GaN Light-emitting Diodes Research Articles

Effects of Mesa Size on Current Spreading and Light Extraction of GaN-Based LEDs

The mesa size effect on light extraction efficiency (LEE) of light-emitting diodes (LEDs) was studied in this work. The mesa area size of three kinds of LEDs that were evaluated include: 350×950 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (small-size embedded electrodes: GaN LED, S-LED), 500×950 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (medium-size embedded electrodes: GaN LED, M-LED), 950×950 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (large-size embedded electrodes : GaN LED, L-LED). This paper not only discusses LEE, but current density and heat dissipation performance as well. The output power and light extraction efficiency at 700 mA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for S-LED, M-LED, and L-LED are 555, 485, and 432 mW and 38.1%, 33.4%, and 29.7%, respectively. The best output power and LEE of S-LED is due to the electron-hole recombination rate increasing. This phenomenon is caused by the greatest current spread and heat dissipation potential.

Reduction of Thermal Resistance and Optical Power Loss Using Thin-Film Light-Emitting Diode (LED) Structure

In this paper, a GaN light-emitting diode (LED) with sapphire structure and a thin-film LED without sapphire structure are characterized in the photoelectrothermal modeling framework for comparison. Starting from the analysis and modeling of internal quantum efficiency as a function of current and temperature of blue LED, this work develops the thin-film LED device model and derives its optical power and the heat dissipation coefficient. The device parameters of the two LED devices with different structural designs are then compared. Practical optical power measurements are compared with theoretical predictions based on the two types of fabricated devices. It is shown that the thin-film LED device has much lower thermal resistance and optical power loss.

Thermally enhanced blue light-emitting diode

We investigate thermoelectric pumping in wide-bandgap GaN based light-emitting diodes (LEDs) to take advantage of high junction temperature rather than avoiding the problem of temperature-induced efficiency droop through external cooling. We experimentally demonstrate a thermally enhanced 450 nm GaN LED, in which nearly fourfold light output power is achieved at 615 K (compared to 295 K room temperature operation), with nearly no reduction in the wall-plug efficiency (i.e., electrical-optical energy conversion efficiency) at bias V&amp;lt;ℏω/q. The LED is shown to work in a mode similar to a thermodynamic heat engine operating with charged carriers pumped into the active region by a combination of electrical work and Peltier heat (phonons) drawn from the lattice. In this optimal operating regime at 615 K, the LED injection current (3.26 A/cm2) is of similar magnitude to the operating point of common high power GaN based LEDs (5–35 A/cm2). This result suggests the possibility of removing bulky heat sinks in current high power LED products thus realizing a significant cost reduction for solid-state lighting.

Theoretical analysis of quasi-random roughened surface on light extraction enhancement and optical field properties of GaN LED

Light extraction has been a limiting factor for the efficiency of light emitting diode (LED). In this study, a roughened surface increasing the light output of LED was demonstrated in numeric simulation. The roughening pattern was generated by imposing definite number of random imperfections on the surface of LED. Numeric calculations approved a remarkable enhancement of light extraction at targeting wavelengths compared with that of LED with unprocessed surface. In addition, optical field properties were also altered in the presence of roughened surface. The study suggested an easier and more flexible solution towards high efficiency LED by reducing complexity in design and allowing a broader margin for tolerance in production.

Synthesis and photo-luminescent properties of new red phosphor SrGe4O9:Mn4+

A series of red-emitting phosphors SrGe4O9 doped with Mn4+ were prepared by the high temperature reaction. The structure, morphology and photo-luminescent (PL) properties of these phosphors were investigated. These phosphors exhibit strong, sharp red emission at 655nm with broad excitation in the UV and blue light regions. Among these as-prepared phosphors, the sample SrGe3.976O9:0.024Mn4+ shows the strongest red emission. The light-emitting diodes (LEDs) were fabricated by coating this red phosphor on blue LED chips, and their electro-luminescent (EL) properties were studies under 20mA current excitation. The results indicated that this phosphor maybe find application on the blue GaN LED chip.

