Mechanical responses of single-crystal ZnO

The Berkovich nanoindentation-induced deformation in single-crystal ZnO was investigated by using cathodoluminescence (CL) imaging and cross-sectional transmission electron microscopy (XTEM) techniques. The load-displacement curves exhibit the clear features of multiple “pop-ins”, albeit somewhat randomly, in the load increasing segment. The XTEM results reveal that the primary plastic deformation is the propagation of dislocations with no evidence of either phase transformation or formation of micro-cracking. The CL images of nanoindentation show the rosette structures typical of the hexagonal system. However, unlike those displayed in similar studies using spherical indenters, the distribution of deformation-induced extended defects/dislocations appears to be more localized for the current Berkovich indentations. Furthermore, the CL spectra recorded at room temperature showed progressive degradation in the near band-edge luminescence intensity with increasing nanoindentation load, indicating severe lattice damages might have occurred.