Higher Radiative Recombination Rate Research Articles

Advantages of AlGaN Tunnel Junction in N-Polar 284 nm Ultraviolet-B Light Emitting Diode

Smart, low cost and environmentally safe aluminum gallium nitride (AlGaN)-based ultraviolet-B light-emitting diodes (UV-B LEDs) are promising in real-world applications including medical as well as agricultural sciences. Higher efficiency droops, low hole injection efficiency, and high operating voltage are the key problems that AlGaN-based UV-B LEDs are facing. In this work, a smart and clean AlGaN-based UV-B LED at 284 nm emission wavelength has been studied. Here an approach is presented to electrically operate the quantum tunnelling probability by exploiting the transported carriers at the interface of p-AlGaN/n-AlGaN/n++-AlGaN tunnel junction (TJ) with moderate Si and Mg-doping levels and optimized thickness with the help of simulation study. The simulation results show that the Augur recombination rate is successfully suppressed and quite a high radiative recombination rate is achieved in the 284 nm N-polar AlGaN-based TJ UV-B LEDs, which is attributed to the improved hole injection toward the MQWs when compared to C-LED (conventional-LED). It is found that C-LED has a maximum IQE (internal quantum efficiency) of 40% under 200 A cm−2 injection current with an efficiency drop of 15%, while the TJ-LED has a maximum IQE of 93% with an efficiency droop of 0%. In addition, TJ-based AlGaN LED emitted power has been improved by 6 times compared to the C-LED structure. The emitted powers of TJ-LED increase linearly under varying current densities, whereas in the case of C-LED, the emitted power changes nonlinearly under varying current densities. This is attributed to the lower Augur recombination rate in the MQWs of N-AlGaN-based TJ UV-B LED. The operating voltages were reduced from 5.2 V to 4.1 V under 200 mA operation, which is attributed to the thickness and doping optimization in TJ and better selection of relatively lower Al-content in the contact layer. N-polar AlGaN-based TJ is explored for UV-B LEDs and the demonstrated work opens the door to epitaxial growth of high-performance UV emitters in MOCVD and MBE for a plethora of biomedical applications.

Large-Area and Efficient Sky-Blue Perovskite Light-Emitting Diodes via Blade-Coating.

Large-area fabrication of perovskite light-emitting diodes (PeLEDs) through mass-production techniques has attracted growing attention due to their potential applications in lighting. Several breakthroughs are made for red/infrared and green emissions. Nevertheless, large-area blue/sky-blue PeLEDs, a requisite color for lighting, have not yet been reported. Here, efficient and large-area sky-blue PeLEDs are fabricated through blade-coating supersaturated precursors. The volume ratio of dimethyl sulfoxide to dimethylformamide is tuned to obtain a supersaturated CsPb(Br0.84 Cl0.16 )3 solution. Blade-coating this supersaturated precursor results in nucleation in the solution phase with much higher nucleation sites, and a faster crystallization rate. The uniform films formed by this approach exhibit smaller grain size, lower trap density, and higher radiative recombination rate. The peak external quantum efficiency of the blade-coated PeLEDs reaches 10.3% with sky-blue emission (489nm). Benefitting from the robustness of this blade-coating technique, large-area sky-blue PeLEDs with a device area of 28 cm2 are also achieved with uniform emission. This work represents a significant step forward toward flat-panel lighting and full-color display for the PeLEDs.

