Conventional Light-emitting Diodes Structure Research Articles

Proposing the n+-AlGaN tunnel junction foranefficient deep-ultraviolet light-emitting diodeat254 nm emission.

Toxic and low-pressure deep-ultraviolet (DUV) mercury lamps have been used widely for applications of surface disinfection and water sterilization. The exposure of pathogens to 254nm DUV radiations has been proven to be an effective and environmentally safe way to inactivate germs as well as viruses in short time. To replace toxic mercury DUV lamps, an n +-A l G a N tunnel junction (TJ)-based DUV light-emitting diode (LED) at 254nm emission has been investigated. The studied conventional LED device has maximum internal quantum efficiency (IQE) of 50% with an efficiency droop of 18% at 200A/c m 2. In contrast, the calculated results show that a maximum IQE of 82% with a 3% efficiency droop under a relatively higher injection current was estimated by employing a 5nm thin n +-A l G a N TJ with a 0.70 aluminum molar fraction. In addition, the TJ LED emitted power has been improved significantly by 2.5 times compared with a conventional LED structure. Such an efficient n +-A l G a N TJ-based DUV LED at 254nm emission might open a new way, to the best of our knowledge, for the development of safe and efficient germicidal irradiation sources.

Electron Blocking Layer Free Full-Color InGaN/GaN White Light-Emitting Diodes

Indium gallium nitride (InGaN) compound semiconductor alloys have been intensively studied and utilized for white light-emitting diodes (LEDs) [1, 2]. Compared to conventional planar structures, III-nitride nanowires can exhibit significant advantages including greatly reduced dislocation densities and polarization fields, due to the effective lateral stress relaxation. Additionally, the nanowire LEDs with the incorporation of quantum dots/disks in the active region may lead to superior carrier confinement, promising for high efficiency LEDs with tunable emission [2]. However, nanowire LEDs still contain several challenges to further improve the quantum efficiency and light output power, which includes inefficient carrier confinement in the active region, nonuniform carrier distribution, and electron overflow. The integration of an electron blocking layer (EBL) in the LED mitigates electron leakage problem to an extent but it also lowers the hole injection into the active region if not designed properly causing poor hole transport problem. Hence, the LED without EBL, with efficient carrier confinement capability, negligible carrier leakage from active region is the optimal choice [3].In this work, we have demonstrated EBL free high-performance phosphor-free InGaN/GaN nanowire white LEDs. The proposed structure is grown on Si (111) substrate by RF plasma-assisted MBE (Veeco Gen II MBE) under nitrogen-rich condition, then fabricated devices for characterization. The proposed LED structure (LED3) consists of a 200 nm thick n-GaN nanowire template, 10 multiple quantum wells (MQWs) of 3 nm GaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) in the active region and a 100 nm thick p-GaN. In addition, we have incorporated 30 nm n-In0.2Ga0.8N QW between the n-GaN and the active region to control electron overflow. Moreover, to utilize the electrons escaped from the active region, a second 10 nm p-In0.2Ga0.9N QW is introduced between the active region and p-GaN. This second QW reduces electron loss to the p-GaN and contributes blue light emission to relatively control the white light emission from the LED device. The schematic diagram of the proposed structure is shown in Fig. 1(a). A comparison has been made with the conventional LED structure (LED1: 200 nm n-GaN, 10 MQWs (3 nm)/MQBs (3 nm), 100 nm p-GaN) and EBL based structure (LED2: 200 nm n-GaN, 10 MQWs (3 nm)/MQBs (3 nm),10 nm p-Al0.1Ga0.9N EBL, 100 nm p-GaN) to understand the enhanced performance of the proposed LED structure. As a result, we observed that proposed LED has better performance in terms of less electron leakage, high hole injection, improved internal quantum efficiency (IQE), external quantum efficiency (EQE) and output optical power as compared to other conventional and EBL based InGaN/GaN LEDs. The resulting device exhibits high IQE of 58.5% with highly stable emission characteristics. The relative EQE and output optical power of the LEDs are shown in Figs. 1(b) and 1(c). Such unique EBL free nanowire LED structure is a promising approach to achieve high-power phosphor-free LEDs for general illumination and display applications.

Improvement of the carrier distribution with GaN/InGaN/AlGaN/InGaN/GaN composition-graded barrier for InGaN-based blue light-emitting diode**Project supported by the Science and Technology Program Project for the Innovation of Forefront and Key Technology of Guangdong Province, China (Grant No. 2014B010121001), the Special Funds for Strategic Emerging Industries of Guangdong Province, China (Grant No. 2012A080304006), the Special Funds for the Innovation of Forefront and

InGaN light-emitting diodes (LEDs) with GaN/InGaN/AlGaN/InGaN/GaN composition-graded barriers are proposed to replace the sixth and the middle five GaN barriers under the condition of removing the electron blocking layer (EBL) and studied numerically in this paper. Simulation results show that the specially designed barrier in the sixth barrier is able to modulate the distributions of the holes and electrons in quantum well which is adjacent to the specially designed barrier. Concretely speaking, the new barrier could enhance both the electron and hole concentration remarkably in the previous well and reduce the hole concentration for the latter one to some extent along the growth direction. What is more, a phenomenon, i.e., a better carrier distribution in all the wells, just appears with the adoption of the new barriers in the middle five barriers, resulting in a much higher light output power and a lower efficiency droop than those in a conventional LED structure.

