To clarify the origin of shallow electron traps in cerium-doped Gd3Al2Ga3O12 (Ce:GAGG), the optical properties of cerium-doped Lu3- xGdxAl2Ga3O12 crystals were investigated. Absorption spectra for x = 3 exhibited a prominent band at 12 000 cm-1 when excited by 3.31-eV ultraviolet light. This band has been previously attributed to shallow electron traps at defect complexes associated with oxygen vacancies. When Gd3+ ions were replaced with Lu3+ ions, the 12 000 cm-1 band weakened and disappeared completely for Ce:Lu3Al2Ga3O12. In addition, thermally stimulated luminescence glow curves were observed. Optically stimulated luminescence indicated that the energy of the conduction band minimum did not change by the presence of Lu3+ ions. Thus, the Gd3+ ions were important for the formation of shallow electron traps in Ce:GAGG. First-principles calculations implied that Gd3+ ions responsible for shallow electron traps formed antisite defects at GAGG octahedral sites. Hence, defect complexes of antisite Gd2+ ions adjacent to oxygen vacancies were the most plausible candidates for shallow electron traps in Ce:GAGG.
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