Effect of Carbon Doping on F-Type Defects in YAG and YAG:Ce Crystals

Ce-doped yttrium aluminum garnet (YAG) is among the efficient rare-earth garnet scintillators and phosphors. This paper addresses a computational and experimental study of complex F-type defect centers formed in YAG by doping with cerium and carbon. The formation energies determined by ab initio calculations of different defects and atomic chemical potentials in YAG:Ce,C are discussed in O-poor and O-rich conditions corresponding to the crystal growth under Ar+CO reducing atmosphere, and post-growth annealing in air. It is shown that as-grown crystals contain numerous positively charged defects responsible for F-type center formation, whereas mainly negative charged defects are remained after the annealing in air. The negatively charged defects located nearby Ce luminescence center occupying Y site may stabilize cerium in the tetravalent state and promote a hole transport to this luminescence center. This describes a high scintillation efficiency of Ce,C-doped garnets. Furthermore, YAG:C and YAG:Ce,C with a high carbon concentration, besides of luminescence peaked at 400 ns ascribed to F⁺-type centers, possess a fast luminescence peaked around 650 nm that may be attributed to the complex defect centers. These unveiled features of the luminescence process in rare-earth garnets may contribute to the development of efficient luminescence materials.