Nanoindentation-induced structural deformation in GaN/AlN multilayers

The mechanical responses of GaN/AlN multilayers grown on Si(111) substrates by using the metalorganic vapor phase epitaxy (MOVPE) were investigated by combining the Berkovich nanoindentation and the cross-sectional transmission electron microscopy (XTEM) techniques. The prominent mechanical parameters such as the hardness and Young's modulus of GaN/AlN multilayers were obtained to be 21.6±0.8 GPa and 285.7±12.1 GPa, respectively by operating the Berkovich nanoindenter with the continuous contact stiffness measurements (CSM) mode. The structural deformation behaviors were delineated by XTEM observations taken in the vicinity regions just underneath the indenter tip. It was revealed that the "pop-ins" observed in the load-displacement curve during loading are due primarily to the activities of dislocation nucleation and propagation. There is no evidence of either phase transformation or micro-crack formation observed in XTEM images of GaN/AlN multilayers, which is consistent with no distinct"pop-out"event displayed in unloading curves.