The Growth of Metal Overlayers on Oxide Surfaces.

The structural and chemical properties of metals on oxides have been analyzed by examining two very different adsorption systems. The first, Ni/SiO_2 , is representative of systems with a rather weak interaction between the metal overlayer and oxide substrate. The second system analyzed, Ti/TiO_2, is an example with very strong reactive adsorbate/substrate interaction. The surface and interface behavior of both systems are investigated using basic thermodynamic and kinetic concepts. The surface diffusion of nickel on a thermally -grown silicon dioxide thin film (5-50A), and the bulk diffusion of Ni through the SiO_2 film into the single crystal silicon substrate have been studied by x-ray photoelectron spectroscopy (XPS), high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). Nickel agglomeration on the oxide occurs in the 100-850K regime, while bulk Ni diffusion through the thin oxide layer occurs in the 700-1050K regime. The onset of bulk Ni diffusion is dependent on oxide thickness; thicker oxides reduce the rate of Ni penetration. Above 950-1100K, the oxide desorbs leaving nickel disilicide on silicon. The study of nickel disilicide island formation on Si(111) (an outgrowth of the Ni/SiO_2 experiments) is reported. The kinetics of this system control a rather interesting series of metastable growth structures. Visually striking NiSi_2 crystallites are observed on the Si(111) surface by AFM. The nickel disilicide islands coalesce following a high temperature anneal (~1260K). The islands differ from those formed at lower temperature in both shape and orientation. These differences are explained by kinetically limited growth accompanying phase and surface segregation of Ni from the bulk silicon wafer, and condensation of a Ni-rich NiSi_{rm 2-x} liquid phase at the surface. Condensation from the liquid phase to NiSi_2 is concluded to be responsible for the structure of the crystallites. High temperature growth conditions lead preferentially to A -type (non-twinned) silicide islands. The adsorption of titanium on titanium dioxide, TiO_2 (110), is studied with XPS, HREELS, and low energy ion scattering (LEIS). XPS data indicate that 4A of Ti deposited at 150K reacts with the substrate to produce about 12A of intermediate oxidation state Ti. HREELS data point toward the formation of a continuous conducting overlayer after only 4A of Ti deposition, even though LEIS data show 20A of Ti is needed to completely cover the oxide. Ar^{+} bombardment of TiO_2 reduces the surface through preferential sputtering of O resulting in a reduced overlayer with an average thickness of 17A.