Caracterização de filmes finos de ZnO:Ho crescidos por spray-pirólise

Abstract

The research investigation on the doping effects of zinc oxide thin films (ZnO) with the rare earth ion Holmium (Ho3+) and the determination of an optimal doping condition for the luminescence of the doping element. The films were deposited by the spray-pyrolysis technique considering nominal rare earth concentrations within the range of 1 to 5%. The structural properties were evaluated by X-ray Diffraction technique (XRD), and morphological and compositional characteristics by Scanning Electron Microscopy (SEM) associated with Energy Spectroscopy Dispersive (EDS). To investigate the optical properties, we employed UV-Vis Absorption Spectroscopy and Photoluminescence Spectroscopy (PL). For PL measurements, the excitation wavelengths were 450 nm and 325 nm, and temperatures around 77 K. The XRD results revealed polycrystalline films in wurtzite hexagonal structure without a preferential orientation with crystallite sizes in the (002) and (101) orientations ​​of 6.9 and 18 nm, respectively, which increases with increasing dopant concentration. SEM micrographs show grains with average size from 1.0 to 2.75 μm. The EDS spectra, in turn, provided the actual concentrations of the rare earth. The UV-Vis absorption showed rare earth absorption in the blue, green, and red regions for samples with 3 to 5% Ho-content. The optical bandgap of ZnO host assumed values ​​between 3.10 and 3.19 eV. The PL spectra showed UV emissions from the host and the dopant emissions in the visible range. Furthermore, the decrease of rare earth luminescence intensity with increasing temperature was attributed to electron-phonon interactions. It was also observed the degeneration in Ho3+ levels, caused by the interaction with the crystalline field of ZnO.