$\mathrm{Co}$-doped $\mathrm{ZnO}\left({\mathrm{Co}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}\right)$ is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial ${\mathrm{Co}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}$ films on ${\mathrm{Al}}_2{\mathrm{O}}_3\left(012\right)$ substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of $\mathrm{Co}\left(x⩽0.35\right)$ can be uniformly incorporated into the film without phase segregation. $\mathrm{Co}$ 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 $\sim 1$ at. % $\mathrm{Al}$. Semiconducting films remain ferromagnetic up to $350\mathrm{K}$. 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 $\mathrm{Co}$ basis was very small $\left(<0.1{\mu }_B∕\mathrm{Co}\right)$, even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the $\mathrm{Co}\left(\mathrm{II}\right)$ ions behave as magnetically isolated $S=3∕2$ ions.

• Received 19 August 2003

DOI:https://doi.org/10.1103/PhysRevB.70.054424