S. Ladas

Oxidation of thin erbium and erbium silicide overlayers in contact with silicon oxide films thermally grown on silicon

Pure Er and co-evaporated Er and Si layers were deposited near room temperature in UHV on SiO2 films grown on Si(100) wafers and were subsequently annealed at increasing temperature up to 1153 K. The samples were characterized in situ by X-ray photoelectron spectroscopy following deposition and each annealing step. The co-evaporated samples were also post-examined by Rutherford backscattering spectroscopy after the final annealing. The results show that both the Er and the ErSix adlayers react readily with the SiO2 upon increasing temperature to give Er2O3, silicon suboxides and elemental silicon. The erbium oxide remains stable up to 1073 K and then transforms back to erbium silicide with a simultaneous loss of oxygen from the surface via the volatile SiO. This behavior is rationalized in terms of a number of solid phase reactions taking place in the overlayer.

SEMICONDUCTING RHENIUM SILICIDE THIN FILMS ON SI(111)

The crystallographic, electronic, and optical properties of thin ReSi2 films (∼20–300 A) have been investigated in situ by low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS), ex situ by glancing incidence x‐ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650u2009°C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed by in situ surface techniques. For very thin films (≲35 A) LEED shows a faint (1×1) pattern after annealing at 750u2009°C, which improves slightly up to ∼900u2009°C. For thick films (∼50–300 A) only a bright background is observed. XPS indicates that the ReSi2 composition is attained upon annealing at 600u2009°C. In the Re‐Si bonding the charge transfer is negligible: the energy positions of the corelevels (Siu20092p and Reu20094f) are the same in the compound and in the elements. As the ener...

Ni/NiO(0 0 1) interface studied by X-ray photoelectron spectroscopy and molecular dynamics simulations

X-ray photoelectron spectroscopy (XPS) results referring to the interface formed upon vapor deposition of Ni atoms on NiO(0 0 1) indicate, that upon annealing the deposited Ni is oxidized forming a NiO overlayer. The oxidation process is activated and becomes very fast above 900 K, resulting in a surface that exhibits identical photoelectron spectrum with the clean substrate. In addition, it was found that during the oxidation process two extra components in the O 1s XPS peak spectra appear, with binding energies above and below the main lattice oxygen peak, manifesting the existence of oxygen atoms carrying charges that deviate from their formal ionic value. Moreover, by molecular dynamics simulations it was found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming a series of NixOy oxides, most of which eventually coalesce into small NiO islands. Consequently, the lower binding energy component of the O 1s XPS peak may be attributed to intermediate oxides with x > y, while the higher binding energy component to intermediate oxides with x < y. # 2003 Elsevier Science B.V. All rights reserved.

X-ray photoelectron spectroscopy study of rhodium particle growth on different alumina surfaces

Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate Rh particle formation on Al2O3 substrates. Rhodium was deposited in situ by electron beam evaporation on single crystalline alumina specimens which had been cleaned by heating in air. The (0001), (1102) and (1120) orientations of alumina were used with no further treatment. In addition, these substrates were also Ar+-ion sputtered before deposition, to modify the surface structure. The Rh growth was monitored by means of the modified Auger parameter (MAP), the binding energy (BE) and the Full Width at Half Maximum (FWHM) of the Rhxa03d5/2 peak. The results for the very early stages of growth on the six substrates exhibit differences which are also reflected in the charge transfer between Rh and alumina, as obtained from the MAP data. Differences in the charge transfer are explained by details of the substrate nature which might differ on individual samples.

Probing the properties of atomic layer deposited ZrO2 films on p-Germanium substrates

Zirconium oxide (ZrO2) thin films of 5 and 25u2009nm thickness were deposited by atomic layer deposition at 250u2009°C on p-type Ge substrates. The stoichiometry, thickness, and valence band electronic structure of the ZrO2 films were investigated by x-ray and ultraviolet photoelectron spectroscopies. For the electrical characterization, metal-oxide-semiconductor (MOS) capacitive structures (Pt/ZrO2/p-Ge) have been fabricated. Capacitance–voltage and conductance–voltage (C–V, G–V) measurements performed by ac impedance spectroscopy in the temperature range from 153 to 313u2009K reveal a typical MOS behaviour with moderate frequency dispersion at the accumulation region attributed to leakage currents. For the determination of the leakage currents conduction mechanisms, current density–voltage (J–V) measurements were carried out in the whole temperature range.