Fabrication and improvement of nanopillar InGaN / GaN light-emitting diodes using nanosphere lithography

Surface-patterning technologies have enabled the improvement of currently existing light-emitting diodes (LEDs) and can be used to overcome the issue of low quantum efficiency of green GaN-based LEDs. We have applied nanosphere lithography to fabricate nanopillars on InGaN/GaN quantum-well LEDs. By etching through the active region, it is possible to improve both the light extraction efficiency and, in addition, the internal quantum efficiency through the effects of lattice strain relaxation. Nanopillars of different sizes are fabricated and analyzed using Raman spectroscopy. We have shown that nanopillar LEDs can be significantly improved by applying a combination of ion-damage curing techniques, including thermal and acidic treatment, and have analyzed their effects using x-ray photoelectron spectroscopy.

Cascaded GaN Light-Emitting Diodes With Hybrid Tunnel Junction Layers

We report the fabrication of cascaded GaN light-emitting diodes (LEDs) with previously reported p++-GaN/i-InGaN/n++-GaN tunnel junction layers (TJLs) and with hybrid TJL. Compared with the conventional LED, it was found that we could enhance the output power by 35% and 80% from the LED with p++-GaN/i-InGaN/n++-GaN TJL and the LED with hybrid TJL. It was also found that the TJ resistances were $6.05 \times 10^{-3}$ and $1.95 \times 10^{-3} \Omega ~\cdot $ cm $^{2}$ for the LED with p++-GaN/i-InGaN/n++-GaN TJL and the LED with hybrid TJL, respectively. It was also found that the use of hybrid TJL could result in smaller efficiency droop. These improvements could all be attributed to the larger polarization charges induced at the AlGaN/InGaN interface which could enhance the tunneling current. Furthermore, it was found that the cascaded GaN LEDs with hybrid TJL were also reliable.

ESD-Reliability Analysis and Strategy of the GaN-Based Light-Emitting Diodes

Compounds such as GaN, ZnSe, and SiC are the compounds that currently hold the most potential in developing blue light-emitting diodes (LEDs) and blue laser diodes (LDs). Speaking of the physical property, the gallium nitride belongs to a direct bandgap material with an obviously super luminous efficiency; therefore, the gallium nitride has the dominate tendency than that of others materials. Although the gallium nitride has excellent physical properties, but in actually it is suffered many challenges during the manufacture process. Especially, it is extremely sensitive to the electrostatic discharge (ESD) threat. In other words GaN diodes generally exhibit very low anti-ESD capabilities when in HBM, MM reversed bias modes. These LEDs in the MM stress situation, its ESD immunity level usually is only about 50-V extremely low anti-ESD ability. Therefore, in this paper, GaN LED DUTs will be stressed and investigated under HBM and MM pulses bombardments, and the aim of this work is to describe a detailed investigation of the factors that limit the robustness of GaN-based LEDs under ESD transient events; finally they will provide some countermeasures in ESD reliability consideration.

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

Nanoscale Epitaxial Lateral Overgrowth of GaN-Based Light-Emitting Diodes on an AlN Nanorod-Array Template

Despite the rapid progress of GaN light-emitting diodes, the quest for better epistructure quality with less defects and relaxed strains to achieve higher internal quantum efficiency has been a popular topic. Here, we propose an AlN nanorod template grown on the sapphire substrate using vapor-liquid-solid method. The corresponding material quality, including voids near nanorods, Raman spectra, and transmission electron microscopy images, indicates significant improvement.

A Simplified Method of Making Flexible Blue LEDs on a Plastic Substrate

A much-simplified method of making flexible GaN blue light-emitting diode (LED) array on a plastic substrate was demonstrated. A sticky elastomeric stamp was first brought into contact with prefabricated GaN LED array on a sapphire substrate. Laser liftoff was applied by shining laser light through the sapphire substrate. The released LED array sitting on the stamp was transferred to a polyethylene terephthalate substrate that was coated with an adhesive layer to finish the fabrication process. Careful investigation of the built-in stress in the GaN LED layer using Raman spectroscopy revealed that the maximum stress that allows for intact GaN LED layer release and transfer was 0.7 GPa. The method drastically simplifies the cumbersome conventional GaN layer transferring method while preserving the original layout of the GaN LED array. Due to its simple and practical characteristics, the method is expected to greatly facilitate the development of versatile transferrable GaN LED applications on various substrates at a much-reduced cost.

