Vacuum ultraviolet dielectric function of ZnFe2O4 thin films

The dielectric function of ZnFe2O4 thin films has been studied in a wide spectral range from 0.5 eV to 9 eV. The observed optical transitions are identified as charge transfer and ligand field transitions. We propose a parametric model based on known model dielectric functions that fits well to the experimental data. The assignment of optical transitions is justified by a thorough comparison with optical data from other materials exhibiting similar transitions. The main contributions to the optical response are found to arise from charge transfer transitions from the O2p to the metal ion 4s and 3d bands. Furthermore weak crystal field transitions are observed. The evolution of the dielectric function as well as the crystal quality of the thin films with respect to growth temperature is further analyzed. A blue shift of the observed optical transitions is related to a relaxation of the lattice toward the bulk and an increase of the crystal quality with increasing growth temperature.