Study of CO adsorption on Rh/alumina model catalysts in dependence on substrate orientation

Abstract The interaction of a CO molecular beam with rhodium vapour-deposited onto (0001), (1–102) and (11–20) α-alumina was investigated. The dependence of the CO sticking coefficient on the sample temperature and the relative CO coverage for different Rh deposits were calculated from the intensity of the reflected CO molecular beam. The diffusion of CO over the uncovered substrate to the metal is discussed in relation to the Rh particle size and density.

The interaction of carbon monoxide with Rh/Al2O3 model catalysts: influence of the support structure

Thermal desorption spectroscopy (TDS) has been used to investigate the interaction of CO with rhodium particles evaporated by electron bombardment onto differently oriented single-crystal alumina supports. Rhodium particle populations of similar particle size and density were deposited onto the (0001), (1-102) and (11-20) α-alumina substrates. The CO TDS results depend on the crystallographic orientation of the support. The CO desorption peak shapes and activation energies, as well as the CO2 production which accompanies CO desorption, are compared. The data set obtained enables a discussion of the influence of the alumina support structure on the adsorptive and catalytic activity of the rhodium particles.

ZrO2 and Al2O3 Thin Films on Ge(100) Grown by ALD: An XPS Investigation

Thin films of aluminium oxide (Al2O3) and zirconium oxide (ZrO2) were prepared by Atomic Layer Deposition (ALD) on p-type (100) germanium substrates. In the present work a detailed analysis of the films by X-ray photoelectron spectroscopy (XPS) in order to investigate their chemical composition is presented. This study is dedicated to an XPS investigation of the principal core levels (Al, Zr, O) of Al2O3 and ZrO2 thin films. In particular, wide scan spectra, detailed scans for the Zr 3d, Al 2p, O 1s, and C 1s regions and related data for zirconia and alumina films are presented and discussed. The results point out the formation of Al2O3 and ZrO2 films with the presence of OH groups and carbon contamination on the surface.

Identification of different surface species of NO adsorbed on Ru(0001) with NEXAFS

The molecular adsorption states of NO on Ru(0001) have been investigated using near-edge X-ray absorption fine structure (NEXAFS) measurements of the oxygen and nitrogen K-edge. Four different molecular states were identified by the σ-resonance position. Assuming that a linear relation between the variation of this resonance position and the change in the intramolecular bond length holds, values for the NO bond lengths in the various molecular adsorption states were derived.

Characterization of ex-situ hydrogenated amorphous SiC thin films by X-ray photoelectron spectroscopy

Abstract The X-ray photoelectron spectroscopy (XPS) characterization of an ex-situ hydrogenated amorphous SiC film shows the following main features for the as-received specimen: (1) carbidic Si2p and C1s states with binding energies (BE) 100.4 ± 0.1 and 282.9 ± 0.1 eV respectively, (2) oxidic Si2p and O1s states with BE 103.2 ± 0.1 and 531.9 ± 0.1 eV respectively, (3) a strong C1s state at BE 286.8 ± 0.1 eV, (4) an O1s state with BE 533.0 ± 0.1 eV, and (5) a small adventitious carbon signal. These results, combined with the corresponding data on the as-grown film before hydrogenation and with the changes accompanying controlled Ar + sputtering, lead to the following model for the film surface: About two monolayers of adventitious carbon cover a 15 A thick, oxygen-containing carbon layer which in turn overlays a 25 A thick SiO 2 layer in contact with the bulk SiC. A good part of the oxidic layer and the oxygen in the carbon layer appear to have been inadvertently produced during the hydrogenation process.