FN ISI Export Format VR 1.0 PT Journal AU Maurice, V Cadot, S Marcus, P TI XPS, LEED and STM study of thin oxide films formed on Cr(110) SO SURFACE SCIENCE AB The growth, thickness, composition and structure of chromium oxide thin films formed by exposing Cr(110) single-crystal surfaces to gaseous oxygen at 300 and 625 K have been investigated by XPS, LEED and STM measurements. The oxide films formed at the two temperatures are significantly different. At 300 K, a granular and non-crystalline oxide is formed, which grows with a constant similar to Cr2O3 stoichiometry up to a limiting thickness of 0.9 nm. The film is hydrated with a water content of 10-20%, which decreases upon annealing. Nuclei of oxide with a lateral dimension of similar to 0.7 nm and a height of similar to 0.2 nm have been observed in the nucleation stage. These nuclei grow predominantly laterally and coalesce to fully cover the substrate surface prior to the thickening stage. At 625 K, a first stage of oxygen adsorption is observed in which stripes 1.5-2.3 nm wide and parallel to the Cr[001] direction are observed after annealing in UHV. They correspond to narrow segments of mixed and close-packed planes of O atoms and ions having a geometry and orientation similar to those of the anions planes in the oxide crystals. Rows of adatoms, possibly Cr3+ ions of oxide nuclei, are observed above the stripes. Thickening at 625 K leads to the formation of a non-crystalline oxide, which grows up to a limiting thickness of 4.6 nm. The presence of Cr3+ vacancies related to a significant cation transport through the oxide film in this temperature regime is detected. After UHV annealing at 825 or 925 K, the film is anhydrous. The Cr3+ vacancies are accumulated at the metal/oxide film interface. The film crystallizes in epitaxy with the substrate in the following orientation: alpha-Cr2O3(0001)parallel to Cr(110) and alpha- Cr2O3[21 (3) over bar 0]parallel to Cr[001]. The STM measurements of the unit cell of the alpha-Cr2O3(0001) surface are consistent with a termination by a cation plane and show three tunneling sites assigned to the various possible locations of the Cr3+ ions at room temperature due to surface diffusion. (C) 2000 Elsevier Science B.V. All rights reserved. TC 5 BP 195 EP 215 PG 21 JI Surf. Sci. PY 2000 PD JUN 20 VL 458 IS 1-3 GA 329UP J9 SURFACE SCI UT ISI:000087927100019 ER PT Journal AU Bontempi, E Depero, LE Sanagaletti, L TI X-ray reflectivity and glancing-incidence diffraction from thin metallic Cr layers SO PHILOSOPHICAL MAGAZINE B-PHYSICS OF CONDENSED MATTER STATISTICAL MECHANICS ELECTRONIC OPTICAL AND MAGNETIC PROPERTIES AB The annealing effects on thin Cr layers deposited by rf sputtering are investigated by X-ray reflectivity and glancing- incidence diffraction. It is shown that annealing induces a phase segregation which dramatically changes the features of the X-ray reflectivity pattern. This study allows us to present and discuss the main features of X-ray reflectivity, which make it a powerful tool for the non-destructive analysis of thin single layers or multilayers and interfaces. TC 2 BP 623 EP 633 PG 11 JI Philos. Mag. B-Phys. Condens. Matter Stat. Mech. Electron. Opt. Magn. Prop. PY 2000 PD APR VL 80 IS 4 GA 303TP J9 PHIL MAG B UT ISI:000086441200018 ER PT Journal AU Stierle, A Koll, T Zabel, H TI Structure and defects of epitaxial Cr2O3(0001) overlayers on Cr(110) SO PHYSICAL REVIEW B AB The structure and defects of thin epitaxial Cr2O3(0001) overlayers on Cr(110) films have been investigated by grazing incidence x-ray diffraction. Cr(110) films were prepared by molecular beam epitaxy before exposing them to a controlled oxygen atmosphere in the temperature range 400-800 K. The experiments reveal over the whole temperature range the same in-plane orientational relationship between Cr2O3(0001) and Cr(110), which is a Nishiyama-Wassermann-type alignment between the oxygen sublattice and the Cr(110) surface. Out-of-plane measurements demonstrate the presence of growth and deformation faults within the Cr-ion and oxygen-ion sublattices. At higher temperatures mainly twin formation takes place, whereas at lower temperatures a random stacking of the Cr-ion layers prevails. We propose that the twin grain boundaries serve as diffusion channels for Cr ions during the oxidation. TC 6 BP 5062 EP 5069 PG 8 JI Phys. Rev. B PY 1998 PD AUG 15 VL 58 IS 8 GA 117GE J9 PHYS REV B UT ISI:000075772500120 ER PT Journal AU Renaud, G TI Oxide surfaces and metal/oxide interfaces studied by grazing incidence X-ray scattering SO SURFACE SCIENCE REPORTS AB Experimental determinations of the atomic structure of insulating oxide surfaces and metal/oxide interfaces are scarce, because surface science techniques are often limited by the insulating character of the substrate. Grazing incidence X- ray scattering (GIXS), which is not subject to charge effects, can provide very precise information on the atomic structure