Shallow electron traps formed by Gd²⁺ ions adjacent to oxygen vacancies in cerium-doped Gd₃Al₂Ga₃O₁₂ crystals

To clarify the origin of shallow electron traps in cerium-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG), the optical properties of cerium-doped Lu_{3- x}Gd_xAl₂Ga₃O₁₂ 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.