Terbium-doped gadolinium garnet thin films grown by liquid phase epitaxy for scintillation detectors

Abstract

Single-crystal films of terbium-doped gadolinium gallium garnet (Gd₃Ga₅O₁₂:Tb) were grown by the isothermal dipping liquid phase epitaxy method on undoped (111)-oriented GGG substrates using PbO/B₂O₃ as a solvent. The effect of the Tb³⁺ doping level (2 to 10 at%) on the growth parameters, structure, composition, morphology, and emission properties of the films under optical and X-ray excitation was systematically studied. The saturation temperature increased almost linearly with the Tb content. The Tb³⁺-doped films exhibit a very low lattice mismatch of less than 0.05% with respect to the GGG substrate. The dopant ions are uniformly incorporated in the layers, with a segregation coefficient close to unity. The conversion efficiency of the films is optimized for a doping level of 6 at% Tb³⁺ in the solution, reaching a maximum light output of 52% with respect to a reference bulk YAG:Ce crystal. The green emission of Tb³⁺ ions at 543 nm matches with the maximum of sensitivity of CCD/CMOS sensors. The luminescence lifetime of the ⁵D₄ Tb³⁺ emitting state amounts to ∼2.3 ms and is weakly dependent on the doping level. Minimum afterglow intensities are reached for the GGG:Tb films, as compared to other currently employed scintillators. Gd₃Ga₅O₁₂:Tb single-crystalline films represent a viable solution for developing novel scintillators providing high efficiency and sub-μm spatial resolution for X-ray imaging.

Fig. 1. As grown GGG:Tb/GGG epitaxies: (a) photographs under natural light (epitaxies no. 3, 7, and 10, with 2 at%, 6 at%, and 10 at% Tb³⁺ in the flux, respectively); (b) a photograph under UV illumination, λ_exc = 313 nm (epitaxy no. 7); (c) a confocal laser microscopy image of an end-facet (epitaxy no. 7) using a ×50 objective.

Fig. 2. Influence of Tb doping level in the solution on the growth parameters of the GGG:Tb epitaxial layers: (a) variation of the growth rate with the growth temperature for various Tb doping levels; (b) range of the used growth temperatures for employed Tb doping levels. Numbers – epitaxy no., see Table 1.

Fig. 3. Crystalline structure of body-centered cubic Gd₃Ga₅O₁₂:Tb (sp. gr. Ia3̄d – O_h¹⁰) after the description reported in the literature:: (left) a fragment of the structure within a single unit-cell (black lines); (right) the coordination polyhedra [Tb|GdO₈], [GaO₆]and [GaO₄].

Fig. 4. Influence of the Tb doping level on the lattice mismatch between the GGG substrates and the doped epitaxial layers: (a) X-ray diffraction (ω, 2θ) scans at the (888) reflection; (b) linear relation between the lattice mismatch and the Tb content.

Fig. 5. Percentages of atoms for the different constituents across the GGG:Tb epitaxy no. 4 with 4 at% Tb³⁺ in the flux: (a) O; (b) Ga; (c) Gd; (d) Tb. Positive positions correspond to the 12.7 μm-thick Tb³⁺-doped layer, and negative ones – to the GGG substrate. Dashed vertical lines – the layer/substrate interface. Solid horizontal lines – the stoichiometric composition for undoped GGG.

Fig. 6. Comparison of the WDS spectra of a GGG substrate and a Tb³⁺-doped layer (epitaxy no. 4 with 4 at% Tb³⁺ in the flux). Symbols – contributions of Gd, Tb and Ga assigned using experimental data reported in the literature.

Fig. 7. Atomic force microscopy images of the as-grown faces of a GGG:Tb layer (epitaxy no. 8): (a and b) the upper face over a 50 × 50 μm² area: (a) 3D rendering, (b) top view; (c and d) the down face over a 50 × 50 μm² area: (c) 3D rendering, (d) top view; (e and f) the down face over a 2.5 × 2.5 μm² area: (e) 3D rendering, (f) top view.

