Extraction Efficiency Of GaN-based Light-emitting Diodes Research Articles

The Light Extraction Efficiency of GaN-based LED with Air-Hole Photonic Crystal Structures

In order to improve the light extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs), a layer of cylindrical air-hole photonic crystal (PC) structure inserted into P-GaN is proposed and investigated numerically. Finite difference time domain (FDTD) method is used to make a series of simulations in the LEE of GaN-based LED with air-hole photonic crystal structure. According to the variable-controlling approach, the PC structure is simulated and optimized. The results of the simulations show that the LEE depends on the PC’s position and relevant structural parameters. When PC is etched in the active layer, and its dielectric constant [Formula: see text][Formula: see text][Formula: see text]m, etching [Formula: see text][Formula: see text][Formula: see text]m and air-hole radius [Formula: see text][Formula: see text][Formula: see text]m, higher LEE is obtained as 44.5%, translated into a 13.6-fold enhancement for the case of a planar LED. The remarkable enhancement is of particular interest for improving LEE of LED and provides a theoretical reference for future LED structure design efforts.

Parameters optimization of nanopatterned sapphire substrates for enhancing light extraction efficiency of GaN-based LEDs

The nanopatterned structures play an important role in enhancing the light extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs). Three-dimensional finite-difference time-domain (3D-FDTD) method was carried out to calculate the LEE enhancement of GaN-based LEDs designed on periodic nanostructures with different geometries. Orthogonal experiment method was employed to obtain the optimal structure parameters and analyze the weighting factor of critical parameters affecting the LEE enhancement. The results of orthogonal experiment showed that the height/depth of the periodic nanostructures is the dominated factor to influence the LEE enhancement, followed by the diameter and the spacing of the periodic nanostructures. It was also found that the far-field intensities of LEDs with periodic convex nanostructures were stronger than those with periodic concave nanostructures. The critical parameters of periodic convex nanostructures were further optimized by single factor method, and their optimal parameters were obtained. The optimal LEE enhancement of GaN-based LEDs grown on cone-shaped nanopillar and column-shaped nanopillar array patterned sapphire substrates are 1.95 and 1.97, respectively. This means the cone-shaped and column-shaped nanopillar structures have almost identical LEE enhancement of GaN-based LEDs.

Improvement of Optical and Electrical Properties of Indium Tin Oxide Layer of GaN-Based Light-Emitting Diode By Surface Plasmon in Silver Nanoparticles

Recently, GaN-based light-emitting diodes (LEDs) have found many applications, including automobile headlights, full-color displays, traffic light lamps, and solid-state lightings. However, the light extraction efficiency of GaN-based LEDs is limited by the total internal reflection of light at the interface between the LED structure and air caused by the large refractive index difference between the GaN film (n=2.5) and air (n=1). Photons with an incident angle lower than the critical angle of 23° are reflected from the interface, leading to low light extraction efficiency. Although the light extraction efficiency has been increased by using indium tin oxide (ITO) as a transparent conducting layer (TCL), further improvements in the TCL performance and the optical output power are still required to realize high efficiency LEDs. In this study, we demonstrate that the ITO layer with Ag nanoparticles can be used as an optically and electrically high performing TCL for LEDs. The transmittance of the ITO layer was improved by the resonance between incident light in the ITO layer and localized surface plasmons (LSPs) at the ITO-Ag interface. The optical output power and electrical characteristics of LEDs were improved by the Ag nanoparticles deposited on the ITO layer of the LEDs. Figure 1(a) shows a schematic diagram of blue LED with Ag nanoparticles deposited on ITO TCL. The LEDs with a blue emission at 465 nm were grown on a c-plane (0001) patterned sapphire substrate by metalorganic chemical vapor deposition. After growth of a GaN nucleation layer, a 2 μm-thick undoped GaN layer and a 2 μm-thick n-GaN layer were grown at 1025 °C. Then, multiple quantum wells, consisting of five periods of undoped InGaN wells and GaN barriers, were grown at 750 °C, followed by growth of a 200 nm-thick p-GaN layer at 980 °C. To fabricate the LEDs, the p-GaN layer was etched by an inductively coupled plasma etching process to expose the n-GaN layer for an n-type ohmic contact. Then, a 150 nm-thick ITO layer was deposited on p-GaN layer as a TCL by electron-beam evaporation. To investigate the effect of the Ag nanoparticles on the ITO layer, LEDs with Ag nanoparticles deposited on ITO layer were also fabricated. To form Ag nanoparticles, an Ag layer with different film thickness was deposited on the ITO layer by electron-beam evaporation, and then annealed at 500 °C for 1 min in a rapid thermal annealing (RTA) system. This was followed by deposition of Cr/Au on the n-GaN layer to form an n-pad electrode and on the ITO layer to form a p-pad electrode. Figure 1(a) also shows atomic force microscopy (AFM) image of the ITO layer with Ag nanoparticles after RTA processing. The surface of an as-deposited Ag layer on the ITO layer is smooth and featureless. However, the thin Ag layer is transformed into nanoparticles, and the particle size is increased by thermal annealing via the Ostwald ripening process. Figure 1(b) shows the optical output power of the blue LEDs with different density and size of Ag nanoparticle deposited on the ITO TCLs as a function of injection current. As shown in Fig. 1(b), the optical output powers of the LEDs with Ag nanoparticles on top of the ITO TCL are higher than that of the LED with the bare ITO TCL. In particular, the optical output power of an LED with Ag nanoparticles (1 nm-thick Ag layer) is increased by 16% at an injection current of 20 mA compared to that of the LED without Ag nanoparticles and the enhancement is increased with increasing the density of Ag nanoparticles. This improvement is mainly attributed to the increased transmittance of ITO by the resonance of the light with the LSPs in the randomly distributed Ag nanoparticles. The electrical properties of the LEDs are also improved by the enhanced conductivity of the ITO TCL caused by the Ag nanoparticles. These results indicate that the Ag nanoparticles/ITO with high transmittance and low resistivity can be used as a high efficiency TCL for LEDs. Figure 1

