Annealing induced void formation in epitaxial Al thin films on sapphire (α-Al2O3)

In this work faceted voids are studied which were induced by solid state dewetting at 600 °C of tetracrystalline Al thin films covered with a native oxide layer. Hexagonally shaped voids are observed in a few locations where Al is uniformly redistributed to the surrounding thin film. Although faceted, the majority of the voids exhibit irregular shapes caused by pinning of distinct sides of the retracting Al thin film. The two different Al|void shapes (hexagonal or irregular) are investigated by site-specific cross-sectional transmission electron microscopy (TEM) analysis. The TEM studies reveal Al|void regions with and without rims and ridges. The presence of rims and ridges is explained by a discontinuous void formation process caused by pinning of the retracting Al film.During annealing, crystallization and a thickness increase of the surface oxide, which is still continuously covering the Al thin film as well as the void, occurs. The surface scale undergoes a phase transformation from the amorphous state to γ-Al2O3, which is confirmed locally on the nanometer scale using scanning TEM techniques including electron energy loss near-edge structure investigations. Spherical aberration corrected atomic column resolved scanning TEM revealed a cube-on-cube orientation relationship between the Al thin film and the γ-Al2O3 surface oxide.

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