The amorphous to crystalline transition of ultrathin (Al,Mg)-oxide films grown by thermal oxidation of AlMg alloys: A high-resolution transmission electron microscopy investigation

The microstructural evolution of ultrathin (<3 nm) oxide films grown on bare Al-based AlMg alloy substrates, by thermal oxidation in the temperature range of 300 to 610 K and at partial oxygen pressures in the range 10⁻⁴–10⁻² Pa, was investigated by high-resolution transmission electron microscopy. Angle-resolved x-ray photoelectron spectroscopy was applied to establish the chemical constitution of the analyzed oxide films (i.e., the overall Al/Mg cationic ratio, as well as the relative depth distributions of Al and Mg in the grown oxide films). The ˜0.8-nm-thick (Al,Mg)-oxide film grown at 300 K is fully amorphous. A gradual development of long-range order in the oxide film sets in for thickening (Al,Mg)-oxide films of relatively high Mg content at T ≥ 475 K. The amorphous-to-crystalline transition proceeds by a phase separation: still predominantly amorphous oxide regions exist next to crystallized oxide regions, which are constituted of an MgO-type of oxide phase with a face-centered-cubic oxygen sublattice and an average lattice parameter of 4.146 ± 0.1 Å.