The oxidation of the (100) surface of the intermetallic alloys Ni$_{3}$Al and CoAl and the growth of Co on the clean and oxidized N$_{3}$Al(100) surface

The aim of this work was the preparation and characterization of thin Al-oxide films an the (100) surface of the intermetallic compounds Ni$_{3}$Al and COAT, as well as the study of Co growth an the clean and oxidized Ni$_{3}$Al(100) surface. The films were characterized by Auger electron spectroscopy (AES), Low Energy Electron Diffraction (LEED), Electron Energy Loss Spectroscopy (EELS) and Scanning Tunneling Microscopy (STM). $\textbf{The clean Ni$_{3}$Al(100) surface}$ The LEED pattern of clean Ni$_{3}$Al(100) Shows a (1 x 1) structure. STM images of the Ni$_{3}$Al(100) surface display flat and large terraces (500 - 1000 A) separated by steps with a step height of 3.5 $\mathring{A}$ which corresponds to the lattice constant of Ni$_{3}$Al and represents a double atomic step. This suggests that different terraces have always the Same termination. $\textbf{Co/Ni$_{3}$Al(100)}$ At low coverage (0 .1 ML) and 300 K, the cobalt deposited an the Ni$_{3}$Al(100) surface shows a two-dimensional growth mode. For deposition of 0.3 ML the nucleation takes also place in the second layer. After deposition of 3.5 ML Co, the surface consists of Co islands with a mean diameter of $\sim$ 90$\mathring{A}$. Annealing at 700 K leads to the growth of large terraces of fcc-Co which are arranged with the (100) plane parallel to the (100) surface of the Substrate. Co is stable an Ni$_{3}$Al(100) up to 750 K when it starts to diffuse into the substrate. At 1100 K, Co is disappeared completely from the surface via diffusion into the Substrate. $\textbf{Al$_{2}$O$_{3}$/Ni$_{3}$Al(100)}$ At room temperature oxygen adsorption an Ni$_{3}$Al(100) leads to the formation of a thin amorphous Al-oxide (a-Al$_{2}$O$_{3}$) layer ($\sim$ 5$\mathring{A}$). Oxidation at 1100 K leads to formation of a well ordered $\gamma'$-Al$_{2}$O$_{3}$ film with a thickness of $\sim$ 10$\mathring{A}$. The STM images of the completely oxide-covered surface exhibit hexagonal superstructures with lattice constants of 18, 24 and 54 $\mathring{A}$. The band gap of Al$_{2}$O$_{3}$ formed an Ni$_{3}$Al(100) amounts for the amorphous film to $\sim$ 3.2 and to $\sim$ 4.3 eV for the well ordered Al$_{2}$O$_{3}$ film, respectively and both are strongly diminished with respect to the bulk values. $\textbf{Co/Al$_{2}$O$_{3}$/Ni$_{3}$Al(100)}$ Co deposited at room temperature an a Al$_{2}$O$_{3}$ film, which was grown an Ni$_{3}$Al(100) at 1100 K, Shows a three dimensional (Volmer-Weber) growth mode. After a nominal deposition of 30 $\mathring{A}$ the 3D cobalt clusters have a mean diameter of 70 $\mathring{A}$ and a roughness of $\sim$ 10 $\mathring{A}$. Annealing at 700 and 900 K leads to a coalescence of the Co clusters, and to a gradually diffusion of Co through the oxide into the substrate. After annealing at 1000 K the entire surface of alumina is Co free. $\textbf{Al$_{2}$O$_{3}$/CoAl(100)}$ Oxygen adsorption at 300 K leads to the formation of an amorphous Al$_{2}$O$_{3}$ film an CoAl(100). Annealing at temperatures between 800 - 1000 K induces a phase transformation from a-Al$_{2}$O$_{3}$ into the $\theta$-Al$_{2}$O$_{3}$ phase, which exhibits a (2x1) structure with respect to the substrate. After annealing at temperatures > 1200 K a transition to $\alpha$-Al$_{2}$O$_{3}$ occurs, while above 1300 K the decomposition and removal of the oxide film from the surface is observed