Photoluminescence of trivalent rare-earth ions codoped in ZnO thin films: Competing site occupation by Eu3+ and Er3+ ions

Abstract

The effect of codoping two kinds of trivalent rare-earth ions in ZnO thin films was investigated in terms of photoluminescence (PL) intensities relative to those of singly doped ones. ZnO films codoped with 1 at.% of Eu3+ and a variable concentration of Er3+ were deposited on Si(100) and SiO2 substrates. With preference to securing emission signals from Eu3+, the deposition was carried out under flow of H2O vapor gas. After the samples were post annealed at various temperatures, their PL spectra upon bandgap excitation at 325 nm were monitored. It was found that the Eu3+ emission signal at 612 nm became steeply attenuated as the Er3+ concentration increased, whereas the Er3+ emission signal was absent from the films deposited on Si(100) substrate. The necessary conditions for Er3+ emission signal at 665 nm to appear were (i) deposition on SiO2 substrate at low H2O pressure, (ii) high Er content close to 3 at.%, and (iii) post annealing above 700 °C. Conditions (i) and (iii) correspond to minimizing the influence of hydrogen-containing (OH and H) species. Thus, the emissions from Eu3+ and Er3+ do not appear together because Eu3+ emission requires a hydrogenated ZnO host crystal, whereas Er3+ emission prefers a hydrogen-free environment. The observation of Er3+ emissions from films on SiO2 but not from films on Si reflected the stiffness of the Si crystal and the flexibility of the SiO2 network. Strain in the ZnO host crystal, which was caused by rare-earth ions substituting at Zn2+ sites, would have been relieved by slightly deforming the amorphous SiO2 network. The concentration quenching for codoping with Eu3+ and Er3+ was greater than that for single doping with Eu3+ or Er3+. We propose an occupation-site-competing model wherein only limited numbers of emission-active Zn2+ sites are available for substitution by Eu3+ and Er3+