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

Experiments and molecular dynamics (MD) simulations showthat crystallizationof amorphous Al2O3 via solid phase epitaxy (SPE)on a (0001), c-plane, & alpha;-Al2O3 substrateforms a metastable & gamma;-Al2O3 polymorph beforetransforming eventually to & alpha;-Al2O3. MDsimulations over a wide range of crystallization temperatures abovethe glass transition point T (g) show thatthe growth velocity of epitaxial & gamma;-Al2O3 follows the Wilson-Frenkel relation. The barriers associatedwith interfacial reorganization processes are minimal, indicatingthat mass transport to the amorphous-crystalline interfaceis the rate-limiting step over the entire temperature range for & gamma;-Al2O3 SPE. The mechanisms of transport depend on thetemperature and have a significant effect in determining the crystallizationvelocity. Above T (g), mass transport iscontrolled by bulk diffusion. Below T (g), the crystallization velocity is faster than predicted from theWilson-Frenkel relation in both the MD and experimental results.X-ray characterization shows that the growth of epitaxial & gamma;-Al2O3 follows an Arrhenius dependence on crystallizationtemperature from 700 to 800 & DEG;C with an apparent activation energyof 3.1 eV. Despite the fact that MD shows essentially no bulk diffusionat temperatures below T (g), SPE growth isnonetheless observed. Our simulations show that this persistent growthis the result of kinetic heterogeneity between bulk and interface,with epitaxial growth governed by enhanced diffusion near the amorphous-crystallineinterface. The rate of interfacial diffusion is computed using a hoppingrate based on the first passage time of atoms moving to neighboringcrystallization sites. The combination of conventional diffusion andinterfacial hopping modes of mass transport within the Wilson-Frenkelmodel provides an accurate estimate of the SPE growth velocity of & gamma;-Al2O3 both above and below T (g).