Luminescence properties of Ce~(3+) and Tb~(3+) co-doped SiO_xN_y thin films: Prospects for color tunability in silicon-based hosts

摘要

In this work, the role of the nitrogen content, the annealing temperature, and the sample morphology on the luminescence properties of Ce~(3+) and Tb~(3+) co-doped SiO_xN_y thin films has been investigated. An increasing nitrogen atomic percentage has been incorporated in the host matrix by gradually replacing oxygen with nitrogen during fabrication while maintaining the Si content unaltered, obtaining a sequential variation in the film composition from nearly stoichiometric SiO_2 to SiO_xN_y. The study of rare earth doped single layers has allowed us to identify the parameters that yield an optimum optical performance from Ce~(3+) and Tb~(3+) ions. Ce~(3+) ions proved to be highly sensitive to the annealing temperature and the nitrogen content, showing strong PL emission for relatively low nitrogen contents (from 0 to 20%) and moderate annealing temperatures (800-1000 ℃) or under high temperature annealing (1180℃). Tb~(3+) ions, on the other hand, displayed a mild dependence on those film parameters. Rare earth co-doping has also been investigated by comparing the luminescence properties of three different approaches: (ⅰ) a Ce~(3+) and Tb~(3+) co-doped SiO_xN_y single layer, (ⅱ) a bilayer composed of two SiO_xN_y single layers doped with either Ce~(3+) or Tb~(3+) ions, and (ⅲ) a multilayer composed of a series of either Tb~(3+) or Ce~(3+)-doped SiO_xN_y thin films with interleaved SiO_2 spacers. Bright green emission and efficient energy transfer from either Ce~(3+) ions or Ce silicates to Tb~(3+) ions has been observed in the co-doped single layer as a consequence of the strong ion-ion interaction. On the other hand, independent luminescence from Ce~(3+) and Tb~(3+) ions has been observed in the Ce~(3+) and Tb~(3+) co-doped bilayer and multilayer, providing a good scenario to develop light emitting devices with wide color tunability by varying the number of deposited films that contain each rare earth dopant. Moreover, the optoelectronic properties of Ce~(3+)- and/or Tb~(3+)-doped thin films have been studied by depositing transparent conductive electrodes over selected samples. An electroluminescence signal according to the rare earth transitions is obtained in all cases, validating the excitation of Ce~(3+) and Tb~(3+) ions upon electron injection. Also, the main charge transport of injected electrons has been evaluated and correlated with the layer stoichiometry. Finally, a simple reliability test has allowed disclosing the origin of the early breakdown of test devices, attributed to the excessive joule heating at filament currents that occur around a region close to the polarization point.