Vertical-injection Light-emitting Diodes Research Articles

The Improvement of Bonding Metal Layers in a GaAs Vertical Structure Light-Emitting Diode Package

This paper presents an investigation into the reliability of substrate bonding between metal layers and gallium arsenide vertical-injection light-emitting diodes (GaAs VLEDs). In this study, molybdenum (Mo) acts as a buffer layer between the GaAs VLED and the aluminum-based metal core printed circuit board (MCPCB). The thermal expansion coefficient (CTE) value of GaAs is approximately equal to that of the Mo layer under the substrate. This is beneficial for slowing the decay of the adhesive bond due to temperature cycling from − 40°C to 125°C for 50 cycles. The experimental results show that the use of the Mo buffer layer greatly improved the electrical and optical performance of the LED packages. The average leakage current was reduced from 33.63 mA to 2.37 mA at the reversed voltage of − 3 V and the increase in light output power was 130% at an injection current of 0.7 A. The results also show that a Mo buffer layer can reduce the risk of cracking on bonding wires after the thermal shock test (TST). These results are useful for improving the aging of GaAs VLEDs.

Vertical LEDs on Rigid and Flexible Substrates Using GaN-on-Si Epilayers and Au-Free Bonding

Vertical-injection light-emitting diodes (VLEDs) were fabricated and demonstrated on mechanically rigid and flexible substrates using GaN-on-Si epilayers and a cost-effective Au-free Cu/Sn bonding method. With a mirror layer between the VLEDs and the receptor Si(100) carrier, $500~\mu \text{m} \,\, \times \,\, 500~\mu \text{m}$ VLEDs emit up to 134-mW optical power at a drive current of 300 mA. The peak wall-plug efficiency was 23% at 1-A/cm2 current injection density. Due to excellent heat dissipation of the metal and Si carrier, the VLED junction temperature was measured to be only 47.5 °C at a working current density of 350 mA/mm2 and increased by 0.27 °C per 1-mA current increment. After release from the rigid substrate, the 40- $\mu \text{m}$ -thick Cu/Sn/Cu bonding layer can also work as a handling substrate to paste LED thin films onto flexible substrates, including plastic and paper. The self-contained VLED structure exhibited the original $I$ – $V$ characteristics and high brightness on various substrates. The results attest to the feasibility of using GaN-on-Si epilayers and Cu/Sn/Cu bonding for a wide range of applications, including low-cost solid-state lighting and flexible illuminations.

Optimized via-hole structure in GaN-based vertical-injection light-emitting diodes

This study examined the electrical and the optical characteristics of GaN-based vertical-injection light-emitting diodes (VI-LEDs) with various numbers of via holes. As the number of via holes was increased from 1 to 11, the operating voltage at 350 mA of the VI-LEDs decreased from 5.4 to 3.7 V, and the light output power increased greatly from 150 to 233 mW. A further increase in the number of via holes up to 23, however, decreased the light output power due to a decrease in the active area, which resulted in an increase in the efficiency droop at high current injection. The light intensity distributions were characterized by using beam profile measurements to assess the current crowding phenomenon of the VI-LEDs according to the number of via holes. The VI-LEDs with 1 and 5 via holes exhibited a non-uniform light intensity distribution whereas the VI-LEDs with 11 via holes displayed a uniform light intensity distribution and an improved efficiency droop due to the current being uniformity spread by the adequate number of via holes, which suggests that in this study, the VI-LEDs with 11 via holes are optimized via the hole structures.

Vertical-Injection AlGaInP LEDs with n-AlGaInP Nanopillars Fabricated by Self-Assembled ITO-Based Nanodots.

The light output power of AlGaInP-based vertical-injection light-emitting diodes (VI-LEDs) can be enhanced significantly using n-AlGaInP nanopillars. n-AlGaInP nanopillars, ~200 nm in diameter, were produced using SiO2 nanopillars as an etching mask, which were fabricated from self-assembled tin-doped indium oxide (ITO)-based nanodots formed by the wet etching of as-deposited ITO films. The AlGaInP-based VI-LEDs with the n-AlGaInP nanopillars provided 25 % light output power enhancement compared to VI-LEDs with a surface-roughened n-AlGaInP because of the reduced total internal reflection by the nanopillars at the n-AlGaInP/air interface with a large refractive index difference of 1.9.

Novel thin-GaN LED structure adopted micro abraded surface to compare with conventional vertical LEDs in ultraviolet light.

