Crystal-phase-specific near-infrared photoluminescence from Er3+-doped Bi2O3 thin films

Abstract

Er3+-doped Bi2O3 films were sputter deposited on Si(100) substrates at room temperature with H2O vapor as an oxygen source gas. Crystal phases appearing after postannealing in an O2 atmosphere included single-phases of α-Bi2O3, γ-Bi2O3, and δ-Bi2O3, as well as a mixed phase of α-Bi2O3 and γ-Bi2O3. Selection of the crystal phase was possible in terms of H2O pressure and postannealing temperature. Photoluminescence spectra from Er3+ ions excited at a laser wavelength of 532 nm revealed distinct spectral features specific to the crystal phases. A clear crystal-field splitting feature consisting of eight emission lines was observed in PL spectra from sufficiently oxidized α-Bi2O3:Er films, indicating that Er3+ ions occupied low-symmetry C2v sites of Bi3+. The optimum annealing temperature for optical activation of Er3+ ions was between 400 and 450 °C. The emission intensity of α-Bi2O3:Er deposited with H2O was seven times higher than that of α-Bi2O3:Er deposited with O2 probably because larger numbers of Er3+ could substitute Bi3+ sites under reduced condition. Increasing deposition temperature lowered the emission intensity due to the loss of OH and H species from the as-deposited films. The emission spectra of γ-Bi2O3:Er were featureless and its emission intensity was one order of magnitude lower than that of α-Bi2O3:Er. The low-temperature phase of δ-Bi2O3:Er turned out to be entirely emission inactive. Reactions at the interface between the Bi2O3 film and the Si substrate were promoted upon postannealing at 500 °C. The resulting Bi2SiO5:Er exhibited a weak emission spectrum with four emission lines, which reflected occupation at tetragonal Bi3+ sites in the (Bi2O2)2− layers.