Radiative Recombination Centers Research Articles

Subpicosecond luminescence rise time in magnesium codoped GAGG:Ce scintillator

Abstract The influence of co-doping of Gd 3 Al 2 GA 3 O 12 :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.

Parameters of ZnO films with p-type conductivity deposited by high-frequency magnetron sputtering

It is demonstrated that a reduction in the defect concentration in ZnO films formed by high-frequency magnetron sputtering allows effective doping with acceptor impurities both in the cation (Li) and anion (N+) sublattices and p-type conductivity with reproducible charge-carrier parameters (concentration and mobility) to be obtained. ZnO films are doped with nitrogen by annealing in a high-frequency gas discharge atmosphere. Hall measurements by the Van der Pauw technique show that the deposition of thin Eu layers on the ZnO film surface increases the concentration and mobility of majority charge carriers. The embedding of metal impurities with different ionic radii (Ag and Au) in the cation sublattice of the ZnO films to compensate misfit stresses makes it possible to enhance the concentration of radiative-recombination centers.

Effect of intrinsic impurities and annealing conditions on dislocation‐related luminescence in self‐ion‐implanted Si

AbstractThe influence of intrinsic impurities (boron, phosphorus) in the original CZ‐Si on the behavior of the D1 dislocation‐related luminescence line has been investigated in the samples with dislocations introduced by Si+ ion implantation upon the change of annealing ambient. It is established that, for the phosphorus‐doped samples, the D1 line does not occur at the annealing at 1100 °C in inert ambient (N2), but its intensity increases with increasing the oxygen content. For the boron‐doped samples, this line arises in the case of annealing in an inert ambient, herewith the intensity practically does not depend on the composition of the annealing ambient. At the annealing in argon ambient at 900‐1100 °C, the maximal photoluminescence intensity is achieved for annealing at 900 °C. The results are interpreted in terms of interaction between boron and point defects as radiative recombination centers located in dislocations atmospheres. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Revisiting radiative deep-level transitions in CuGaSe2 by photoluminescence

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10 K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined from the temperature quenching depend on the excitation level and show unexpected values of 600 meV and higher. The thermal activation of non-radiative processes can explain the distortion of the ionization energies. Both the deep levels play a major role as radiative and non-radiative recombination centers for electrons and can be detrimental for photovoltaic applications.

Enhancement of Luminescence of Colloidal Ag2S Quantum Dots by Thionine Molecules

Enhancement of IR luminescence (1205 nm) of colloidal Ag2S quantum dots (QDs) with an average size of 2.5 ± 0.3 nm was detected upon excitation in the absorption band of thionine dye molecules (530–610 nm). It is found that the observed effect occurs during a hybrid association of Ag2S QDs with monomers of the cationic thionine (Th + ) molecule. It is concluded that the photosensitization of IR luminescence of colloidal Ag2S QDs is realized due to a resonance nonradiative transfer of electronic excitation energy directly to the centers of radiative recombination from the excited Th + molecules.

Blue iridium complexes as electron trapping sites and efficient recombination centres in poly(N-vinylcarbazole) seen by spectrally resolved thermoluminescence

Abstract Spectrally resolved thermoluminescence (SRTL) of poly(N-vinylcarbazole) (PVK) doped with blue iridium (III) complexes: bis(4,6-difluorophenyl)-pyridinato-N,C2)(picolinate) (FIrpic) or tris[2–(2,4–difluorophenyl)pyridine] (Ir(Fppy)3) is studied and compared with photoluminescence results. Monochromatic thermoluminescence (TL) curves registered in the temperature range 15–320 K indicate that besides trapping sites localised on the PVK matrix, deep electron traps on the blue iridium complexes molecules play a very important role. After doping with a small amount of FIrpic, a shift of dominant TL peak to higher temperature was observed. It shows the existence of slightly deeper traps in comparison with those in the neat PVK. This is a result of interaction of trapped carriers with randomly oriented high permanent dipoles of FIrpic. The SRTL experiments for different concentration of FIrpic provide evidence that the dopant molecules act as emission centres which effectively compete with other radiative recombination centres present in PVK matrix. Even at 0.1 wt% of the dopant molecules embedded in PVK, the emission originates almost exclusively from triplet excitons of the blue iridium complex. In addition, the highest intensity of the predominant maximum of TL occurs when the concentration of electron traps located on the dopant molecules is similar to the concentration of hole traps formed on the matrix. The studies confirm that host–guest system works very effectively as emission system in the case when the dopant molecules are not only efficient emitters but they act also as trapping sites.

