Kinetics and Mechanism of γ-Al2O3 Solid Phase Epitaxy on c-Plane α-Al2O3

Experiments and molecular dynamics (MD) simulations show that crystallization of amorphous Al 2 O 3 via solid phase epitaxy (SPE) on a (0001), c-plane, α-Al 2 O 3 substrate forms a metastable γ-Al 2 O 3 polymorph before transforming eventually to α-Al 2 O 3 . MD simulations over a wide range of crystallization temperatures above the glass transition point T g show that the growth velocity of epitaxial γ-Al 2 O 3 follows the Wilson–Frenkel relation. The barriers associated with interfacial reorganization processes are minimal, indicating that mass transport to the amorphous–crystalline interface is the rate-limiting step over the entire temperature range for γ-Al 2 O 3 SPE. The mechanisms of transport depend on the temperature and have a significant effect in determining the crystallization velocity. Above T g, mass transport is controlled by bulk diffusion. Below T g, the crystallization velocity is faster than predicted from the Wilson–Frenkel relation in both the MD and experimental results. X-ray characterization shows that the growth of epitaxial γ-Al 2 O 3 follows an Arrhenius dependence on crystallization temperature from 700 to 800 °C with an apparent activation energy of 3.1 eV. Despite the fact that MD shows essentially no bulk diffusion at temperatures below T g, SPE growth is nonetheless observed. Our simulations show that this persistent growth is the result of kinetic heterogeneity between bulk and interface, with epitaxial growth governed by enhanced diffusion near the amorphous–crystalline interface. The rate of interfacial diffusion is computed using a hopping rate based on the first passage time of atoms moving to neighboring crystallization sites. The combination of conventional diffusion and interfacial hopping modes of mass transport within the Wilson–Frenkel model provides an accurate estimate of the SPE growth velocity of γ-Al 2 O 3 both above and below T g .