Optical properties of ZnCoO films and nanopowders
  ZnCoO is one of the most studied and promising semiconductor materials for spintronics applications. In this work we discuss optical and electrical properties of ZnCoO films and nanoparticles grown at low temperature by either Atomic Layer Deposition or by a microwave driven hydrothermal method. We report that doping with Cobalt quenches a visible photoluminescence (PL) of ZnO. We could observe a visible PL of ZnO only for samples with very low Co fractions (up to 1%). Mechanisms of PL quenching in ZnCoO are discussed. We also found that ZnO films remained n-type conductive after doping with Co, indicating that a high electron concentration and Cobalt 2+ charge state can coexist. 
E. Wolska, M. {\L}ukasiewicz, J.D. Fidelus, W. {\L}ojkowski, E.   Guziewicz, M. Godlewski
cond-mat.mtrl-sci