Cerium-doped mixed garnet-type single crystals (GdxY1-x)3Al2Ga3O12 with different yttrium content have been fabricated and studied as a prospective scintillating material enabling improvement of ...scintillation properties by tuning the composition of the matrix-building crystal. The influence of the matrix composition on the emitting Ce ion is studied using linear and time-resolved nonlinear optical absorption and time-resolved photoluminescence spectroscopy. The study of photoluminescence at resonant excitation revealed a composite origin of Ce3+ emission band. This behavior is interpreted by the contribution of Ce3+ ions located in inequivalent positions expected due to the disorder caused in the garnet-type lattice of a mixed crystal by compositional fluctuations. The substitution of gadolinium by yttrium in the lattice results not only in an emission blue shift and in decreased splitting of the lowest doublet 5d state of Ce3+ but also in an increased separation between the lowest doublet level 5d1 and the lowest triplet 5d3 level, as well as in changing the rates of intracenter and extracenter energy relaxation.
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•Photoluminescence spectrum is composite due to inequivalent positions of Ce3+ ions.•Decay time of Ce3+ emission decreases from 55 ns at 1.9 eV to 40 ns at 2.5 eV.•Substitution of Gd by Y decreases 5d1-5d2 splitting of Ce3+ and increases 5d1-5d3 separation.•Introduction of Y in GAGG lattice increases Ce3+ intracenter relaxation time.•Capture of trapped electrons to nonradiative recombination centers is enhanced in GYAGG.
A model for the dynamics of a coupled system of free and localized carriers in semiconductors with strong carrier localization is suggested. Kinetic Monte Carlo technique is exploited for ...simulations. The model is verified by fitting the simulated and experimental temperature dependences of photoluminescence (PL) band intensity, peak position, and band width, and the carrier density dependence of PL efficiency in AlGaN quantum wells. The influence of carrier localization conditions on the dominating carrier migration and recombination processes is revealed. The efficiency droop effect is shown to be caused by peculiarities of carrier localization without significant influence of Auger recombination.
Coincidence time resolution (CTR) of scintillation detectors based on Ce- and Mg-codoped Gd 3 Al 2 Ga 3 O 12 (GAGG) scintillation crystals and high-density silicon photomultipliers (SiPMs) is shown ...to be 165 ps (full width at half maximum) for 511-keV <inline-formula> <tex-math notation="LaTeX">\gamma </tex-math></inline-formula>-quanta, approaching that achieved by using LSO scintillators. To study the prospective for further improvement of the time resolution, the population of the emitting Ce centers was investigated by optical pump and probe technique using selective photoexcitation and probing by a white light continuum with subpicosecond time resolution. The importance of free electron trapping for excitation transfer to emitting Ce ions was revealed. The influence of transfer delay on the scintillation response time is described, and the dynamics of electron relaxation to the lowest excited level of Ce ion is studied experimentally and analyzed by taking into account intracenter relaxation and relaxation via conduction band. The influence of electron diffusivity on the rise time of the population of the emitting level is described. It is shown that codoping of GAGG:Ce by magnesium even at the level as low as 10 ppm efficiently decreases the scintillator response time by enhancing the electron diffusivity.
A new inorganic scintillation material based on Ba-Gd silica glass doped with cerium (BGS) is fabricated and studied. With the highest light yield among heavy glasses at the level of 2500 ph/MeV and ...fast scintillation response, the new scintillator ensures a good coincidence time resolution of < 230 ps FWHM for 511 keV γ-quanta from a 22Na source and SiPM readout. In addition to good performance in γ-quanta detection, the material demonstrates capability for efficient detection of low-energetic neutrons. The scintillator is produced by exploiting the standard industrial glass technology, which allows for an unlimited scaling up the conversion of raw material into a high-quality scintillator at a high rate. The glass can be casted in application-specific molds, so minimizing the material losses. The presented glass scintillator has potential for further improvement of its light output and scintillation response time.
Radiation detectors based on multiple single crystalline scintillating fibers are promising for next generation of heterogeneous electromagnetic calorimeters to be exploited in high-luminosity ...colliders for high-energy physics experiments. These novel fiber-based detectors consist of long single crystalline rods placed in heavy metal absorber, thus, the light propagation conditions in these fiber-like structures are of crucial importance for obtaining uniform response to electromagnetic shower. In this work, the uniformity of Czochralski-grown cerium-doped gadolinium aluminum gallium garnet fibers was studied by employing confocal optical microscopy. Growth related defects were observed on a micrometer scale. Light attenuation length was estimated and found to vary in nominally identical fibers. The influence of crystal growth defects and surface polishing quality on light propagation is discussed.