The impact of ZnO nanotube on the performance of hybrid inorganic/organic light-emitting diode as a single-mode ring-core UV waveguide

After a systematic survey in hybrid inorganic/organic light-emitting heterostructure devices based on ZnO in the last decay, in this novel work, the impact of the single-mode ring-core waveguide based on ZnO nanotube (NT)/MEH-PPV for ultraviolet organic light-emitting diode (UV-OLED) application has been carefully scrutinized for the first time. The proposed structure has been fabricated, simulated and compared with conventional ZnO nanorod (NR)/MEH-PPV structure. To synthesize ZnO NTs, the as-grown chemical bath deposited ZnO NRs have been etched in KCL solution in various molar (M) concentration, etching time and etching temperature. The optimized etching condition is obtained in 1 M concentration of KCL solution, 4 h etching time and 90 °C temperature. The structural properties (such as strain, stress and texture coefficient), electrical properties (such as band gap energy) and optical properties (such as Urbach energy, absorbance and photoluminescence spectra) of ZnO NTs have been investigated, systematically. In continue, hybrid UV-OLEDs have been fabricated based on ZnO NRs and ZnO NTs. According to the results, ZnO NT-based OLED depicts superior electrical and optical results including lower turn-on voltage (11.2 V < 15 V) and higher UV peak in electroluminescence spectra with respect to ZnO NR-based device. To acquire more enlightenment about UV emission mechanism, the proposed devices have been simulated through Silvaco TCAD and Lumerical FDTD software. The results from simulations illustrate great agreement with experimental results. Higher radiative recombination rate, higher Purcell factor and single-mode waveguiding effect of ring-core ZnO NT lead to major superiority of the ZnO NT-based UV-OLED fabricated and simulated in this work.

Calculating the Effect of AlGaN Dielectric Layers in a Polarization Tunnel Junction on the Performance of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes.

In this work, AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) with AlGaN as the dielectric layers in p+-Al0.55Ga0.45N/AlGaN/n+-Al0.55Ga0.45N polarization tunnel junctions (PTJs) were modeled to promote carrier tunneling, suppress current crowding, avoid optical absorption, and further enhance the performance of LEDs. AlGaN with different Al contents in PTJs were optimized by APSYS software to investigate the effect of a polarization-induced electric field (Ep) on hole tunneling in the PTJ. The results indicated that Al0.7Ga0.3N as a dielectric layer can realize a higher hole concentration and a higher radiative recombination rate in Multiple Quantum Wells (MQWs) than Al0.4Ga0.6N as the dielectric layer. In addition, Al0.7Ga0.3N as the dielectric layer has relatively high resistance, which can increase lateral current spreading and enhance the uniformity of the top emitting light of LEDs. However, the relatively high resistance of Al0.7Ga0.3N as the dielectric layer resulted in an increase in the forward voltage, so much higher biased voltage was required to enhance the hole tunneling efficiency of PTJ. Through the adoption of PTJs with Al0.7Ga0.3N as the dielectric layers, enhanced internal quantum efficiency (IQE) and optical output power will be possible.

Temperature-Dependent Photoluminescence of Manganese Halide with Tetrahedron Structure in Anti-Perovskites.

The temperature-dependent photoluminescence (PL) properties of an anti-perovskite [MnBr4]BrCs3 sample in the temperature range of 78–500 K are studied in the present work. This material exhibits unique performance which is different from a typical perovskite. Experiments showed that from room temperature to 78 K, the luminous intensity increased as the temperature decreased. From room temperature to 500 K, the photoluminescence intensity gradually decreased with increasing temperature. Experiments with varying temperatures repeatedly showed that the emission wavelength was very stable. Based on the above-mentioned phenomenon of the changing photoluminescence under different temperatures, the mechanism is deduced from the temperature-dependent characteristics of excitons, and the experimental results are explained on the basis of the types of excitons with different energy levels and different recombination rates involved in the steady-state PL process. The results show that in the measured temperature range of 78–500 K, the steady-state PL of [MnBr4]BrCs3 had three excitons with different energy levels and recombination rates participating. The involved excitons with the highest energy level not only had a high radiative recombination rate, but a high non-radiative recombination rate as well. The excitons at the second-highest energy level had a similar radiative recombination rate to the lowest energy level excitons and a had high non-radiative recombination rate. These excitons made the photoluminescence gradually decrease with increasing temperature. This may be the reason for this material’s high photoluminescence efficiency and low electroluminescence efficiency.

