  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.
