Phase Selection and Structure of Low-Defect-Density γ-Al₂O₃ Created by Epitaxial Crystallization of Amorphous Al₂O₃

A multistep phase sequence following the crystallization of amorphous Al₂O₃ via solid-phase epitaxy (SPE) points to methods to create low-defect-density thin films of the metastable cubic γ-Al₂O₃ polymorph. An amorphous Al₂O₃ thin film on a (0001) α-Al₂O₃ sapphire substrate initially transforms upon heating to form epitaxial γ-Al₂O₃, followed by a transformation to monoclinic θ-Al₂O₃, and eventually to α-Al₂O₃. Epitaxial γ-Al₂O₃ layers with low mosaic widths in X-ray rocking curves can be formed via SPE by crystallizing the γ-Al₂O₃ phase from amorphous Al₂O₃ and avoiding the microstructural inhomogeneity arising from the spatially inhomogeneous transformation to θ-Al₂O₃. A complementary molecular dynamics (MD) simulation indicates that the amorphous layer and γ-Al₂O₃ have similar Al coordination geometry, suggesting that γ-Al₂O₃ forms in part because it involves the minimum rearrangement of the initially amorphous configuration. The lattice parameters of γ-Al₂O₃ 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-Al₂O₃ can minimize the epitaxial elastic deformation of γ-Al₂O₃ during crystallization.