of oxide surfaces: roughness, relaxation and reconstruction. It is also well adapted to analyze the atomic structure, the registry, the misfit relaxation, elastic or plastic, the growth mode and the morphology of metal/oxide interfaces during their growth, performed in situ. GMS also allows the analysis of thin films and buried interfaces, in a non-destructive way, yielding the epitaxial relationships, and, by variation of the grazing incidence angle, the lattice parameter relaxation along the growth direction. On semi-coherent interfaces, the existence of an ordered network of interfacial misfit dislocations can be demonstrated, its Burger's vector determined, its ordering during in situ annealing cycles followed, and sometimes even its atomic structure can be addressed. Careful analysis during growth allows the modeling of the dislocation nucleation process. This review emphasizes the new information that GIXS can bring to oxide surfaces and metal/oxide interfaces by comparison with other surface science techniques. The principles of X-ray diffraction by surfaces and interfaces are recalled, together with the advantages and properties of grazing angles. The specific experimental requirements are discussed. Recent results are presented on the determination of the atomic structure of relaxed or reconstructed oxide surfaces. A description of results obtained during the in situ growth of metal on oxide surfaces is also given, as well as investigations of thick metal films on oxide surfaces, with lattice parameter misfit relaxed by an array of dislocations. Recent work performed on oxide thin films having important physical properties such as superconductivity or magnetism is also briefly reviewed. The strengths and limitations of the technique, such as the need for single crystals and surfaces of high crystalline quality are discussed. Finally, an outlook of future prospects in the field is given, such as the study of more complex oxide surfaces, vicinal surfaces, reactive metal/oxide interfaces, metal oxidation processes, the use of surfactants to promote wetting of a metal deposited on an oxide surface or the study of oxide/liquid interfaces in a non-UHV environment. (C) 1998 Elsevier Science B.V. All rights reserved. TC 60 BP 1 EP + PG 89 JI Surf. Sci. Rep. PY 1998 VL 32 IS 1-2 GA 115EX J9 SURF SCI REP UT ISI:000075651500001 ER PT Journal AU Theis-Brohl, K Zoller, I Bodeker, P Schmitte, T Zabel, H Brendel, L Belzer, M Wolf, DE TI Temperature- and rate-dependent RHEED oscillation studies of epitaxial Fe(001) on Cr(001) SO PHYSICAL REVIEW B AB Reflection high-energy electron diffraction (RHEED) intensity studies were performed during the growth of thin Fe layers on vicinal Cr(001)/Nb(001)/Al2O3(1 (1) over bar 02) substrates. The results are compared with those of recent molecular-beam epitaxy (MBE) growth models. General agreement is found as concerns the linear relationship between the logarithm of the number of RHEED oscillations and the inverse growth temperature. In agreement with theory the RHEED oscillation damping time is found to depend algebraically on the growth rate. However, contrary to expectations, the RHEED oscillations vanish faster at higher growth temperatures and lower growth rates. This behavior can be explained by a change in the growth mode from layer-by-layer to step flow. Numerical simulations in which step bunch melting during the Fe growth on the Cr buffer is assumed reproduce well the present experimental results. [S0163-1829(98)02708-8]. TC 7 BP 4747 EP 4755 PG 9 JI Phys. Rev. B PY 1998 PD FEB 15 VL 57 IS 8 GA YZ177 J9 PHYS REV B UT ISI:000072228000079 ER PT Journal AU Schad, R Bahr, D Falta, J Belien, P Bruynseraede, Y TI Structural analysis of Fe/Cr superlattices and their components SO JOURNAL OF PHYSICS-CONDENSED MATTER AB Structural analysis of metallic superlattices is essential for understanding their interesting transport properties like the giant magnetoresistance (GMR) effect. We present a detailed analysis of the [Fe (4.8 Angstrom)/Cr (13 Angstrom)](50) superlattice which shows a record GMR value. The analysis is done by simulating its x-ray diffraction spectrum measured with a synchrotron x-ray source. The ingredients for this simulation are obtained from simulations of x-ray diffraction spectra of single Fe and Cr films. This way we reduce the number of free parameters resulting in reliable values for the interfaces roughness. We also show that x-ray diffraction spectra of systems with low contrast in electron density can be quantitatively analysed. TC 6 BP 61 EP 66 PG 6 JI J. Phys.