Surface Of Light-emitting Diodes Research Articles

Planarization of p-GaN surfaces on MOCVD grown V-defect engineered GaN-based LEDs

The large polarization barriers between the quantum wells and quantum barriers in long-wavelength GaN-based light-emitting diodes (LEDs) inhibit their performance by requiring excess driving voltages to reach standard operating current densities. Lateral injection of carriers directly into quantum wells is required to circumvent this issue. V-defects are naturally occurring inverted hexagonal defects with semipolar 101¯1-plane sidewalls generated on surface depressions from threading dislocations. LEDs engineered to intentionally generate V-defects below the active region of the LED can achieve lateral carrier injection through the V-defect sidewalls and have already been able to demonstrate world record wall-plug efficiencies for LEDs in the green-red wavelengths. V-defects can be enlarged during kinetically limited growth where the growth rate of the c-plane GaN is faster than that of their sidewalls, leaving them unfilled. We report on the metal organic chemical vapor deposition growth conditions required to fill in V-defects with p-GaN during epitaxial growth of the LED post the active region. Circular transmission length measurements of Pd/Au contacts processed on p-GaN surfaces with various amounts of unfilled V-defects showed no significant difference in their sheet resistance and specific contact resistance. J–V measurements of LEDs grown with varying unfilled V-defect densities showed no significant difference in the forward bias regime. However, in the reverse bias regime, catastrophic breakdown occurred at markedly lower voltages for samples with larger unfilled V-defect densities. This suggests that unfilled V-defects may act as hotspots for device failure, and planarizing LED surfaces may help prevent early degradation of LED devices.

FDTD Simulation to Investigate the Effect of Nanopattern on Light Extraction Distribution of ZnS:Cu LED

Light Emitting Diode (LED) technology is very common in everyday life. Due to the large refractive index difference between LED material and air, many excited photons from the active layer cannot be passed on to the air, thereby Total Internal Reflection (TIR) will occur and reducing the Light Extraction Efficiency (LEE) of LED. The use of nanopatterns on the LED surface can help trapped photons to get out of the LED by changing the direction and angle of photons’ propagation. Because the shape of the nanopattern will affect the increase in the LEE of LEDs and its fabrication is expensive, simulations are needed to design and study the effect of the shape and size of the nanopattern on the LEE of LEDs. In this study, FDTD simulations will be carried out on nano-sized grating patterns on the ZnS:Cu LED surface and then compared with experimental results to see the accuracy of the simulation results using Ansys Lumerical FDTD. The simulation results obtained in light emission distribution are consistent with the real measurement results. The simulation results are also used to evaluate the performance of the LEDs with and without the nanopattern on the surface.

Machine vision inspection of early failure and line-shaped defects of blue InGaN/GaN light emitting diodes soaked in liquid nitrogen for cryogenic tests

Cryogenic tests were conducted on blue InGaN/GaN light-emitting diodes (LEDs) by immersing the device in liquid nitrogen (LN2). To study the degradation of the LEDs, a combination of optical, electrical, and material characterizations were performed on the LN2-soaked device. The results indicate that noticeable emission spectral shifts were observed during the LED immersion due to lattice deformation. Moreover, clustering of cracks was generated in the cross-section, and line-shaped defects appeared on the surface of the GaN LED, which was different from the defects on AlInGaP LEDs after LN2 immersion. To track the trend of device damage, computer-assisted OpenCV-Python software was used to zoom in on nuanced variations in certain areas of the emission images. Early degradation of certain areas can be detected by computer-assisted machine vision, which has the potential to provide an early warning and protective circuits for the system consisting of the initial degraded LEDs. Reliability studies on LN2-soaked InGaN/GaN LEDs show promise in studying the reliability issues of GaN device applications in Arctic regions and space exploration.

Short-Wave Ultraviolet-Light-Based Disinfection of Surface Environment Using Light-Emitting Diodes: A New Approach to Prevent Health-Care-Associated Infections.

