Light Extraction Efficiency Research Articles

Realization of high-efficiency AlGaN deep ultraviolet light-emitting diodes with polarization-induced doping of the p-AlGaN hole injection layer

We investigate the polarization-induced doping in the gradient variation of Al composition in the p-Al0.75Ga0.25N/Al x Ga1−x N hole injection layer (HIL) for deep ultraviolet light-emitting diodes (DUV-LEDs) with an ultra-thin p-GaN (4 nm) ohmic contact layer capable of emitting 277 nm. The experimental results show that the external quantum efficiency (EQE) and wall plug efficiency (WPE) of the structure graded from 0.75 to 0.55 in the HIL reach 5.49% and 5.04%, which are improved significantly by 182% and 209%, respectively, compared with the structure graded from 0.75 to 0.45, exhibiting a tremendous improvement. Both theoretical speculations and simulation results support that the larger the difference between 0.75 and x in the HIL, the higher the hole concentration that should be induced; thus, the DUV-LED has a higher internal quantum efficiency (IQE). Meanwhile, as the value of x decreases, the absorption of the DUV light emitted from the active region by the HIL is enhanced, reducing the light extraction efficiency (LEE). The IQE and LEE together affect the EQE performance of DUV-LEDs. To trade off the contradiction between the enhanced IQE and decreased LEE caused by the decrease in Al composition, the Al composition in the HIL was optimized through theoretical calculations and experiments.

Light extraction enhancement of light-emitting diodes by nanopillar hierarchical structures

Patterning of p-GaN and/or indium-tin-oxide (ITO) surfaces is investigated to improve light extraction efficiency for GaN-based light-emitting diodes (LEDs). A nanopillar-patterned p-GaN layer, a nanopillar-patterned ITO contact layer, and dual nanopillar-patterned p-GaN and ITO layers are fabricated. When compared with conventional LEDs, LEDs with the nanopillar-patterned p-GaN, nanopillar-patterned ITO, and dual nanopillar-patterned p-GaN and ITO layers showed light output enhancement by 50%, 90%, and 120%, respectively. The light extraction enhancement mechanism of these nanopillar-patterned nanostructures is investigated via the 3D finite-difference time-domain method. The results show that the LED with the dual nanopillar-patterned p-GaN and ITO layers offers the best light extraction efficiency, which could stimulate further efforts to increase the escape light cone effectively and overcome total internal reflection. This methodology for synchronous nanopatterning of two light-emitting interfaces is believed to provide a major light extraction enhancement.

(Invited) Enhanced Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes Using Photonic Crystal Structures

High efficiency ultraviolet (UV) light-emitting diodes (LEDs) are in high demand for a wide range of applications [1-3]. Due to its wide bandgap, covering from deep UV to near visible, AlGaN semiconductor has been intensively studied for high efficiency UV LEDs. However, the performance of AlGaN UV LEDs is still limited, especially in the deep UV wavelength region, due to large dislocation density and extremely inefficient p-type doping in AlGaN film. Therefore, AlGaN UV LEDs have been suffering from low internal quantum efficiency (IQE), poor light extraction efficiency (LEE), and low external quantum efficiency (EQE). Alternatively, AlInN alloy is relatively unexplored for light-emitters even though it holds great potential application in UV and visible light-emitting devices. Recent studies have reported that InAlN has a large optical gain for deep UV LEDs. In this context, we have successfully developed AlInN nanowire LEDs with strong emission in the UV wavelength regime. The AlInN nanowires achieved a high IQE of >50% and significantly strong Transverse Magnetic (TM) polarized emission at 299nm.The AlInN nanowire LED structures were spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II molecular beam epitaxy (MBE) system. The AlInN nanowire was grown on GaN nanowire template. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. The GaN nanowire segments were grown at ~ 800 °C, nitrogen flow rate of 0.5-1.0 sccm, and forward plasma power of ~ 400 W. However, the growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm. The MBE grown AlInN under nitrogen rich condition offers an effective approach to eliminate the composition inhomogeneity in the AlInN. The nanowires are vertically aligned to the Si substrate with quite uniform dimension, as illustrated in Figure 1(a). Figure 1(b) presents the structure of a single AlInN nanowire LED. The photoluminescence (PL) spectra of AlInN nanowires were measured using a 266 nm diode-pumped solid-state laser as the excitation source. The peak emissions vary from 280nm to 365nm by varying the Al/In composition in the AlInN layers. The peak emission is shifted to shorter wavelength when the substrate temperature gets increase which is attributed to the increased In adatom desorption at higher growth temperature, resulted in the reduced In composition in the AlInN segment. The AlInN/GaN nanowire exhibits relatively high IQE which is estimated of ~ 52% at room temperature [3, 4]. Illustrated in Figure 1(c), the UV nanowire LED holds strong emission spectra at ~299 nm with a negligible shift in the peak wavelength, attributing to the high crystalline quality and reduced quantum-confined Stark effect in the AlInN nanowires. The LED has a low turn-on voltage of ~5V and low reverse leakage current less, i.e., ~0.5 µA at -8.6V. Moreover, as shown in Figure 1(d), we have demonstrated that the LEE of the AlInN nanowire LEDs could reach ~ 63% using hexagonal photonic crystal nanowire structures which is significantly higher compared to that of the random nanowire arrays [5]. This study paves the way for the development of high-power ultraviolet light-emitters using AlInN nanowire arrays.

