Revealing the mechanism of crystal orientation-dependent surface morphology and topography evolution in single-crystal ZnO using nanoindentation

Single-crystal zinc oxide (ZnO) is an important optoelectronic material, but its micro/nanoscale mechanical behavior is poorly understood. In this study, nanoindentation tests were performed on various crystalline planes of ZnO crystals using a Berkovich indenter at different indenter orientations. The surface and subsurface characteristics of the different crystal planes were analyzed and compared. It was found that indentation on the (0001) plane activated both basal and pyramidal slip systems, which suppressed pile-up. Dislocation pinning caused pop-in events during loading and promoted edge crack formation during unloading due to high elastic recovery. Indentation on the (1−100) and (11−20) planes activated basal and prismatic slip systems. As these systems cannot accommodate dislocation glide along the [0001] direction, corner cracks easily initiated. Meanwhile, dislocations glided toward the surface with minimal resistance, thereby facilitating outward material flow and pile-up formation along the [11−20] and [1−100] directions. Indenter orientation variation did not alter slip system activation for any plane and had minimal impact on plastic flow trends. No phase transformation was detected on the indented surfaces, while dislocation-induced luminescence quenching was observed. This study reveals how crystal orientation governs surface evolution in ZnO, offering guidance for machining method development and processing parameter optimization.