Ultraviolet (UV)-C irradiation is a promising method for microbial eradication on surfaces. Major developments have taken place in UV-C light-emitting diodes (LEDs) technology. In this study, we examined the suitability of UV-C LED-based surface disinfection in hospitals. We tested the efficacy of UV-C LED surface treatment on different microorganisms dried on a carrier surface or in a liquid solution. The influences of soiling, shading, surface material, radiation wavelength, microbial load and species on the disinfection performance were investigated. UV-C LED caused a reduction of >5 log10 levels of E. coli, S. aureus and C. albicans, whereas 3 log10 reduction was observed for G. stearothermophilus spores. The components of the medium led to a reduced UV-C LED efficiency compared to buffered solutions. We observed that the microbial load and the roughness of the carrier surface had a major influence on the UV-C LED disinfection efficiencies, whereas shading had no impact on inactivation. This study showed that UV-C is suitable for surface disinfection, but only under certain conditions. We showed that the main factors influencing microbial inactivation through UV-C light (e.g., intrinsic and extrinsic factors) had a similar impact when using a UV-C LED radiation source compared to a conventional UV-C lamp. However, the potential of LEDs is contributed by their adjustable wavelength and customizable geometry for the decontamination of medical devices and surfaces, and thereby their ability to overcome shading effects.

Color Conversion Light-Emitting Diodes Based on Carbon Dots: A Review.

This paper reviews the state-of-the-art technologies, characterizations, materials (precursors and encapsulants), and challenges concerning multicolor and white light-emitting diodes (LEDs) based on carbon dots (CDs) as color converters. Herein, CDs are exploited to achieve emission in LEDs at wavelengths longer than the pump wavelength. White LEDs are typically obtained by pumping broad band visible-emitting CDs by an UV LED, or yellow–green-emitting CDs by a blue LED. The most important methods used to produce CDs, top-down and bottom-up, are described in detail, together with the process that allows one to embed the synthetized CDs on the surface of the pumping LEDs. Experimental results show that CDs are very promising ecofriendly candidates with the potential to replace phosphors in traditional color conversion LEDs. The future for these devices is bright, but several goals must still be achieved to reach full maturity.

Design and Performance of Ultraviolet 368-nm AlGaN-Based Flip-Chip High-Voltage LEDs with Epitaxial Indium Tin Oxide/Al Reflective Mirror and Symmetry Electrode Arrangement

In this letter, we describe the design and fabrication of high-power AlGaN-based ultraviolet (UV) flip-chip high-voltage light-emitting diodes (LEDs) operating at 368 nm with an epitaxial indium tin oxide (ITO)/Al reflecting mirror and symmetry electrode layout. Metal-organic chemical vapor deposition (MOCVD) was used to grow an ITO thin film as a transparent electrode on the LED surface. At 365 nm, epitaxial ITO thin films exhibited a transmittance of up to 93.6%. Additionally, the epitaxial ITO/Al reflective mirror has a reflectance of 81.2% at 365-nm. To investigate the electrical characteristics, four types of HV-LED micro-cells were constructed with varying n-type mesa structures and p-type interconnect electrodes. We demonstrated a forward voltage (Vf) of 7.86 V at 350 mA with a 2 × 2 mico-cells high-voltage ultraviolet 368-nm flip-chip LED after optimising electrode structure and device process.

Enhanced light extraction efficiency via double nano-pattern arrays for high-efficiency deep UV LEDs

This work reports on the packaging structure of double-layer nano-pattern arrays (NPAs) for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs), which can significantly enhance the light extraction efficiency (LEE), a major device performance bottleneck. The double-layer NPAs were fabricated on the surface of the flip-chip DUV LED by etching sapphire and coating the fluoropolymer resin, which can alleviate the total internal reflection (TIR) at the top interfaces and enhance light extraction on sidewalls, leading to the great improvement of both TE and TM-polarized light. As a result, the 273 nm DUV LED with double-layer NPAs realized 28.3% enhancement in light output power (LOP) compared with the reference device without the NPA at large current of 100 mA. High peak external quantum efficiency (EQE) and wall-plug efficiency (WPE) of 5.19% and 4.38% were demonstrated, respectively. Combined with the finite element analysis (FEA), it is further confirmed that the double-layer NPAs have higher external coupling efficiency to enhance LEE. The proposed LEE enhancement strategy comes from the combination of nanostructured materials and packaging technology, which is cost-effective and meets mass production, providing efficient value in practical applications of UV devices.

