Backbond States Research Articles

Diffraction refinement of localized antibonding at the Si(111)7×7surface

We report an experimental refinement of the local charge density at the Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ surface using a combination of x-ray and high-energy electron diffraction. This method can be generally applied to the charge-density refinement at surfaces of other materials. By perturbing about a bond-centered pseudoatom model, we find experimentally that the adatom electrons occupy antibonding-like backbond states with the atoms below. We are also able to refine a charge transfer of $0.26\ifmmode\pm\else\textpm\fi{}0.04\text{ }{e}^{\ensuremath{-}}$ from each adatom to the underlying layers, in agreement with full-potential density-functional theory calculations.

Valence Surface Electronic States on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.

Oxide Formation on Si(100)-2×1 Surfaces Studied by Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy

The oxide formation on Si(100)-2×1 surfaces at room temperature has been studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The change in electronic structures due to oxidation has been discussed based on the spatially resolved STS spectra. It was found in the initial stage of oxidation that the local density-of-states (LDOS) of back bond states and dangling bond sites decreases dramatically at the oxidation-induced bright protrusions. The STS spectra of the 2×1 region with an oxygen exposure of 4.5 L suggest that oxygen atoms adsorb on the back bond sites. At the average SiO2 thickness of 0.4 nm, the surface states of Si(100)-2×1 vanish completely and the energy width of the gap near the Fermi level becomes very close to the band gap of bulk Si. In addition, the observed LDOS is spatially uniform in spite of the presence of bumps in the STM image. The STS spectra indicate the formation of the valence band and the conduction band of SiO2 at the oxide thickness of 0.4 nm.

Occupied and unoccupied surface states on the Si(111) sqrt 3 x sqrt 3 :B surface.

The Si(111)√3×√3:BR30 o surface has been studied with k-resolved inverse photoemission and angle-resolved photoemission. In the unoccupied region of the band structure, one strongly dispersing surface state is observed and in the occupied region, three surface states are observed. The unoccupied surface state is identified as an empty dangling-bond state and two of the occupied surface states are identified as backbond states of the adatom

Kinetics of the adsorption of atomic oxygen (N 2O) on the Si(001)2 × 1 surface as revealed by the change in the surface conductance

The adsorption behaviour of N 2O on the Si(001)2 × 1 surface at 300 K substrate temperature has been investigated by measuring in situ the surface conductance during the reaction process. For comparison we monitored in the same way the adsorption of O 2 on the same surface which ultimately leads to the flat band situation. The adsorption of atomic oxygen as released by decomposition of the N 2O molecule, in contrast with molecular oxygen, was found to result in an increase of the band bending. The difference in behaviour of the change of the surface conductance between the two solid-gas reactions can be explained by considering that the adsorption of O 2 will also remove deep-lying backbond states in addition to the dangling bond (DB) and dimer bond (DM) related surface states. It is well known that only the DB and DM surface states are affected by N 2O. The surface conductance measurements (SCM) presented in this paper complement our previous spectroscopic differential reflectivity measurements and Auger electron spectroscopic results for the system Si(001)2 × 1 + N 2O; we have found evidence that the second step of the proposed three-stage adsorption process of atomic oxygen can be divided into two substages. From our SCM data we could derive that the distance between the valence band edge and the Fermi energy of the clean Si(001)2 × 1 surface is 0.32 ± 0.02 eV, which is in agreement with previous photoemission results.

Optical response of Si(111)-7 × 7

The optical properties of the Si(111)-7 × 7 surface are studied by means of a surface differential reflectometer. The optical reflectivity of the clean surface is first compared to that of the native oxide-covered Si surface, then to that of the oxygen-exposed surface. The exact surface optical response function of the Si(111)-7 × 7 surface and its changes upon adsorption of oxygen are determined by use of an electromagnetic surface model. A main peak is observed at 3.9 eV and is interpreted as due to electronic transitions from the back-bond states of the Si adatoms to a bulk conduction band. Secondary maxima at lower energies, due to transitions involving the dangling bond states, are also observed. Indications on the oxygen chemisorption mode are finally deduced.