In this study, novel thin-GaN-based ultraviolet light-emitting diodes (NTG-LEDs) were fabricated using wafer bonding, laser lift-off, dry etching, textured surface, and interconnection techniques. Placing PN electrodes on the same side minimized the absorption caused by electrodes in conventional vertical injection light-emitting diodes (V-LEDs) and the current spreading was improved. The light output power (700 mA) of the NTG-LEDs was enhanced by 18.3% compared with that of the V-LEDs, and the external quantum efficiency (EQE) of the NTG-LEDs was also relatively enhanced by 20.0% compared with that of a reference device. When the current operations were 1,500 mA, the enhancements of the light output power and EQE were 27.4% and 27.2%, respectively. Additionally, the efficiency droop was improved by more than 15% at the same current level.

Enhanced performance of nitride-based ultraviolet vertical-injection light-emitting diodes by non-insulation current blocking layer and textured surface.

For the purpose of light extraction and efficiency enhancement, the nitride-based ultraviolet vertical-injection light-emitting diodes (UV-VLEDs) with non-insulation current blocking layer (n-CBL) and optimized textured surface were fabricated. The optical and electrical characteristics were investigated in this n-CBL UV-VLED. Furthermore, the efficiency of optimized structure was improved by 5 ~ 6 times compared to our reference.

Nitride-based micron-scale hexagonal pyramids array vertical light emitting diodes by N-polar wet etching

In this work, we reported the fabrication of nitride-based hexagonal pyramids array (HPA) vertical-injection light emitting diodes (V-LEDs) by N-polar wet etching. The performance of HPA V-LEDs devices was significantly improved with 30% higher internal quantum efficiency compared with conventional roughened broad area V-LEDs. The simulated extraction efficiency by finite difference time domain method was 20% higher than typical roughened V-LEDs. The reversed leakage current of HPA V-LEDs was reduced due to better crystal quality, which was confirmed by conductive atomic force microscopy measurement. Furthermore, the efficiency droop for HPA V-LEDs were substantially alleviated.

Hexagonal Pyramids Shaped GaN Light Emitting Diodes Array by N-polar Wet Etching

ABSTRACTIn this work, we investigated the influence of N-polar wet etching on the properties of nitride-based hexagonal pyramids array (HPA) vertical-injection light emitting diodes (V-LEDs). The cathodeluminescence images showed the randomly distribution of hexagonal pyramids with isolated active regions. The transmission electron microscopy images demonstrated the reduced density of threading dislocations. The IQE was estimated by temperature dependence of photoluminescence, which showed 30% increase for HPA V-LEDs compared with broad area (BA) V-LEDs. The improved extraction efficiency was verified by finite difference time domain simulation, which was 20% higher than that of roughened BA V-LEDs. The electrical properties of HPA V-LEDs were measured by conductive atomic force microscopy (CAFM) measurements. HPA V-LEDs exhibited much lower leakage current due to the improved crystal quality.

High Extraction Efficiency of GaN-Based Vertical-Injection Light-Emitting Diodes Using Distinctive Indium–Tin-Oxide Nanorod by Glancing-Angle Deposition

The enhanced light extraction and reduced forward voltage of a GaN-based vertical injection light emitting diode (VI-LED) with an indium–tin-oxide (ITO) nanorod array were demonstrated. The ITO nanorod array was fabricated by the glancing-angle deposition method. The employment of ITO nanostructures amplified not only the broadband transmission but also the current spreading. The optical output power of GaN-based VI-LEDs with ITO nanorods was enhanced by 50% compared with a conventional VI-LED at an injection current of 350 mA. The extraction efficiency was dramatically raised from 62 to 93% by the surface ITO nanorods. We also optimized the extraction efficiency of the GaN-based VI-LED with an ITO nanorod array by tuning the thickness of the n-GaN top layer via three-dimensional finite difference time domain (3D-FDTD) simulation.

High Extraction Efficiency of GaN-Based Vertical-Injection Light-Emitting Diodes Using Distinctive Indium–Tin-Oxide Nanorod by Glancing-Angle Deposition

High Extraction Efficiency of GaN-Based Vertical-Injection Light-Emitting Diodes Using Distinctive Indium–Tin-Oxide Nanorod by Glancing-Angle Deposition

Double Photonic Quasi-Crystal Structure Effect on GaN-Based Vertical-Injection Light-Emitting Diodes

GaN-based thin-film vertical-injection light-emitting diodes (VLEDs) with a double 12-fold photonic quasi-crystal (PQC) structure formed using nanoimprint lithography (NIL) and inductively coupled plasma reactive-ion etching (ICP-RIE) are fabricated and presented. At a driving current of 20 mA and with a chip size of 350 ×350 µm2, our thin-film LED with a double-12-fold-PQC structure gave a light output power of 40.5 mW, which is an increase of 77% when compared with the output power of a VLED without a PQC structure at a peak wavelength of 460 nm. In addition, the corresponding light radiation patterns show a narrow beam shape due to the strong guided light extraction on the n-GaN surface and the reflected light effect of the PQC structure formed in the vertical direction on the p-GaN surface.