Enhanced performance of photonic crystal GaN light-emitting diodes with graphene transparent electrodes.

The two-dimensional (2D) triangle lattice air hole photonic crystal (PC) GaN-based light-emitting diodes (LED) with double-layer graphene transparent electrodes (DGTE) have been produced. The current spreading effect of the double-layer graphene (GR) on the surface of the PC structure of the LED has been researched. Specially, we found that the part of the graphene suspending over the air hole of the PC structure was of much higher conductivity, which reduced the average sheet resistance of the graphene transparent conducting electrode and improved the current spreading of the PC LED. Therefore, the work voltage of the DGTE-PC LED was obviously decreased, and the output power was greatly enhanced. The COMSOL software was used to simulate the current density distribution of the samples. The results show that the etching of PC structure results in the degradation of the current spreading and that the graphene transparent conducting electrode can offer an uniform current spreading in the DGTE-PC LED.PACS: 85.60.Jb; 68.65.Pq; 42.70.Qs

Effect of the Quantum-Well Shape on the Performance of InGaN-Based Light-Emitting Diodes Emitting in the 400–500-nm Range

The electrical and optical properties of InGaN–GaN light-emitting diodes (LEDs) emitting in the 400–500-nm range and having a v-shaped quantum well (VSQW) and a u-shaped quantum well (USQW) are numerically investigated using APSYS simulation program. The simulation results showed that the devices containing VSQW have superior performance in terms of optical power and internal quantum efficiency droop compared to those with USQW. The optical power of the LEDs containing USQW increases gradually and reaches a maximum at 460 nm; however, the optical power of the LEDs with VSQW improves gradually, and the maximum is obtained in a window from 420 to 436 nm as a result of radiative recombination enhancement. The simulation results suggest that the higher performance of the VSQW is due to piezoelectric field reduction and an enhancement of electron and hole wave functions overlap.

Insights into annealing-induced ohmic contact formation at graphene/p-GaN interface with a NiOx contact layer

The effect of rapid thermal annealing on the graphene/p-GaN interface with a thin NiOx contact layer in a GaN light-emitting diode (LED) was investigated. Raman and ultraviolet photoemission measurements revealed charge transfer in graphene on a NiOx contact layer. Rapid thermal annealing at temperatures above 350 °C induced the formation of metallic Ni with a simultaneous increase of Ni3+ ions within the NiOx layer covered by the graphene. As a result, the graphene electrode on a NiOx/p-GaN surface of a blue GaN LED formed a low-resistance ohmic contact with a specific contact resistance of 5.3 × 10–4 ohm cm2. The ensuing LED chip offered a low forward voltage, comparable to that of the Ni/Au counterpart. This is attributed to a combination of distinct phenomena arising due to a work function modulation in graphene, increase in the carrier concentrations at the near surface region incited during the formation of NiOx, and thermal reduction induced modifications within the NiOx contact layer. At elevated annealing temperature, however, oxidation of graphene led to poor current spreading and thus a low optical output, but both these constraints were eliminated by using a few layer graphene.

Compatibility of the selective area growth of GaN nanowires on AlN-buffered Si substrates with the operation of light emitting diodes

AlN layers with thicknesses between 2 and 14 nm were grown on Si(111) substrates by molecular beam epitaxy. The effect of the AlN layer thickness on the morphology and nucleation time of spontaneously formed GaN nanowires (NWs) was investigated by scanning electron microscopy and line-of-sight quadrupole mass spectrometry, respectively. We observed that the alignment of the NWs grown on these layers improves with increasing layer thickness while their nucleation time decreases. Our results show that 4 nm is the smallest thickness of the AlN layer that allows the growth of well-aligned NWs with short nucleation time. Such an AlN buffer layer was successfully employed, together with a patterned SiOx mask, for the selective-area growth (SAG) of vertical GaN NWs. In addition, we fabricated light-emitting diodes (LEDs) from NW ensembles that were grown by means of self-organization phenomena on bare and on AlN-buffered Si substrates. A careful characterization of the optoelectronic properties of the two devices showed that the performance of NW-LEDs on bare and AlN-buffered Si is similar. Electrical conduction across the AlN buffer is facilitated by a high number of grain boundaries that were revealed by transmission electron microscopy. These results demonstrate that grainy AlN buffer layers on Si are compatible both with the SAG of GaN NWs and LED operation. Therefore, this study is a first step towards the fabrication of LEDs on Si substrates based on homogeneous NW ensembles.