Understanding the Luminescence Characteristics of Ultraviolet InGaN/AlGaN Multiple Quantum Wells with Different In Gradients

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity.

One-dimensional semiconductor nanostructures grown on two-dimensional nanomaterials for flexible device applications

This Perspective presents a review of current research activities on one-dimensional (1D) semiconductor nanostructures grown on two-dimensional (2D) nanomaterials for flexible electronic and optoelectronic device applications. For hybrid-dimensional nanostructures, 1D nanostructures exhibit excellent material characteristics, including high carrier mobility and radiative recombination rate as well as long-term stability, while 2D layers show good optical transparency, mechanical flexibility, and electrical characteristics. Accordingly, the versatile and fascinating properties of 1D nanostructures grown on 2D layers enable the fabrication of high-performance optoelectronic and electronic devices even in transferable, flexible, or stretchable forms. We initially present a variety of 1D/2D hybrid nanostructures and their preparation methods, followed by a discussion of techniques for fabricating aligned 1D nanostructure arrays on 2D layers and their heterostructures. Furthermore, we discuss a wide range of devices based on the 1D/2D hybrid nanostructures. These devices include light-emitting devices, photodetectors, transistors, and pressure sensors. Several important issues, including 1D/2D junction properties and device fabrication processes for device applications, are also addressed. We conclude with personal remarks on the issues and perspectives for research on 1D/2D hybrid nanostructures for more sophisticated device applications.

High Output Power GaN-Based Green Resonant-Cavity Light-Emitting Diodes With Trapezoidal Quantum Wells

Green resonant-cavity light-emitting diode (RCLED) has great potential in optical communication but suffers low efficiency. In this study, GaN-based flip-chip green RCLED incorporated with nitrogen face-oriented inclination asymmetric trapezoidal quantum wells (NOAT-QWs) is proposed to enhance the light-output power (LOP). Samples with NOAT-QWs and normal symmetric square quantum wells (SS-QWs) were fabricated and characterized. Although their electrical characteristics and emission spectra are similar, the LOP and emission efficiency are significantly improved. At a driving current of 450 mA, the improvement of LOP for RCLED with NOAT-QWs can be as high as 1.44 times in comparison to the sample with SS-QWs. The unprecedented high output power of 115 mW and the narrow full-width-at-half-maximum (FWHM) of 6.4 nm in the emission spectrum give great potential in optical communication. The inherent mechanism was investigated by the finite element analysis, from which the simulation results match well with the experimental measurements. The simulation results reveal that the NOAT-QWs are beneficial in alleviating the quantum confined stark effect and facilitating easier hole carriers’ flow across the barriers, leading to more electron–hole wave function overlaps and higher radiative recombination rate.

Halide Double‐Perovskite Light‐Emitting Centers Embedded in Lattice‐Matched and Coherent Crystalline Matrix

AbstractThrough first‐principles calculations, it is found that two lattice‐matched halide double‐perovskites, Cs2NaBiBr6 and Cs2AgBiBr6, have a type‐I band alignment and can form highly miscible alloys in which the disordering makes the bandgaps become direct and activates the direct transition from the valence to conduction band edge, leading to a strong optical absorption and high radiative recombination rate. The bandgaps of the alloys are tunable in a wide range of 1.93–3.24 eV, while the lattice constants remain unchanged. This advantage inspires the design of a coherent crystalline matrix based on Cs2(Na,Ag)BiBr6 alloys, in which the Ag‐rich and narrower‐bandgap regions are embedded in the Na‐rich and wide‐bandgap region with lattice‐matched and coherent interfaces. The type‐I band alignment drives the photogenerated excitons into the narrower‐bandgap Ag‐rich regions, so the regions become light‐emitting centers with a high photoluminescence quantum yield (PLQY). The bandgaps of the Ag‐rich regions are tunable, so the color of emitted light can be adjusted, making a broadband emission possible. Such kind of coherent crystalline matrix with high‐PLQY and broadband emission can also be fabricated based on the alloys of other lattice‐matched halide double‐perovskites, demonstrating the flexibility of band structure engineering in the coherent heterostructures of various halide double‐perovskites.