(Invited, Digital Presentation) High-Efficiency and Stable Perovskite LEDs and Displays with Nanophotonic Methods

Perovskite light-emitting diodes (PeLEDs) have experienced rapid development in the past 8 years. The external quantum efficiencies (EQEs) of red, green, and near-infrared (NIR) PeLEDs have all surpassed the 20% milestone. Meanwhile, the blue PeLEDs also achieved EQEs higher than 10% and are catching up quickly with PeLEDs of other colors.However, there are still two key problems remaining within the PeLEDs for their further development, namely the light extraction problem and the short lifetime problem. Therefore, in this report, we will discuss our recent work targeting addressing the above two critical problems.First, our work on improving the light extraction efficiency by deploying the nanophotonic substrates will be introduced. Second, the improvement of the device’s operational lifetime at high luminance conditions with perovskite nanowires embedded in a porous alumina template will be discussed. Third, a full evaporation method that is compatible with industrialization will be shown. Last but not least, our endeavors on the displays based on perovskite materials will be covered.

Photonics design theory enhancing light extraction efficiency in quantum dot light emitting diodes

The external quantum efficiency (EQE) of quantum dot light emitting diodes (QLEDs) needs improvement for more power-efficient devices. One of the main limitations is the low light extraction efficiency (LEE). Generally, only 20% of the light that is generated inside the emissive layer makes its way out of the device into air, with the rest being lost to waveguide and substrate modes and surface plasmon polaritons. Different photonics structures have been previously tested to help extract the light that is trapped inside the device. Here we report a photonics design which is a combination of nanopillars and grating structures for improving the LEE of QLEDs. The effect of changing the nanopillar height, radius and material has been studied. It was found that ZnO nanopillars of 500 nm pitch, 200 nm height and 400 nm width alongside 150 nm width and pitch grating structure can increase the LEE at 460 nm by 50% and at 640 nm by 20%. It was also found that different materials can help extract light at different wavelengths. TiO2 nanopillars increased the extraction efficiency at ∼590 nm region which was not observed by the other materials. As around 19% of the world’s electricity consumption is due to lighting applications, increasing the LEE can significantly reduce the power consumption.

Enhancing opto-thermal performances of white laser lighting by high reflective phosphor converter

Phosphor-converted white laser lighting has been considered as a promising candidate in next-generation high-brightness lighting. However, the color converter confronts the inevitable roadblock of low light extraction efficiency and poor thermal stability in reflective white laser diodes (LDs). Herein, a high reflective converter of phosphor-in-glass film-BN layer-Al substrate (PiG-BN-Al) was prepared for high-brightness white laser lighting. The effects of laser power and excitation time on opto-thermal performances of PiG-BN-Al-based converter were investigated and compared with the traditional PiG-Al-based converter. Attributing to the high reflectivity of BN glass composite and the high thermal conductivity of Al substrate, the PiG-BN-Al-based converter achieves a luminous flux of 458.6 lm at a laser power of 3.15 W, which is about 70 % higher than the PiG-Al-based converter. Furthermore, the PiG-BN-Al-based converter still remains low surface temperature of 46.4 °C under the continuous laser excitation. The PiG-BN-Al-based converter provides an effective strategy to enhance the opto-thermal performances of white laser lighting. • A high reflective PiG-BN-Al-based converter was prepared for white laser lighting. • The PiG-BN-Al-based converter presents high luminous flux of 458.6 lm. • The PiG-BN-Al-based converter shows thermal stability and low surface temperature. • The converter enhances the opto-thermal performances of white laser lighting.