Fig. 8. μ-Raman spectra of a GGG substrate and layer doped with 2 at% Tb³⁺ (epitaxy no. 3). The inset shows a close look at the Raman peak at 741 cm⁻¹ for a GGG substrate and layers doped with 2 at% and 6 at% Tb³⁺ (epitaxies no. 3 and 6, respectively). λ_exc = 514 nm. Numbers – Raman frequencies in cm⁻¹.

Fig. 9. Transmission spectra of GGG:Tb epitaxies no. 3, 5 and 7 with 2 at%, 4 at%, and 6 at% Tb³⁺ in the flux, respectively, in the UV-visible spectral range. Symbols – contributions of Gd³⁺ and Tb³⁺ ions, and compensation of the oxygen vacancies V_O²⁻, dashed grey line – theoretical limit T₀ set by Fresnel losses after Wood and Nassau.

Fig. 10. Excitation and luminescence properties of Tb³⁺ as dopant in GGG layers grown by LPE: (a) partial energy-level scheme of Tb³⁺ (solid black lines) and Gd³⁺ ions (dashed orange lines) after Carnall et al.,arrows – 4f–4f transitions in absorption and emission of Tb³⁺, NR – multiphonon non-radiative relaxation, grey area – conduction band of Tb³⁺, blue areas – excited configuration 4f⁷5d¹, LS and HS – low- and high-spin states, respectively; (b) photoluminescence excitation spectrum of GGG:Tb epitaxy, λ_lum = 543 nm, * – Gd³⁺ 4f–4f excitation lines, diamonds – Tb³⁺ inter-configurational 4f⁸ → 4f⁷5d¹ transitions; (c) photoluminescence spectrum of Tb³⁺ ions, λ_exc = 488 nm, square – Eu³⁺ impurities in the GGG substrate; (d) μ-luminescence mapping across the end-facet of the GGG:Tb/GGG epitaxy monitoring the peak intensity at 544 nm (the ⁵D₄ → ⁷F₅ transition). The detailed (b) excitation and (c) luminescence spectra are available in the ESI materials.

Fig. 11. Photoluminescence decay curves from the ⁵D₄ state of Tb³⁺ ions in GGG:Tb layers with various doping levels (epitaxies no. 3, 5 and 6, with 2 at%, 4 at%, and 6 at% Tb³⁺ in the flux, respectively). λ_exc = 488 nm, λ_lum = 543 nm. Circles – experimental data, lines – linear fit, τ_lum – luminescence lifetime of the ⁵D₄ emitting state of Tb³⁺ ions.

Fig. 12. Radioluminescence spectra of Tb³⁺ ions in GGG epitaxial layers doped with 2 at%, 4 at%, and 6 at% Tb³⁺ in the flux, respectively: (a) UV-blue emission from the ⁵D₃ manifold; (b) visible emission from the ⁵D₄ manifold. X-ray illumination, 8 keV. Squares – Eu³⁺ impurities in the GGG substrate. The inset in (a) shows the partial energy-level scheme of Tb³⁺ after Carnall et al.,arrows – 4f–4f transitions in emission from the ⁵D₃ manifold, NR – multiphonon non-radiative relaxation.

Fig. 13. Afterglow properties of various rare-earth-doped garnet single crystals and single-crystalline films: (a) influence of the exposure time to X-rays on the afterglow of a GGG:Tb epitaxial film and a GGG:Tb single crystal, for exposure times of 0.1 s, 1 s and 10 s; (b) comparison of the afterglow for miscellaneous garnet single crystals and SCFs doped with various rare-earth ions, namely Ce³⁺, Eu³⁺ and Tb³⁺ (10 s exposure time). X-ray illumination, 8 keV.

Fig. 14. Influence of Tb doping level in the solution on the relative light output of the GGG:Tb epitaxial layers. Arrows – general evolution of the growth rate and temperature during the elaboration of the epitaxies. A 500 μm thick YAG:Ce single crystal scintillator was used as a reference.