Solid-phase epitaxy of a cavity-shaped amorphous alumina nanomembrane structure on a sapphire substrate

A stripe-shaped cavity structure was formed with amorphous alumina membrane on a sapphire substrate and its solid phase epitaxy (SPE) to crystalline sapphire membrane was investigated by transmission electron microscopy (TEM). The SPE process occurred through two-stage phase transformation from amorphous to γ-Al2O3 and subsequently to α-Al2O3. SPE of γ-Al2O3 at around 780 °C was initiated from the interface between amorphous alumina and sapphire substrate with an epitaxial relationship of [0 0 0 1]sapphire//[1 1 1]γ-Al2O3. Besides SPE, it was observed that nanocrystalline γ-Al2O3 were formed in the upper part of the membrane after an incubation time. The subsequent phase transformation of the alumina membrane from γ- to α- Al2O3 was observed at higher temperatures above 920 °C. The SPE of α-Al2O3 began at the corner region of the membrane and proceeded along the whole membrane. The whole alumina membrane was successfully transformed into single crystalline α-Al2O3 by SPE after annealing at 1063 °C for 2.5 h. The SPE kinetics was investigated by precise measurement of the thicknesses of epitaxially transformed γ- and α-Al2O3 layers in TEM images, resulting in activation energies of 3.1 eV and 3.9 eV for the SPE from amorphous to γ-phase and that from γ- to α-phase, respectively.