S. L. Molodtsov

Growth of cobalt and cobalt disilicide on Si(100)

The growth of Co on Si(100) and the initial stages of the formation of cobalt suicides have been studied by means of a multitechnique approach. Adsorption of Co on Si(100) at room temperature does not result in reaction and formation of a few ML-thick CoSi2 overlayer, contrary to adsorption on (111) surfaces. Rather, a layer-by-layer growth of metallic Co with some Si interdiffused is observed. The formation of CoSi2 requires annealing to 350°C, a temperature much lower than in the (111) surface. Annealing to 600° C results in additional Si-enrichment at the surface produced by disruption of the CoSi2 overlayer. The thin CoSi2 “template” layer, which is crucial to achieve epitaxial growth, contains Si at the outer surface, as demonstrated by chemical titration.

Effects of adatom concentration on Au/Si(111) and Si/Au interface formation

Effects of Au(Si) adatom concentration on Au/Si(Si/Au) interface formation were investigated by photoemission and partial-electron-yield spectroscopy using synchroton radiation, as well as by Auger-electron spectroscopy. Both interfaces are found to be reactive. The process of Au/Si(111) interface formation can be divided into three stages: for very low Au coverages, θ<2 A, Au forms a rather unreacted adlayer, in the intermediate range, 2 < θ < 25 A, a reacted layer with a stoichiometry close to Au3Si is found; the high-coverage region is governed by the formation of an unreacted metallic Au phase beneath an about one reacted monolayer. For Si/Au the formation of a reacted phase is observed from the first stage of Si deposition. A model for the formation of both interfaces in terms of Au spd/Si sp hybridization is presented.

Photoemission study of Lafullerite and Lagraphite interfaces

A photoemission study of reactive La/C 60 and La/graphite interfaces reveals compound formation as described by the intercalation of La into the C-derived matrices. At La coverages below ∼ 4 A nonmetallic phases are formed for the La/C 60 system, which are characterized by the successive filling of LUMO-derived electronic states. At higher La coverages, (LUMO + 1)-derived states get occupied leading to a thermally unstable phase that decomposes to the nonmetallic phase with a single filled LUMO-derived band upon annealing to 600 K. The La/graphite system forms intercalation compounds, which in analogy to the La-C 60 compounds can be described by filling of C-p π* -derived states due to charge transfer from La.

Initial stages of MgO/Si and Si/MgO interface formation

The formation of MgO/Si(111) and Si/MgO(111) interfaces was investigated by Auger-electron and photoelectron spectroscopy using synchrotron radiation. These interfaces are found to be rather abrupt. A chemically reacted buffer layer of about 1 monolayer thickness was found between bulk MgO and Si. It is concluded that the Mg-O or Si-Si bonds are not broken in the buffer layer, with no interdiffusion of the components.

Resonant photoemission study ofK-derived valence-band states in K X C60

The electronic structure of K-doped C60 was investigated by photoemission (PE) and X-ray absorption near-edge structure (XANES) studies at the C-1s and K-2p thresholds. In addition, information on the local K-derived partial density of states in superconducting K3C60 was obtained by resonant PE at the K-2p1/2 threshold. The experimental observations support a complete charge transfer from K to C60 and we clearly observe a finite density of states atEF. From resonant PE, occupied states with K-p, d character could be identified in the binding-energy region from 1.5 to 8 eV below, but not directly at the Fermi level. This partial-density-of-states structure agrees well with the results of our band-structure calculations based on the local-density approximation.

Unusual Temperature-Dependent Behavior of the Bi2Sr2CaCu2O8+δEnergy Gap

The electronic structure close to the Fermi level of Bi2Sr2CaCu2O8+δwas studied in the superconducting state by high resolution angle-integrated, resonant photoemission spectroscopy as a function of temperature and submonolayer Au coverage. In addition to the opening of an energy gap below Tc, described by a reduced gap parameter 2Δ (20K)/kBTc ≈ 8, a relatively sharp density of states feature is discovered within the gap. Resonant enhancement at a photon energy of 18 eV indicates that this feature is oxygen-derived. Its relative spectral weight decreases with temperature, and also under submonolayer gold coverage. This suggests that it either originates from oxygen atoms in the Bi-O planes exposed at the cleaved (001) surface, or possibly the ‘apex’ oxygen atoms situated immediately below.