High angle annular dark field and electron energy loss spectroscopy study on the substitution and distribution of cobalt in ZnO by multilayer growth

Abstract

Co-doped ZnO films were synthesized by ion beam sputtering using multilayer (ZnO/Co) growth. Both the distribution and the chemical states of Co in ZnO can be well controlled by varying the ratio of the nominal layer thickness of ZnO to Co. Transmission electron microscopy indicated that all of the as-deposited Zn1−x(Co)xO films were polycrystalline with a (0002) preferred orientation. The local microstructures and chemical states were identified by Z-contrast imaging and electron energy loss spectroscopy. In ZnO (1.5 nm)/Co (0.1 nm), homogeneous Co-doped ZnO was observed to have been formed through interdiffusion. However, decreasing or increasing the thickness of ZnO leads to the formation of Co clusters in the ZnO matrix or Zn1−x(Co)xO multilayers, respectively. For ZnO thickness≧1.5 nm, Co is substituted for Zn, and its valence state is 2+. All Co-doped ZnO films show room-temperature ferromagnetic behavior, which appears to depend strongly on the Co distribution.