-Condes. Matter PY 1998 PD JAN 12 VL 10 IS 1 GA YV398 J9 J PHYS-CONDENS MATTER UT ISI:000071819000007 ER PT Journal AU Muller, M Oechsner, H TI In situ STM and AES studies on the oxidation of Cr(110) SO SURFACE SCIENCE AB A chromium(110) surface was exposed to various doses of oxygen at substrate temperatures between 50 and 880 degrees C. The resulting oxygen concentration was determined by AES and the respective surface topography imaged by in situ STM. Depending on the temperature at constant exposure conditions, the oxygen concentration displays a maximum around 380 degrees C and a pronounced minimum around 760 degrees C. For oxygen exposure at fixed temperatures, at first different surface phases are formed with a stretched hexagonal symmetry at 380 degrees C and a relaxed one above 700 degrees C. In this stage the oxygen concentration increases only slightly. With ongoing exposure, characteristic stripes with an initial distance around 23 Angstrom occur with an orientation parallel to the [001] direction of the Cr substrate. At 380 degrees C, stripe formation starts at arbitrary places on the terraces, and above 700 degrees C mainly at the steps of the surface. In that stage a rapid increase of the oxygen uptake is observed. The formation of regularly arranged oxide stripes is explained by a model which is based on the interference pattern of a hexagonal oxygen layer as in Cr2O3 with the Cr(110) surface. When the oxygen exposure is further increased, the stripes grow together cover the entire terraces and pile up one upon another in a parallel staggered arrangement. The thickening of the initial oxide stripes is also discussed in some detail. (C) 1997 Elsevier Science B.V. TC 2 BP 269 EP 278 PG 10 JI Surf. Sci. PY 1997 PD OCT 8 VL 387 IS 1-3 GA YC848 J9 SURFACE SCI UT ISI:A1997YC84800034 ER PT Journal AU Stierle, A Zabel, H TI Oxidation of ultrathin Fe(110) layers on Cr(110) SO SURFACE SCIENCE AB We have investigated the oxidation of a 1.3 nm thick (110)- oriented Fe film grown epitaxially on Cr(110) at 708 K. The oxidation was characterized by X-ray reflectivity measurements before and after oxidation in an ultrahigh vacuum chamber. The final oxide thickness was larger than expected if only the Fe layer had been oxidized. Auger electron spectroscopy and surface X-ray diffraction indicate that on top of the Fe-oxide layer, an additional Cr2O3 layer with [0001] orientation was formed. This provides evidence that the Cr-metal ion is the diffusing species during oxidation. In contrast, a 2.2 nm thick Cr cap layer on Fe(110) transforms at 620 K into a 4.6 nm thick Cr2O3(0001) film which protects the Fe layer below against oxidation. This difference is due to the fact that Fe ion diffusion through the Cr-oxide layer is more difficult than through the Fe-oxide layer because of the smaller defect density in the former. (C) 1997 Elsevier Science B.V. TC 6 BP 310 EP 317 PG 8 JI Surf. Sci. PY 1997 PD AUG 10 VL 385 IS 2-3 GA YB535 J9 SURFACE SCI UT ISI:A1997YB53500025 ER PT Journal AU Stierle, A Zabel, H TI Oxidation induced roughening during Cr2O3(0001) growth on Cr(110) SO SURFACE SCIENCE AB X-ray reflectivity measurements on the oxidation of epitaxial Cr(110) films are reported, providing the roughness for the oxide surface and metal/oxide interface as a function of the oxidation temperature. Up to the temperature of T-R similar to 725 K the oxide film thickness in quasi-equilibrium increases linearly with temperature and the oxide surface and metal/oxide interface remain smooth on the order of 0.2-0.4 nm. At higher temperatures the oxide surface roughens and cavities are formed at the metal/oxide interface. The transition from a layer-by- layer like to a three-dimensional island-like growth mode can be explained in terms of epitaxial growth models. At the roughening temperature the Mott-field aided diffusion current of the metal ions through the oxide layer exceeds the surface diffusion current, resulting in a rough oxide surface. (C) 1997 Elsevier Science B.V. TC 8 BP 167 EP 177 PG 11 JI Surf. Sci. PY 1997 PD AUG 1 VL 385 IS 1 GA XZ720 J9 SURFACE SCI UT ISI:A1997XZ72000018 ER PT Journal AU Stierle, A Zabel, H TI Kinetics of Cr2O3 growth during the oxidation of Cr(110) SO EUROPHYSICS LETTERS AB We have studied the oxidation of single crystalline Cr(110) films prepared by molecular beam epitaxy (MBE) in the temperature range from 400 K to 700 K. Epitaxial alpha-Cr2O3 layers in [0001] orientation are formed over the whole temperature range. We monitor the oxidation by X-ray reflectivity measurements providing information on the oxide layer thickness, its defect density and the roughness of the oxide surface and the metal/oxide interface for different times and temperatures. After a rapid increase of the oxide thickness in the early growth stage, we observe a saturation of the oxide thickness. This saturation thickness depends markedly on the oxidation temperature, indicative for an oxidation process which is controlled by the Cr-ion diffusion. The activation energy for the cation diffusion is determined to be 1.4 eV, which is smaller by a factor of two as compared to the value found for bulk interstitial diffusion. TC 12 BP 365 EP 370 PG 6 JI Europhys. Lett. PY 1997 PD FEB 10 VL 37 IS 5 GA WK109 J9 EUROPHYS LETT UT ISI:A1997WK10900009 ER