The aim of this work has been to improve the time resolution of radiation detectors for future high-energy physics experiments and medical imaging applications. Ce-doped oxyorthosilicate Lu2SiO5:Ce ...(LSO) and mixed oxyorthosilicate Lu1.6Y0.4SiO5:Ce (LYSO) have been investigated as prospective scintillators for such high-time-resolution applications. A differential optical absorption technique with sub-picosecond time resolution upon selective excitation of Ce3+ ions to different excited states has been adopted to study carrier dynamics in these scintillators, and coincidence time resolution measured using 511 keV γ-quanta has been exploited to test their timing properties. A delay in population of the emitting level of Ce3+ has been observed, and is interpreted in terms of electron trapping, which is more pronounced in mixed yttrium-containing LYSO crystals due to composition fluctuations. It is shown that the delay, which affects the luminescence response time, can be eliminated by co-doping of LYSO:Ce with calcium at concentrations as low as 5 ppm. The faster kinetics of electron transfer correlates with a better coincidence time resolution. Thermalization and spatial distribution of non-equilibrium carriers has been studied theoretically to link the results obtained by the time-resolved differential optical absorption technique with the behavior of the non-equilibrium carriers generated by irradiation.
•The time resolution of LYSO is more affected by carrier trapping than that of LSO.•Carrier trapping in LYSO is enhanced by fluctuations in the compound composition.•Ca-co-doping, even at a concentration of 5 ppm, improves the response time of LYSO:Ce.•Femtosecond pump-probe spectroscopy is a useful tool for improving Ce-doped scintillators.
The influence of co-doping of Gd3Al2GA3O12:Ce (GAGG:Ce) scintillator with magnesium on the rise time of luminescence response was studied in two GAGG:Ce crystals grown in nominally identical ...conditions except of Mg co-doping in one of them. Time-resolved photoluminescence spectroscopy and free carrier absorption techniques were exploited. It is evidenced that the Mg co-doping decreases the rise time down to sub-picosecond domain. Meanwhile, the light yield decreases by ∼20%. Thus, the feasibility of exploitation of the fast rise edge in luminescence response for ultrafast timing in scintillation detectors is demonstrated. The role of Mg impurities in facilitating the excitation transfer to radiative recombination centers is discussed.
•Photoluminescence rise time is compared in GAGG:Ce and GAGG: Ce,Mg.•Sub-picosecond photoluminescence rise time is revealed in GAGG:Ce,Mg.•The role of Mg in facilitating the excitation transfer is discussed.
Dynamics of radiative and nonradiative recombination of non-equilibrium carriers is investigated in thick AlGaN epitaxial layers with Al content ranging from 0.11 to 0.71. The internal quantum ...efficiency (IQE) in the epilayers was obtained using two approaches: either estimated from PL measurements or calculated using the recombination coefficients of a simple ABC model, retrieved by fitting the kinetics of light induced transient gratings (LITG). At photoexcited carrier densities below ~1019 cm−3, both approaches provided similar IQE values indicating that the simple ABC model is applicable to analyze carrier recombination at such carrier densities. The increase in IQE at higher carrier densities slowed down for the values extracted from PL considerably faster than for those obtained from LITG transients. This discrepancy is explained in terms of the mixed nature of the rate coefficient B caused by the onset of the density-activated nonradiative recombination at high carrier densities.
Dynamics of two distinct bands present in the photoluminescence spectra of non-polar m-plane InGaN/GaN multiple quantum wells (MQWs) was studied by using photoluminescence (PL), cathodoluminescence ...(CL) and differential transmission (DT) spectroscopy. The competition between these bands caused the large shift of peak emission wavelength with increasing excitation. DT measurements allowed attributing the high-energy PL band to the optical transitions between the ground QW states, while the low-energy PL band was assigned to the recombination of localized carriers. CL measurements confirmed the wide spot-to-spot dispersion of deep localized states and suggested that this disorder occurs on a small spatial scale. PL measurements showed that the distribution of localized states is very sensitive both to indium content and to structure parameters. Temperature-dependent PL studies revealed strong carrier-phonon interaction.
•Double-peaked emission spectra in non-polar m-plane InGaN MQWs.•Large energy difference (up to 300 meV) between the two PL bands.•High-energy band attributed to transitions between ground QW states.•Low-energy band attributed to recombination of localized carriers.•Wide and asymmetric PL spectra shape due to strong carrier-phonon interaction.
The band offset parameters of low-boron-content BGaN/GaN heterojunctions have been studied using x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) in BxGa1−xN epilayers (x 0.043) ...grown on GaN/sapphire and AlN/sapphire templates. A staggered-gap (type-II) band alignment has been identified at the BGaN/GaN heterojunction by XPS. A study of the red shift of deep-level-related yellow PL band and the band gap shrinkage of BGaN epilayers with increasing boron content confirmed the type-II band alignment and enabled us to estimate that the ratio of the conduction-to-valence band discontinuity is 57:43. It is also shown that the band gap bowing of the BGaN alloy system is accommodated in the conduction band.
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