Self-Assembled Two-Dimensional Surface Structures for Beam Shaping of GaN-Based Vertical-Injection Light-Emitting Diodes

Enhanced light extraction and beam shaping of GaN-based vertical-injection light-emitting diodes (VI-LEDs) employing biomimetic surface structures were demonstrated. The biomimetic surface structures were fabricated using self-assembled polystyrene nanospheres serving as a monolayer mask, and followed by anisotropic inductively coupled plasma reactive ion etching. The light output power of the VI-LEDs with the patterned structures exhibited an efficiency enhancement factor of 68% at a driving current of 350 mA, compared to those without any surface structures. The structures also resulted in a modified heart-shaped radiation pattern, which is preferable for backlight applications in flat panel displays.

Enhanced light output power of GaN-based vertical-injection light-emitting diodes with a 12-fold photonic quasi-crystal by nano-imprint lithography

GaN-based thin-film vertical-injection light-emitting diodes (VLEDs) with a 12-fold photonic quasi-crystal (PQC) by nano-imprint lithography (NIL) are fabricated and presented. At a driving current of 20 mA and with a chip size of 350 µm × 350 µm, the light output power of our thin-film LED with a 12-fold PQC structure reaches 41 mW. This result is an enhancement of 78% when compared with the output power of a VLED without a PQC structure. In addition, the corresponding light radiation pattern shows a narrower beam shape due to the strong guided light extraction effect by the formed PQC structure in the vertical direction.

Improving Light Output Power of the GaN-Based Vertical-Injection Light-Emitting Diodes by Mg$^{+}$ Implanted Current Blocking Layer

A method for forming a current blocking layer (CBL) by ion implantation in GaN-based vertical-injection light-emitting diodes (VI-LEDs) was proposed. It was found that the use of CBL in VI-LEDs can effectively reduce the current crowding effect and enhance the light output power. The uniform emission intensity distribution of VI-LEDs with CBL was demonstrated by electroluminescence measurements. Experimental results show that the wall-plug efficiency was enhanced by 12.3% at an injection current of 20 mA, compared to that of VI-LEDs without CBL, and by 56.2% compared to that of conventional LEDs. The device simulation results reveal that the current path can be blocked by CBL, resulting in high light extraction efficiencies and large current densities within the effective emission region of active layers.

Efficiency Enhancement and Beam Shaping of GaN–InGaN Vertical-Injection Light-Emitting Diodes via High-Aspect-Ratio Nanorod Arrays

The enhanced light extraction and collimated output beam profile from GaN-InGaN vertical-injection light-emitting diodes (VI-LEDs) are demonstrated utilizing high-aspect-ratio nanorod arrays. The nanorod arrays are patterned by self-assembled silica spheres, followed by inductively coupled-plasma reactive ion etching. The fabricated nanorod arrays not only provide an omnidirectional escaping zone for photons, but also serve as waveguiding channels for the emitted light, resulting in a relatively collimated beam profile. The light output power of the VI-LED with nanorod arrays is enhanced by 40%, compared to a conventional VI-LED. The measured far-field profiles indicate that the enhancement is mainly along the surface normal direction, within a view angle of 20deg.

Investigation of GaN-based vertical-injection light-emitting diodes with GaN nano-cone structure by ICP etching

The GaN-based thin-film vertical-injection LEDs (VLEDs) with GaN nano-cone structures are fabricated and presented. Under the process conditions of fixed Cl 2/Ar flow rate of 10/25 sccm and ICP/bias power of 200/200 W, the GaN nano-cone structures are self-assembly formed with variable density of 1.5 × 10 7 to 1.4 × 10 9 cm −2 and variable depth of 0.56–1.34 μm when varying the ICP chamber pressure. At a driving current of 350 mA and with chip size of 1 mm × 1 mm, the light output power of our thin-film LED with a specific GaN nano-cone structure reaches 224 mW which is enhanced by 160% when compared with the output power of conventional VLED. In addition, the corresponding light radiation pattern shows much higher light intensity due to the strong light scattering effect by the formed nano-cone structure.

Further Enhancement of Nitride-Based Near-Ultraviolet Vertical-Injection Light-Emitting Diodes by Adopting a Roughened Mesh-Surface

In this letter, the nitride-based near-ultraviolet (NUV) vertical-injection light-emitting diodes (VLEDs) with roughened mesh-surface are proposed and demonstrated by a combination of pattern sapphire substrate, wafer bonding, laser lift-off, and chemical wet etching processes. With the help of adopting a roughened mesh-surface, the light-output power (at 350 mA) of the NUV-VLEDs could be further enhanced about 20% as compared with that of the conventional NUV-VLED.