Growth and microstructural stability of epitaxial Al films on (0001) α-Al2O3 substrates Original Research Article

Al films were grown epitaxially on single-crystal α-Al2O3 substrates by magnetron sputtering and molecular beam epitaxy, respectively. The microstructure and thermal stability of these films were analysed in detail using X-ray diffraction methods and electron microscopy techniques. The films consist of two twin-related growth variants, related by a 180° rotation around the <111> film normal resulting in a {111} Al || (0001) α-Al2O3, and ± <1̄10> Al || <101̄0> α-Al2O3 orientation relationship. The Al variants are separated by Σ3 {21̄1̄} Al twin boundaries possessing a rigid body translation of the {111} Al planes across the boundary plane in order to reduce their energy. Motion of the twin boundaries was observed by annealing plan-view samples in situ in a transmission electron microscope. The twin boundaries advance in jerky motion at velocities of several μm/s at temperatures of ~400 °C, resulting in grain coarsening. In all cases, heat treatments resulted in increased area fraction of one twin variant, which finally will result in single-crystal films upon further annealing.