γ-Al2O3 formation from pulsed-laser irradiated sapphire

Abstract

High-resolution transmission electron microscopy was used to examine the effect of laser irradiation of sapphire (single crystalline α-Al2O3). It was observed that laser-melted material resolidified as γ-Al2O3 epitactically grown on the (0001) plane of α-Al2O3 having an orientation relationship (0001)α//(111)γ and [011̄0]α//[11̄0]γ. The interface between unmelted α-Al2O3 and resolidified γ-Al2O3 is atomically flat with steps of one to a few close-packed oxygen layers. The high thermal gradient in the α-Al2O3 did not produce any interfacial dislocations. However, pronounced lattice distortions existed in the resolidified γ-Al2O3. It is argued that the very rapid cooling rate present during solidification prevents the ordering of Al atoms that is required for the formation of the stable α-Al2O3 phase.