Enhanced light extraction efficiency of GaN-based LED fabricated by multi-chip array

In this study, we report the enhanced light extraction efficiency of light emitting diode (LED) devices by employing multi-chip arrayed LED structures. We analyze the optoelectronic property of multi-chip arrayed LED structures with 16, 36 and 64 LED chips in the same 1 × 1 mm2 device as well as 2 × 2 and 3 × 3 mm2 devices. When compared with a conventional large area single-chip LED structure with the same device size, the multi-chip arrayed LED structures generally exhibit enhanced light extraction efficiency and electrical efficiency owing to the reduced current crowding effect. The effect of chip and device size on the LED performance is discussed.

Enhanced Light Extraction Efficiency of GaN-Based LED with ZnO Nanorod Grown on Ga-Doped ZnO Seed Layer

High density and narrow zinc oxide nanorod (ZnO NR) arrays (aspect ratio ?18.9) were hydrothermally grown on gallium-doped ZnO (GZO) seed layer (3 wt% Ga-doping). Light-emitting diode (LED) fabricated with ZnO NR arrays grown on GZO seed showed marked 19% and 70% increases in light output power at 20 mA of driving current compared to that of LED with ZnO NR arrays grown on ZnO seed layer and reference LED. The significant enhancement of light output power is attributed to high density and surface-to-volume ratios of NR arrays as well as to enhanced multiple photons scattering at the NR surface.

Light extraction efficiency of GaN-based LEDs with non-periodic and periodic sub-wavelength structures

Antireflective non-periodic and periodic sub-wavelength structures (SWSs) are fabricated on indium tin oxide (ITO) layers deposited on p-GaN layers of GaN-based green light-emitting diodes (LEDs). The light output powers of the LEDs using non-periodic and periodic SWSs are increased by 18% and 39% at a 20-mA input current, respectively, compared to LEDs with flat ITO layers. Periodic ITO SWSs show a larger enhancement of output power because they have more effective profiles of the graded refractive index. The output power of periodic SWS LEDs is improved in all angular directions, indicating that the improved output power is attributed to lower total internal reflection and lower Fresnel reflection.

Enhanced Light Extraction Efficiency of GaN-Based LEDs With 3-D Colloidal-Photonic-Crystal Bottom Reflector

In this letter, self-assembled polystyrene nano-spheres with a diameter of 193 nm were used to fabricate the 3-D colloidal-photonic-crystal (CPC) bottom reflector for blue GaN-based light-emitting diodes (LEDs). Experimentally, the key point is the hydrophilic treatment of substrate and immediately to disperse the suspension into the substrate at 30 °C. The equilibrium between the particle transport and crystallization in the aqueous solution helps CPCs to form a faced-centered-cubic phase when the solution temperature decreases. This will lead to the improvement of reflectivity due to reduced defects, such as dislocations and vacancies, in the CPCs. The light output intensity of the LEDs fabricated with a well-controlled 3-D-CPC bottom reflector increases by 219% compared to that of conventional LEDs without it.

Light extraction efficiency of GaN-based LED with pyramid texture by using ray path analysis

We study three different gallium-nitride (GaN) based light emitting diode (LED) cases based on the different locations of the pyramid textures. In case 1, the pyramid texture is located on the sapphire top surface, in case 2, the pyramid texture is locate on the P-GaN top surface, while in case 3, the pyramid texture is located on both the sapphire and P-GaN top surfaces. We study the relationship between the light extraction efficiency (LEE) and angle of slant of the pyramid texture. The optimization of total LEE was highest for case 3 among the three cases. Moreover, the seven escape paths along which most of the escaped photon flux propagated were selected in a simulation of the LEDs. The seven escape paths were used to estimate the slant angle for the optimization of LEE and to precisely analyze the photon escape path.

