Semiconducting surface reconstructions of p-type Si(100) substrates at 5 K

We report scanning tunneling microscopy (STM) studies of the technologically important Si(100) surface that reveal at 5 K the coexistence of stable surface domains consisting of the p(2 x 1) reconstruction along with the c(4 x 2) and p(2 x 2) reconstructions. Using highly resolved tunneling spectroscopic measurements and tight binding calculations, we prove that the p(2 x 1) reconstruction is asymmetric and determine the mechanism that enables the contrast variation observed in the formation of the bias-dependent STM images for this reconstruction.     

Influence of sublayer atoms on Si(100) surface reconstructions

Influence of sublayer atoms on Si(100) surface reconstruction has been examined with density functional theory and molecular dynamic simulation. We found that the displacements of sublayer atoms under the buckled dimer affect the motions of their neighboring atoms and thus play an important role in determining the surface reconstructions. The present results reveal the relationship between the surface dimer reconstruction and the motion of the sublayer atoms and provide an account for experimental observations of Si(100) surface reconstructions at very low temperatures.     

Field ion microscope observations of surface reconstructions on Pt(100) and Ir(100)

The reconstruction of the Pt(100) and Ir(100) plane has been studied by field ion microscopy. Small clusters of atoms produced by low-temperature field evaporation transform to new structures at temperatures above 270 K for Pt and 470 K for Ir. The atomic structure of reconstructed images is not resolved, but the shapes of the clusters suggest reconstructions consistent with current models based on low energy electron diffraction studies. Field evaporation of the reconstructed surfaces indicates that the restructuring extends to the second layer of atoms.     

Photoemission yield spectroscopy of electronic surface states on germanium (111) surfaces

The electronic surface states of cleaved and annealed Ge(111) surfaces have been investigated by photoemission yield spectroscopy and contact potential measurements on a set of differently doped samples. On the 2 × 1 cleaved surface, a surface state band centered about 0.7 eV below the valence band maximum is found. The variations of the work function with the doping level show that an empty surface state band exists above the Fermi level. After annealing at temperatures of the order of 350°C, this surface exhibits a 2 × 8 superstructure. A new surface state band is then found closer to the valence band maximum. This variation of the surface state distribution is correlated to a change in the surface potential. The variation of the electronic characteristics upon oxygen adsorption are also reported and an evaluation of the sticking coefficient is made for both structures.     

Hydrogen-induced reconstruction on W(100) and Mo(100) by surface infrared spectroscopy

Infrared absorption and low-energy electron-diffraction measurements of H adsorbed on W(100) and Mo(100) show that on each surface, distinct wavenumbers characterize the H-substrate stretching modes associated with the different long-range structures of the complicated T-θ phase diagram. Hydrogen is bonded at a two-fold bridge site at all temperatures and coverages investigated and the wavenumber of the symmetric stretch mode, v₁, is determined by the local geometry, i.e. the substrate dimer length. Analysis of the coverage dependence of the v₁ wavenumber shows that, at low coverages (θ ≲0.3), the effective H-H interactions are very different for the two substrates, leading to a uniform H layer on W(100) and to island formation on Mo(100). In general, the phase transitions are continuous on W(100), with regions of intermediate structures, and first order on Mo(100), with regions of coexisting phases.     

Infrared reflection-adsorption spectroscopy of W(100)-H at 100 K

Using infrared reflection-absorption spectroscopy in conjunction with the broad-band technique of Fourier-transform interierometry, changes m the absorption spectrum of W(100) have been investigated as a function of H coverage at 100 K from 900 to 3900 cm⁻¹. The changes measured upon initial adsorption indicate the removal of intrinsic-surface states 0.36 eV below the Fermi surface as well as the establishment of the unreconstructed-surface electronic absorption at 0.15 eV. The appearance of the 0.15 eV mode upon initial adsorption implies that the surface largely switches to an unreconstructed state at low coverages, consistent with H-atom immobility at low temperatures. For higher coverages [> 0.7 monolayer (ML) (2.0 ML saturation)] the measured reflectivity changes show that free-carrier surface scattering dominates the coverage induced variations. It is seen that infrared measurements of these surface-scattering variations can lead to determination of the effective plasma frequency ohmsp of the metal.     