Highly Efficient Quantum Dot Light‐Emitting Diodes by Inserting Multiple Poly(methyl methacrylate) as Electron‐Blocking Layers

AbstractThis work presents a new device architecture integrating multiple poly(methyl methacrylate) (PMMA) electron‐blocking layers (EBL) in quantum dot light‐emitting diodes (QD‐LEDs). The device utilizes red‐emitting CdSe/ZnS QD with a novel structure where multiple PMMA EBLs are sandwiched between a pair of QD layers. A systematic optimization of QD‐LED structures has shown that a device including two PMMA and three QD layers performs the best, achieving a current efficiency of 17.8 cd A−1 and a luminance of 194 038 cd m−2. Numerical simulation of a simplified model of the proposed QD‐LED structure verifies that the structure consisting of two PMMA and three QD layers provides significant improvement in electroluminescent intensity. The simulation provides further insight into the origin of the effect of the PMMA EBL by showing that the addition of PMMA EBL reduces the electron leakage from the active QD region and enhances electron confinement, leading to an increased electron concentration in the QD active layers and a higher radiative recombination rate. The experimental and theoretical studies presented in this work demonstrate that multiple layers of PMMA can act as efficient EBLs in the fabrication of QD‐LEDs of improved performance.

Numerical Investigation on the Enhanced Performance of N-Polar AlGaN-Based Ultraviolet Light-Emitting Diodes With Superlattice p-Type Doping

N-polar AlxGa1−xN-based 365-nm ultraviolet (UV) light-emitting diodes (LEDs) with a hole-supplier layer composed of p-type Al0.2Ga0.8N/GaN superlattice (SL) doping or bulk doping structures are numerically investigated over Ga-polar UV-LEDs counterpart. The simulation results demonstrate that N-polar LEDs are favorable in mitigating the negative effect of the polarization-induced electric field. Thus, it exhibits superiority in promoting carriers confinement capability and suppressing electrons overflow over Ga-polar. As a result, the internal quantum efficiency, light output power, and electroluminescence intensity are significantly improved. In addition, both Ga-polar and N-polar devices exhibit the increased hole concentrations and higher radiative recombination rate within quantum wells after adopting SL p-doping structure. From the simulated energy band diagrams, the polarization-induced higher acceptor ionization rates at the heterostructure interfaces are observed. Evidently, the UV-LEDs based on N-polar SL p-doping structure are very effective to improve its performance, which is promising in achieving high-performance UV-LEDs of the future fabrication.

On the Carrier Transport for InGaN/GaN Core-Shell Nanorod Green Light-Emitting Diodes

In this paper, we model and investigate the carrier transport for the core-shell nanorod (NR) structured green light-emitting diodes (LEDs) for which the InGaN/GaN multiple quantum wells are grown on the nonpolar surface of the NR. Our results show the absence of polarization fields in the m-plane quantum wells for the core-shell NR LEDs, which manifest the flat energy band condition, the improved electron injection efficiency, and the high electron-hole wave functions overlap for the quantum wells, leading to the high radiative recombination rate and the enhanced quantum efficiency. We further find that the quantum efficiency is also affected by the vertical charge injection for the core-shell NR LEDs. The vertical charge injection is more sensitive to the doping concentration and the thickness of the p-GaN layer than of the n-GaN layer. Moreover, the increase of the NR height also leads to the nonuniform vertical charge injection.

