Investigation of the near surface mechanical behavior of single crystal zinc oxide by nanoindentation

The near surface mechanical behavior of single crystal ZnO was examined using nanoindentation. The effect of the surface preparation technique was studied by comparing (0001) surfaces that had been prepared by etching, chemomechanical polishing, and mechanical polishing with 1/4 μm and 1 μm diamond abrasive. The chemomechanical polished and etched surfaces exhibited pop-in whereas the mechanical polished surfaces did not. Measured hardness values were found to be dependent on surface preparation, with an increased hardness observed with increased surface preparation severity. All surfaces exhibited an increase in hardness as the depth decreased i.e., the indentation size effect. The elastic modulus was found to not depend on the surface preparation and the values were consistent with calculated values obtained with the inclusion of the piezoelectric effect. To study the variation of near surface mechanical behavior with crystallographic direction, chemomechanical polished (0001¯), {101¯0}, and {112¯0} surfaces were examined. The elastic modulus, hardness, and load at which pop-in occurred for the (0001¯) surface was found to be similar to the (0001) surface. The {101¯0} and {112¯0} surfaces exhibited pop-in at a similar load as for the (0001) and (0001¯) surfaces but the displacement during the pop-in on the prismatic surfaces was about twice that of the polar surfaces. Measured hardness values of the prismatic surfaces were about half that of the polar surfaces over the depth range investigated. The effects of broad band illumination on near surface mechanical behavior were measured for etched (0001) surfaces and chemomechanical polished {101¯0} surfaces. For both surfaces, sample illumination caused an increase in the load at which pop-in occurred and hardness for contact depths less than 100 nm. For the (0001) surface hardness increased 7–20% with sample illumination and for the {101¯0} surface hardness increased 4–9%. The use of scanning Kelvin probe microscopy with sample illumination to observe indentation dislocation rosettes was demonstrated. The rosette pattern consisted of a central zone around the indentation and six “arms” emanating from the indentation along the (112¯0) directions. The length of the longest rosette arm was found to be proportional to the square root of the maximum load of the indentation.