Indentation-induced dislocations and cracks in (0001) freestanding and epitaxial GaN

The (0001) surface of freestanding GaN and an (0001) oriented epitaxial GaN layer of 3 μm thickness on sapphire have been deformed at room temperature using a Vickers indenter. The samples were indented with two different orientations of the indenter with loads in the range from 0.02 to 4.90 N and 0.10 to 4.90 N, respectively. Dislocations and cracks at the indentations were observed by means of scanning electron microscopy, cathodo-luminescence, optical microscopy and transmission electron microscopy. Dislocations occur at all indentations for the loads used in the investigations. In both materials, the dislocation arrangement corresponds to the symmetry of the indented surface and the orientation of the indenter has only a marginal influence. Higher loads lead to radial cracks at the corners of the indentations and lateral cracks beneath the surface. The crack system is predominantly determined by the symmetry and orientation of the indenter. The dislocation arrangement and the crack system in freestanding GaN and an epitaxial layer of GaN on sapphire are compared.