Observation of the phase-boundary controlled formation of NiAl<inf>2</inf> O<inf>4</inf> from a single-crystal NiO thin film and single-crystal αAl<inf>2</inf>O<inf>3</inf> substrate

High-quality NiO thin films have been grown on single-crystal α-Al2O3 substrates (0001) orientation by pulsed-laser ablation, forming essentially the idealized solid-state reaction geometry-a single-crystal film in intimate contact with a single-crystal substrate. These reaction couples have been characterized by cross-section transmission electron microscopy and scanning electron microscopy both before and after being heated in air to induce the solid-state reaction (i.e., Ni-spinel formation). The NiO films consisted of two twin variants which were found to conform to the underlying substrate surface steps. The substrate surface steps were produced by heat-treating the substrates prior to thin film deposition. Using this reaction geometry, it has been found that the initial reaction of the spinel takes place where twin boundaries in the NiO films meet the substrate. The initial reaction corresponds to the nucleation of the spinel. This interpretation is supported by the fact that the reaction proceeded faster up the NiO twin boundaries than elsewhere along the reaction layer (i.e., nucleation of the spinel is easier at twin boundaries in the NiO film). Scanning electron microscopy has been used with the present thin-film reaction geometry to measure reaction layer width along interfaces up to 2 mm long.