Photoemission Spectroscopy Spectra Research Articles

Electronic structure and electron transport properties of amorphous CaAlGa and CaMgGa alloys

Both electronic structure and electron transport properties of the amorphous CaAlGa and CaMgGa alloys have been studied by measurements of the low temperature specific heat, the resistivity and the X-ray photoemission spectroscopy (XPS) spectra. The electrical resistivity ϱ 300 K at 300 K and the electronic specific heat coefficient γ exp are found to increase with increasing Ga concentration in both amorphous Ca 60Al 40− x Ga x and Ca 60Mg 40− x Ga x (0 ⩽ x ⩽ 30) alloy systems. The XPS valence band spectra clearly showed that the introduction of Ga atoms results in the split band structure in both systems. We explain that the Al(Mg) 3s states and Ga 4s states are probably isolated from the p states, which form a narrow band across the Fermi level. This unique electronic structure gives rise to a resistivity at 300 K higher than 600 μω cm, while prossessing a large electronic specific heat coefficient exceeding 5 mJ mol −1 K −2. This results in an electron diffusion coefficient as small as 0.19 m 2 s −1, the smallest value so far found in amorphous alloys.

Electronic and atomic structure of Al xLa 70− xNi 30 amorphous alloys

X-ray photoemission spectroscopy (XPS) spectra, electrical resistivity, optical reflectivity and extended X-ray absorption fine structure (EXAFS) of Al x La 70− x Ni 30 (0≤ x ≤ 40) amorphous alloys have been measured. The XPS spectra show that the peak of the Ni 3d-band lies at a higher binding energy and has a narrower width than that of pure Ni, as a result of the hybridization of the Ni 3d-band with La and Al bands. The density of states at the Fermi level, N( E F), decreases as the Al content increases. The decrease of N( E F) is consistent with the results of the electrical resistivity. From the reflectivity spectra and EXAFS data, it was found that above x = 30 at.% the hybridization of Ni and Al becomes dominant rather than that of Ni and La, implying that the interaction between Ni and Al is comparatively strong. These results have been discussed in relation to chemical short-range order.

Theory of core level X-ray photoemission and photoabsorption in Ti compounds

We analyze the Ti2p X-ray photoemission spectroscopy (XPS) and photoabsorption spectroscopy (XAS) spectra in formally tetravalent Ti compounds such as TiO 2, SrTiO 3 and BaTiO 3, by means of a TiO 6 cluster model, paying particular attention to the interplay between the Ti3dO2p hybridization and the intra-atomic multiplet coupling. We also show that the satellite peaks observed in the Ti2p XPS and XAS spectra can be understood as charge transfer satellites, although they have until now been ascribed to the exciton satellite which originates from the excitation from the O2p to O3s states.

Calculation of Carbonaceous Overlayer Thickness in XPS to Yield Substrate Stoichiometries

A method has been developed which calculates the thickness of the hydrocarbon overlayer from x-ray photoemission spectroscopy (XPS) spectra. The method is specific to each sample and utilizes the relative intensity of the C 1 s peak and all other peaks. Attenuations calculated with this overlayer thickness yield more accurate substrate stoichiometries. A rough-sample model is also developed which not only explains previous anomalies in the thickness of hydrocarbon overlayer calculated but is capable of yielding further accuracy in substrate stoichiometries.

Electronic structure and electron transport properties of Al-based (Ni67X33)1-xAlx(X identical to Ti, Zr and La) amorphous alloys

A large number of (Ni67X33)1-xAlx(X identical to Ti, Zr and La; 0<or=x<or=0.85) ternary amorphous alloys have been synthesised by the melt-spinning technique in combination with the sputtering method as a supplementary tool. The electronic structure was studied through measurements of the low-temperature specific heats and the X-ray photoemission spectroscopy (XPS) spectra. For comparison, the amorphous Al100-x-ySixNiy (10<or=x<or=35, 10<or=y<or=20) alloys were also prepared and their valence band structure was studied by XPS and soft-X-ray spectroscopy. As electron transport properties, the authors measured the electrical resistivity, the Hall coefficient and the thermoelectric power over a wide temperature range below 300 K. They revealed that the electronic structure at the Fermi level changes from the d- to sp-electron-dominant states, as the Al content increases across x=0.6. This drastic change in the electronic structure is found to be reflected in various electron transport properties and the crystallisation temperature. A reversal in the sign of the Hall coefficient and the change in the characteristic feature of the temperature dependence of the electrical resistivity have been discussed in relation to the electronic structure involved.

Removal of surface relaxation of Cu(110) by hydrogen adsorption

It is now established that the termination of a metal crystal by a vacuum causes oscillatory relaxation of the interplanar distances near the surface, even when there is no lateral reconstruction. Since the details of the driving forces for this oscillatory relaxation are not fully understood at the present time, we have undertaken low-energy electron diffraction (LEED)–IV experiments and calculations to study the structure of(1×1) Cu(110) as a function of atomic hydrogen concentration adsorbed at 90 K. At this temperature high-resolution electron energy-loss spectroscopy reveals that the hydrogen lies in inhomogeneous sites within the (110) troughs. At most coverages LEED indicates that the hydrogen is disordered. Hydrogen induced changes in the work function and in angle-resolved ultraviolet photoemission spectroscopy spectra are small, indicating a weak interaction between the hydrogen and copper substrate. This one-dimensional lattice gas causes a continuous shift in the Cu(110) interplanar distances, resulting eventually in a bulk lattice spacing between the top two layers. Subsurface lattice spacings change at a slower rate. We conclude that the simple model of an electrostatic driving force created by a lateral charge redistribution [see M. W. Finnis and V. Heine, J. Phys. F 4, L37 (1974)] can explain the nature of the experimental results. The adsorption of hydrogen reduces the charge-density corrugation of the surface, and eliminates the charge redistribution and related surface relaxation of the copper substrate.

Cobalt disilicide epitaxial growth on the silicon (111) surface

Initial stages of Co${\mathrm{Si}}_{2}$ formation on the silicon (111) surface were investigated by low-energy electron diffraction (LEED), $x$-ray photoemission spectroscopy (XPS), and angle-resolved ultraviolet photoemission spectroscopy (ARUPS) techniques under ultrahigh-vacuum conditions. At room temperature, a small amount of Co evaporated onto Si reacts strongly with Si atoms to form cobalt silicide. With increasing coverage ($\ensuremath{\Theta}&gt;4$ monolayers) results from surface techniques indicate an enrichment of the metal in the probed region and the ultraviolet photoemission spectroscopy spectrum resembles that of a Co bulk metal. The behavior of these metal-silicon interfaces with annealing at \ensuremath{\sim}600\ifmmode^\circ\else\textdegree\fi{}C under ultrahigh-vacuum conditions was also examined. In the submonolayer range, two LEED superstructures, $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ and 2\ifmmode\times\else\texttimes\fi{}1, are found, while at higher coverage the formation of an epitaxial Co${\mathrm{Si}}_{2}$ film can be achieved. First experimental ARUPS measurements of the interface are presented and compared to very recent calculations.