Deformation behavior during nanoindentation of epitaxial ZnO thin films on sapphire substrate

Nanoindentation studies were carried out on epitaxial ZnO thin films on (0001) sapphire substrates grown by radio frequency magnetron sputtering. A single discontinuity (‘pop-in’) in the load–displacement curve was observed at a specific depth (13−16 nm) irrespective of the film thickness. The physical mechanism responsible for the ‘pop-in’ event in these epitaxial films may be due to the nucleation, propagation and interaction behavior of the glissile threading dislocations during mechanical deformation. Indentation well below the critical depth was found to be plastic deformation behavior (residual impression of 4 nm).