Thermal Evolution of Band Edge States in ZnO Film as a Function of Annealing Ambient Atmosphere

RF-sputtered ZnO films were annealed under various annealing ambient atmospheres, including a vacuum, air, and water vapor. The physical and electrical properties of ZnO films annealed in various ambient atmospheres, were studied as a function of annealing temperature. The carrier concentration was dramatically increased, and the mobility was decreased when the films were annealed in a vacuum or water vapor. Even though the annealing ambient atmosphere and temperature were different, the preferred orientation and crystallization of the annealed ZnO films are maintained. However, two distinct band edge states below the conduction band, observed by spectroscopic ellipsometry measurement, undergo a thermal change as a function of annealing ambient atmosphere and these changes are correlated to changes in carrier concentration and mobility.