Integration of Molybdenum Disulfide with Light-Emitting Diode for Reversible NO2 Sensing

Introduction Two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors have recently been applied to gas sensing applications given their unique physical and chemical properties [1-3]. However, the reversible and reliable gas detection at room temperature is not completely successful yet due to insufficient recovery of TMDCs that adsorbed the target analytes in an air environment. In order to improve the recovery of the reacted TMDCs, application of external thermal energy and illumination of UV light during the sensing have been reported [4,5]. However, the excessive supply of thermal energy could degrade the sensing response, and the UV light source may have a harmful effect on the human body. Moreover, processes to integrate the TMDCs on desired surfaces or substrates usually require complex, time-consuming steps.This paper reports a sensitive and reversible NO2 sensor based on 2D molybdenum disulfide (MoS2) that is integrated on a surface of light-emitting diode (LED). The MoS2 layers are locally coated on the surface of LED by simply hand-drawing from the MoS2 pellet, which can be rapidly completed. The drawn MoS2 layers serve as a NO2 sensitive material where its electrical resistance changes upon the adsorption of NO2. In parallel, visible light illumination from the LED generates excitons in the MoS2 during the sensing cycles, resulting in the increased number of electrons that can respond to the NO2 molecules, which eventually improves the sensing responses. At the same time, the increased number of holes in the MoS2 by the light illumination could combine with the electrons trapped by the NO2 molecules, thereby releasing the adsorbed NO2 and thus improving the recovery. In addition, heat dissipated by the LED, which is inevitable in solid-state lighting, provides thermal energy to promote the desorption of adsorbed NO2 molecules, further improving the recovery rate. Unlike the previous works that usually activate the sensing materials by global illumination, the MoS2-integrated LED is able to effectively improve the response and the recovery of NO2 sensing by simultaneous optical and thermal stimuli. Results and Conclusions The MoS2 layers are formed on the surface of LED by hand-drawing of the pellet. This process provides a simple and cost-effective way to integrate the MoS2 onto the desired surface. Raman spectrum confirms the successful coating of the MoS2 with distinguishable in-plane and out-of-plane vibration modes. It is noted that the LED can be used for its original purpose of light emission even after the MoS2 coating. The electroluminescence center of the LED is ~625 nm, which covers the B exciton transition of the MoS2 measured by UV-vis spectrophotometer. We experimentally found the optimal power to operate the LED, which leads to the largest response of the MoS2 toward the NO2. Compared to the response at dark condition (~40%) under 10 ppm NO2 exposure, the response of MoS2 reaches ~131% at 400 mW of input power of the LED. In addition, the recovery rate is much improved to ~90% under 400 mW LED illumination due to increased heat dissipation from the LED. This optimal response might originate from the balance between the exciton generation in the MoS2 and the NO2 desorption by the thermal energy. The measured sensitivity is ~10.8%/ppm at the NO2 concentration range from 1 to 10 ppm under 400 mW LED illumination, which would be enough for early detection of NO2 exposure. Thus, our approach can be applicable to the sensitive, reversible gas sensing while routine and usual lighting of the LED in daily life.

Influence of structural and surface properties of MgO thin film as a heat spreader on high power LED performance

Thermal management difficulties in light emitting diodes (LEDs) incredibly impacts LEDs performance, reliability, and lifespan. To find a lasting solution to LEDs thermal management difficulties, MgO thin film was deposited on Cu substrate and used as heat spreader for LED package. Influence of structural and surface properties on thermal performance of LED was investigated by XRD, FESEM and AFM. Optimum LED thermal performance was recorded from the LED mounted to MgO heat spreader with low microstrain of 7.3 × 10–3, uniform particle size (34 nm), surface roughness (4.5 nm) and minimum peak-valley distance (1.90 nm). Thermal conductivity and thermal transient characterization illustrated that 0.6 M MgO thin film showed 15.73 W/mK, higher junction temperature difference (18.06 °C) with higher total thermal resistance difference (3.35 K/W). Improved illumination and reduction in LED surface temperature were recorded using MgO heat spreader from optical and thermal infrared analysis. Therefore, magnesium oxide thin film would be suggested as a heat spreader for enhancing LED’s thermal management.

