Phase Selection and Structure of Low-Defect-Density γ-AlO Created by Epitaxial Crystallization of Amorphous AlO.

A multistep phase sequence following the crystallization of amorphous AlO via solid-phase epitaxy (SPE) points to methods to create low-defect-density thin films of the metastable cubic γ-AlO polymorph. An amorphous AlO thin film on a (0001) α-AlO sapphire substrate initially transforms upon heating to form epitaxial γ-AlO, followed by a transformation to monoclinic θ-AlO, and eventually to α-AlO. Epitaxial γ-AlO layers with low mosaic widths in X-ray rocking curves can be formed via SPE by crystallizing the γ-AlO phase from amorphous AlO and avoiding the microstructural inhomogeneity arising from the spatially inhomogeneous transformation to θ-AlO. A complementary molecular dynamics (MD) simulation indicates that the amorphous layer and γ-AlO have similar Al coordination geometry, suggesting that γ-AlO forms in part because it involves the minimum rearrangement of the initially amorphous configuration. The lattice parameters of γ-AlO are consistent with a structure in which the majority of the Al vacancies in the spinel structure occupy sites with tetrahedral coordination, consistent with the MD results. The formation of Al vacancies at tetrahedral spinel sites in epitaxial γ-AlO can minimize the epitaxial elastic deformation of γ-AlO during crystallization.