Solid-Phase Epitaxial Growth of an Alumina Layer Having a Stacking-Mismatched Domain Structure of the Intermediate γ‑Phase

Solid-phase epitaxy (SPE), a solid-state phase transition of materials from an amorphous to a crystalline phase, is a convenient crystal growing technique. In particular, SPE can be used to grow α-Al₂O₃ epitaxially with a novel structure that provides an effective substrate for improved performance of light-emitting diodes (LEDs). However, the inevitable two-step phase transformation through the γ-Al₂O₃ phase hinders the expected improved crystallinity of α-Al₂O₃, and thereby further enhancement of LED performance. Herein, we provide a fundamental understanding of the SPE growth mechanism from amorphous to metastable γ-Al₂O₃ using transmission electron microscopy (TEM) and density functional theory (DFT) calculations. The nanobeam precession electron diffraction technique enabled clear visualization of the double-positioning domain distribution in the SPE γ-Al₂O₃ film and emphasized the need for careful selection of the viewing directions for any investigation of double-positioning domains. Void and stacking fault defects further investigated by high-resolution scanning TEM (STEM) analyses revealed how double-positioning domains and other SPE growth behaviors directly influence the crystallinity of SPE films. Additionally, DFT calculations revealed the origins of SPE growth behavior. The double-positioning γ-Al₂O₃ domains randomly nucleate from the α-Al₂O₃ substrate regardless of the α-Al₂O₃ termination layer, but the large energy requirement for reversal of the γ-Al₂O₃ stacking sequence prevents it from switching the domain type during the crystal growth. We expect that this study will be useful to improve the crystallinity of SPE γ- and α-Al₂O₃ films.