Near and far-field analysis of Fresnel structure applied in the LED surface

In this contribution, we present light emitting diode (LED) with Fresnel structure implemented in the LED surface. Fresnel structure in one-dimensional arrangement consisting of lines unequally distributed with square root of distance were etched directly in the LED emitting surface using electron beam lithography with the etched depth app. 400 nm. The Fresnel LED was studied in near-and far-field measurements. Near-field measurements of the structure show enhanced emission from patterned lines in comparison with the unpatterned surface. This local enhancement contributes to the effect of the Fresnel structure on the overall intensity and it causes far-field broadening for great angles. The asymmetric intensity profile is documented by goniophotometer measurements and it is in well agreement with the simulation.

A theoretical modeling analysis for triboelectrification controlled light emitting diodes

Abstract In this paper, we demonstrate the use of triboelectric nanogenerator (TENG) as a mean of mechanical light triggering to control InGaN-based light-emitting diodes (LEDs). Light extraction from the LED is two successive steps process. First, the voltage produced by the TENG is used to control the gate-to-source current of a MOSFET transistor through adjusting the transistor channel width and length. The second step is forwarding the drain-source current resulting from MOSFET transistor to the LED as its injection current to induce spontaneous emission from the LED surface to the air. Three LED colors are considered: red, green and blue. Significant emitted power from these InGaN-based LEDs in the RGB wavelength band is observed for both P-MOSFET and N-MOSFET transistor configurations. The emitted optical spectrum is controlled by optimizing the combined TENG-MOSFET-RGB LED geometry; dimensions and the bias voltage between the drain and source terminals of the MOSFET transistor. With recent advances in TENGs as an energy harvesting technology, it is expected that this study offers an approach to enhance the light extraction of various LED devices. With the enhancements in the performance of optoelectronic devices, the field of tribo-phototronics has attracted more attention, and in this work, we introduce the first theoretical framework, to the best of our knowledge, based on finite element modeling. This study provides significant insights into the working principles of tribo-Phototronic devices as well as guidelines for future device design.

Enhanced Optical Output of Near-Ultraviolet Light-Emitting Diodes by a Monolayer of Nanospheres

The optical output of near-ultraviolet (NUV) light-emitting diodes (LEDs) was improved by including a monolayer of hexagonal close-packed polystyrene (PS) nanospheres. PS nanospheres with different sizes were deposited on the indium tin oxide layer of the NUV LEDs. The electroluminescence results showed that the light extraction efficiency of the NUV LEDs was increased by the inclusion of PS nanospheres, and the maximum optical output enhancement was obtained when the size of the nanospheres was close to the light wavelength. The largest enhancement of the optical output of 1.27-fold was obtained at an injection current of 100 mA. The enhanced optical output was attributed to part of the incident light beyond the critical angle being extracted when the exit surface of the NUV LEDs had a PS nanosphere monolayer. This method may serve as a low-cost and effective approach to raise the efficiency of NUV LEDs.

Graphene-Perfluoroalkoxy Nanocomposite with High Through-Plane Thermal Conductivity Fabricated by Hot-Pressing.

