Investigations on the morphological evolution of zinc oxide nanostructures and their optical properties

Abstract

The morphological evolution of ZnO nanostructures grown under various argon sputtering pressures and substrate temperatures has been systematically investigated. A surface phase diagram depicting compact two-dimensional thin films to various types of one-dimensional structures as a function of pressure and temperature is constructed. Structural studies of ZnO nanostructures reveal the preferential orientation along the c-axis of a hexagonal wurtzite crystal structure with a highly crystalline nature. However, the as-grown ZnO nanostructures slightly experience compressive strain, which increases with the argon sputtering pressure. Exciton recombination and point defects have been extensively analyzed by temperature-dependent photoluminescence spectroscopy. The observation of donor-bound exciton emission and its replicas reveals the high optical quality of the nanostructures. Nevertheless, the observed visible emission indicates the presence of point defects which significantly depend on the deposition pressure. The characteristic visible emission bands at 2.28 and 2.02 eV provide strong evidence for the existence of oxygen vacancies and interstitial sites in ZnO nanostructures irrespective of growth temperature and pressure.