We
report the results of the investigation of Ga-doping effects
on the electronic structure of the yttrium–aluminum garnet.
The bandgap structure was studied both experimentally using thermally
stimulated luminescence technique and theoretically by first-principles
density functional theory calculations. We observe a nonlinear decrease
in the conduction band minimum with respect to the vacuum referred
binding energy with an increase in Ga doping and argue that the effect
can be explained by taking into account different influences of the
distorted crystal field on the molecular orbitals of the crystal unit
cell. The reported nonlinear behavior is important for the band engineering
of scintillators based on multicomponent garnets.
Afterglow is an important phenomenon in luminescent materials and can be desired (e.g. persistent phosphors) or undesired (e.g. scintillators). Understanding and predicting afterglow is often based on analysis of thermally stimulated luminescence (TSL) glow curves assuming the presence of one or more discrete trap states. Here we present a new approach for the description of the time-dependent afterglow from TSL glow curves using a model with a distribution of trap depths. The method is based on the deconvolution of the energy dependent density of occupied traps derived from TSL glow curves using Tikhonov regularization. To test the validity of this new approach, the procedure is applied to experimental TSL and afterglow data for Lu 1 Gd 2 Ga 3 Al 2 O 12 :Ce ceramics co-doped with 40 ppm of Yb 3+ or Eu 3+ traps. The experimentally measured afterglow curves are compared with simulations based on models with and without the continuous trap depth distribution. The analysis clearly demonstrates the presence of a distribution of trap depths and shows that the new approach gives a more accurate description of the experimentally observed afterglow. The new method will be especially useful in understanding and reducing undesired afterglow in scintillators.
Light
yield, time response, afterglow, and thermoluminescence of
Ce-doped garnet scintillators and persistent luminescent materials
are controlled by a complex interplay between recombination and trapping/detrapping
processes. Extensive research has contributed to a good qualitative
understanding of how traps, impurities, and the presence of Ce4+ affect the materials properties. In this work we present
a quantitative model that can explain the thermoluminescence and afterglow
behavior of complex garnets. In particular, the model allows the determination
of capture rates and effective capture radii for electrons by traps
and recombination centers in Lu1Gd2Ga3Al2O12:Ce garnet ceramics. The model relies
on solving a set of coupled rate equations describing charge carrier
trapping and recombination in garnet ceramics doped with Ce and also
codoped with a known concentration of an intentionally added electron
trap, Yb3+. The model is supported by analysis of a complete
set of experimental data on afterglow, rise-time kinetics, and X-ray
excited luminescence which show that thermoluminescence/afterglow
are governed by trapping/detrapping processes following interactive
kinetics with dominant recombination channel. The underlying reason
for dominant recombination is the presence of a small fraction of
Ce4+ (≈2 ppm in the 0.2% Ce-doped sample) which
have a very high capture cross section (≈2.7 Å effective
radius) because of the Coulomb attractive nature of this recombination
center. The quantitative insights on capture cross sections and concentrations
of Ce4+ help to better understand the optical properties
of Ce-doped garnet scintillators and persistent luminescent materials
and serve in optimizing synthesis procedures by tuning the Ce3+/Ce4+ ratio by codoping with divalent cations
and annealing in an oxygen-containing atmosphere.
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