Impact of the structural properties of holmium doped ZnO thin films grown by sol-gel method on their optical properties

Abstract Polycrystalline Ho (1, 3, 5 at %) doped ZnO thin films were prepared by sol–gel method. The films show a pure wurtzite crystalline structure. The particle size decreases from 29 to 9 nm when the Ho concentration increases from 0 to 5 at %. They present a homogeneous morphology for the specimens containing 1 and 3 at % of Ho. At 5 at % Ho the morphology of the film changes and the particles agglomerate in larger clusters. The UV–Vis transmission was found higher than 70% and a decreasing of the band gap from 3.28 to 3.22 eV was observed with the increase of the Ho concentration. The 5F5 →5I8 Ho transition at ~ 662 nm can be easily identified in the photoluminescence (PL) spectra, using an UV excitation source. A combined analysis of the photoluminescence properties at the room temperature and of the morphological and structural characteristics of the films allows us to directly link the Ho emission to the Ho incorporation in ZnO matrix.