Epitaxial aluminum-doped zinc oxide thin films on sapphire: II, Defect equilibria and electrical properties

The electrical transport properties of epitaxial ZnO films grown on sapphire with different surface orientation, as a function of partial pressure of oxygen, were measured. The carrier concentration at equilibrium was found to change from a pO(2)- 1/4 to a pO(2)-3/8 dependence with increasing oxygen partial pressure. The partial pressure dependence was consistent with zinc vacancies being the rate-controlled diffusive species. The carrier concentration in ZnO films grown on A-, C-, and M-plane sapphire are the same but that of R-plane sapphire is lower. Electron Hall mobility measurements for all the substrate orientations exhibit a transition from 'single-crystal' behavior at high carrier concentrations to 'polycrystalline' behavior at low ones. The trap density at the grain boundaries is deduced to be ≈ 7 × 1012 /cm2. The electron mobility varies with the substrate orientation on which the ZnO films at constant carrier concentration were grown.