With the rapid development of electronics and portable devices, polymer nanocomposites with high through-plane thermal conductivity (TC) are urgently needed. In this work, we fabricated graphene nanosheets−perfluoroalkoxy (GNs−PFA) composite sheets with high through-plane TCs via hot-pressing followed by mechanical machining. When the GNs content exceeded 10 wt%, GNs were vertically aligned in the PFA matrix, and the through-plane TCs of nanocomposites were 10–15 times higher than their in-plane TCs. In particular, the composite with 30 wt% GNs exhibited a through-plane TC of 25.57 W/(m·K), which was 9700% higher than that of pure PFA. The composite with 30 wt% GNs was attached to the surface of a high-power light-emitting diode (LED) to assess its heat-dissipation capability. The composite with vertically aligned GNs lowered the LED surface temperature by approximately 16 °C compared with pure PFA. Our facile, low-cost method allows for the large-scale production of GNs–PFA nanocomposites with high through-plane TCs, which can be used in various thermal-management applications.

Effect of a thin Au and ZnO layer on optical properties of 1D PhC structures patterned in LED surface

1D patterns with different periods were fabricated by Electron Beam Direct Write (EBDW) lithography with the aim to improve the Light Emitting Diode (LED) efficiency, i.e. the light out coupling. Three sets of 1D PhC LED samples with bare PhC, thin Au, and ZnO layers were prepared. The highest Light Extraction Enhancement (LEE) was achieved for 600 nm Photonic Crystals (PhC) period in the case of bare and Au deposited PhC, and for 500 nm PhC period in the case of ZnO deposited PhC, respectively. LEE was studied by measuring L-I characteristics of untreated reference LEDs compared to patterned LEDs.

Qualitative analysis of coupling effect of fluid velocity distribution in microchannels on the performance of the LED water cooling system

PurposeThe purpose of this paper is to ensure adequate thermal management to remove and dissipate the heat produced by a light-emitting diode (LED) and to guarantee reliable and safe operation.Design/methodology/approachA three-dimensional (3-D) computational fluid dynamics (CFD) model was used to analyze the distribution of fluid velocities among microchannels at four different aspect ratios.FindingsThe results showed that at the same inlet flow rate, the larger the aspect ratio of the microchannels, the better the uniformity of the internal fluid velocity and thus better the heat dissipation performance on the surface of the high-power LED chip. In addition, the thermal performance of a high-power LED water cooling system with four different aspect ratios’ microchannel structures is further studied experimentally. Specifically, the coupling effect between the fluid velocity distribution in the microchannels and the heat dissipation performance of a high-power LED water cooling system is qualitatively analyzed and compared with the simulation results of the fluid velocity distribution. The results fully demonstrated that a larger aspect ratio of the microchannels results in better heat dissipation performance on the surface of the high-power LED chip.Originality/valueOptimizing the structural parameters to facilitate a relatively uniform velocity distribution to improve the water cooling system performance may be a key factor to be considered.

Improvement of light extraction efficiency in GaN-based light-emitting diodes by addition of complex photonic crystal structure

To improve the light extraction efficiency of a GaN-based light-emitting diode (LED), a new complex periodic photonic crystal structure containing two kinds of hemispheres was designed. This complex photonic crystal structure is arrayed on the surface of the planar LED, and a finite-difference time-domain method was used to optimize the hemisphere materials (ZnO, GaN, and SiC) and the structural parameters of the complex photonic crystal (the size of the hemisphere radius and the lattice constant of complex structure). A high improvement of light extraction efficiency is achieved in complex photonic crystal LED, translated into an 8.3-fold enhancement for the case of a planar LED.

Illuminance and heat transfer characteristics of high power LED cooling system with heat sink filled with ferrofluid

In the case of a high power LED (light emitting diode) cooling system using the ferrofluid, the illuminance characteristics, including the illuminance and the illuminance efficacy, as well as the heat transfer characteristics, including the temperature and the thermal resistances, were experimentally investigated under various input voltages and magnetic fields and compared with air and water working fluids. The temperatures of the LED surface, the heatsink surface, and the LED junction of the high power LED cooling system with ferrofluid all increased with a rise in the input voltages. The illuminance of the high power LED cooling system with the ferrofluid also increased with increases in the input voltage until an elapsed time of 1200 sec, but the illuminance of the high power LED cooling system dramatically decreased at 19.3 V, due to the increased power consumption. The junction temperature and the illuminance efficacy of the high power LED cooling system with the magnet decreased at all input voltages by 9.7% and increased by 6.9%, respectively, compared to those without the magnet. The high power LED cooling system with the ferrofluid at an elapsed time of 3600 sec showed 67.5% and 66.1% higher illuminances than those of the air and the water, respectively. As a result, the cooling and optical performances could be improved using the ferrofluid as the cooling liquid, and could be further enhanced by applying the external magnetic field. In addition, the designed LED cooling system with the thin fin heat sink filled with the ferrofluid is suggested as a potential vehicle headlamp considering the illuminance for visibility and effective cooling.

