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

Al films were grown epitaxially on single-crystal alpha-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 180degrees rotation around the <111> film normal resulting in a I I I I I Al parallel to (0001) alpha-Al2O3, and +/- <(1) over bar 10> Al parallel to <10 (1) over bar0> alpha-Al2O3 orientation relationship. The Al variants are separated by Sigma3 {2 (1) over bar(1) over bar} 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 mum/s at temperatures of similar to400degreesC, 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. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.