Optical properties of ZnO doped with Cobalt ions

While doping with rare earth ions is used for emission activation, doping with transition metal ions is often used to get specific magnetic properties of a given host material. Recently investigations of transition metal doped materials focused on chances of achieving a room temperature ferromagnetic response. This is because carrier mediated room temperature ferromagnetic order was theoretically predicted for ZnO doped with Mn or Co ions. Such order is required for some of spintronics applications. To realize RT FM both Mn and Co should stay in 2+ charge state, expected when Mn/Co substitute zinc in ZnO. Both ZnMnO and ZnCoO alloys show a strong absorption band, which appears below ZnO band gap transitions. The origin of this absorption in ZnCoO is discussed in the present work. We show based on the results of photoluminescence and photo-ESR investigations that the broad absorption band is related to Co photo-ionization.