(Invited) Enhanced Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes Using Photonic Crystal Structures

In this paper, we report on the enhanced light extraction efficiency (LEE) of AlInN nanowire ultraviolet light-emitting diodes (LEDs) at an emission wavelength of 283 nm using the surface passivation approach and hexagonal photonic crystal structures. Several dielectric materials including SiO2, Si3N4, HfO2, AlN, and BN, have been investigated as the surface passivation layer for the AlInN nanowire LEDs. The LEDs using these dielectric materials show significantly improved LEE compared to that of the unpassivated ultraviolet nanowire LEDs. With a 35nm Si3N4 as surface passivation, the AlInN LED could achieve a LEE of ~ 42.6%, while the unpassivated LED could only have an average LEE of ~ 25.2%. Moreover, the LEE of the AlInN nanowire LEDs could be further increased using hexagonal photonic crystal structures. The periodically arranged nanowire LED arrays could reach up to 63.4% which is almost two times higher compared to that of the random nanowire LEDs. Additionally, the AlInN nanowire ultraviolet LEDs exhibit highly transverse-magnetic polarized emission.

Analytical and numerical design of a hybrid Fabry-Perot plano-concave microcavity for hexagonal boron nitride.

An efficient single-photon emitter (SPE) should emit photons at a high rate into a well-defined spatio-temporal mode along with an accessible numerical aperture (NA) to increase the light extraction efficiency that is required for effective coupling into optical waveguides. Based on a previously developed experimental approach to fabricate hybrid Fabry–Perot microcavities (Ortiz-Huerta et al. Opt. Express 2018, 26, 33245), we managed to find analytical and finite-difference time-domain (FDTD) values for the, experimentally achievable, geometrical parameters of a hybrid plano-concave microcavity that enhances the spontaneous emission (i.e., Purcell enhancement) of color centers in two-dimensional (2D) hexagonal boron nitride (hBN) while simultaneously limiting the NA of the emitter. Paraxial approximation and a transfer matrix model are used to find the spotsize of the fundamental Gaussian mode and the resonant modes of our microcavity, respectively. A Purcell enhancement of 6 is found for a SPE (i.e., in-plane dipole) hosted by a 2D hBN layer inside the hybrid plano-concave microcavity.

On the impact of the beveled mesa for GaN-based micro-light emitting diodes: electrical and optical properties.

In this report, the impact of different mesa designs on the optical and electrical characteristics for GaN-based micro-light emitting diodes (µLEDs) has been systematically and numerically investigated by using TCAD simulation tools. Our results show that an enhanced light extraction efficiency can be obtained by using beveled mesas. The inclined mesa angles can more effectively reflect the photons to the substrate, and this helps to extract the photons to free air for flip-chip µLEDs. However, it is found that the current injection is influenced by inclination angles for the investigated µLEDs, such that the beveled mesas make stronger charge-coupling effect and increase the electric field magnitude in the multiple quantum wells at the mesa edge, so that the carriers cannot be effective consumed by radiative recombination. As a result, this gives rise to stronger defect-induced nonradiative recombination at mesa surfaces. Therefore, there are tradeoffs between the LEEs and IQEs when changing the beveled angle, to maximize external quantum efficiency for GaN-based µLEDs, the beveled mesa angle shall be carefully designed and optimized.

Deep‐Ultraviolet LEDs Incorporated with SiO2‐Based Microcavities Toward High‐Speed Ultraviolet Light Communication

AbstractOptical wireless communication (OWC) in the deep‐ultraviolet (DUV) band requires an efficient DUV light source with large bandwidth characteristics. In this work, a feasible approach is reported to enlarge the light output power as well as the bandwidth of a DUV light‐emitting diode (LED) by embedding a SiO2‐based microcavity on which an aluminum (Al) reflector is simultaneously deposited. Consequently, on the one hand, the microcavity with the Al‐reflector can facilitate photon escape from the LED to increase the light extraction efficiency, thus enhancing the light output power of the devices. On the other hand, the LED incorporated with a microcavity structure exhibits a reduced resistance–capacitance time constant, leading to an increase in the modulation bandwidth of the LED. Strikingly, the DUV LED incorporated with microcavities represents a significant enhancement of light output power by nearly 30% at 80 mA while exhibiting a higher modulation bandwidth of 12% in comparison to the conventional LED without microcavities. Thus, the implementation of the microcavity and Al reflector on top of a classic LED can enlarge the light output power and modulation bandwidth, eventually facilitating to establish viable high‐speed OWC systems.

