Visible up-conversion in ZnO:Er and ZnO:Er:Yb thin films deposited by RF magnetron sputtering under different O-2-rich atmospheres has been studied. Conventional photoluminescence (325 nm laser source) and up-conversion (980 nm laser source) have been performed in the films before and after an annealing process at 800 degrees C. The resulting spectra demonstrate that the thermal treatment, either during or post-deposition, activates optically the Er3+ ions, being the latter process much more efficient. Moreover, the atmosphere during deposition was also found to be an important parameter, as the deposition under O-2 flow increases the optical activity of Er+3 ions. In addition, the inclusion of Yb3+ ions into the films has shown an enhancement of the visible up-conversion emission at 660 nm by a factor of 4, which could be associated to either a better energy transfer from the F-2(5/2) Yb level to the I-4(11/2) Er one, or to the prevention of having Er2O3 clustering in the films. (C) 2014 Elsevier B.V. All rights reserved.