Post-annealing effects on the structural and optical properties of vertically aligned undoped ZnO nanorods grown by radio frequency magnetron sputtering

We report the nature of point defects associated with the visible transitions and X-ray photoelectron emissions of post-growth annealed ZnO nanorods under vacuum and air atmospheres. The ZnO nanorods are vertically aligned along the c-axis with a hexagonal cross section. The compressive strain in the as-grown ZnO nanorods has been completely relaxed by the post-growth annealing under vacuum. The relative quantity of oxygen deficiencies in the as-grown and post-annealed ZnO nanorods is calculated from the X-ray photoelectron spectra. Despite high oxygen deficiencies, the intense bi-donor bound exciton emission with narrow full width at half maximum reflects good optical quality of the vacuum annealed ZnO nanorods. The additional green and red emissions are attributed to electron transitions owing to the oxygen mediated defects in the nanorods.