Angle resolved ultraviolet photoemission spectroscopy study of the electronic structure of InSb(111) surfaces along the [110] azimuth

The analysis of non-normal photoemission spectra of the polar A(111) and B (1̄1̄1̄) surfaces of InSb along the [110] azimuth indicates strong contribution from bulk features. Transitions common to both surfaces around 1.4, 2.6, 3.3, and 6.2 eV are associated with critical points in the bulk band structure. Umklapp scattering of photoelectrons with reciprocal vectors of the InSb(1̄1̄1̄)(3×3) surface is identified for transitions around 3.3 eV. Modifications in the spectra of this surface after Sn coverage suggest contribution from a surface resonance band around 1.4 eV; bands around 4.3 and 4.9 eV dispersing within a gap of the projected bulk band structure calculated by a realistic empirical tight-binding method are tentatively assigned to backbond states.

Electronic properties of the Cs-GaAs(110) interface at monolayer coverage

The electronic properties of the interface obtained by a monolayer deposition of Cs onto GaAs(110) are studied theoretically by modelling the overlayer with a thin jellium slab of appropriate electron density. The calculations are carried out self-consistently with a pseudopotential approach for two different substrate geometries, corresponding to the ideal and the relaxed configuration of the surface atoms. In both cases a band of Cs-induced states is found in the gap which hybridizes with the dangling and back bond states of the GaAs surface. The change in the work function upon Cs deposition is calculated and found for both the substrate geometries in good agreement with the experimental data. Comparison with electron energy loss and photoemission experiments indicates that the model with the surface in the relaxed configuration is the most appropriate one.

Electronic structure of tin dioxide surfaces

We present the results of a theoretical study of the surface electronic structure of the ideal (110), (001), and (100) faces of tin dioxide, a semiconductor crystallizing with the bulk rutile structure. The surface electronic structure is determined using a scattering-theoretic method in which the bulk electronic structure is described by a tight-binding Hamiltonian including first- and second-nearest-neighbor interactions. The results are obtained in terms of surface bound states, surface resonances, and surface densities of states, which are wavevector, layer, atom, and orbital resolved. All this information allows a detailed discussion of the origin, the nature, and the localization of the obtained surface features. Common trends are found for the three faces. The optical band gap is found to be nearly free of surface states. The dominant features are backbond states in the lower valence-band region and Sn $s$-derived states in the lower conduction-band region. We find one of each for the (001) and (100) faces and two of each for the (110) face which contains two tin atoms per surface unit cell. The upper valence-band region leads only to weak, O $p$-derived resonances. All these features are governed by the relatively strong ionicity of Sn${\mathrm{O}}_{2}$. The differences in the results obtained for the three faces are interpreted in terms of the coordination of the surface cations.

Surface electronic structure of rutile-type semiconductors: SnO 2(110) and TiO 2(110)

We have investigated the electronic structure of the ideal (110) surfaces of the rutile structure compounds SnO 2 and TiO 2 using the scattering theoretic method. The results are presented in terms of surface bound states, surface resonances and wave vector resolved densities of states. For SnO 2(110), the dominant features are two backbond states in the stomach gap of the main valence band and two Sn-s derived states in the lower conduction band region. For TiO 2(110), we obtain no occupied surface states in the gap region, in agreement with UPS measurements on defectless TiO 2(110) surfaces. The creation of the surface leads essentially to O-p resonances in the valence band region and Ti-d resonances in the conduction band region.

Surface electronic structure of SnO 2(110)

We report theoretical investigations on the surface electronic structure of the (110)-face of SnO 2, a semiconductor of rutile bulk structure. Starting with a tight binding Hamiltonian for the bulk, we determine the surface electronic structure using the scattering theoretic method. As results we obtain the surface bound states, the surface resonances and the wave-vector resolved surface layer densities of states. The dominant features are two backbond states in the stomach gap of the main valence band and two Sn- s derived states in the lower conduction band region. In the upper valence band region, only weak resonances occur, like in other materials with relatively strong ionicity.