Image-potential-induced surface state at Si(100)

We have identified a surface state on Si(100) (2×1) at a binding energy of 0.69±0.05 eV with respect to the vacuum level. Band-structure calculations within the GW method reveal that almost 80% of the probability density of the resonance is located in front of the surface. We therefore assign the surface resonance to an image-potential state. It has a lifetime of about 10 fs and contributes significantly to two-photon photoemission from Si(100). PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

Surface reconstructions of InGaAs alloys

The surface reconstructions of In_xGa_1−xAs alloys grown by molecular beam epitaxy on the (0 0 1) surfaces of GaAs and InAs have been studied by reflection high-energy electron diffraction and scanning tunnelling microscopy. A surface phase diagram is presented for the nominally strain-free alloy as a function of substrate temperature and alloy composition, and structural models for the commonly observed 3× reconstructions are discussed. Two new, electronically stable structural models are described that account for the transition of the In_xGa_1−xAs surface alloy from a c(4 × 4) to an asymmetric 3× reconstruction and that are fully consistent with all current experimental evidence.     

High-resolution electron energy-loss spectroscopy at epitaxially grown GaAs(100)

High-Resolution Electron Energy-Loss Spectroscopy (HREELS) is shown to be a very sensitive tool to investigate the space-charge regime of n-respectively p-type semiconductors. The most simple model we applied to fit experimental spectra is based on a step-like distribution of free carriers with the Drude dielectric response function. In this case, the dispersion of surface plasmon excitations is neglected, but it is considered in the Thomas-Fermi and the Debye-Hückel models. We use these models to fit HREELS-spectra, obtained from heavily Si-doped GaAs(100), which was grown by Molecular Beam Epitaxy (MBE). A comparison shown that the Drude model overestimates both the free-carrier concentration and the plasmon damping factor. The use of a more realistic smooth free-carrier profile, obtained by the self-consistent solution of the Schrödinger and Poisson equations, leads to plasmon excitations with lower frequencies. Besides Ohmic damping, the calculations show that Landau damping should be incorporated in order to obtain a better fit, particularly at intermediate frequencies.     

Anomalous bismuth-stabilized (2x1) reconstructions on GaAs(100) and InP(100) surfaces

First-principles phase diagrams of bismuth-stabilized GaAs- and InP(100) surfaces demonstrate for the first time the presence of anomalous (2x1) reconstructions, which disobey the common electron counting principle. Combining these theoretical results with our scanning-tunneling-microscopy and photoemission measurements, we identify novel (2x1) surface structures, which are composed of symmetric Bi-Bi and asymmetric mixed Bi-As and Bi-P dimers, and find that they are stabilized by stress relief and pseudogap formation.     

DC-electric-field-modified second-harmonic generation at the Si(100) surface

1 resonances for clean and H covered surfaces shift as a function of the dc field in agreement with experiment. This suggests the presence of built-in electric fields whose strength depends on the H coverage, and which are strongly localized in the subsurface region. Received: 20 September 1998     

Ellipsometric spectroscopy of the ZnO nonpolar(1\bar 100) surface

Ellipsometric spectroscopy has been performed on nonpolar ZnO surfaces in the spectral range 1.5 eV<ħω<4.0 eV. Absolute measurements with two different crystal orientations in air allow the determination of the optical constantsn _∥,k _∥ andn _⊥,k _⊥ for light polarized parallel and perpendicular to thec-axis. The ellipsometric angles Δ and ψ are changed remarkably on ultrahigh vacuum cleaved surfaces near the band gap energy of ∼3.4eV when oxygen or atomic hydrogen is adsorbed or when the crystal is annealed. This observation is interpreted in terms of a field effect on the optical constants in the space charge layer.     

Photoemission studies of the alkali-covered Ge (111) surface

Photoelectron energy distribution measurements have been used to monitor changes in the surface potential of the Ge(111) surface as a function of cesium and sodium coverage. The specimen was sputter-cleaned, intrinsic (45 ohm-cm) germanium. It was found that the surface was p-type throughout the entire stable coverage range. In the case of cesium deposition, strong evidence of emission from occupied surface states was observed for coverages less than 0.15 monolayer. Exposure of the cesium-covered specimen to oxygen caused the surface to become less p-type. Structure in the surface potential versus coverage curve for sodium correlates with structure in the work function versus coverage curve and with a surface reconstruction observed by LEED.