The effect of Gd on structural, optical and magnetic properties of ZnO has been investigated in a large concentration range. On the one side ZnO substrates have been ion-implanted with Gd leading to an average volume concentration of 5 x 1018 -1020 /cm3 . On the other side GdxZn1-xO-films have been sputtered reactively on sapphire substrates yielding Gd concentrations of 1.3 to 16% . Ion implantation of ZnO with RE atoms like Gd usually leads to a distortion of the lattice which has been simulated by theory  and evidenced with XRD. In the sputtered films it could be shown with XLD that Gd mainly substitutes for Zn in the lattice. With increasing Gd percentage the lattice distortion becomes massive and secondary phases occur. Eventhough there are many defects in the RE doped ZnO, there is no long range magnetic order, even for high concentrations of 16% Gd in ZnO. This was confirmed on a global scale with SQUID and element selective with XMCD. The occurrence and shift of defect bands in ZnO due to doping will be shown with PL. Finally the results for Gd:ZnO will briefly be compared with those for Sm, Ho and Eu implanted ZnO substrates.
 V. Ney et al, J. Appl. Phys. 104, 083904 (2008)
 V. Ney et al, Phys. Rev. B 85, 235203 (2012)
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