Photostimulated Defect Creation Processes in the Undoped and Bi³⁺‐Doped Ca₃Ga₂Ge₃O₁₂ Garnets

Abstract

Bismuth-doped compounds, due to their excellent luminescence performance and tunable luminescence properties, attract increasing attention as promising materials for applications in different fields, e.g., in fiber lasers, broadband optical amplifiers, bioimaging, scintillators, light-emitting diodes, and solar cells (see, e.g., refs. [1-12] and references therein). Among them, the materials where the trivalent Bi 3þ ion substitutes for a trivalent rare-earth ion were investigated in most detail (see, e.g., review articles [1][2][3][4]13] and references therein). However, due to the relatively high cost and limited resources of rare earths, their high supply disruption risk, and the environmental issues related to their mining and refinement, these materials should be replaced by highly effective rare-earth-free compounds. Recently, these problems were analyzed in detail, and alternative materials suitable for various lighting applications were considered (see, e.g., Pavel et al. and Ricci [14,15] and references therein).In this work, the Ca 3 Ga 2 Ge 3 O 12 :Bi garnet was chosen for investigation as one of the rare-earth-free compounds. In the crystal lattice of Ca 3 Ga 2 Ge 3 O 12 , the Ca 2þ , Ga 3þ , and Ge 4þ ions are located in the dodecahedral, octahedral, and tetrahedral sites and surrounded by eight, six, or four oxygen ions, respectively. [16,17] The first detailed investigation of undoped and Bi 3þ -doped Ca 3 Ga 2 Ge 3 O 12 garnets by the X-ray diffraction and time-resolved photoluminescence spectroscopy methods was conducted in our recent work. [18] It was found that in the luminescence spectrum of the undoped Ca 3 Ga 2 Ge 3 O 12 sample, the main complex emission band is peaking at 2.74 eV and excited around 5.04 eV and in the band-to-band absorption region (with E exc ≥ 5.5 eV). It was shown that this emission is not of exciton-like origin and suggested that the 5.04 eV excitation results in the charge-transfer transitions from the oxygen O 2À levels at the top of the valence band (VB) to the ground state of intrinsic defects.In the luminescence spectrum of Ca 3 Ga 2 Ge 3 O 12 :Bi, the Bi 3þrelated ultraviolet (UV) and visible (vis) emission bands were observed under excitation in the complex %4.45 eV absorption band of Bi 3þ ions corresponding to the 1 S 0 ! 3 P 1 electron transitions of a free Bi 3þ ion. From the temperature dependences of the luminescence decay kinetics, the origin, structure, and parameters of the relaxed excited states (RESs) corresponding to each emission band were determined. It was concluded that the UV and vis emissions are of different origin. Four UV emission bands of Ca 3 Ga 2 Ge 3 O 12 :Bi (located around 3.9 and 3.06 eV) were shown to arise from the triplet RES of Bi 3þ ions having different nearest surroundings; i.e., these bands correspond to the 3 P 1,0 ! 1 S 0 transitions of a free Bi 3þ ion. Indeed, the spin-orbit splitting energies of their triplet RES are rather large and vary from 56 to 100 meV, which is characteristic for the triplet