The surface electronic structure of Ge(111)c(2x8) was studied by experimental techniques [low-energy electron diffraction, scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES)] and theoretical band-structure calculations. Bias-dependent STM images exhibit two different types of adatoms (A(T),A(R)) and rest atoms (R-T,R-R) confirming the presence of asymmetries within the c(2x8) cell. The ARPES study resulted in a more detailed picture of the surface electronic structure of the Ge(111)c(2x8) surface compared to earlier studies. The energy dispersion curves showed the presence of seven surface bands labeled A1, A2, A2(), A3, A4, A4(), and A5. The experimental surface bands were compared to the calculated band structure of the full c(2x8) unit cell. The most important results are (i) we have identified a split surface-state band in the photoemission data that matches a split between R-T and R-R derived rest atom bands in the calculated surface band structure. This allows us to identify the upper A2 band with the R-R and the lower A2() band with the R-T rest atoms. (ii) The uppermost highly dispersive band (A1) originates from states below the adatom and rest atom layers and should not be confused with rest atom bands A2 and A2(). (iii) The bias-dependent changes in the adatom/rest atom contrast in the experimental STM images were closely reproduced by simulated STM images generated from the calculated electronic structure. (iv) A split was observed in the back-bond derived surface band at higher emission angles (A4 and A4()).

The electronic structure of H/Ge(111)1×1 was investigated using angle-resolved photoelectron spectroscopy. Spectra were measured along the high-symmetry lines of the 1×1 surface Brillouin zone. In the Γ̅ −K̅ −M̅ direction, two surface states, labeled a and a^′, were found in the lower and upper band-gap pockets. The a and a^′ surface states are associated with the Ge-H bonds and the Ge-Ge backbonds, respectively. In the Γ̅ −M̅ direction, only the Ge-H surface state, a, can be identified. It is found in the band-gap pocket around the M̅ point. The two hydrogen-induced surface states on H/Ge(111)1×1 show strong similarities with the corresponding surface states on H/Si(111)1×1. Results from H/Ge(111)1×1 and H/Si(111)1×1 are compared in this Brief Report.

The relation between annealing temperature and surface photovoltage (SPV) shifts on the Si(111)7×7 surface of lightly n-doped substrates has been studied by core-level and valence-band photoelectron spectroscopies at 100 K. The SPV shift was found to depend strongly on the annealing temperature and the photon flux. Between 900 and 1150 °C the magnitude of the SPV shift shows a general decrease with annealing temperature. After a narrow plateau, the SPV shift becomes positive for annealings at 1250 and 1270 °C. As a consequence, the adatom surface state of the 7×7 surface appears above the Fermi level. The unexpected SPV shift can be explained by the formation of a p-type layer during high-temperature annealing of the Si sample. The role of boron and carbon contaminations has been discussed in this context in the literature. By correlating the SPV shifts with the C 1s and B 1s core-level signals, we conclude that carbon, but not boron, is involved in the formation of the p-type layer. Further, our results show that the annealing temperature plays a crucial role when binding energies are determined from photoemission spectra at low temperature. The effect is of particular importance in the study of surface band-gap openings related to phase transitions at low temperature.

The atomic and electronic structures of the Ag∕Si(111)√3×√3 surface are currently under debate. By employing angle-resolved valence-band spectroscopy, the surface band dispersions around the K̅ point of the Ag∕Si(111)√3×√3 surface have been investigated in detail. Contrary to a recent study, we conclude that the S₂ and S₃ surface state bands do not show any detectable split at 100 K. Thus, photoemission spectra at both room temperature and 100 K show only a single peak at the K̅ point without any direct evidence of a split. Calculated band structures for the inequivalent triangle (IET) model show a gap at the K̅ point in contrast to the honeycomb-chain-trimer (HCT) model. We find, however, that there is no real contradiction between our photoemission data and the IET model provided the energy gap of the latter model is small as indicated by a recent calculation [ Phys. Rev. B 70 245431 (2004)].

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.

