A comparison of the mechanical properties and the impact of contact induced damage in a- and c- axis ZnO single crystals

Nanoindentation studies were conducted on a-axis oriented ZnO single crystals. The mechanical properties and deformation mechanisms were monitored and compared to previously determined data from c-axis material. Hardness and modulus values reveal that a-axis ZnO is significantly softer than c-axis material (hardness of 2 ± 0.2 GPa), and behaves more plastically. Additionally, the influence of contact induced damage on the defect structure of a-axis material was examined using cathodoluminescence spectroscopy and monochromatic imaging to monitor the luminescence from indent sites. Deformation directly under the indent site enhanced the occurrence of red luminescence, and was attributed to a native defect in ZnO that has a higher formation energy than the defects responsible for the green and yellow visible defect bands, which were present in ZnO during growth and clustered to the indent site during annealing.