Output Power Of Light-emitting Diodes Research Articles

(Invited) High Output Power Deep Ultraviolet Light-Emitting Diodes with Hemispherical Lenses Fabricated Using Room Temperature Bonding

We reviewed the structural and optical properties of high output power 255 and 280 nm light-emitting diodes (LEDs) bonded with hemispherical lenses made of inorganic materials. [This is described as a full paper (and not a review). Is this your intended meaning?][This is my intended] The optimal LED structure with a lens to improve the output power for deep ultraviolet LEDs was designed using Monte Carlo simulation. The LED chips were bonded to sapphire lenses at room temperature using either atomic diffusion bonding or surface activated bonding. The bonding of the lenses to the LEDs is experimentally shown to improve the light extraction efficiency, and the light output power of the LEDs with lenses was significantly higher than that of conventional structure LEDs. The output power of a 255 nm LED with a lens was 2.8 times larger than that without a lens, with a maximum external quantum efficiency of 4.56%, and that of the 280 nm LED with a lens was larger by a factor of 2.3.

Simulation and Experimental Study on Barrier Thickness of Superlattice Electron Blocking Layer in Near-Ultraviolet Light-Emitting Diodes

The optical performance and relevant physical properties of near-ultraviolet (NUV) GaN-based light-emitting diodes (LEDs) are investigated. Specifically, the influence of traditional AlGaN bulk electron blocking layer (EBL) and AlGaN/GaN superlattice (SL) EBL with various thicknesses of AlGaN layers on NUV LEDs is explored. It is indicated from the band diagrams, electrostatic field profile, electron reflecting and hole transmitting spectra, and carrier concentrations profile that the use of a thin AlGaN layer of AlGaN/GaN SL EBL is beneficial to the electron confinement and hole injection in the active region, which results in the high internal quantum efficiency and low efficiency droop at high injection current. Moreover, the experimental results show that replacing the traditional AlGaN bulk EBL with the AlGaN/GaN SL EBL can markedly improve the optical performance. When compared with the NUV LED with traditional AlGaN bulk EBL, the output power of the NUV LED with the proposed AlGaN/GaN SL EBL increases from 13.5 to 48.7 mW at 100 mA.

Femtosecond laser direct writing of microholes on roughened ZnO for output power enhancement of InGaN light-emitting diodes.

A significant enhancement of light extraction efficiency from InGaN light-emitting diodes (LEDs) with microhole arrays and roughened ZnO was experimentally demonstrated. The roughened ZnO was fabricated using an Ar and H2 plasma treatment of ZnO films pre-coated on a p-GaN layer. When followed by a femtosecond laser direct writing technique, a periodic array of microholes could be added to the surface. The diameter of the microhole was varied by changing the output power of the femtosecond laser. Compared to conventional LEDs on the same wafer, the output power of LEDs with roughened ZnOs and a microhole (diameter of 2 μm) array was increased by 58.4% when operated with an injection current of 220 mA. Moreover, it was found that LEDs fabricated with roughened ZnO and the microhole array had similar current-voltage (I-V) characteristics to those of conventional LEDs and no degrading effect was observed.

High-performance InGaN-based green light-emitting diodes with quaternary InAlGaN/GaN superlattice electron blocking layer.

In this study, high-performance InGaN-based green light-emitting diodes (LEDs) with a quaternary InAlGaN/GaN superlattice electron blocking layer (QSL-EBL) have been demonstrated. The band structural simulation was employed to investigate the electrostatic field and carriers distribution, show that the efficiency and droop behavior can be intensively improved by using a QSL-EBL in LEDs. The QSL-EBL structure can reduce the polarization-related electrostatic fields in the multiple quantum wells (MQWs), leading to a smoother band diagram and a more uniform carriers distribution among the quantum wells under forward bias. In comparison with green LEDs with conventional bulk-EBL structure, the light output power of LEDs with QSL-EBL was greatly enhanced by 53%. The efficiency droop shows only 30% at 100 A/cm2 comparing to its peak value, suggesting that the QSL-EBL LED is promising for future white lighting with high performance.

Enhanced light output power of InGaN-based amber LEDs by strain-compensating AlN/AlGaN barriers

We investigated the effect of a combined AlN/Al0.03Ga0.97N barrier on InGaN-based amber light-emitting diodes (LEDs) grown by metalorganic vapor-phase epitaxy. InGaN-based multiple quantum wells with a combined AlN/Al0.03Ga0.97N barrier showed intense photoluminescence with a narrow full-width at half-maximum. The amber LEDs with a combined AlN/Al0.03Ga0.97N barrier achieved a light output power enhanced approximately 2.5-fold at 20mA compared to that of the LED with a combined AlN/GaN barrier, owing to the reduction of defects in InGaN active layers. Thus, the efficiency of high-In-content InGaN-based LEDs can be improved in the spectrum range of amber.

