Comparison of quenching mechanisms in Gd3Al5−xGaxO12:Ce3+ (x = 3 and 5) garnet phosphors by photocurrent excitation spectroscopy

Abstract

In this work we present the results of photocurrent excitation spectroscopy (PCE) of Gd₃Al₂Ga₃O₁₂:Ce³⁺ (GAGG:Ce³⁺) and Gd₃Ga₅O₁₂:Ce³⁺ (GGG:Ce³⁺) performed at temperatures ranging from 100 to 500 K supplemented by spectroscopic measurements (steady state and time resolved photoluminescence spectroscopy) performed at temperatures ranging from 10 to 500 K and at high pressure up to 300 kbar. The PCE spectra contain bands related to transitions from the ground state ²F_5/2 of the 4f¹ electronic configuration to the crystal field split states related to the 5d¹ electronic configuration of Ce³⁺. This implicates the presence of the autoionization process – transfer of electrons from the localized, excited states of Ce³⁺ to the conduction band (CB), directly linked to luminescence quenching of Ce³⁺. The mechanism of autoionization of GAGG:Ce³⁺ and GGG:Ce³⁺ was determined to be different on the grounds of differences in temperature dependence of photocurrent intensity. The latter system exhibits autoionization, which occurs when all of the 5d excited states are degenerated with the CB, whereas in the former system, the autoionization process is thermally assisted with an activation energy barrier (distance to the edge of the CB) of approximately 1600 cm⁻¹. In GGG:Ce³⁺ the degeneracy of 5d¹ states of Ce³⁺ was lifted by application of high pressure, shifting the edge of the CB up and exposing Ce³⁺ luminescence at 20 kbar. Further spectroscopic analyses of the pressure–temperature dependence of the luminescence decay time as well as the temperature dependence of photocurrent intensity of GGG:Ce³⁺ have independently shown existence of a luminescence quenching state located approximately 600 cm⁻¹ below the CB, attributed to the impurity trapped exciton.