Probability Of Radiative Recombination Research Articles

Influence of 120 MeV Au Ion Irradiation on Phase Transition, Surface, and Optical Properties of Lead‐Free (K,Na)NbO3 Films

High energy ion beam irradiation is an important tool to engineer the properties of multifunctional materials for diverse applications. The present study reports the impact of 120 MeV Au ion beam irradiation on structural and luminescence properties of (K,Na)NbO3 (KNN) films. The increase in ion fluence has resulted in crystalline‐to‐crystalline and crystalline‐to‐amorphous phase transitions at room temperature. The dense grain‐like morphology of crystalline films gets transformed into clusters of nano‐sized grains after irradiation, and the grain boundaries are diffused at higher fluence. X‐ray photoelectron spectroscopy results indicate the evaporation of K‐species from the surface after irradiation. The relative elemental ratio and film's thickness were estimated using Rutherford backscattering spectrometry, and subsequently, KNN/Si interface analysis was also carried out. The increase in optical band gap after irradiation is correlated to the suppression of extended band tails. An intense photoluminescence emission in the UV region is observed, and the intensity increases abruptly after the crystalline‐to‐amorphous phase transition, where the grain boundaries merge with nearby ones. The impact of high electronic energy loss in KNN enhanced the probability of radiative recombination. The obtained results demonstrate the possible use of KNN for optoelectronic applications.

Excitons in metal halide perovskite nanoplatelets: an effective mass description of polaronic, dielectric and quantum confinement effects.

A theoretical model for excitons confined in metal halide perovskite nanoplatelets is presented. The model accounts for quantum confinement, dielectric confinement, short and long range polaron interactions by means of effective mass theory, image charges and Haken potentials. We use it to describe the band edge exciton of MAPbI3 structures surrounded by organic ligands. It is shown that the quasi-2D quantum and dielectric confinement squeezes the exciton radius, and this in turn enhances short-range polaron effects as compared to 3D structures. Dielectric screening is then weaker than expected from the static dielectric constant. This boosts the binding energies and radiative recombination probabilities, which is a requisite to match experimental data in related systems. The thickness dependence of Coulomb polarization and self-energy potentials is in fair agreement with sophisticated atomistic models.

Optimization of photonic crystal structure by FDTD method to improve the light extraction efficiency in silicon

Silicon photonics technology is on its last hurdle to complete the system with active research on enhancing the light extraction efficiency of silicon. Silicon has a very poor radiative emission efficiency due to its indirect bandgap, thus making the probability of radiative recombination to be very low. The periodic quantum confinement structures such as photonic crystals (PhCs) can be tailored to increase the radiative recombination probability at the desired wavelengths. In this paper, we have optimized the silicon photonic crystal (Si PhCs) parameters to obtain luminescence at silicon bandgap and telecommunication wavelength using FDTD method. The extraction of photons is mostly conquered in the silicon structure that has lattice constant, a: 480 nm with radius, r: 140 nm and a: 500 nm with r: 160 nm. We have also observed the emissive photon luminescence that emerges in the most stable state of optical communication wavelength at 1310 nm.

Commercially available chromophores as low-cost efficient electron injection layers for organic light emitting diodes

Further advancements in organic light emitting diodes (OLEDs) using commercially available, low-cost materials is of high significance. Here, we report the application of commercially available chromophores as room temperature processed electron injection layers (EILs) in the conventional OLED architecture. The facile solution-processing of these chromophores, namely, 4‐dimethylamino‐4′‐nitrostilbene (DANS) and 1‐(4‐(dimethylamino)phenyl)‐6‐phenyl‐1,3,5‐hexatriene, along with the presence of nitrogen atom in their structure, which is expected to induce the formation of a negative interfacial dipole at the cathode interface, have motivated their use as EILs. Improved performance of the OLEDs using these chromophores was obtained. Especially, the OLED using DANS exhibited the highest luminous efficiency, power efficiency and external quantum efficiency values of 8.7 cd A−1, 6.75 lm W−1 and 2.9%, respectively, which represented a significant improvement compared to the reference device without the EIL (1.2 cd A−1, 0.7 lm W−1 and 0.4%, respectively). A variety of experimental and simulated results demonstrated that this enhancement is attributed to increased electron injection leading to balanced electron and hole currents, especially in the DANS embedding device. Additionally, we calculated significant broadening of the emission zone profile across the entire organic emitter in the devices using the chromophores, thus increasing the probability of radiative recombination and photon emission.