Effect of Reassembled Remote Phosphor Geometry on the Luminous Efficiency and Spectra of White Light-Emitting Diodes With Excellent Color Rendering Property

This paper presents a scattered photon extraction (SPE) structure with a reassembled remote phosphor layer. The reassembled remote phosphor layer of the SPE structure was fabricated by combining green and red phosphor pieces with varying phosphor geometries, reducing the reabsorption of green emission by the red phosphor. The experimental results revealed that the SPE structure light-emitting diodes (LEDs) with the green and red phosphors in the remote phosphor layer demonstrated a high color rendering property. Moreover, with the same green and red phosphor weights and ratio in the remote phosphor layers, the output power and the luminous flux of the SPE structure LED with a reassembled remote phosphor layer were higher than those of SPE structure LEDs with mixed remote phosphor layers. The reduction in the reabsorption of green emission by red phosphor in the SPE structure with the reassembled remote phosphor layer was responsible for the high output power, and indicated a high correlated color temperature of the SPE structure with the reassembled remote phosphor layer. It was also found that the green emission with corresponding output power of the SPE structure LED containing the reassembled remote phosphor layer decreased with an increase in the piece number.

Improved output power and low forward voltage for GaN-based light emitting diodes with an annealed ITO transparent conducting layer

This study performs various thermal treatments on Indium Tin Oxide (In2O3 + SnO2) films, which are the transparent conducting layers for GaN light emitting diodes (LEDs). Structural characterization by high-resolution transmission electron microscopy (HRTEM) indicates that the ITO films have the least crystalline grain boundary and greatest grain size after thermal treatment at 580 °C for 10 min, which gives a minimum forward voltage of LEDs of 2.95 V and the maximum light output power of LEDs is 177.51 mW at 125 mA. The high output intensity in the near horizontal direction is also greater than that for treatment at other temperatures.

Influence of In content in InGaN barriers on crystalline quality and carrier transport of GaN-based light-emitting diodes

The influence of In content in InGaN barriers on the crystalline quality and carrier transport of GaN-based light-emitting diodes (LEDs) is studied by numerical and experimental investigations. The optimal In content of InGaN barriers is hence obtained. It is found that carrier concentration and crystalline quality degradation are a pair of opposite influential factors as In content increases. In content of 1.2% is optimal because it is the balance value at which a huge gain of carrier concentration is achieved without crystalline degradation. In content of 1.2% in InGaN barriers leads to a remarkable enhancement in both the light output power and external quantum efficiency (EQE) of LEDs. In such cases, the LED’s light output power and the EQE increase by 15.4% and 10.3% at a current of 70 mA, respectively. This work demonstrates the possibility of achieving high-performance LEDs with an aggravated efficiency droop, and is of great interest for the commercial development of GaN-based LEDs.

InGaN/GaN‐based green‐light‐emitting diodes with an inserted InGaN/GaN‐graded superlattice layer

In this study, InGaN‐based green light‐emitting diodes (LEDs) with a graded‐superlattice (GSL) layer inserted between the 3 µm thick n‐type GaN and the multiple quantum wells (MQWs) were studied numerically and experimentally. The simulated results indicate that the use of GSL inserting layer consisting of 12‐stacked InGaN/GaN layers leads to the enhancement of electron injection into the MQWs resulting in the increase of radiative recombination rate, suppressing the electron overflow to the p‐GaN side. The photoluminescence and output power of the GSL LEDs show a 73.5 and 42.5%, respectively, in a comparison with the reference LEDs that has no inserting layer. This improvement with the GSL LEDs indicates that the GSL insertion layer acts not only as a stress‐relaxing buffer layer releasing the residual stress in MQWs, but also as an electron cooler enhancing the electron capture rate in MQWs.

Internal quantum efficiency and carrier injection efficiency of c‐plane, and InGaN/GaN‐based light‐emitting diodes

The electroluminescence (EL) output power of c‐plane InGaN/GaN‐based light‐emitting diodes (LEDs) is much higher than that of semipolar and LEDs at the same operation current. In order to elucidate the reasons for this behavior, we have fitted the pulsed EL data by the well‐known ABC model to extract the internal quantum efficiency (IQE) and the carrier injection efficiency (CIE) to clarify which parameter weighs more for the poor EL output power of the semipolar LEDs. The CIE shows large differences, 78%, 4%, and 4% for the c‐plane, and LEDs, respectively, whereas the IQE values are fairly the same for all three structures. The fit of resonant photoluminescence (PL) data at room temperature confirms the similar IQE values for all three structures. The CIE was increased from 4% to 10% for the planar LED with better electrical conductivity of the p‐GaN layer.

