Concentration dependence of the Er~(3+) visible and infrared luminescence in Y_(2-x)Er_xO_3 thin films on Si

摘要

Y_(2-x)Er_xO_3 thin films, with x varying between 0 and 0.72, have been successfully grown on crystalline silicon (c-Si) substrates by radio-frequency magnetron cosputtering of Y_2O_3 and Er_2O_3 targets. As-deposited films are polycrystalline, showing the body-centered cubic structure of Y_2O_3, and show only a slight lattice parameter contraction when x is increased, owing to the insertion of Er ions. All the films exhibit intense Er-related optical emission at room temperature both in the visible and infrared regions. By studying the optical properties for different excitation conditions and for different Er contents, all the mechanisms (i.e., cross relaxations, up-conversions, and energy transfers to impurities) responsible for the photoluminescence (PL) emission have been identified, and the existence of two different well-defined Er concentration regimes has been demonstrated. In the low concentration regime (x up to 0.05, Er-doped regime), the visible PL emission reaches its highest intensity, owing to the influence of up-conversions, thus giving the possibility of using Y_(2-x)Er_xO_3 films as an up-converting layer in the rear of silicon solar cells. However, most of the excited Er ions populate the first two excited levels ~4I_(11/2) and ~4I_(13/2), and above a certain excitation flux a population inversion condition between the former and the latter is achieved, opening the route for the realization of amplifiers at 2.75 μm. Instead, in the high concentration regime (Er-compound regime), an increase in the nonradiative decay rates is observed, owing to the occurrence of cross relaxations or energy transfers to impurities. As a consequence, the PL emission at 1.54 μm becomes the most intense, thus determining possible applications for Y_(2-x)Er_xO_3 as an infrared emitting material.