Micro-structure changes induced by femtosecond laser on the surface of GaN multilayer film grown on Si substrate

The light-emitting-diode (LED) surface of GaN epilayers grown on Si substrate was irradiated by a femtosecond laser (λ = 800 nm). The morphology of LED surface after laser irradiation was measured by optical microscope and scanning electron microscopy. It was found that many radial cracks appeared in the film on account of thermal expansion and then became more numerous as the number of laser pulses increased. In addition, the changes of the micro structure of GaN and Si substrate were studied by means of Micro Raman Spectroscopy. The results showed the presence of polycrystalline silicon and the decomposition of GaN had taken place in the laser spot or the region with the large laser fluence. Comparatively, in the areas with the low laser fluence or the edge of laser spot, the characteristic peaks of GaN films and the AO phonon peak of Si had an obvious blueshift, which most likely was the result of compressive stress between GaN layers and Si substrate. The maximum values of the compressive stress were 0.74 GPa and 1.4 GPa, respectively. Furthermore, with the increase of the laser fluence, the original Si crystalline structure recovered gradually. The above study showed that the stress, the transformation and the deformation play a predominant role in the femtosecond laser etching process of LED.

(Invited) The Effects of Zinc-Tin-Oxide Nanosphere Monolayer on Improvement of Light Extraction in Light-Emitting Diodes

Over the last two decades, III-nitride-based light-emitting diodes (LEDs) have been widely used for our daily life, such as illumination, various backlight units for full-color displays, and automotive headlamps. In spite of the versatile advantages of GaN-LEDs, relatively low external quantum efficiency is still a problem to expand their application fields. For the viable solution to unravel pending issues in solid state light-emitting research fields, many efforts have been conducted. In this study, we report on the applications of zinc-tin-oxide (ZTO) nanoparticles and monolayer as a light extraction layer for GaN-based LEDs. The ZTO layers formed on top of an LED epi-structure with a variable Sn-ratio, which was deposited by the spin coating method. The transmission spectra of the ZTO layers with Zn to Sn ratios of 1:1 (ZTO-I) and 1:5 (ZTO-II) exhibited optical transmittances of 98% and 88% in the visible region, respectively. The electroluminescence (EL) and light power-current-voltage (L-I-V) measurements show that double ZTO layers consisting of various Zn to Sn ratios led to enhanced light extraction from blue LEDs. Finally, we will discuss the effects of ZTO monolayer transferred onto the p-GaN surface on the external quantum efficiency of LEDs. The ZTO monolayers were prepared by using the ZTO nanospheres synthesized by precipitation solution method. The transferred ZTO nanosphere monolayers on top surface of LED are very effective way to improve the light extraction efficiency.

Enhanced Performance of GaN-based Ultraviolet Light Emitting Diodes by Photon Recycling Using Graphene Quantum Dots

Graphene quantum dots (GQDs) with an average diameter of 3.5 nm were prepared via pulsed laser ablation. The synthesized GQDs can improve the optical and electrical properties of InGaN/InAlGaN UV light emitting diodes (LEDs) remarkably. An enhancement of electroluminescence and a decrease of series resistance of LEDs were observed after incorporation of GQDs on the LED surface. As the GQD concentration is increased, the emitted light (series resistance) in the LED increases (decreases) accordingly. The light output power achieved a maximum increase as high as 71% after introducing GQDs with the concentration of 0.9 mg/ml. The improved performance of LEDs after the introduction of GQDs is explained by the photon recycling through the light extraction from the waveguide mode and the carrier transfer from GQDs to the active layer.