The GaAs(0 0 1)- c(4 × 4) surface: A new perspective from energy loss spectra (original) (raw)
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Electronic anisotropy of the GaAs(001) surface studied by energy loss spectroscopy
Microelectronics Journal, 2003
High-Resolution Electron-Energy-Loss Spectroscopy (HREELS) has been applied to investigate the anisotropy of the GaAs(001)-c(4 £ 4) and b2(2 £ 4) reconstructions. Measurements have been performed on high-quality samples grown in situ by Molecular Beam Epitaxy. The loss intensity is different in the directions parallel and perpendicular to dimers, particularly close to the fundamental gap. We construct relative difference intensity spectra which can be directly compared with the differential reflectivity spectra of the RAS spectroscopy. A one-to-one correspondence is found between experimental and calculated electronic transitions up to about 3 eV. The surface anisotropy given by EELS is about two orders of magnitude higher than that measured optically. The contributions to the anisotropy originate entirely from a few atomic layers beneath the surface. In the b2 phase we find direct evidence of transitions involving the dimers of the top atomic layer which are well separated by those involving bulk states modified by the surface.
LEED structural analysis of GaAs(001)-c(4×4) surface
Surface Science, 2004
The tensor LEED analysis of the intensities of electron beams diffracted from the GaAs(0 0 1)-c(4 · 4) grown by molecular beam epitaxy (MBE) has been performed. Surface structures with symmetrical and asymmetrical 3-dimer models in the topmost layer have been investigated. The best-fit structure with central dimer compressed with respect to the As 4 molecule by 20% has been found. Model with asymmetrically arranged dimers fits experimental data better than that with symmetrical alignment.
Photoemission from α and β phases of the GaAs(001)-c(4×4) surface
Surface Science, 2009
We prepared aand b surface phases of GaAs(0 0 1)-c(4 Â 4) reconstruction by molecular beam epitaxy (MBE) using As 4 and As 2 molecular beams, respectively, and examined them by angle-resolved ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation as an excitation source. The UPS valence band spectra and the XPS 3d core level data show pronounced differences corresponding to the surface composition and the atomic structure of the two phases, as proposed in the literature. In UPS, the b phase is characterized by an intensive surface state 0.5 eV below the top of the valence band at low photon energy, while an analogous peak in the a phase spectra is missing. The surface state is interpreted in terms of dangling bonds on As dimers. The As3d and Ga3d core level photoelectron lines exhibit phase-specific shapes as well as differences in the number, position and intensity of their deconvoluted components. The location of various atoms in the surface and subsurface layers is discussed; our photoemission results support models of the b phase and the a phase with As-As dimers and Ga-As heterodimers, respectively.
Reflection electron-energy-loss investigation of the H-GaAs(110) surface
Physical Review B, 1984
The versatility of electron-energy-loss spectroscopy for investigating the electronic as we11 as the structural properties of a bulk sample is shown. The technique should be considered a powerful tool for ranging from low to high energies in the same experimental apparatus. The electronic properties of palladium are interpreted in detail within band-structure framework and confirmed through a systematic investigation of adjacent transition metals. The structural properties obtained by analyzing extended energy-loss fine structures above shallow core levels are presented in the form of the radial distribution function. These extended x-ray-absorption fine-structurelike oscillations detected above the N&3 and M45 core levels of Pd support the signal interpretation scheme of the high-energy-loss range.
Angle - resolved photoemission study of two phases of the GaAs(100)-c(4×4) surface
Journal of Physics: Conference Series, 2008
We prepared two surface phases of the Ga(001)-c(4x4) reconstruction (α, β) by molecular beam epitaxy (MBE) using As 4 and As 2 molecular beams and examined the surfaces by angle-resolved ultraviolet photoelectron spectroscopy (UPS) and core level photoelectron spectroscopy with the synchrotron radiation as the excitation source. It is demonstrated that the photoelectron spectroscopy can distinguish between the phases. Appearance of intensive surface state 0.5 eV below the top of the valence band at lower energies is linked to the presence of the β-phases on the surface while in the α-phase spectra the peak is missing. Both As 3d and Ga 3d photoelectron lines show substantial differences between the phases in line shapes as well as in their deconvoluted components. The 3d data are in agreement with different surface composition and atomic structure of both phases.
