Spherical nanoindentation and deformation mechanisms in freestanding GaN films

In this work, spherical nanoindentation-with nanoindenter radius of 1 or 13.5 mu m-was used to explore the deformation behavior of GaN freestanding films with two orientations, C (basal) and A (prismatic), grown by hydride vapor phase epitaxy. For the C plane, all the results shown herein and most of those in the literature can be explained by invoking the activation of basal slip alone. The wide distribution of pop-in stresses in the C plane is believed to depend on initial surface and/or near surface defect concentrations. By converting the nanoindentation load-displacement data to indentation stress-strain curves and comparing those for the A and C planes after the pop-ins, we conclude that basal slip is also implicated in the deformation of the A plane. The elastic moduli, determined from spherical nanoindentation, depend on the indenter size. In the C plane repeated spherical nanoindentations, to the same stress, result in reversible, hysteretic loops that are attributed to the formation of incipient kink bands and/or the to-and-fro motion of mobile dislocation walls. (c) 2007 American Institute of Physics.