Epitaxial Growth and Properties of Cobalt-Doped ZnO on α-Al2O3 Single-Crystal Substrates

Co-doped ZnO (Co x Zn 1- x O) is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial Co x Zn 1– x O films on Al 2 O 3 (012) substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co( x ≤ 0.35) can be uniformly incorporated into the film without phase segregation. Co is found to be in the +2 oxidation state, independent of x, by both surface-sensitive core-level x -ray photoemission and bulk-sensitive optical absorption spectroscopies. This material can be grown n -type by the deliberate incorporation of oxygen vacancies, but not by inclusion of ~1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small (