| Abstract [eng] |
Cerium-doped lutetium aluminum garnet (LuAG:Ce) exhibits high light yield and strong resistance to ionizing radiation. These properties make this material attractive for applications in high-energy particle detectors, medical diagnostics, and radiation safety systems. However, one of the major limitations of LuAG:Ce scintillators is the relatively long scintillation decay time caused by antisite defects such as Luₐₗ formed in the crystal lattice. Long decay times may lead to signal overlap, reducing the temporal resolution of detectors and distorting the measured particle energy. In addition, the detector dead time increases, preventing the registration of subsequent signals. One possible solution to this problem is the growth of scintillator crystalline layers using the liquid-phase epitaxy (LPE) method. Due to the lower growth temperature, this technique reduces the probability of defect formation and enables the growth of monocrystalline layers. Another possible approach is the co-doping of garnet scintillators with Mg²⁺ ions. Magnesium ions accelerate recombination processes and introduce non-radiative recombination pathways, in this way shortening the luminescence decay time. The aim of this Master’s thesis was to investigate the dependence of the photoluminescence properties of liquid-phase epitaxy grown lutetium aluminum garnet doped with cerium (LuAG:Ce) on magnesiun co-doping concentration at different temperatures using different experimantal techniques. During the study, six LuAG:Ce,Mg samples with magnesium concentrations ranging from 0 to 3000 ppm relative to lutetium were investigated. Surface area images of the samples were acquired to estimate defect density. Confocal microscopy measurements were performed to evaluate defect size, topography, and photoluminescence properties. Absorption spectra were measured using a UV–Vis spectrometer. Photoluminescence decay kinetics were measured in the temperature range from 73 to 750 K by exciting the Ce 5d₁ ir 5d₂ levels using a Streak camera system, photoluminescence spectra were recorded using a time-integrated photoluminescence setup. Study results showed that liquid-phase epitaxy grown cerium and magnesium doped lutetium aluminum garnet exhibited hexagonal and triangular surface pit defects, which were likely formed due to the BaO-B₂O₃-BaF₂ flux used during crystal growth. The defect area and density decreased with increasing magnesium concentration in the samples. Increasing Mg concentration resulted in enhanced absorption in the ultraviolet spectral region and reduced absorption corresponding to the 4f → 5d₁ and 4f → 5d₂ transitions, which is associated with the stabilization of cerium in the Ce⁴⁺ charge state. The photoluminescence intensity under 5d₂ excitation showed a stronger dependence on magnesium concentration and temperature, supporting the hypothesis that Ce³⁺ - Mg²⁺ pairs reduce the activation energy and introduce non-radiative recombination pathways. Furthermore, the photoluminescence decay time decreased with increasing magnesium concentration and temperature, which is likely related to the stabilization of Ce⁴⁺ due to improved recombination efficiency and the formation of Ce³⁺ - Mg²⁺ pairs that create additional non-radiative recombination channels. |