Enhanced carrier injection in InGaN/GaN multiple quantum wells LED with polarization-induced electron blocking barrier

In this report, we designed a light emitting diode (LED) structure in which an N-polar p-GaN layer is grown on top of Ga-polar In0.1Ga0.9N/GaN quantum wells (QWs) on an n-GaN layer. Numerical simulation reveals that the large polarization field at the polarity inversion interface induces a potential barrier in the conduction band, which can block electron overflow out of the QWs. Compared with a conventional LED structure with an Al0.2Ga0.8N electron blocking layer (EBL), the proposed LED structure shows much lower electron current leakage, higher hole injection, and a significant improvement in the internal quantum efficiency (IQE). These results suggest that the polarization induced barrier (PIB) is more effective than the AlGaN EBL in suppressing electron overflow and improving hole transport in GaN-based LEDs.

Light emission characteristics of blue strain-compensated InGaN/InGaN/InGaN light-emitting diodes

The light emission characteristics of blue strain-compensated InGaN/InGaN light emitting diodes (LEDs) grown on InGaN substrates were investigated using the multiband effective mass theory. The radiative recombination rate of the strain-compensated LED structure is shown to be larger than that of the conventional LED structure due to the decrease in the lattice mismatch with the substrate. Also, the light emission distribution in quantum wells (QWs) for the strain-compensated LED structure is found to be much more uniform than that for the conventional LED structure. As a result, the simulation results suggest that the efficiency droop is significantly improved when a conventional GaN barrier and substrate are replaced by an InGaN barrier and substrate. Thus, a possibility exists to employ a strain-compensated QW to improve the optical performance of InGaN-based blue LEDs.

Carrier Transport Improvement in Blue InGaN Light-Emitting Diodes Via Reduced Polarization Using a Band-Engineered Electron Blocking Layer

This study numerically investigates the effect of using a new electron blocking layer (EBL) for blue InGaN light-emitting diodes (LEDs) to improve hole injection efficiency and electron confinement. Simulation results suggest that the carrier transportation behavior of the EBL can be appropriately modified by adept control of the graded AlGaN layer. Furthermore, when compared with the conventional LED structure, the redesigned LED with graded AlGaN layer shows a slight improvement in forward voltage Vf and a significant enhancement in light output power. The redesigned LED can achieve an exceptional increment of 106.6% in light output power at 100 mA when compared with conventional LED. The observed improvement in the photoelectric performance of blue LEDs is primarily due to the reduced polarization effect at the last-barrier/EBL interface, as a result of the graded Al composition in EBL.

Enhanced performance of InGaN light-emitting diodes with InGaN and composition-graded InGaN interlayers

The effects of InGaN light-emitting diodes (LEDs) with InGaN and composition-graded InGaN interlayers in the space of multiple quantum wells and electron blocking layer are studied numerically. The electrostatic field, energy band diagrams, carrier concentrations, light–current–voltage performances, and internal quantum efficiency (IQE) are investigated. Simulation results show that the light output power and IQE are both largely improved over the conventional LED structure due to the improvement in hole injection efficiency and electron blocking capability, especially for the LED with composition-graded InGaN interlayer.

An improvement of light extraction efficiency for GaN-based light emitting diodes by selective etched nanorods in periodic microholes

We have demonstrated the enhancement of a GaN-based light emitting diode (LED) by means of a selective etching technique. A conventional LED structure was periodically etched, to form periodic microholes. It showed an improvement of the light extraction efficiency (LEE) of approximately 15%, compared to that of a conventional LED. Furthermore, nano-sized rods inside the microholes were randomly formed by using a powder mask, resulting in an LEE of 43%. From the result of confocal scanning electroluminescence measurement, the light emission arises mainly from the vicinity of the nanorods in the periodic microholes. Therefore, we found that nanorods randomly distributed in periodic microholes in a LED structure play a significant role in the reduction of total internal reflection, by acting as photon wave-guides and scattering centers. This method would be valuable for the fabrication of high efficiency GaN-based LED, in terms of technical simplification and cost.

Solution-processed Li–Al layered-double-hydroxide platelet structures for high efficiency InGaN light emitting diodes

High-oriented Li-Al layered double hydroxide (LDH) films were grown on an InGaN light-emitting diode (LED) structures by immersing in an aqueous alkaline Al(3+)- and Li+-containing solution. The stand upward and adjacent Li-Al LDH platelet structure was formed on the LED structure as a textured film to increase the light extraction efficiency. The light output power of the LED structure with the Li-Al LDH platelet structure had a 31% enhancement compared with a conventional LED structure at 20 mA. The reverse leakage currents, at -5V, were measured at -2.3 × 10(-8) A and -1.0 × 10(-10)A for the LED structures without and with the LDH film that indicated the Li-Al LDH film had the insulated property acted a passivation layer that had potential to replace the conventional SiO2 and Si3N4 passivation layers. The Li-Al LDH layer had the textured platelet structure and the insulated property covering whole the LED surface that has potential for high efficiency InGaN LED applications.