Efficient phonon cascades in WSe2 monolayers

Energy relaxation of photo-excited charge carriers is of significant fundamental interest and crucial for the performance of monolayer transition metal dichalcogenides in optoelectronics. The primary stages of carrier relaxation affect a plethora of subsequent physical mechanisms. Here we measure light scattering and emission in tungsten diselenide monolayers close to the laser excitation energy (down to ~0.6 meV). We reveal a series of periodic maxima in the hot photoluminescence intensity, stemming from energy states higher than the A-exciton state. We find a period ~15 meV for 7 peaks below (Stokes) and 5 peaks above (anti-Stokes) the laser excitation energy, with a strong temperature dependence. These are assigned to phonon cascades, whereby carriers undergo phonon-induced transitions between real states above the free-carrier gap with a probability of radiative recombination at each step. We infer that intermediate states in the conduction band at the Λ-valley of the Brillouin zone participate in the cascade process of tungsten diselenide monolayers. This provides a fundamental understanding of the first stages of carrier–phonon interaction, useful for optoelectronic applications of layered semiconductors.

Kinetics of the Luminescence Response of Self-Assembled Ge(Si) Nanoislands Embedded in Two-Dimensional Photonic Crystals

The results of studies of the spectral and kinetic characteristics of the photoluminescence of photonic crystals formed on the basis of structures with self-assembled Ge(Si) nanoislands are reported. The experimentally observed enhancement of the photoluminescence-signal intensity of the nanoislands in the spectral range 1.1–1.6 μm due to interaction with the radiative modes of photonic crystals in the vicinity of the Γ point of the Brillouin zone and the effect of such interaction on the probability of radiative recombination in Ge(Si) nanoislands are considered.

Composition optimization of lead-free double perovskite Cs2AgIn1-xBixCl6 for efficient and stable photoluminescence

Recently, lead-free double perovskites is increasingly focused in view of the nontoxicity and stability compared to lead-halide perovskites. In this study, we optimized the composition of lead-free double perovskite Cs2AgIn1-xBixCl6 nanocrystals (NCs) via an antisolvent recrystallization method at room-temperature. The pure Cs2AgInCl6 NCs show a low emission at 520 nm with a low photoluminescence quantum yield (PLQY) of 1.2%, which was assigned as the parity-forbidden direct band transition. By alloying 20%mol Bi3+ with In3+, broad-band orange emission peaking at 575 nm with the PLQY of 28.9% was obtained. The increasing of PL efficiency was considered mainly come from the breaking of the parity-forbidden direct band transition and improved crystal quality, which enhances the radiative recombination probability and decreases the defects density of the NCs. To further understand the exciton recombination dynamics, time-resolved PL and femtosecond TA technique were performed. And, a detailed photo-induced charge carrier dynamics process, especially the charge carrier trapping, recombination, and transition were confirmed and plotted. Moreover, the resulting Cs2AgIn0.8Bi0.2Cl6 products demonstrate a remarkable stability against heat, ultraviolet light irradiation, and moisture. The results suggest that the lead-free double perovskite Cs2AgIn0.8Bi0.2Cl6 is potentially an attractive candidate for solid-state lighting application.

Modification of Luminescence from Dual-Emission Molecules by Plasmonic Surfaces

Phosphorescence from heavy-atom-free organic conjugated molecules is weak at room temperature due to the low probability of radiative recombination from the triplet state. In this study, the relati...

Enhanced red light emission from dense Si quantum dot-based silicon oxynitride light-emitting diodes with reduced efficiency droop

Strong red light emission from silicon oxynitride-based (SiOxNy-based) light-emitting diodes (LEDs) was investigated. The introduction of dense Si quantum dots (Si QDs) into SiOxNy not only increases carrier injection efficiency but also results in a significant enhancement of more than 300% in the light emission efficiency compared with SiOxNy-based LEDs without Si QDs. Moreover, the efficiency droop phenomenon is remarkably suppressed in the SiOxNy-based LED containing dense Si QDs. Analysis of the dominant recombination process indicates that the improved performance of the SiOxNy-based LEDs results from the increased bimolecular radiative recombination probability.