Investigation of whether uniform carrier distribution in quantum wells can lead to higher performance in InGaN light-emitting diodes

InGaN/GaN light-emitting diodes (LEDs) with graded-composition multiple quantum wells (GQWs) were designed and theoretically investigated. In the GQW LED structure, the indium composition in In x Ga1−x N QW was decreased sequentially in increments, $$\Delta x$$ , along the [0001] orientation. Across the composition range, the GQW LEDs have high carrier injection efficiencies and enhanced uniform carrier distributions in the multiple quantum wells because of the band structure. When $$\Delta x = 0.01$$ , the light output power the GQW LED was enhanced by 4.2 % compared with that of the original LED, which corresponds to an increase of IQE of 4.5 % at 180 mA. However, when $$\Delta x$$ increased to 0.02, the light output power of the GQW LEDs was reduced by 3.1 % compared with that of original LED, which corresponds to a decrease of IQE of 3.3 % at 180 mA. Results from the simulations indicated that the device performance was not only related to the carrier injection efficiency and the carrier distribution, but was also related to the balance of electrons and holes in the QWs.

Well and Polarization Effects on Carrier Distribution and Interband Transitions in NUV Light-Emitting Diodes

The illumination efficiency and the relevant physical mechanism of near-ultraviolet (NUV) AlGaN-based light-emitting diodes (LEDs) are investigated numerically. In particular, the interrelationship of quantum well (QW) thickness and degree of polarization, and the relevant influence on the light output power of NUV LEDs are systematically studied. Simulation results indicate that the use of wider QWs with less polarization field is beneficial in suppressing the Auger recombination by reducing the carrier density. However, the structure with wider QWs suffers from severer spatial separation of electron and hole wave functions within the QW, which is more sensitive to the degree of polarization in its optical performance. To resolve this problem, the quaternary Al0.1In0.05Ga0.85N is proposed as the material of quantum barriers in wide QWs, in which the polarization mismatch between the QW and the barrier is reduced and the relevant quantum-confined Stark effect is relieved consequently.

Confocal electroluminescence investigations of highly efficient green InGaN LED via ZnO nanorods

The light emission mechanism of green InGaN-based light-emitting diodes (LEDs) coated with ZnO nanorods (ZnO-nanorods/InGaN LEDs) was investigated by confocal scanning electroluminescence (EL) spectroscopy (CSEM). The ZnO nanorods were grown by a hydrothermal method on the surface of fully fabricated InGaN LEDs. The vertical alignment and high crystallinity of the ZnO nanorods were confirmed by scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, micro-Raman spectroscopy, and high-resolution transmission electron microscopy. Measurement of the light output as a function of the injection current, far-field radiation pattern, and current versus voltage curves revealed that the light output power of the green InGaN LEDs with ZnO nanorods was enhanced by approximately 60% compared to that of conventional green InGaN LEDs without ZnO nanorods. However, the electrical properties of the ZnO-nanorods-coated device were not appreciably affected by the nanorods. To analyse the light emission mechanism of the ZnO-nanorods/InGaN LEDs, CSEM was used to observe the EL intensity distributions at different focal planes.

External stress effects on the optical and electrical properties of flexible InGaN-based green light-emitting diodes.

Flexible InGaN-based green light emitting diodes (LEDs) were fabricated by transferring epilayer to a flexible polyimide substrate with laser lift-off (LLO) and double-transfer technologies. We present a method of increasing light output power in flexible LEDs without modifying their epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect by reducing piezoelectric polarization that results from compressive stress in the GaN epilayer. The compressive stress is relaxed due to the external stress induced by increasing bending displacement of flexible substrate. The light output power of the flexible LED at an injection current of 150 mA is increased by approximately 42.2%, as the external bending went to the case of effective length of 15 mm. The experimental results demonstrated that applying external tensile stress effectively compensates for the compressive strain and changes the piezoelectric field in the InGaN/GaN MQWs region, thereby increases the probability of radiative recombination.

