Optical emission and electron capture of rare-earth trivalent ions located at distinct sites in thin films

We present photoluminescence and decay of photo excited conductivity data for sol-gel SnO2 thin films doped with rare earth ions Eu3+ and Er3+, a material with nanoscopic crystallites. Photoluminescence spectra are obtained under excitation with several monochromatic light sources, such as Kr+ and Ar+ lasers, Xe lamp plus a selective monochromator with UV grating, and the fourth harmonic of a Nd:YAG laser (4.65 eV), which assures band-to-band transition and energy transfer to the ion located at matrix sites, substitutional to Sn4+. The luminescence structure is rather different depending on the location of the rare-earth doping, at lattice symmetric sites or segregated at grain boundary layer, where it is placed in asymmetric sites. The decay of photo-excited conductivity also shows different trapping rate depending on the rare-earth concentration. For Er-doped films, above the saturation limit, the evaluated capture energy is higher than for films with concentration below the limit, in good agreement with the different behaviour obtained from luminescence data. For Eu-doped films, the difference between capture energy and grain boundary barrier is not so evident, even though the luminescence spectra are rather distinct.