Effect of Coupled Quantum-Dot Insertion on the Radiative Recombination Probability of Wurtzite InGaN/GaN Quantum Dots

Light emission charactersitcs of single and coupled wurtzite (WZ) InGaN/GaN quantum-dot (QD) structures were investigated by using an effective mass theory. Strain components in the well of the coupled QD are found to be smaller than those in the well of the single QD structure. In particular, the reduction effect in ϵzz is dominant and, as a result, the polarization component Izz for the strain tensor ϵzz is largely reduced for the coupled QD. The coupled QD structure shows much larger peak intensity than the single QD structure because the interband transition probability between electrons and holes is enhanced owing to the reduced internal field.

On the Synthesis and Photoluminescence and Cathodoluminescence Properties of CdSe, CdTe, PbS, InSb, and GaAs Colloidal Quantum Dots

Quantum dots of the group of wide-gap and narrow-gap semiconductors are synthesized and investigated under identical conditions, which makes it possible to perform the comparative analysis and modeling of the mechanisms of radiative recombination and luminescence, for which a stable exciton bond between the electron and hole has an important role. Exciton states are unstable for quantum dots without a shell and narrow-gap semiconductors, which leads to a substantial decrease in the probability of radiative recombination and, correspondingly, the quantum yield of luminescence. The experimental values of the spectral position of the luminescence maximum for quantum dots with clear manifestation of the exciton recombination mechanism noticeably shift to the long-wavelength region with respect to the calculated ones. In calculations and analysis, we use the effective electron mass for bulk semiconductors. The observed good correspondence of the calculated values of the maximum and spectral band with the experiment can mean that quantum dots have a long-range order crystalline structure similar to that one observed in single crystals and polycrystals.

Electroluminescence from h-BN by using Al2O3/h-BN multiple heterostructure.

Two-dimensional (2-D) hexagonal boron nitride (h-BN) has attracted considerable attention for deep ultraviolet optoelectronics and visible single photon sources, however, realization of an electrically-driven light emitter remains challenging due to its wide bandgap nature. Here, we report electrically-driven visible light emission with a red-shift under increasing electric field from a few layer h-BN by employing a five-period Al2O3/h-BN multiple heterostructure and a graphene top electrode. Investigation of electrical properties reveals that the Al2O3 layers act as potential barriers confining injected carriers within the h-BN wells, while suppressing the electrostatic breakdown by trap-assisted tunneling, to increase the probability of radiative recombination. The result highlights a promising potential of such multiple heterostructure as a practical and efficient platform for electrically-driven light emitters based on wide bandgap two-dimensional materials.

Impact of microscopic In fluctuations on the optical properties of InxGa1-xN blue light-emitting diodes assessed by low-energy X-ray fluorescence mapping using synchrotron radiation

Among the III-nitride semiconductors, InxGa1-xN is a key material for visible optical devices such as light-emitting diodes (LEDs), laser diodes, and solar cells. Light emission is achieved via electron-hole recombination within the InxGa1-xN layer. When InxGa1-xN-based blue LEDs were first commercialized, the high probability of electron-hole radiative recombination despite the presence of numerous threading dislocations was a mystery. Extensive studies have proposed that carrier localization in nanoscopic potential fluctuations due, for example, to the immiscibility between InN and GaN or random alloy fluctuations is a key mechanism for the high emission efficiency. In actual LED devices, not only nanoscopic potential fluctuations but also microscopic ones exist within the InxGa1-xN quantum well light-emitting layers. Herein we map the synchrotron radiation microbeam X-ray fluorescence of InxGa1-xN blue LEDs at a sub-micron level. To acquire weak signals of In, Ar, which is in the air and has a fluorescent X-ray energy similar to that of In, is evacuated from the sample chamber by He purge. As a result, we successfully visualize the spatial In distribution of InxGa1-xN layer nondestructively and present good agreement with optical properties. Additionally, we demonstrate that unlike nanoscopic fluctuations, microscopic In compositional fluctuations do not necessarily have positive effects on device performance. Appropriately controlling both nanoscopic and microscopic fluctuations at the same time is necessary to achieve supreme device performance.