Optical anisotropy spectra of GaAs(001) surfaces
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1992
We present theoretical studies of the optical anisotropy spectra of GaAs(OOl) surfaces with different reconstructions at growth temperatures. The Ga-rich (4 X 2) missing~dimer surface, three different phases (a, (3, and y) of the As-rich (2X4) surface, and an As-rich c(4X4) surface are considered. Total energy calculations within a nearest-neighbor tight-binding model are performed to determine the surface geometry for each reconstruction considered. The difference between reflectance spectra with [110] and [110] polarization for these surfaces is analyzed and compared to available data.
Reflectance-difference spectroscopy of (001) GaAs surfaces in ultrahigh vacuum
Physical Review B, 1992
Reflectance-difference spectroscopy (RDS) is employed to study in situ the (4X2), (1X6), (4X6), (3X1), (2X4)-a, (2X4)-P, (2X4)-y, c(4X4), and d(4X4) reconstructions of (001) GaAs surfaces prepared in ultrahigh vacuum (UHV) by molecular-beam epitaxy and simultaneously characterized by reflection high-energy electron diffraction (RHEED). Reproducibility of the data is excellent. %'ith the aid of previous theoretical calculations, we interpret characteristic spectral features at 1.9, 2.6, and 4.2 eV in terms of electronic excitations involving surface dimers of Ga, As, and As, respectively. Because RD couples to local electronic structure rather than to long-range order, RD spectra not only determine surface reconstructions but also provide details not accessible by RHEED, such as the existence of As dimers in the (1 X 6), (4X6), and (3 X 1}reconstructions and of the fractional coverage within a given reconstruction. Our data show that the (3X1), (1X6},and (4X6) reconstructions are at least partly determined by kinetics, since they can only be obtained by following specific heating or cooling procedures under very low As4 flux. More generally, it is possible to employ this optical technique to determine surface atomic and electronic structure. Because RD spectra can be obtained with the surface in any transparent ambient, the database that we have established here provides a new approach for elucidating surface reconstructions of (001) GaAs and hence the dynamics of surface reactions in non-UHV environments.
Surface Science, 1982
Electronic structures of several atomic wires on an H-terminated Si͑100͒2ϫ1 surface have been examined by using first-principles calculations within the local-density-functional approach. Several dangling-bond ͑DB͒ wires, which are constructed by extracting H atoms from the surface, have been examined and found to have different characteristics depending on their structures. Electronic states near the Fermi energy are localized around the wire on the atomic scale in DB wires along the dimer rows on the surface, while they are much more delocalized around a DB wire in the direction across the dimer rows. Ga adsorbate atomic wires, which are formed by Ga adsorbates around the above wires, have also been examined. Several metastable geometries of Ga adsorbates were found. It was found that formation of Ga dimers was stable on this surface.
GaAs(001) surface reconstructions: geometries, chemical bonding and optical properties
Applied Surface Science, 2002
We re-examine the GaAs(0 0 1) surface by means of first-principles calculations based on a real-space multigrid method. The cð4 Â 4Þ; ð2 Â 4Þ and ð4 Â 2Þ surface reconstructions minimize the surface energy for anion-rich, stoichiometric and cation-rich surfaces, respectively. Structural models proposed in the literature to explain the Ga-rich GaAs(0 0 1) ð4 Â 6Þ surface are dismissed on energetic grounds. The electronic properties of the novel zð4 Â 2Þ structure are discussed in detail. We calculate the reflectance anisotropy of the energetically most favoured surfaces. A strong influence of the surface geometry on the optical anisotropy is found. #
GaAs(001): Surface Structure and Optical Properties
physica status solidi (a), 2001
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 Â 4) and b2(2 Â 4) as well as for the stoichiometric a2(2Â4) and the Ga-rich z(4 Â 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 E 1 critical point energy, characteristic of the b2(2 Â 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 Â 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the z(4 Â 2) model for the surface geometry.