EPITAXIAL ALUMINUM-DOPED ZINC-OXIDE THIN-FILMS ON SAPPHIRE .2. DEFECTEQUILIBRIA AND ELECTRICAL-PROPERTIES

The electrical transport properties of epitaxial ZnO films grown on di fferent orientations of sapphire substrates have been measured as a fu nction of partial pressure of oxygen, After equilibration, the carrier concentration is found to change from a p(O2)(-1/4) to a p(O2)(-3/8) dependence with increasing oxygen partial pressure, The partial pressu re dependence is shown to be consistent with zinc vacancies being the rate-controlling diffusive species, In addition, the carrier concentra tion in ZnO films grown on A-, C-, and M-plane sapphire are the same b ut that of R-plane sapphire is systematically lower, Electron IIall mo bility measurements as a function of carrier concentration for all the substrate orientations exhibit a transition from ''single-crystal'' b ehavior at high carrier concentrations to ''polycrystalline'' behavior at low carrier concentrations, This behavior is attributed to the eff ective height of potential barriers formed at the low-angle grain boun daries in the epitaxial ZnO films, The trap density at the grain bound aries is deduced to be similar to 7 x 10(12)/cm(2). The electron mobil ity, at constant carrier concentration, varies with the substrate orie ntation on which the ZnO films were grown. The difference is attribute d to the difference in dislocation density in the films produced as a result of lattice mismatch with the different sapphire orientations.