Structural and chemical characterization of Co-doped ZnO layers grown on Si and sapphire

Ferromagnetism at room temperature has been experimentally observed for several doped oxidic semiconductors (Co:TiO2, Mn:TiO2, Co:ZnO, Mn:ZnO, etc.) [1], but its origin is still under discussion. Therefore, Co(3 at%)-doped ZnO layers of approximately 150 nm thickness were grown on (001)Si and (001)sapphire substrates at 500°C by magnetron sputtering. For each substrate the Zn0.94Co0.06O layers were deposited under oxygen-poor and -rich conditions by using Ar and Ar/O2, respectively, for sputtering and post-annealing in O2 to ensure oxygen saturation for samples prepared under oxygen rich conditions. The magnetic properties of the Co-doped samples were investigated by SQUID (superconducting quantum interference device) magnetometry. Only samples that were grown under oxygen-poor conditions with no oxygen post-annealing on sapphire show a small ferromagnetic contribution to the paramagnetic background, while oxygen-rich ones and those on Si substrate do not, even at low temperatures. To correlate the magnetic behavior with both structural and chemical materials properties, transmission electron microscopy (TEM) including high-resolution and analytical TEM (HRTEM/AEM) have been applied.