Improved extraction efficiency of CsPbBr3 perovskite light-emitting diodes due to anodic aluminum oxide nanopore structure

In this work, we investigate the improvement in the performance of a CsPbBr3 perovskite light-emitting diode (PeLED) due to an anodic aluminum oxide (AAO) nanopore structure. The AAO structure in the CsPbBr3 PeLED structure can improve the light extraction efficiency of CsPbBr3 PeLEDs in two ways: the emission light in the side direction being redirected to the normal direction due to the light scattering effect caused by aluminum oxide nanopores and the effective emission area as a result of the rough surface of the AAO structure. The peak luminance, current efficiency, and external quantum efficiency (EQE) were 11,460 cd/m2, 2.03 cd/A, and 0.69% at a bias of 6.0 V, respectively. For comparison, the luminance, current efficiency, and EQE values of CsPbBr3 PeLEDs with the AAO structure using 50 V of pore-expanding voltage demonstrated improvements of 282%, 190%, and 1280%, respectively, over CsPbBr3 PeLEDs without the AAO structure.

Impact of photon recycling on the light extraction from metal halide perovskite light emitting diodes

The impact of photon recycling on the efficiency of light extraction from metal halide perovskite light emitting diodes is quantified using a novel modelling framework based on a detailed-balance compatible Green dyad approach. Analysis of photon modes contributing to internal emission and iterative evaluation of re-absorption and re-emission processes is performed for single perovskite layers in absence and presence of a metallic reflector and under consideration of associated parasitic absorption losses. Finally, the approach is employed to characterize the emission characteristics of a realistic multilayer device stack in dependence of the emitter thickness.

Optical Efficiency Improvement of Chip-on-Board Design LEDs with TiO2/Silicone Packaging Coating

Since TiO2 nanoparticles and silicon composites include a strong scatter influence, they are well-known for improving scattered lighting in LED packets. To improve the optic quality of LEDs packaged with chip-on-board (COB), a thin layer made of high-concentration TiO2 and silicon glue is added to the primary packing layer. COB LEDs’ light extraction efficiency (LEE) rises up to 65% when the key encapsulation includes just silicone, according to the findings of experiments. As a coating of TiO2 and silicone is added, however, the increase in LEE is dependent on the TiO2 concentration. The LEE can be increased from 6% to 24% as the concentration of nanoparticles drops to 0.035 g/cm3. Furthermore, at a mean correlated color temperature (CCT) of around 8500 K, the TiO2/silicone compounds layer will assist in lowering the angular correlated color temperature (CCT) variance between 900 and 470 K within the -90° to 90° observing angle range.

AlGaN Ultraviolet Micro-LEDs

AlGaN-based UV LEDs have proven to have broad applications in many fields including sterilization, disinfection, purification, and phototherapy, but the performance still needs to be further improved. Benefiting from advantages such as better current spreading, lower self-heating effect and higher light extraction efficiency achieved by the reduced LED size, UV micro-LEDs are expected to improve quantum efficiency and thus can expand more potential applications. In this article, performance enhancement techniques and applications of UV micro-LEDs will be reviewed, providing an outlook for further development of AlGaN-based UV-LEDs. The micro-LED format offers possibilities to address fundamental issues that UV LEDs confront in general, but also demonstrates specific characteristics and performance advantages opening up new areas of application including high-speed optical communication, time-resolved fluorescence lifetime measurement, optical pumping, direct writing and charge management.

Light Efficiency Investigation of Ultraviolet Light-Emitting Diodes Combined With Polarized Emission and Packaging Structure

Ultraviolet light-emitting diodes (UV-LEDs) have attracted extensive applications in resin curing, medical treatment, biochemical inspection, and disinfection. However, the UV-LEDs suffer from the main bottleneck of low light efficiency, which depends on the polarized emission and packaging structure of UV-LEDs simultaneously. In this work, we investigated the light efficiency of UV-LEDs combined with polarized emission and packaging structure. Four types of UV-LEDs (UVC-A, UVC-B, UVB, and UVC) without a lens (UV-Ref.), with a flat lens (UV-FL), and with a hemispherical lens (UV-HL) were fabricated, respectively. Through investigating the light output power, far-field emission pattern, and electrical field intensity distribution of UV-LEDs, the UV-HL packaging structure displays the highest emission intensity and light output power in the whole UV band. Compared with the UVC-A-Ref. and UVC-A-FL structures, the enhancement ratios of the light output power of UVC-A-HL structure are 28.9% and 46.5%, respectively. Furthermore, the light extraction efficiency of the TM-polarized mode in the UVC-A-HL structure is enhanced by 29.6% compared with the UVC-A-Ref. structure. The results indicate that the light extraction of UV-LEDs is greatly affected by the polarized emission and packaging structure. Our work opens an efficient guidance to enhance the light efficiency of various UV-LEDs with appropriate packaging structure.