The adsorption and reaction processes of physisorbed oxygen molecules on a Si(111)-(7×7) surface have been investigated using time-resolved O 1s core-level photoemission measurements at 45 K. Physisorbed oxygen molecules are only observed at 45 K and lower temperatures on a Si(111)-(7×7) surface. At the dosage when the dangling bonds are saturated by chemisorbed oxygen, the coverage of the physisorbed species increases drastically. This result indicates that oxygen species, which are chemisorbed on top of adatoms, modifies the potential energy curve for an oxygen molecule approaching the surface such that physisorbed oxygen molecules are stabilized. Further, the longer lifetime at a higher dosage indicates that an intermolecular force plays a role for the stabilization of this species. Taking these results into account, an oxidation stage-dependent gas-surface interaction for an oxygen molecule approaching the Si(111) surface is suggested.

Adsorption of atomic hydrogen has been studied by scanning tunneling microscopy (STM) and photoelectron spectroscopy with a focus on the different adsorption sites provided by the Si(111) 7×7 surface. At low temperature, the hydrogen atoms adsorb preferentially on adatoms while at elevated temperatures the rest atoms are the first to become hydrogen terminated. The hydrogen-terminated rest atoms are no longer visible in the STM images and the surrounding adatoms appear brighter compared to the clean 7×7 surface. This indicates that there is a local charge transfer back to the adatoms from the rest atoms. Three kinds of modified triangular subunit cells of the 7×7 reconstruction have been identified corresponding to one, two, and three hydrogen-terminated rest atoms, respectively. A detailed study of the apparent height using STM line profiles through the adatom and rest atom positions on the surface is presented. These line profiles show a characteristic and reproducible variation of the apparent heights of the adatoms for the different kinds of triangular subunit cells and the changes are interpreted in terms of charge transfer. The very local nature of the charge transfer is concluded from the fact that only the hydrogen termination of neighboring rest atoms is significantly affecting the apparent height of an adatom.

The electronic structure of the Ca/Si(111)-(3×2) surface has been investigated by angle-resolved photoelectron spectroscopy. Five surface states, none of which crosses the Fermi level, were observed in the bulk band gap, and one surface state was observed in a bulk band pocket. The dispersion features of three of the surface states in the band gap agree well with results from monovalent atom adsorbed Si(111)-(3×1) surfaces along the chain direction. The close resemblance indicates that the origins of the surface states are the same as or quite similar to those of the (3×1) surface. The two other states observed in the band gap have not been reported in the literature, and they are interpreted as surface states that occur on Ca/Si(111)-(3×2) due to the lower coverage (1/6 monolayer of Ca). Further, based on the finite surface state dispersion in the direction perpendicular to the Ca chains, we conclude that the electronic character of this surface is not completely one dimensional.

High-resolution photoemission provides important information about the electronic and atomic structure of surfaces. To make full use of the high-energy resolution that is available at many synchrotron radiation facilities, it is important to lower the phonon induced broadening by reducing the sample temperature. Another equally important factor is the sample quality. Sample inhomogeneities may have a significant detrimental effect on the line widths of the core-levels masking essential information. The surfaces discussed in this paper include Si(111)7 x 7, Si(111)1 x 1:As and Si(111)root3 x root3:Ag. The Si(111)root3 x root3:Ag surface is a good example of the importance of the sample preparation and characterization. A tiny amount of additional Ag atoms on top of the root3 x root3 layer leads to a significant broadening of the apparent core-level widths. (C) 2004 Elsevier B.V. All rights reserved.

We have investigated the initial oxidation stage of an Si(111)-(7 × 7) surface using valence-band photoemission measurements. As the oxygen exposure increases, the intensities of the dangling bond states of adatoms (S 1) and rest atoms (S2) decrease. Among the four oxygen-induced states, three originate from the orbitals of adsorbed oxygen species, and one originates from the dangling bonds of adatoms with more than one oxygen atom adsorbed into its back-bond. Taking the dosage-dependent intensity of this modified dangling bond state into account, we conclude that the first adsorption site of oxygen is the back-bond of an adatom. © 2004 Elsevier B.V. All rights reserved.