Effect of oxygen pressure and post-annealing on the properties of reactively sputtered zinc oxide thin films

Abstract

• ZnO thin films are grown at room temperature on glass using radio-frequency sputtering. • As-grown ZnO films have wurtzite crystalline structure and direct bandgap behavior. • ZnO grown in O-rich and O-deficient plasmas shows annealing-sensitive crystallinity. • Bandgap energy evolution with deposition duration and post-annealing is investigated. • ZnO films with stable morphology and optical behavior are grown under some conditions. Using radio-frequency sputtering in the reactive regime at a fixed radio-frequency power of 200 W, we deposited zinc oxide thin films at room temperature on glass substrates in argon-oxygen sputtering atmospheres with different sputtering durations, and various oxygen contents from 20% to 70%. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and transmittance spectrophotometry were used to probe structural, morphological, chemical and optical properties of as-grown and post-annealed thin films in air at medium temperatures (up to 500 °C). The as-grown samples were found to present a hexagonal crystalline structure with a major (0 0 2) orientation and direct bandgap energies ranging from 3.27 to 3.3 eV. The post-annealing results in the improvement of crystallinity of ZnO thin films deposited at low oxygen contents and in the deterioration then the recrystallization of films deposited in oxygen-rich plasma. The optimal properties and thermal stability were obtained for 30% of oxygen in the sputtering environment, with an annealing-induced shrinkage of the bandgap smaller than 100 meV.