Enhanced light output of scintillators by ZnO nanorod arrays

Scintillators are widely used in national security, industrial and medical fields. The light extraction efficiency of scintillator is quite low due to the total internal reflection. Therefore, enhancement the light output of scintillator is highly important. In this paper, we use ZnO nanorod (NR) arrays fabricated by a simple hydrothermal growth route to enhance the light extraction efficiency of Bi 4 Ge 3 O 12 (BGO) crystal scintillator. The light output results demonstrate that the light extraction efficiency of BGO with ZnO NRs is significantly enhanced, with an enhancement ratio of 4.1 and 3.5 at 510 nm wavelength under 266 nm laser and X-ray excitation, respectively. Our numerically calculated light transmissivity spectra and electric field intensity distributions of BGO with and without ZnO NRs by finite-difference time-domain (FDTD) simulation reveal that the ZnO NRs could reduce the total internal reflection and allow more light to be extracted, resulted in the enhancement of light output of BGO. • BGO with ZnO nanorods modifications yields a significant enhancement of light output. • The enhancement of light output in wide wavelength and angle ranges is demonstrated. • The mechanism of light extraction is revealed by numerical simulations.

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.

Performance improvement of InGaN-based multiple quantum wells embedded nanoporous n-GaN layers

InGaN-based multiple quantum wells (MQWs) embedded nanoporous (NP) GaN layers were fabricated by metal-organic chemical vapor deposition (MOCVD) and followed by electrochemical (EC) etching in oxalic, HF or HNO3 electrolytes. The oxalic-etching and HF-etching do not lead to enhanced luminescence of MQWs, which should be contributed to the damage of InGaN/GaN stacking layers. For the HNO3-etching, the photoluminescence (PL) strength of MQW layer is increased by a factor of 0.52 compared with the as-grown sample, which should be contributed to stress relaxation, light scattering and reflection of quasi vertical nanopore array, and amplify spontaneous emission by resonant cavity. If the nanopore walls in NP-GaN layer are smoothed by the annealing in NH3 atmosphere, the light extraction efficiency of MQWs can be further improved. The study provides a useful method for improving the light extraction efficiency of GaN-based light emitting devices.

Nanostructuration of YAG:Ce Coatings by ZnO Nanowires: A Smart Way to Enhance Light Extraction Efficiency.

In this study, we report on the enhancement of the light extraction efficiency of sol–gel-derived Y3Al5O12:Ce3+ (YAG:Ce) coatings using ZnO nanowire (NW) arrays. The ZnO NWs were grown by hydrothermal synthesis from a ZnO seed layer directly deposited on a YAG:Ce coating. Highly dense and vertically aligned ZnO NW arrays were evidenced on the top of the YAG:Ce coating by electron microscopy. A photoluminescence study showed that this original design leads to a different angular distribution of light together with an increase in emission efficiency of the YAG:Ce coating upon blue excitation, up to 60% more efficient compared to a non-structured YAG:Ce coating (without NWs). These improvements are ascribed to multi-scattering events for photons within the structure, allowing them to escape from the phosphor layer by taking optical paths different from those of the non-structured coating. This strategy of light extraction enhancement appears to be very promising, since it uses soft chemical processes and cheap ZnO NWs and is applicable to any sol–gel-derived luminescent coating.

Toward high-bandwidth yellow-green micro-LEDs utilizing nanoporous distributed Bragg reflectors for visible light communication

In this study, high − 3 dB bandwidth yellow-green InGaN/GaN micro-LEDs grown on polar c-plane GaN substrates are realized by using nanoporous distributed Bragg reflectors, which can increase light extraction efficiency and serve as strain-relaxed buffers to mitigate the quantum-confined Stark effect, resulting in improved external quantum efficiency. Moreover, atomic layer deposition technology is introduced for surface defect passivation, thereby reducing the leakage current. As a result, the device exhibits the highest − 3 dB bandwidth up to 442 MHz and a data transmission rate of 800 Mbit/s at a current density of 2.5 kA / cm 2 with on–off keying modulation, and holds great promise for future high-speed visible light communication applications.