Tridimensional morphology and kinetics of etch pit on the {0 0 0 1} plane of sapphire crystal

The tridimensional morphology and etching kinetics of the etch pit on the C-{0 0 0 1} plane of sapphire crystal (α-Al2O3) in molten KOH were studied experimentally. It was shown that the etch pit takes on tridimensional morphologies with triangular symmetry same as the symmetric property of the sapphire crystal. Pits like centric and eccentric triangular pyramid as well as hexagonal pyramid were observed, but the latter is less in density. In-depth analyses show the side walls of the etch pits belong to the {1 1¯ 0 2¯} family, and the triangular pit contains edges full composed by Al3+ ions on the etching surface so it is more stable than the hexagonal pit since its edges on the etching surface contains Al2+ ions. The etch pits developed in a manner of kinematic wave by the step moving with constant speed, which is controlled by the chemical reaction with activation energy of 96.6 kJ/mol between Al2O3 and KOH.

Graphical abstractSchematic showing the atomic configuration of the predicted side walls of regular triangular pyramid shaped etch pit on the C-{0 0 0 1} plane of sapphire crystal.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Observed the tridimensional morphology of etch pits. ► Figured out the atomic configuration origin of the etch pits. ► Quantitatively determined the etch rates of the etch pits.