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 gamma-Al2O3 polymorph. An amorphous Al2O3 thin film on a (0001) alpha-Al2O3 sapphire substrate initially transforms upon heating to form epitaxial gamma-Al2O3, followed by a transformation to monoclinic 0-Al2O3, and eventually to alpha-Al2O3. Epitaxial gamma-Al2O3 layers with low mosaic widths in X-ray rocking curves can be formed via SPE by crystallizing the gamma-Al2O3 phase from amorphous Al2O3 and avoiding the microstructural inhomogeneity arising from the spatially inhomogeneous transformation to theta-Al2O3. A complementary molecular dynamics (MD) simulation indicates that the amorphous layer and gamma-Al2O3 have similar Al coordination geometry, suggesting that gamma-Al2O3 forms in part because it involves the minimum rearrangement of the initially amorphous configuration. The lattice parameters of gamma-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 gamma-Al2O3 can minimize the epitaxial elastic deformation of gamma-Al2O3 during crystallization.