Photoplastic anisotropy in nanoindentation of wurtzite ZnO single crystals

Anisotropy plays a crucial role in understanding and optimizing the properties of materials with directional dependencies. The hexagonal wurtzite structure, which is a typical crystal structure in compound semiconductors, demonstrates pronounced anisotropy, especially in its response to external stimuli. Recently, mechanical behavior under light illumination has attracted increasing interest especially in semiconductor compounds. In this study, we investigated the anisotropy of illumination effects on the nanomechanical properties of wurtzite ZnO. Four surface orientations—(0001), (0001) 45° off, (11¯00), and (21¯1¯0)—were subjected to nanoindentation creep and nanoindentation hardness tests under controlled light illumination. The indentation depth during nanoindentation creep under light illumination was consistently smaller than that in darkness for all surface orientations, confirming that light suppresses indentation creep deformation, but to different degrees depending on the surface orientation. This suggests that the activated slip systems and the distribution of dislocations play a crucial role in modulating dislocation behavior under light illumination. The nanoindentation hardness followed the trend on the four surface orientations: (0001) > (0001) 45° off > (11¯00) > (21¯1¯0), reflecting anisotropic behavior in nanomechanical properties. Second and subsequent pop-in events were extracted, exhibiting different behaviors depending on the surface orientations, and may play a key role in determining the anisotropy in nanoindentation hardness. Our findings contribute to a comprehensive understanding of the plastic anisotropy under light control in wurtzite ZnO.