Annealing study of hydrothermally grown ZnO wafers

The annealing behaviour of ZnO has been studied using x-ray diffraction (XRD) and atomic force microscopy (AFM). Hydrothermally grown ZnO substrates were annealed in the range of (500–1050°C), both in vacuum and different atmospheres/pressures, e.g., ambient air, pure oxygen, or in the presence of ZnO powder. Annealing at 0.5 bar oxygen results in degradation of both surface roughness and bulk crystallinity as measured by AFM and XRD. Increasing the oxygen pressure up to 1 bar improves the diffraction peaks but the surface remains rough. The best annealing results when considering stabilization/improvement of bulk and surface properties of ZnO are obtained for annealing in air at 680–960°C, and for annealing in the presence of ZnO powder at 960°C. The last result is discussed in terms of the establishment of a gas/solid equilibrium during annealing at the cost of the ZnO powder with its much higher surface to volume ratio compared to that of the wafer. In addition, these samples exhibit very flat, step-like terraces on the Zn-face terminated surface (root mean square roughness on terraces is in the range of 0.15 nm).