Recombination dynamics of excitons in ZnO/ZnMgO multiple quantum wells grown on silicon substrate

Time-resolved photoluminescence measurements were taken at various temperatures to investigate the behavior of excitons in ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) grown on Si substrate using epitaxial Lu2O3 buffer layer by pulsed laser deposition. The temperature dependence of the radiative decay times of the emissions from both the ZnO wells and the Zn0.9Mg0.1O barriers is analyzed. The radiative lifetime of the well excitons exhibits linear dependence till room temperature, indicating that the excitons are effectively confined in the quantum wells and do not thermalize to 3D excitons. The short radiative lifetime of ~1.15 ns indicates high radiative recombination rate at room temperature. The radiative lifetime of the barrier excitons shows approximately T1/2 dependence on temperature. In combination with the spectral dependence of the PL lifetime, it indicates localization of excitons in the barrier due to compositional fluctuation. The short radiative lifetime at room temperature and the weak localization energy of ~9 meV suggest that our MQWs are of high optical quality.

Optical and Electrical Properties of GaN-Based Light Emitting Diodes Grown on Micro- and Nano-Scale Patterned Si Substrate

We investigate the optical and electrical characteristics of the GaN-based light emitting diodes (LEDs) grown on micro- and nano-scale patterned silicon substrate (MPLEDs and NPLEDs). The transmission electron microscopy images reveal the suppression of threading dislocation density in InGaN/GaN structure on nano-pattern substrate due to nano-scale epitaxial lateral overgrowth. The plan-view and cross-section cathodoluminescence mappings show less defective and more homogeneous active quantum-well region growth on nano-porous substrates. From temperature-dependent photoluminescence (PL) and low temperature time-resolved PL measurement, NPLEDs have better carrier confinement and higher radiative recombination rate than MPLEDs. In terms of device performance, NPLEDs exhibit smaller electroluminescence peak wavelength blue shift, lower reverse leakage current and decrease in efficiency droop when compared with the MPLEDs. These results suggest the feasibility of using NPSi for the growth of high quality and power LEDs on Si substrates.

Investigation of efficiency droop for InGaN-based UV light-emitting diodes with InAlGaN barrier

The efficiency droop in InGaN-based UV light emitting device (LED) with AlGaN and InAlGaN barrier is investigated. Electroluminescence results indicate that the light performance of quaternary LEDs can be enhanced by 25% and 55% at 350 mA and 1000 mA, respectively. Furthermore, simulations show that quaternary LEDs exhibit 62% higher radiative recombination rate and low efficiency degradation of 13% at a high injection current. We attribute this improvement to increasing of carrier concentration and uniform redistribution of carriers.

Inorganic nanostructures grown on graphene layers

This article presents a review of current research activities on the hybrid heterostructures of inorganic nanostructures grown directly on graphene layers, which can be categorized primarily as zero-dimensional nanoparticles; one-dimensional nanorods, nanowires, and nanotubes; and two-dimensional nanowalls. For the hybrid structures, the nanostructures exhibit excellent material characteristics including high carrier mobility and radiative recombination rate as well as long-term stability while graphene films show good optical transparency, mechanical flexibility, and electrical conductivity. Accordingly, the versatile and fascinating properties of the nanostructures grown on graphene layers make it possible to fabricate high-performance optoelectronic and electronic devices even in transferable, flexible, or stretchable forms. Here, we review preparation methods and possible device applications of the hybrid structures consisting of various types of inorganic nanostructures grown on graphene layers.

Optical properties of (1 1¯ 0 1) semi-polar InGaN/GaN multiple quantum wells grown on patterned silicon substrates

We present a study of high quality (11¯01) GaN films and the InGaN/GaN multiple quantum wells (MQWs) using epitaxial lateral overgrowth (ELO) technique by atmospheric pressure metal organic chemical vapor deposition (MOCVD). The smooth coalescence of the stripes and surface morphology was measured by scanning electron microscope (SEM) and atomic force microscopy (AFM). Due to reduction in internal electric field, semipolar InGaN/GaN MQWs have higher radiative recombination rate from time-resolved photoluminescence (TRPL) measurement. In addition, from degree of polarization (DOP) measurement, we observed higher polarization ratio attributed to the induced anisotropic compressive strain.