The electrical transport properties of epitaxial films which had been grown onto variously oriented sapphire substrates were measured as a function of the O partial pressure. After equilibration, the carrier concentration was found to change from a P-1/4 to a P-3/8 dependence with increasing O partial pressure. This pressure dependence was shown to be consistent with the assumption that Zn vacancies were the rate-controlling diffusive species. In addition, the carrier concentration in films which had been grown onto A-, C- or M-plane sapphire were the same, while that in films which had been grown onto K-plane sapphire was lower. Electron Hall mobility measurements revealed a transition from single-crystal behavior, at high carrier concentrations, to polycrystalline behavior at low carrier concentrations. This behavior was attributed to the effective height of potential barriers which formed at low-angle grain boundaries in the epitaxial films. The trap density at the grain boundaries was deduced to be equal to about 7 x 1012/cm2. The electron mobility varied with the orientation of the substrate upon which the film was grown. This difference was attributed to differences in dislocation density which were produced by lattice mismatch with the various sapphire orientations.

Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: II, Defect Equilibria and Electrical Properties. Srikant, V., Sergo, V., Clarke, D.R.: Journal of the American Ceramic Society, 1995, 78[7], 1935-49