Trifluoroacetate induced small-grained CsPbBr3 perovskite films result in efficient and stable light-emitting devices

Quantum efficiencies of organic-inorganic hybrid lead halide perovskite light-emitting devices (LEDs) have increased significantly, but poor device operational stability still impedes their further development and application. All-inorganic perovskites show better stability than the hybrid counterparts, but the performance of their respective films used in LEDs is limited by the large perovskite grain sizes, which lowers the radiative recombination probability and results in grain boundary related trap states. We realize smooth and pinhole-free, small-grained inorganic perovskite films with improved photoluminescence quantum yield by introducing trifluoroacetate anions to effectively passivate surface defects and control the crystal growth. As a result, efficient green LEDs based on inorganic perovskite films achieve a high current efficiency of 32.0 cd A−1 corresponding to an external quantum efficiency of 10.5%. More importantly, our all-inorganic perovskite LEDs demonstrate a record operational lifetime, with a half-lifetime of over 250 h at an initial luminance of 100 cd m−2.

Синтез, фото- и катодолюминесцентные свойства коллоидных квантовых точек CdSe, CdTe, PbS, InSb, GaAs

AbstractQuantum dots of the group of wide-gap and narrow-gap semiconductors are synthesized and investigated under identical conditions, which makes it possible to perform the comparative analysis and modeling of the mechanisms of radiative recombination and luminescence, for which a stable exciton bond between the electron and hole has an important role. Exciton states are unstable for quantum dots without a shell and narrow-gap semiconductors, which leads to a substantial decrease in the probability of radiative recombination and, correspondingly, the quantum yield of luminescence. The experimental values of the spectral position of the luminescence maximum for quantum dots with clear manifestation of the exciton recombination mechanism noticeably shift to the long-wavelength region with respect to the calculated ones. In calculations and analysis, we use the effective electron mass for bulk semiconductors. The observed good correspondence of the calculated values of the maximum and spectral band with the experiment can mean that quantum dots have a long-range order crystalline structure similar to that one observed in single crystals and polycrystals.

(Invited) Enhanced Light Output Power from Eu-Doped GaN Narrow-Band Red Light-Emitting Diodes By Actively Controlling Photon Fields

While InGaN/GaN blue and green light-emitting diodes (LEDs) are commercially available, the search for an efficient red LED based on GaN is ongoing. This is essential for the monolithic integration of the three primary colors and the development of nitride-based small-size, full-color, high-resolution displays. We have succeeded in the realization of a red LED using Eu-doped GaN (GaN:Eu) grown by organometallic vapor phase epitaxy [1]. Making use of the intra-4f shell transitions of Eu3+ ions, our LED features an ultra-narrow linewidth of less than 1 nm at room temperature, and unprecedented thermal stability of 1 pm/K. By gradual optimization of the Eu excitation mechanism, we have increased the light output power above 1 mW at 20 mA [2]. One of limiting factors for enhanced light output power is the relatively long radiative lifetime of the Eu emission in GaN:Eu (∼300 µs). According to the Fermi’s golden rule, modifying the spontaneous emission rate of Eu ions can be achieved by increasing the photonic density of states (PDOS) at the frequency of spontaneous emission, as already demonstrated with a planar Fabry-Perot cavity. We have boosted the output power by actively manipulating radiative recombination probability at the atomic level of the Eu ions, which can be achieved through control of their photon fields in micro- and nano-cavities. In a GaN:Eu layer embedded in a microcavity consisting of an AlGaN/GaN distributed Bragg reflector (DBR) and a Ag reflecting mirror, a 21-fold increase of the Eu emission intensity was obtained under optical pumping at room temperature [3]. Furthermore, in a preliminary LED with a microcavity consisting of ZrO2/SiO2 and AlInN/GaN DBRs, the output power was enhanced by 5 times [4]. In this talk, current status of the red LED and strategies for further enhancement of the light output power are reviewed. A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, Y. Fujiwara, Appl. Phys. Exp. 2, 071004 (2009).B. Mitchell, V. Dierolf, T. Gregorkiewicz, and Y. Fujiwara, submitted to J. Appl. Phys.T. Inaba, D. Lee, R. Wakamatsu, T. Kojima, B. Mitchell, A. Capretti, T. Gregorkiewicz, A. Koizumi, Y. Fujiwara, AIP Adv. 6, 045105 (2016).T. Inaba, K. Shiomi, J. Tatebayashi, and Y. Fujiwara, in preparation.