Analysis of Thermoluminescence Glow Peaks in β-Irradiated TlGaSeS Crystals

Thermoluminescence properties of TlGaSeS layered single crystals were investigated in the temperature range of 280‐720 K. Thermoluminescence glow curve exhibited three peaks with maximum temperatures of 370, 437, and 490 K. Curve fitting, initial rise and peak shape methods were used to determine the activation energies of the trapping centers. All applied methods resulted with energies around 0.82, 0.91, and 0.99 eV. Dose dependence of the thermoluminescence intensity was also examined for the doses in the range of 0.7‐457.6 Gy. Peak maximum intensity of the observed peak around 370 K showed an increase up to a certain dose and then a decrease at higher doses. This non-monotonic dose dependence was discussed under the light of a reported model in which dierent kinds of competition between radiative and nonradiative recombination centers during excitation or heating stages of the thermoluminescence process are explained. DOI: 10.12693/APhysPolA.129.1165 PACS/topics: 78.60.Kn, 71.55.Ak, 61.82.Fk

Suppression of the thermal quenching of photoluminescence in irradiated silicon heterojunction solar cells

We report on the decrease in photoluminescence (PL) intensity with the increase of the sample temperature (thermal quenching) in crystalline silicon and its suppression after ion irradiation. The crystalline silicon surface was passivated with intrinsic and doped hydrogenated amorphous silicon (a‐Si:H) layer stacks as for making silicon heterojunction solar cell precursors. Low energy argon ion irradiation, in the range between 5 and 17 keV, was used for controlled defect formation either in the thin a‐Si:H top layer or at the interface with the crystalline silicon beneath. The irradiation defects introduce radiative recombination centers in the a‐Si:H as can be measured from PL at low temperature. Moreover, the irradiation and annealing result in a strong modification of the thermal quenching of the PL intensity up to 500 K, showing evidence for the strong reduction of thermally activated non‐radiative recombinations at the amorphous‐crystalline interface. This result can have implications in the field of crystalline silicon surface passivation.

Optical properties of Ge nanostructures embedded into porous alumina matrices

The effect of structural properties and the morphology of Ge@PAA nanocomposites synthesized by thermal deposition of Ge on porous anodic alumina matrices with different porosity on the band gap value and PL properties is investigated. PL in the range of 330–520 nm may be due to F and F2 luminescent centers in the surface of nanopores of PAA matrices. The density of electronic states in this interface depends on the temperature of matrices during deposition and on the surface morphology. The role of radiative recombination centers is played by the broken bonds in a thin intermediate Gex–Aly–Oz layer in the Ge/oxide matrix interface. No obvious effect of the crystalline structure of germanium nanoparticles on the PL maximum position is observed, but the spatial localization of electron–hole pairs of small-sized Ge crystallites of which nanoparticles consist leads to an increase in the optical band gap.

Spectrally resolved thermoluminescence versus electroluminescence spectra of PVK doped with 1 wt % of Ir(btp)2(acac)

