Effects of Laser Irradiation on the Microstructure and Surface Morphology of Zinc Oxide Doped with Different Additives

Zinc oxide (ZnO) disc samples doped with copper oxide as the main impurity were prepared and irradiated for 2 min by 1.064 μm semi-train of 20 ps pulses having energy of 80 mJ in a spot size of 5 mm diameter and average pulse power density of about 1.3 GW/cm². X-ray diffraction reveals a large increase in the average crystallite size after laser irradiation for 2 min, which leads to a consequent large decrease in the average dislocation as it is inversely proportional to the square of the crystallite size. Scanning electron microscopy images show the presence of inter-granular phase and CuO particles between the ZnO grain boundaries. They also show a noticeable decrease of ZnO grain size after laser irradiation for 2 min due to grain melting. Atomic force microscopy shows increase in the roughness after laser irradiation. The laser irradiation of samples also resulted in the destruction of ZnO surface hexagonal structure with large increase of inter-granular spaces, which may be due to grain melting. The laser-induced grain melting also resulted in increasing roughness and in forming pores, cracks and particulates with dimensions comparable to average grain size. The binding energies of Zn 2p, Cu 2p, Cr 2p and O 1s core levels of ZnO varistor surfaces were determined for one sample by X-ray photoelectron spectroscopy.