Fabrication of oxide-based nano-patterned sapphire substrate to improve the efficiency of GaN-based of LED

In this study, an oxide-based nano-patterned sapphire substrate (ONPSS) was used as the substrate for a nitride-based light emitting diode (LED) in order to enhance the LED’s internal quantum efficiency and light extraction efficiency. The ONPSS was fabricated by a direct spin-on-glass printing technique, which is simple, easy, and relatively low-cost technique. Conventional PSSs are generally fabricated by photolithography and a sapphire etching process. However, the process reported here, it is possible to fabricate an oxide-based PSS without the sapphire etching process. After a GaN-based blue LED device was grown on the ONPSS, we measured the photoluminescence and electroluminescence intensity to confirm the light extraction efficiency and internal quantum efficiency of the LED. Compared to a GaN LED grown on an unpatterned sapphire, the ONPSS-based LED exhibited a 100% increase in light output power without electrical degradation.

Optimization by a genetic algorithm of the light-extraction efficiency of a GaN light-emitting diode

Light extraction from light-emitting materials is fundamentally limited by internal reflections due to the high dielectric-constant contrast between the material that produces the light and the emergent medium. These internal reflections can however be reduced significantly by a well-designed texturation of the surface of the emitting material. We used a genetic algorithm to determine optimal geometrical and material parameters for this texturation, the objective being to maximize the extraction of light of a GaN light-emitting diode. This study, which was restricted to two-dimensional texturations, shows that symmetric triangles actually correspond to the optimal shape. The dielectric constant of the material used for this texturation should ideally have the same dielectric constant as the GaN. The optimal texturation determined in this work leads to a light-extraction efficiency of 11.1%, which improves significantly the value of 3.7% obtained with a flat surface and the value of 5.7% achieved in previous work.

GaN Light-Emitting Diodes for up to 5.5-Gb/s Short-Reach Data Transmission Over SI-POF

We report a high-speed data transmission over a 1-mm diameter step-index polymer optical fiber (SI-POF) with GaN light-emitting diodes (LEDs) operating at 435 and 475 nm. The modulation bandwidths of the LEDs at 40 mA were 180 and 300 MHz, respectively. A maximum bit rate of 5.5 Gb/s was achieved with a 475-nm GaN LED at −1.8-dBm fiber-coupled power over a 1-m SI-POF employing 4-pulse-amplitude modulation format. Compared with the performance of a high-speed red resonant-cavity LED under the same test setup, the chromatic dispersion reduces the potential of the GaN LED-based system when the transmission distance is longer than 30 m.

Mechanisms of the Asymmetric Light Output Enhancements in $a$ -Plane GaN Light-Emitting Diodes With Photonic Crystals

The unique properties of nonpolar GaN light-emitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.

Finite-difference time-domain analysis on light extraction in a GaN light-emitting diode by empirically capable dielectric nano-features

Enhancement of light extraction in GaN light-emitting diode (LED) by addressing an array of nanomaterials is investigated by means of three dimensional (3D) finite-difference time-domain (FDTD) simulation experiments. The array of nanomaterials is placed on top of the GaN LED and is used as a light extraction layer. Depending on its empirically capable features, the refractive index of nanomaterials with perfectly spherical (particle) and hemispherical (plano-convex lens) shapes were decided as 1.47 [Polyethylene glycol (PEG)] and 2.13 [Zirconia (ZrO2)]. As a control experiment, a 3D FDTD simulation experiment of GaN LED with PEG film deposited on top is also carried out. Different light extraction profiles between subwavelength- and over-wavelength-scaled nanomaterials addressed GaN LEDs are observed in distributions of Poynting vector intensity of the light extraction layer–applied GaN LEDs. In addition, our results show that the dielectric effect on light extraction is more efficient in the light extraction layer with over-wavelength scaled features. In the case of a Zirconia particle array (ϕ = 500 nm) with hexagonal closed packed (hcp) structure on top of a GaN LED, light extraction along the normal axis of the LED surface is about six times larger than a GaN LED without the extraction layer.