Abstract Spectrally resolved thermoluminescence (SRTL) of poly( N –vinylcarbazole) (PVK) doped with 1 wt% of bis[2-(2′-benzothienyl)pyridinato-N,C 3’ ](acetylacetonate) iridium (Ir(btp) 2 (acac)) is studied and compared with electroluminescence and photoluminescence results. Monochromatic thermoluminescence (TL) curves registered in the temperature range 15–320 K indicate that besides trapping sites localised on the PVK matrix, deep traps on Ir(btp) 2 (acac) molecules play a very important role. The SRTL experiments provide evidence that the dopant molecules act as emission centres which effectively compete with other radiative recombination centres present in PVK matrix. The SRTL spectra are compared with electroluminescence spectra of the single layer polymer light emitting diodes based on the same compounds. From the comparison of the SRTL and EL spectra results one can notice that the same radiative recombination mechanisms occur under both optical excitation at low temperature and electrical excitation. Even at 1 wt% of the dopant molecules embedded in PVK, the emission originates almost exclusively from triplet excitons of Ir(btp) 2 (acac) molecules. The studies prove that host–guest system works very effectively in the case when the dopant molecules are not only efficient emitter but they act also as trapping sites.

Estimation of band gap of muscovite mineral using thermoluminescence (TL) analysis

Abstract In this article thermoluminescence (TL) mechanism in muscovite mineral has been reported in detail. The trap spectroscopy of the mineral has been studied from the TL glow curve of X-ray irradiated sample. A stable TL peak has been observed at around 347 K in the glow curve, however after annealing the sample above 573 K an additional peak is observed at around 408 K. In the TL emission spectra an emission peak has been observed at around 447 nm at TL peak maximum temperatures. The glow curves are analyzed by T m − T stop analysis, peak shape method and fractional glow technique. Analyses showed that there is a trap center and a radiative recombination center at depth around 0.71 and 2.78 eV from the conduction band. These two localized centers do not affected by annealing up to 773 K. However, annealing at 573 K (or above) a new electron trap center was found to generate at depth around 1.23 eV. This significant result has been confirmed with the help of phototransfer phenomena observed under UV irradiation. Due to UV irradiation on the excited sample, the transfer of trapped charges from the deeper trap level (1.23 eV) to the shallow level (0.71 eV) has been observed. Based on the analysis a schematic band diagram of muscovite crystal has been proposed. With the help of the band model, radiative as well as non-radiative recombination mechanisms have been discussed. The band gap of the material has been estimated to be around 5.09 eV and verified.

Observation of applied bias-dependent dot-like luminescence in GaInN-based light-emitting diodes

Abstract Forward and reverse bias-induced dot-like luminescence was investigated by evaluating the X-ray rocking curve, micro-photoluminescence (PL), and micro-electroluminescence (EL) of GaN-based light-emitting diodes (LEDs) grown on a normal sapphire substrate (NSS) and a patterned sapphire substrate (PSS). The current–voltage (I–V) characteristics indicated that the forward and reverse leakage currents of the PSS-based LEDs were lower than those of the NSS-based LEDs, which was consistent with the forward and the reverse bias-induced dot-like luminescence densities. These results show that both types of dot-like luminescence are affected by crystal defect-related leakage currents. The micro-PL intensity of the reverse bias-induced dot-like emission region was lower than that of the normal region, whereas the PL intensity of the forward bias-induced dot-like emission region was equal to that of other normal region. In particular, the forward and reverse bias-induced dot-like luminescence was observed in different regions, indicating that the evolutions of the dot-like luminescence under the reverse and the forward bias originated from defect-related leakage paths as nonradiative and radiative recombination centers, respectively.

Practical concept of an all-optical hot carrier solar cell

The all-optical hot carrier solar cell (aoHCSC) is an intriguing device concept which circumvents HC thermalization by feeding HCs into local radiative recombination centers. These have transition energies above the HC absorber (HCA) bandgap and are located within the HCA to match the HC ballistic mean free path, suppressing HC cooling as major loss mechanism. HC energy extraction proceeds by photon emission. We propose a technologically feasible concept of the aoHC energy converter (aoHCEC) which feeds into a conventional solar cell with its bandgap matching the emitted photons. Using real materials, the concept builds upon waveguides within a HCA which consist of highly polar direct bandgap material to promote radiative carrier recombination.

