Electronic properties of the polar GaAs(111)As surface

Photoemission yield spectroscopy (PYS) has been used to investigate the electronic properties of the polar GaAs(111)As surface cleaned by ion bombardment with subsequent annealing at 770 K under UHV conditions of about 10⁻⁷ Pa. The elemental composition was recorded simultaneously by means of Auger electron spectroscopy (AES). After the above described cleaning procedure As has been completely removed from the first surface monolayer which corresponds to the (1×1) atomic surface structure whereas the values of the work function ø and absolute band bending were 4.00±0.02 and −0.16±0.06 eV respectively. Moreover two filled electronic surface state bands were observed which have been described as the Ga-derived surface state band and As-derived “back-bond” surface state band respectively.     

Electronic structure of graphene on Ni(111) and Ni(100) surfaces

Physics of the Solid State - A comparative investigation of graphene prepared by cracking of propylene (C3H6) on nickel surfaces with different orientations, Ni(111) and Ni(100), has been carried...     

Nature of versatile chemisorption on TiC(111) and TiN(111) surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR’s) at the Fermi level and X2p-derived SR’s deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR’s in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

Ab initio calculations of the CaTiO3 (111) polar surfaces

The stoichiometric terminations and nonstoichiometric terminations have been constructed for CaTiO₃ (111) surface. The cleavage and surface energies, surface grand potential and surface electronic structure have been calculated for the two main classes of terminations using ab initio plane wave ultrasoft pseudopotential method based on Local Density Approximation (LDA). The results show that the stoichiometric terminations are unstable compared with the nonstoichiometric terminations and the polarity compensation achieved through the modification of the surface stoichiometry is more effective than that by the anomalous filling of the surface states. In the O and Ca chemical environments, only CaO₂ and TiO terminations can be formed; the CaO₃ and Ti terminations cannot be stabilized, even in very O-rich chemical environment.     

Segregation of tin on (111) and (100) surfaces of copper

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (√3 × √3)R30° structure for the (111) surface and a p(2 × 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to ΔH = ?(53 ± 5) kJ/g-atom.     

Stability of rocksalt (111) polar surfaces: beyond the octopole

Stable polar oxide surfaces must be simultaneously electrostatically compensated and in thermodynamic equilibrium with the environment. As a paradigm, the MgO(111)-p(2x2) reconstructed surface is shown to involve combinations of Mg-covered terminations with peculiar insulating electronic structure, favored in O-poor conditions, and the O-terminated octopole, stabler in more O-rich environments. Such a picture, which could not have been foreseen by either experiments or simulations separately, goes beyond the Wolf model and reconciles the theory with the experimental data taken in variable thermodynamic conditions.     

Electronic structure of clean and metal chemisorbed diamond (111) surfaces

The electronic density of clean and Ga-chemisorbed diamond (111) surfaces have been calculated using a cluster Bethe-lattice method. The results have been compared with those obtained for Ge (111) surfaces. A comparatively flat dangling bond band appears in both the clean and chemisorbed diamond surfaces in contrast to the other homopolar material surfaces. The metal-induced states appear on the top of the valence band as well as on the bottom of the conduction band.     

Electronic structure of H chemisorbed on Si(111) surfaces

We study the electronic density of states of hydrogen chemisorbed (111) Si surface. We analyse two situations: one single chemisorbed hydrogen atom in an otherwise clean surface and a complete monolayer of hydrogen chemisorbed at the surface. The method of calculation is based on an extension of the cluster-Bethe lattice approximation developed by the authors to study surfaces. Our results for the monolayer are in good agreement with UPS data, as well as with other theoretical calculations.     

Theoretical study of oxygen chemisorption on (111) and (100) silicon surfaces

The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especially for the (111) surface.     

Structural and physical properties of BiFeO3 thin films epitaxially grown on SrTiO3（001） and polar（111） surfaces

The influence of surface polarity on the structural properties of BiFeO3 （BFO） thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 （STO） （100） and polar （111） surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar （001） or polar （111） surfaces perform very differently. BFO/STO （001）is a fully strained tetragonal phase with orientation relationship （001）[100]BFOII（001）[100]STO, while BFO/STO （111） is a rhombohedral phase. BFO/STO （111） has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO （001） and STO （111） are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

Angular dependence of auger electron emission from Cu (111) and (100) surfaces

Experimental angular emission profiles of the M_2,3M_4,5M_4,562 eV Cu Auger electrons from clean copper (100) and (111) surfaces in several azimuths are presented. A simple single scattering theory to account for elastic scattering of the Auger electrons by other ion cores in the solid is presented, and calculations have been performed to assess the importance of this process in contributing to the observed angular dependence. These calculations produce angular structure having a similar magnitude and temperature dependence to that observed experimentally, and some featural similarity in peak positions or shapes. It appears that elastic scattering is an important source of angular dependence, and that studies of adsorbed species on surfaces should provide a very sensitive method of surface structure determination.