (2 4) translational symmetry has been proposed [4, 5]. In solid-source molecular beam epitaxy a (2 4) diffraction pattern shows up under both Ga and P supply [6]. In addition, (4 4) spots have been observed. A slightly different behaviour is found for samples grown by metal-organic vapour phase epitaxy or chemical beam epitaxy as well as for GaP(001) surfaces prepared by thermal desorption of a protective arsenic/phosphorous double layer cap under ultrahigh vacuum conditions [7–9]. After annealing to 690 K the de-capped GaP(001) surfaces show a (2 1)=(2 2)-like LEED pattern. Annealing to higher temperatures (785 K) leads to a (2 4) translational symmetry. The many-step annealing procedure is accompanied by a transition from a more P-rich to a more Ga-rich GaP(001) surface. Previous total-energy (TE) calculations [8, 9] show the stability of several (2 4) reconstructions. For Ga-rich surfaces they are characterized by single Ga–Ga or mixed Ga–P dimers on top of a complete Ga layer. For lower Ga contents one Ga pair is missing in this layer and one (d structure) or two P dimers (b2 structure) terminate the surface. (4 2) reconstructions with Ga or P dimers in the uppermost atomic layer are unstable. The comparison of measured and calculated reflectance anisotropy spectra support the mixed-dimer reconstruction of the Ga-rich GaP(001) surface [8, 9]. This has recently been confirmed by a comparison of calculated and measured scanning-tunneling microscopy (STM) images [10]. The (2 4) mixed-dimer structure agrees with recent measurements of the surface core-level shifts [5]. The structure of the (2 1)=(2 2) reconstruction of the GaP(001) surface remains unknown. Auger electron spectroscopy indicates a P-rich surface. The similarities with

Theory for Modeling the Optical Properties of Surfaces

Abstract: We summarize the theoretical framework and some of the computational methods currently used for the calculation of the optical properties of surfaces within the “ab initio” scheme. Applications and examples are given for calculations including self-energy, excitonic, and local-field effects, with an emphasis on convergence problems and on the approximations related to the pseudopotential approach.

Towards a Complete Many-Body Description: Optical Response of Real Surfaces

Abstract: We demonstrate the potential of recently developed total-energy and electronic-structure methods for the calculation of optical properties of real surfaces. We show that many-body effects such as quasiparticle shifts and electron–hole interaction can now fully be taken into account. As a prototypical property the optical reflectance anisotropy is discussed. Surface examples are large stoichiometry-dependent reconstructions of InP(001) and the hydrogen-passivated Si(110) face.