The impact of 1 wt% of Ir(ppy)3 on trapping sites and radiative recombination centres in PVK and PVK/PBD blend seen by thermoluminescence

Abstract Thermoluminescence (TL) of poly(N-vinylcarbazole) (PVK) and PVK + 40 wt% of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) blend, both doped with 1 wt% of fac-tris(2-phenylpiridine) iridium (Ir(ppy)3), were studied. TL spectra, registered in temperature range 15–320 K, reveal that trapping sites localised on the matrices and on Ir(ppy)3 molecules exist in both investigated systems. The traps localised on Ir(ppy)3 have depth about 0.4 eV and they dominate in the doped PVK. At the same concentration of the dopant molecules in the PVK/PBD matrix, the traps located on the matrix dominated. Contribution of deep traps on Ir(ppy)3 is much smaller. After doping, a shift of TL peak, associated with the release of carriers from the matrix traps, to higher temperature was observed. It indicates on the presence of slightly deeper traps as compared with those in the neat matrices. The effect is caused by the interaction of trapped carriers and randomly oriented high permanent dipoles of Ir(ppy)3. In addition, the experiments of spectrally resolved TL (SRTL) provide evidence that the presence of the dopant creates a new channel of energy transfer. Even at concentration of 1 wt% of Ir(ppy)3, these molecules act as emission centres which effectively compete with other centres of radiative recombination present in PVK and PVK/PBD systems.

Temperature evolution of carrier dynamics in GaNxPyAs1−y−xalloys

The temperature dependence of carrier dynamics in GaNxAs1−yPy alloys has been investigated by time resolved photoluminescence. This investigation has shown that the decay time constant does not change significantly up to 100 K, and then starts to decrease rapidly above this temperature. Additionally, the decay times at the high-energy side of the spectrum decrease faster than those at the low-energy side. The effects have been explained by the interplay between carrier capture by radiative and nonradiative recombination centers. Detailed simulations show that the effect of carrier localization in the investigated materials is better described by double-scale potential fluctuations that are related to (i) distribution of localized states energy and (ii) bandgap fluctuations. In addition, it was observed that the increase in nitrogen concentration leads to a shorter decay time at room temperature, which is attributed to a larger concentration of non-radiative recombination centers. Furthermore, a post-growth annealing step leads to a longer decay time at room temperature, which is attributed to a reduction in non-radiative recombination centers. At low temperatures, the role of non-radiative centers is suppressed, and therefore the decay time does not differ significantly for samples with either different nitrogen concentrations or in both the as-grown and annealed samples.

Photoluminescence of Cr-doped TiO2 induced by intense UV laser excitation

Abstract Nanocrystalline anatase ( A ) and rutile ( R ) TiO 2 were synthesized by the thermal hydrolysis and then were surface doped with Cr ions by adsorption. The samples were characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, optical absorption and photoluminescence (PL) methods. It is found that the PL emission and optical absorption near the band gap edge occur with the same electronic transitions in TiO 2 . The Cr 3+ doping leads to the blue and red shifts of the PL peaks in A /Cr TiO 2 due to the Burstein–Moss and band tailing effects, respectively. The Auger type non-radiative recombination in R /Cr TiO 2 quenches the PL emission intensity. For anatase samples at low concentrations of Cr ions (0.5 at%) the PL emission increases compared to the undoped A TiO 2 due to the formation of additional radiative recombination centers. At higher contents of Cr (1.0 at%), the PL intensity decreases due to the concentration quenching effect.

InGaN/GaN multiple quantum well for fast scintillation application: radioluminescence and photoluminescence study

We prepare InGaN/GaN multiple quantum well (MQW) structure by metal–organic vapour phase epitaxy and characterize it by fine XRD measurements. We demonstrate its suitability for scintillator application including a unique measurement of wavelength-resolved scintillation response under nanosecond pulse soft x-ray source in extended dynamical and time scales. The photoluminescence and radioluminescence were measured: we have shown that the ratio of the intensity of quantum well (QW) exciton luminescence to the intensity of the yellow luminescence (YL) band IQW/IYL depends strongly on the type and intensity of excitation. Slower scintillation decay measured at YL band maximum confirmed the presence of several radiative recombination centres responsible for wide YL band, which also partially overlap with the QW peak. Further improvements of the structure are suggested, but even the presently reported decay characteristics of the excitonic emission in MQW are better compared to the currently widely used single crystal YAP:Ce or YAG:Ce scintillators. Thus, such a type of a semiconductor scintillator is highly promising for fast detection of soft x-ray and related beam diagnostics.

