Structure and electronic properties of antimony films on the Mo(110) surface

The structure and electronic properties of antimony on the Mo(110) surface are investigated over a wide range of coverages. In the submonolayer range, p(2×1), p(1×1), (1×3), and (1×2) adsorbate structures matched to the substrate are formed at room temperature. For coverages larger than a monolayer, three-dimensional antimony crystals whose orientation is determined by the substrate grow on the surface. Annealing of the system at temperatures higher than 1000 K leads to the formation of structures that are not observed upon condensation. The results of analyzing the electron energy-loss spectra jointly with the work function of the surface suggest the formation of surface molybdenum-antimony alloys.     

Quasiparticle corrections to the electronic properties of anion vacancies at GaAs(110) and InP(110)

We propose a new method for calculating optical defect levels and thermodynamic charge-transition levels of point defects in semiconductors, which includes quasiparticle corrections to the Kohn-Sham eigenvalues of density-functional theory. Its applicability is demonstrated for anion vacancies at the (110) surfaces of III-V semiconductors. We find the (+/0) charge-transition level to be 0.49 eV above the surface valence-band maximum for GaAs(110) and 0.82 eV for InP(110). The results show a clear improvement over the local-density approximation and agree closely with an experimental analysis.     

Oxygen adsorption and the surface electronic structure of GaAs (110)

The surface electronic structure of cleaved GaAs (110) is found to be very sensitive to small amounts of adsorbed oxygen. For example, adsorbing oxygen on only a few percent of the surface Ga or As atomic sites can produce changes of a factor of two in the surface electronic structure. Thus, long range effects must be involved, and these are associated with rearrangement of the surface atoms.     

Surface states on GaAs(110)

The electronic structure of the (110) surface of GaAs is recalculated using the relaxation geometry recently obtained from analyzing elastic low-energy electron diffraction intensity data and a self-consistent ab initio pseudopotential approach. Better agreement is found for the occupied surface states compared with photoemission data, giving support for the new structural model. The influence of convergence of the plane-wave expansion and relativistic effects on the surface states is also examined.     

The epitaxial growth of Fe on GaAs(110): Development of the electronic structure and interface formation

The epitaxial growth of ultra-thin Fe films on GaAs(110) at a substrate temperature of 175° C has been studied by spin-resolved and spin-integrated photoemission with synchrotron radiation. Formation and evolution of the interface region have been followed for incremental Fe coverages Θ between 0.1 and 75 ML. The ordered growth of the overlayer is accompanied by reactive intermixing for metal coverage up to 15 ML followed by further As outdiffusion. The surface is ferromagnetically ordered by 6 ML.     

AB initio calculation of the atomic and electronic structure for Sb adsorbed on GaAs(110)

We report results obtained by a systematic study of Sb adsorption on the relaxed GaAs(110) surface, using density-functional theory within the local-density approximation (LDA) and norm-conserving, fully separable, ab-initio pseudopotentials. The GaAs(110) surface is simulated by a slab geometry wherein the atomic structure of the Sb atoms at the preferred adsorption positions and the uppermost substrate layer is optimized by minimizing the total energy, in contrast to previously reported theoretical approaches obtaining the surface bandstructure for given geometrical equilibrium structures. Sb coverages of Q=1/2 and Θ=1 are considered. We give a detailed analysis of the total-energy surface of the Sb/GaAs(110) system and identify stable and metastable adsorption sites. The resulting equilibrium geometries are discussed: We interpret these results in terms of the Sb-Sb interaction within the chains parallel to the [1¯11] direction and of possible structural instabilities in such chains. The atomic positions are compared with results of LEED analysis, stating an overall agreement except the buckling of the chain atoms. The resulting electronic properties (surface bandstructure, photothreshold, Schottky barrier) are discussed within the context of experimental data available from STM, photoemission spectroscopy, etc.     

Cluster model approach for electronic structure of Si and Ge(111) and GaAs(110) surfaces

For the purpose of exploring how realistic a cluster model can be for semiconductor surfaces, extended Huckel theory calculations are performed on clusters modeling Si and Ge(111) and GaAs(110) surfaces as prototypes. Boundary conditions of the clusters are devised to be reduced. The ideal, relaxed, and reconstructed Si and Ge(111) surfaces are dealt with. Hydrogen chemisorbed (111) clusters of Si and Ge are also investigated as prototypes of chemisorption systems. Some comparison of the results with finite slab calculations and experiments is presented. The cluster-size dependence of the calculated energy levels, local densities of states, and charge distributions is examined for Si and Ge(111) clusters. It is found that a 45-atom cluster which has seven layers along the [111] direction is large enough to identify basic surface states and study the hydrogen chemisorption on Si and Ge(111) surfaces. Also, it is presented that surface states on the clean Si and Ge(111) clusters exist independent of relaxation. Further, the calculation for the relaxed GaAs(110) cluster gives the empty and filled dangling-orbital surface states comparable to experimental data and results of finite slab calculations. The cluster approach is concluded to be a highly useful and economical one for semiconductor surface problems.     

Surface phonons in GaAs(110)

Surface phonon dispersion curves have been measured along the <001>, <11&#x0304;2>, and <11&#x0304;0> azimuths of GaAs(110). Features of note include a very low frequency (5.5 meV at zone boundary) surface acoustic mode in the first two directions; this may arise through the known (1 × 1) reconstruction of this surface. A higher frequency surface mode (7.3–8.8 meV, depending on azimuth) is seen in all directions. The helium scattering intensities are greatly influenced by bound state resonances. A careful survey of the selective adsorption signatures in extremely high resolution scans of polar angle, azimuthal angle, and incident beam wavevector allows the bound state energies to be determined with some confidence. Initial results indicate energies of roughly 1, 2, and 4 meV.     

Growth of Sb overlayers on GaAs(110)

The GaAs(110)-Sb system is studied with AES, EELS, LEED, ellipsometric spectroscopy and SEM. As indicated by EELS Sb atoms are adsorbed first on Ga sites. The AES spectra can be explained by assuming a simultaneous growth of multiple layers on top of a well ordered homogeneous first monolayer (MSM growth mode). The results of ellipsometric spectroscopy confirm the inhomogeneity of the Sb-film as proposed by the MSM mode. Desorption experiments and EELS demonstrate a strong chemical bonding between the first Sb monolayer and the substrate.     

On the electronic structure of cleaved GaAs(110) surfaces exposed to formic acid

GaAs(110) surfaces cleaved in UHV and exposed to HCOOH have been studied by work function measurements (Kelvin method), electron energy loss spectroscopy (ELS) and by low energy electron diffraction (LEED). From the different changes of the work function on n- and p-type material information about intrinsic and extrinsic surface states is derived. In the loss spectra the adsorbed formate species causes a loss near 9 eV. The intensity of the loss near 20 eV generally ascribed to an excitonic transition from the Ga 3d core level into surface states is reduced only by a factor of two after saturation with HCOOH. This might be related to the c(2 × 2) superstructure observed in LEED, which suggests a saturation coverage of half a monolayer.