Competing Growth Pathways of γ- and α-Al2O3 during Solid-Phase Epitaxy: The Critical Role of Interfacial Reconstruction

The crystallization of amorphous aluminum oxide (Al2O3) via solid-phase epitaxy (SPE) on a (0001), c-plane α-Al2O3 substrate forms a metastable γ-Al2O3 polymorph, followed by a sequence of other metastable polymorphs before eventually transforming into thermodynamically stable α-Al2O3. Using molecular dynamics (MD) simulation together with on-the-fly probability enhanced sampling (OPES), the free energy profiles for the epitaxial growth of α-Al2O3 and γ-Al2O3 on a c-plane α-Al2O3 substrate are calculated. MD simulation shows that the SPE growth of γ-Al2O3 is mechanistically simple, with a minimal free energy barrier for interfacial rearrangement. In contrast, the SPE growth of α-Al2O3 is complicated, involving an unstable intermediate, which involves aluminum sublattice rearrangement, and a γ-like interfacial layer (denoted as γ*), which lowers the interfacial free energy of the newly crystallized α-Al2O3. Comparing the free energy profiles of the two competing pathways, we find that the epitaxial growth of α-Al2O3 is thermodynamically favored yet kinetically hindered. In contrast, the epitaxial growth of γ-Al2O3 requires merely overcoming a sliding fault barrier, which is expected to be easily achieved under annealing. Consequently, polymorph sequence and phase selection of γ-Al2O3 are achieved largely due to the presence of the γ* interfacial layer, which highlights the importance of interface reconstruction during SPE in polymorphic systems.