Defect study of TiO2 nanorods grown by a hydrothermal method through photoluminescence spectroscopy

Abstract This paper describes the photoluminescence study of titanium dioxide (TiO2) nanorods grown by a hydrothermal synthesis method on the surface of fluorine doped tin oxide (FTO) coated glass. The effects of growth conditions including: reaction time, precursor concentration and adding NaCl to hydrothermal solution on the structural and optical properties of productions are examined by using SEM, XRD, TEM and room temperature photoluminescence measurements. Also, the performance of the TiO2 nanorods as a photoanode of dye sensitized solar cells is investigated. The different excitation energies and intensities are chosen to verify the discrete electronic state of radiative recombination centers in nanorods. The photoluminescence studies show the profound effect of the excitation light energy and intensity on the emission spectra. The nanorods have extended emission spectra from UV to visible region. The near band edge emission is observed at 3.04±0.01 eV. Also, the spectra consist of high emission peaks around at 3.44 and 3.14 eV and other emissions at 2.90±0.01, 2.83±0.02, 2.70±0.01, 2.56, 2.36±0.03, 2.30±0.02, 2.03, and 1.66 eV. The photoluminescence study of TiO2 nanorods and photoconversion efficiency of dye sensitized solar cells indicate a correlation among some PL emission peaks, the aspect ratio and efficiency of photoanode based solar cells.

Near-infrared emission from ZnO nanorods grown by thermal evaporation

We report the growth of ZnO nanorods on Si/SiO2 subtrates by the thermal evaporation method at different distances (substrate temperatures) from vapor source to substrates. SEM images showed that morphologies of nanorods were significantly affected by distance from the substrate to vapor source. Energy dispersive X-ray spectroscopy (EDS) spectra present change of the ratio of zinc to oxygen in ZnO nanostructures as the substrate temperature varied. X-ray diffraction patterns revealed that the prepared ZnO nanorods are preferentially oriented in the c-axis at lower substrate temperature. The shift towards small angle of the XRD pattern peaks is consistent with the presence of the redundant zinc and the lack oxygen in the ZnO lattice. The photoluminescence (PL) spectra of the ZnO nanorods show beside the near band edge UV emission, a very broad emission ranges from green to near-infrared (NIR). The NIR emission is interpreted as due to the transition of carriers between radiative recombination centers related to Zn interstitials and oxygen interstitials.

Optical and electrical properties of dislocations in plastically deformed GaN

Abstract Optical and electrical properties of fresh dislocations in GaN bulk crystals deformed plastically at elevated temperatures were reviewed. A dislocation band model was proposed. The fresh dislocations of ( a /3) [ 1 2 ¯ 10 ] -type edge dislocations on the ( 10 1 ¯ 0 ) prismatic plane induced several photoluminescence peaks at around 1.8, 1.9 and 2.4 eV, which implies the formation of radiative recombination centers of the dislocations. Simultaneously, near-band-edge (3.48 eV) photoluminescence intensity decreased remarkably for a high-density of non-radiative recombination centers originating in deformation-induced abundant Ga-vacancy related clusters. The intensity variation of yellow luminescence with plastic deformation and subsequent annealing did not relate to the native property of dislocations. Variation of optical absorption dependent on the strain in plastically deformed GaN was understood in a model of the Franz–Keldysh effect by the electric fields associated with charged dislocations (~5.8 e/nm). Scanning spreading resistance microscopic images showed many spots with high conductivity around the induced dislocations, showing electrical conduction along dislocations according to the Frenkel–Poole mechanism.