Characterization of the well-aligned ZnO nanorod structure on a pulsed laser deposited AlZnO seed layer

ZnO nanorod structures on InGaN light-emitting diode (LED) structures were grown by inserting the aluminum zinc oxide (AZO) seed layers through a sputtering and a pulsed laser deposition (PLD) processes. The well-aligned ZnO nanorods on the PLD-AZO seed layer had a higher crystalline quality and a preferred orientation along (002) plane because the X-ray intensity of (002) peak was stronger than that of (100) and (101) peaks. The inter-planar spacing in a high resolution transmission electron microscopy image is approximately 0.26nm corresponding to (002) plane of the ZnO nanorod structure. The strong photoluminescence intensity of the ZnO nanorods on the PLD-AZO seed layer was observed at 378nm. The light output power had a 54% enhancement for the InGaN LEDs with the ZnO nanorod structures on the PLD-AZO seed layer compared with a conventional LED structure.

InGaN-Based Light-Emitting Diodes With a Sawtooth-Shaped Sidewall on Sapphire Substrate

An InGaN light-emitting diode (LED) with a cone-shaped GaN structure and a sawtooth-shaped sapphire sidewall structure was fabricated through a laser-drilling process. The fabricated procedures consisted of a laser scribing/drilling process, a wet etching process, and a chip cleaving process. In the treated LED structure with the laser-drilling sawtooth-shaped sidewall, the light output power had a 16% enhancement compared to a conventional LED structure with a laser-scribing sidewall. A periodic high light emission intensity, with a 2.6- μm width spaced at regular intervals of 3.8 μm, was observed on the treated LED sidewall structure corresponding to the laser-drilling patterns. The LED structure consists of a laser-drilling sidewall and a cone-shaped GaN structure that increases the light extraction efficiency for high efficiency InGaN LED applications.

InGaN light emitting diodes with a laser-treated tapered GaN structure

InGaN light-emitting diode (LED) structures get an air-void structure and a tapered GaN structure at the GaN/sapphire interface through a laser decomposition process and a lateral wet etching process. The light output power of the treated LED structure had a 70% enhancement compared to a conventional LED structure at 20 mA. The intensities and peak wavelengths of the micro-photoluminescence spectra were varied periodically by aligning to the air-void (461.8nm) and the tapered GaN (459.5nm) structures. The slightly peak wavelength blueshift phenomenon of the EL and the PL spectra were caused by a partial compressed strain release at the GaN/sapphire interface when forming the tapered GaN structure. The relative internal quantum efficiency of the treated LED structure (70.3%) was slightly increased compared with a conventional LED (67.8%) caused by the reduction of the piezoelectric field in the InGaN active layer.

An AlN Sacrificial Buffer Layer Embedded into the InGaN Light Emitting Diodes for a Chemical Lateral Etching Process

High chemical lateral etching rate at AIN buffer/sacrificial layer embedded into GaN/patterned-sapphire substrate of InGaN-based light emitting diode (LED) was achieved. An air-void structure was observed at a truncated triangular patterned sapphire that provided an empty space to increase the lateral etching rate of the AIN sacrificial layer. A 30 μm wide lateral etched region was observed around the LED chip. After a chemical lateral wet-etching process on AlN sacrificial layer and a bottom-up N-face etching process on GaN epitaxial layer, the stable crystallographic etching planes were formed as GaN {1011} planes. The treated LED structure had a higher light-output power and a smaller divergent angle compared with conventional LED structure that had the potential application for the chemical lift-off process.

Enhancement of Light Extraction from GaN-Based Light-Emitting Diodes by Coating Surface with Al2O3 Powder

GaN-based light-emitting diodes (LEDs) were fabricated by coating Al2O3 powder on a p-GaN and a semitransparent p-contact metal surface to increase the extraction probability of the internal reflected photons through the Al2O3 powder. The density of the ∼300-nm-diameter Al2O3 powder was about 1.9 ×1013 cm-2 on both surfaces. The forward voltages of the LEDs coated with Al2O3 powder on the p-GaN surface and the semitransparent p-contact metal surface were 4.15 and 3.42 V at 20 mA, respectively. The light output powers of both LED structures with Al2O3 powder coated on the p-GaN and semitransparent p-contact metal surfaces were increased by almost 30% compared with the conventional LED structure without Al2O3 powder coating.

Fabrication of reflective GaN mesa sidewalls for the application to high extraction efficiency LEDs

AbstractWe proposed and demonstrated a modified GaN light‐emitting diode (LED) structure. The structure was made by integrating angled sidewalls to an otherwise conventional LED structure. Various GaN sidewall angles were obtained by adjusting etch conditions and the etched surface roughness was ∼2 nm. Near‐field emission patterns indicated that the angled sidewalls efficiently deflect photons that are initially guided laterally within the GaN epilayer into the off‐surface direction. For sidewall angle of 30°, total surface emission strength was improved by a factor exceeding 3. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)