Improved light extraction efficiency of GaN-based LEDs with patterned sapphire substrate and patterned ITO

To improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs), periodic semisphere patterns with 3.5μm width, 1.2μm height, and 0.8μm spacing were formed on sapphire substrate by dry etching using BCl3/Cl2 gas chemistry. The indium tin oxide (ITO) transparent conductive layer was patterned by wet etching to reduce the total internal reflection existing along between p-GaN, ITO, and air. At 350mA injection current, the high power LED by integrating patterned sapphire substrate with patterned ITO technology exhibited a 36.9% higher light output power than the conventional LEDs.

Enhancement of light extraction in GaN-based light-emitting diodes using rough beveled ZnO nanocone arrays

A remarkable enhancement of light extraction efficiency in GaN-based blue light-emitting diodes (LEDs) with rough beveled ZnO nanocone arrays grown on the planar indium tin oxide (ITO) layer is reported. The light output power of LEDs with rough beveled ZnO nanocone arrays was increased by about 110% at 20 mA compared with conventional LEDs with planar ITO. The light extraction efficiency of GaN-based LEDs with rough-beveled ZnO nanocones is measured much greater than with smooth-surface hexagonal ZnO nanorods. The light-ray tracing analysis showed that ZnO nanocones with rough surfaces enlarge the light escape cone of GaN-based LEDs and have a greater advantage for extracting light compared with ZnO nanorods.

Improvement of photon extraction efficiency of GaN-based LED using micro and nano complex polymer structures

A micro- and nanoscale complex structure made of a high refractive index polymer (n = 2.08) was formed on the ITO electrode layer of an edge-emitting type GaN blue light-emitting diode (LED), in order to improve the photon extraction efficiency by suppressing total internal reflection of photons. The nanoimprint lithography process was used to form the micro- and nanoscale complex structures, using a polymer resin with dispersed TiO2 nano-particles as an imprint resin. Plasma processing, such as reactive ion etching, was used to form the micro- and nano-scale complex structure; thus, plasma-induced damage to the LED device can be avoided. Due to the high refractive index polymeric micro- and nanostructure on the ITO layer, the electroluminescence emission was increased up to 20%, compared to an identical LED that was grown on a patterned sapphire substrate to improve photon extraction efficiency.

Direct Indium Tin Oxide Nanoparticle Printing Technique for Improvement of Light Extraction Efficiency of GaN-Based LEDs

To enhance the light extraction efficiency of the GaN-based light emitting diode (LED), indium tin oxide (ITO) nanoparticle photonic crystal patterns are fabricated on the surface of the GaN-based blue LED device using the direct printing technique of the ITO nanoparticles. According to electroluminescence (EL) measurements, the EL intensity of the GaN-based blue LED with photonic crystal patterns is 28% higher than an identical LED without photonic crystal patterns. Printing the ITO nanoparticles eliminates the need for a plasma etching process of the ITO layer so that the current–voltage characteristics do not degrade.

Enhancement of electroluminescence in GaN-based light-emitting diodes using an efficient current blocking layer

Enhancement of electroluminescence in GaN-based light-emitting diodes (LEDs) was achieved using an efficient current blocking layer formed by postannealing. When a LED chip with Ni∕Au pad on Ni∕Au transparent p contact was annealed at 500 °C, the electroluminescence of the LED chip increased by 55%. The specific contact resistivity of metal contact below the p pad significantly increased due to indiffusion of Au and Ni atoms from the p pad to the contact interfacial region. As a result, an efficient current blocking layer could be formed below the p pad, enhancing the light output and decreasing the reverse leakage current of the LED chip. This result suggests that a further increase in the extraction efficiency of GaN-based LEDs can be easily obtained using the postannealing.

Extraction Efficiency of GaN-Based LEDs

In this work, we investigate the extraction efficiency for UV emitting rectangular 300 x 300 μm 2 gallium nitride (GaN) based light emitting diodes (LEDs) by simulation with a raytracer tool. It is shown that the extraction efficiency depends strongly on slight variations of the absorption in the GaN layers. Furthermore, the influence of the substrate shape is studied. For standard rectangular sapphire substrate based LEDs the calculated extraction efficiency is 12.4%, whereas for silicon carbide substrate based devices the higher refractive index causes a lower efficiency (4.5%). Using a shaped SiC substrate the extraction efficiency can be improved to 17.2% and 19.9% for a sapphire substrate. The influence of geometric design parameters like sidewall angle are analyzed as well.

Extraction Efficiency of GaN-Based LEDs

In this work, we investigate the extraction efficiency for UV emitting rectangular 300 x 300 μm 2 gallium nitride (GaN) based light emitting diodes (LEDs) by simulation with a raytracer tool. It is shown that the extraction efficiency depends strongly on slight variations of the absorption in the GaN layers. Furthermore, the influence of the substrate shape is studied. For standard rectangular sapphire substrate based LEDs the calculated extraction efficiency is 12.4%, whereas for silicon carbide substrate based devices the higher refractive index causes a lower efficiency (4.5%). Using a shaped SiC substrate the extraction efficiency can be improved to 17.2% and 19.9% for a sapphire substrate. The influence of geometric design parameters like sidewall angle are analyzed as well.