Yu. Zorenko

Luminescence of F + and F centers in YAlO3

In YAlO3 crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ=30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F+ centers in the band at 355 nm, with τ=2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F+ and F centers in oxides of the Al2O3-Y2O3 system is analyzed on the example of the compounds α-Al2O3, YAlO3, and Y3Al5O12. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed.

Luminescence properties of phosphors based on Tb3Al5O12 (TbAG) terbium-aluminum garnet

The processes of excitation energy transfer in phosphors based on single-crystal Tb3Al5O12:Ce (TbAG:Ce) and Tb3Al5O12:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb3+ cations) to Ce3+ and Eu3+ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb3+ ions to Ce3+ and Eu3+ ions and from Ce3+ ions to Eu3+ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb3+ cation sublattice.

The α-particle excited scintillation response of the liquid phase epitaxy grown LuAG:Ce thin films

Liquid phase epitaxy grown Lu3Al5O12:Ce (LuAG:Ce) 20μm thick films and plate cut from the bulk Czochralski-grown LuAG:Ce crystal were prepared for comparison of photoelectron yield (PhY) and PhY dependence on shaping time (0.5–10μs). Am241 (α particles) was used for excitation. At the 0.5μs shaping time, the best film shows comparable PhY with the bulk sample. PhY of bulk material increases noticeably more with shaping time than that of the films. Energy resolution of films is better. Influence of Pb2+ contamination in the films (from the flux) and antisite LuAl defect in bulk material is discussed.

Mechanism of dissipation of the excitation energy in garnet oxides doped with rare-earth ions with 4f-5d transitions

Mechanisms of dissipation of the excitation energy in garnet crystal phosphors Y3Al5O12 and Y3Ga5O12 doped with rare-earth ions with interconfiguration transitions are studied depending on the Ga concentration.

Electronic structure of Ce3+ multicenters in yttrium aluminum garnets

Low temperature, infrared, and visible-ultraviolet absorption spectra of yttrium aluminum garnet (YAG) bulk crystals and epitaxial layers doped with Ce are presented. In the region of intra-configurational 4f–4f transitions, the spectra of the bulk YAG crystals exhibit existence of at least two different Ce3+ related centers, a major one associated with Ce in regular positions substituting yttrium and also additional center, due to so called antisite positions in the garnet host, i.e., ions in the Al positions. Crystal field analysis based on exchange charge model exhibit excellent agreement with the experimental data for major Ce3+ center.

New scintillation detectors based on oxide single crystal films for biological microtomography

Scintillation characteristics of the Lu3Al5O12-based single crystal films, grown on the Y3Al5O12 (YAG) substrate by liquid phase epitaxy, were explored and improved for the application in biological microtomography as X-ray image screens. To achieve a high spatial resolution, the film must be of about 1mm thickness, a large effective atomic number Zeff being necessary for a sufficient absorption in the X-ray range. The Lu3Al5O12:Ce 3+ garnet (ZeffE60) is a proper heavy material for such films. However, a stable growth of this film on the YAG substrate is hampered by the lattice mismatch between the film and substrate. This mismatch was duly reduced with the use of specially chosen isoelectronic impurities which simultaneously enhance the light yield of the film, and the overlap of its luminescence spectrum with the spectral sensitivity of the photoreceiver (CCD camera). r 2003 Elsevier Science B.V. All rights reserved. PACS: 07.85.Tt; 29.40.M; 68.55; 78.66

Development of Novel UV Emitting Single Crystalline Film Scintillators

The work is dedicated to the development of new types of UV-emitting scintillators based on single crystalline films (SCF) of aluminum perovskites and garnets grown by the liquid phase epitaxy method. The peculiarities of growth and properties of the following three types of UV SCF scintillators are considered in this work: i) Ce-doped SCF of Y-Lu-Al-perovskites with the Ce3+ emission in the 360-370 nm range of the decay time of 16-17 ns; ii) Pr-doped SCF of Y-Lu-Al garnets with the Pr3+ emission in the 300-400 nm range of the decay time of 13-17 ns; iii) La3+ or Sc3+ doped SCF of Y-Lu-Al-garnets, emitting in the 290-400 nm range due to formation of the LaY,Lu, ScY,Lu and ScA1 centers with decay time of the order of hundreds of nanoseconds. Preliminary test of these SCFs scintillators for visualization of X-ray images at European Synchrotrons Radiation Facility are presented as well.

Luminescence of Sc3+ and La3+ isoelectronic impurities in Lu3Al5O12 single-crystal films

The time-resolved luminescence and luminescence excitation spectra, and luminescence decay kinetics at 8 and 300 K of Lu3A15O12 (LuAG) single-crystal films doped with Sc3+ and La3+ isoelectronic impurities and excited by synchrotron radiation are investigated. It is established that the La3+ isoelectronic impurity in the ⨑ub;c⫂ub; positions of the garnet lattice forms LaLu3+ luminescence centers emitting in the band with λmax = 280 nm and the decay time of the main component τ = 300 ns at 300 K. The Sc3+ isoelectronic impurity located in the ⨑ub;c⫂ub; and (a) positions of the LuAG lattice forms two luminescence centers, ScLu3+ and ScAl3+, emitting in the bands with λmax = 290 nm and τ = 240 ns and λmax = 335 nm and τ = 375 ns, respectively, at 300 K. It is shown that the luminescence excitation of the La3+ and Sc3+ isoelectronic impurities in LuAG single-crystal films occurs through radiative decay of excitons localized near LaLu3+, ScLu3+, and ScAl3+ centers. The energies of formation of these excitons are determined to be 6.8, 6.88, and 7.3 eV, respectively. It was found that the excited state of the excitons genetically related to the LaLu3+, ScLu3+, and ScAl3+ enters has two radiative levels with different transition probabilities. This configuration leads to the presence of fast (2.3–8.4 ns) and slow (150–375 ns) main components in the luminescence of the centers formed by isoelectronic impurities in garnets.

Luminescence of excitons and antisite defects in Lu3Al5O12:Ce single crystals and single-crystal films

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce3+ ion) in phosphors based on Lu3Al5O12:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the LuAl3+ type and vacancy-type defects. The luminescence band with λmax = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λmax = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λmax = 253.5 nm occurs predominantly near LuAl3+ ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λmax = 325 nm (τ = 540 ns), which is excited in the band with λmax = 175 nm, is due to the radiative recombination of electrons with holes localized near LuAl3+ ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce3+ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce3+ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f-5d absorption band of Ce3+ ions with λmax = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce3+ ions is significantly smaller due to a low concentration of these types of defects.

Growth and luminescence properties of single-crystalline films of RAlO3 (R = Lu, Lu-Y, Y, Tb) perovskite

The conditions and the mechanisms of crystallization by liquid-phase epitaxy of undoped and Ce3+-doped RAlO3 (R = Lu, Lu-Y, Y, Tb) single-crystalline films on YAlO3 single-crystalline substrates at a considerable mismatch between the lattice constants of the single-crystalline films and the substrate are analyzed. The maximum values of this mismatch at which the stable growth of the single-crystalline films is observed are determined. It is demonstrated that transition layers are present between the substrates and the grown single-crystalline films in which the difference between the lattice parameters of the single-crystalline film and the substrate is diminished. The optical and luminescence characteristics of the undoped and Ce3+-doped RAlO3 (R = Y, Y-Lu, Lu, Tb) single-crystalline films, as well as the scintillation characteristics of the (Lu-Y)AlO3: Ce single-crystalline films under excitation by alpha-particles of the 239Pu source (5.15 MeV), are investigated.