Effects of indium surfactant on growth and characteristics of (112¯2) plane AlGaN-based multiple quantum wells

High quality semi-polar (112¯2) plane AlGaN-based multiple quantum wells (MQWs) were successfully grown on (101¯0) m-plane sapphire substrates with metal-organic chemical vapor deposition (MOCVD) technology and the effects of indium (In) surfactant on the structural and optical properties of the AlGaN-based MQWs were investigated intensively. The characterization results revealed that the surface morphology as well as the crystalline quality for the semi-polar (112¯2) plane AlGaN MQWs could be improved remarkably by adopting In as surfactant during the MOCVD growth process. Furthermore, the integrated MQWs-related excition emission peak intensity and the radiative recombination probabilities in MQWs could be increased as well with the help of In-surfactant, resulting an enhanced internal quantum efficiency.

Effects of Introduction of InGaN Quantum Structures on Structural and Optical Properties of InGaN Nanocolumns

We systematically investigate and compare the structural and optical properties of InGaN nanocolumns (NCs) and InGaN/GaN single quantum structures (SQSs) fabricated on top of GaN NCs as functions of the GaN column diameter. Each InGaN/GaN SQS comprises an In‐rich InGaN single quantum dot at the center of a semipolar Ga‐rich InGaN single quantum well. The InGaN NCs and InGaN/GaN SQSs qualitatively exhibit similar optical properties. However, the specific diameters at which the photoluminescence spectra change from single‐peak emission patterns to double‐peak patterns are different. Based on scanning transmission electron microscopy observations of the InGaN layers formed on the GaN NCs with different growth times, a model for the growth of InGaN on GaN NCs is proposed and the origin of the difference in the specific diameters is elucidated. Finally, when QSs are introduced to the InGaN‐based NCs, the radiative recombination probability increases while the nonradiative and surface recombination probabilities decrease, thus leading to high emission efficiency.

Polychromatic emission from polar-plane-free faceted InGaN quantum wells with high radiative recombination probabilities

We fabricated three-dimensional (3D) InGaN quantum wells (QWs) on semipolar GaN substrates by selective area epitaxy. It was found that polar-plane-free 3D structures can be fabricated with a particular mask pattern on the plane and that InGaN/GaN QWs on these structures exhibit facet-dependent polychromatic emission properties. In addition, time-resolved photoluminescence spectroscopy indicated that the radiative recombination lifetimes of the obtained 3D QWs without the polar (0001) plane are much shorter than those of conventional 3D QWs with the (0001) plane, particularly at long wavelengths.

Time-Resolved Infrared Spectroscopy Directly Probes Free and Trapped Carriers in Organo-Halide Perovskites

Free carrier dynamics in organo-halide perovskites can directly reveal information about their carrier lifetimes and indirectly reveal information about trap state distributions, both of which are critical to improving their performance and stability. Time-resolved photoluminescence (TRPL) spectroscopy is commonly used to probe carrier dynamics in these materials, but the technique is only sensitive to radiative decay pathways and may not reveal the true carrier dynamics. We used time-resolved infrared (TRIR) spectroscopy in comparison to TRPL to show that photogenerated charges relax into free carrier states with lower radiative recombination probabilities, which complicates TRPL measurements. Furthermore, we showed that trapped carriers exhibit distinct mid-infrared absorptions that can be uniquely probed using TRIR spectroscopy. We used the technique to demonstrate the first simultaneous measurements of trapped and free carriers in organo-halide perovskites, which opens new opportunities to clarify how...