Anisotropic microhardness and crack propagation in epitaxially grown GaN films

The anisotropic character of microhardness of cubic and hexagonal GaN films epitaxially grown on silicon and sapphire, respectively, is investigated. A series of Vickers indentations demonstrate anisotropic indentation induced crack propagation in cubic GaN, whereas in the hexagonal material no cracks are formed for indentations with the same load. Directions of easy crack propagation are observed. A series of Knoop indentations is conducted in order to inspect the probable microhardness orientation dependence relative to the main crystallographic directions of films. The measurements reveal an orientation dependence of microhardness at room temperature only in hexagonal GaN films. The indentations are made with an angular interval of five degrees and the microhardness-orientation curve is obtained. The curve has a periodicity close to sixty degrees.