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

Abstract

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 À4 -10 À2 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 AE 0.1 Å .

Interface thermodynamics of ultra-thin, amorphous oxide overgrowths on AlMg alloys

“…Results obtained in the present research project [15] demonstrate that ultra-thin (<3 nm) oxide films grown on AlMg alloy surfaces by thermal oxidation at T < 450 K are amorphous and of uniform thickness. Therefore, in the following treatment, the focus is on the growth of a homogeneous amorphous oxide film, {oxide}, of uniform thickness, h {oxide} , on a single-crystalline AlMg alloy substrate, hAlMgi, by e.g.…”

“…8. It follows that h crit fMgOg decreases from about 5 nm at T $ 690 K to about 2 nm at T $ 900 K. However, at temperatures T > 700 K and thicknesses > 1 nm, the crystalline modification of the oxide overgrowth is preferred [9,12,13,15], which is not considered here.…”

“…G cell Fig. 4; not regarding the possible formation of corresponding crystalline oxide phases for thicker films formed at elevated temperatures T > 500 K [5,[11][12][13]15,21], see further below Fig. 8.…”

“…without a native oxide) AlMg substrates at T < 450 K have been shown recently to be amorphous as well and to contain both Al and Mg in an oxidized chemical state by high-resolution transmission electron microscopy (HR-TEM) analysis [15] and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) [9,15,16]. For thicker oxide films grown at more elevated temperatures (i.e.…”

“…For thicker oxide films grown at more elevated temperatures (i.e. T > 450 K), these amorphous oxide films gradually transform into bulk-thermodynamically preferred crystalline-MgO [9,15].…”