Studies of trapping and luminescence phenomena in yttrium aluminum garnets
Varney, Christopher Richard
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Rare-earth-doped yttrium aluminum garnet (YAG) crystals are important photonic materials with a wide range of applications. The optical properties and performance of these crystals are largely governed by exciton dynamics, which is greatly affected by the presence of defects. In this work, the optical and scintillation properties of undoped and rare-earth-doped YAG crystals were studied in conjunction with thorough characterization of defects. First, a comprehensive study of optical properties, including absorption and luminescence, was carried out. Color centers were found to be present in both undoped and doped crystals. A new x-ray luminescence spectrometer was developed and installed to investigate both luminescence and scintillation properties in a new way. Defects that trap excitons were studied by thermoluminescence spectroscopy. Deep and shallow traps were identified by high and low thermoluminescence measurements and their energy levels in the band gap were calculated. Positron annihilation lifetime spectroscopy was carried out for the first time on YAG to provide information about defect types, structures, and concentrations. The positron measurements revealed the presence of isolated aluminum vacancies and defect complexes of aluminum and oxygen vacancies. They also revealed the dependence of defect structure on growth atmosphere and post-growth treatments. This knowledge gained from thermoluminescence and positron lifetime measurements elucidated the effects various defects have on scintillation properties and suggested ways to control them. Lastly, a new fast scintillator with good energy resolution is discussed.