Light enhancement of surface nano-textured GaN based light emitting diodes using self-assembled Ni nano-masks

Light enhancement of GaN based light emitting diodes (LEDs) has been investigated by texturing the top p-GaN surface. Nano-textured LEDs have been fabricated using self-assembled Ni nano mask during dry etching process. Experimental result was further compared with simulation data. Three types of LEDs were fabricated: Conventional (planar LED), surface nano-porous (porous LED) and surface nano-cluster (cluster LED). Compared to planar LED there were about 100% and 54% enhancement of light output power for porous and cluster LED respectively at an injection current of 20mA. Moreover, simulation result showed consistency with experimental result. The increased probability of light scattering at the nano-textured GaN-air interface is the major reason for increasing the light extraction efficiency.

Enhancement of the Optical Output Power of InGaN/GaN Multiple Quantum Well Light-Emitting Diodes by a CoFe Ferromagnetic Layer

We report the increase of the optical output power of InGaN/GaN multiple quantum well (MQW) flip-chip blue light-emitting diodes (LEDs) using cobalt–iron (CoFe) ferromagnetic layers. The CoFe alloy layer is deposited on a p-ohmic reflector of the flip-chip LEDs to apply a magnetic field in the MQWs. The optical output power of LEDs with a CoFe layer after magnetization is increased by 23% at an injection current of 20 mA compared with LEDs that have a CoFe layer before magnetization. The time-resolved photoluminescence spectra and magnetic field simulations indicated that the improvement of the optical output power of the LEDs is attributed to an enhanced radiative recombination rate in the MQWs by the additional drift of carriers in the MQWs due to the gradients of the magnetic field of the CoFe ferromagnetic layer.

Numerical Simulation and Experimental Analysis of Current Spreading Length in Nitride-Based Light-Emitting Diodes Prepared on 10- $\mu \text{m}$ -Thick n-GaN Template

Numerical and experimental demonstrations were performed in this letter to enhance the current spreading length of nitride-based light-emitting diodes (LEDs) with a 10- $\mu \text{m}$ -thick n-GaN template on an AlN/high-aspect ratio patterned sapphire substrate template via hydride vapor phase epitaxy. At an injection current of 20 mA, the output powers were 4.34 and 6.39 mW for a conventional LED and an LED with a 10- $\mu \text{m}$ -thick n-GaN template, respectively. The larger LED output power is attributed to the enhanced current spreading length, which improved the heat dissipation ability and the improved crystal quality.

Relationship between threading dislocations and the optical properties in GaN-based LEDs on Si Substrates

Owing to the large lattice mismatch between Si and Al(Ga)N, GaN-based structures grown on Si(111) substrates usually have a high density of threading dislocations, which act as non-radiative recombination centers. In this work, we analyze the relationship between threading dislocations and the internal quantum efficiency of GaN-based light-emitting diodes (LEDs) by using various characterization methods such as atomic force microscopy, transmission electron microscopy, cathodoluminescence, photoluminescence, and electroluminescence measurements. Non-radiative recombination centers are found to have a direct effect on the optical properties of optoelectronics devices such as LEDs. Reducing the density of the threading dislocations is demonstrated to be a key parameter in improving the output power of LEDs grown on Si substrates.

Ni-assisted Fabrication of GaN Based Surface Nano-textured Light Emitting Diodes for Improved Light Output Power

Light enhancement of GaN based light emitting diodes (LEDs) have been investigated by texturing the top p-GaN surface. Nano-textured LEDs have been fabricated using self-assembled Ni nano mask during dry etching process. Experimental results were further compared with simulation data. Three types of LEDs were fabricated: Conventional (planar LED), Surface nano-porous (porous LED) and Surface nano-cluster (cluster LED). Compared to planar LED there were about 100% and 54% enhancement of light output power for porous and cluster LED respectively at an injection current of 20 mA. Moreover, simulation result showed consistency with experimental result. The increased probability of light scattering at the nano-textured GaN-air interface is the major reason for increasing the light extraction efficiency.

Enhanced performances of AlGaInP-based light-emitting diodes with Schottky current blocking layers**Project supported by the National Natural Science Foundation of China (Grant No. 11204009) and the Natural Science Foundation of Beijing, China (Grant No. 4142005).

A new epitaxial structure of AlGaInP-based light-emitting diode (LED) with a 0.5-μm GaP window layer was fabricated. In addition, indium tin oxide (ITO) and localized Cr deposition beneath the p-pad electrode were used as the current spreading layer and the Schottky current blocking layer (CBL), respectively. The results indicated that ITO and the Schottky CBL improve the total light extraction efficiency by relieving the current density crowding beneath the p-pad electrode. At the current of 20 mA, the light output power of the novel LED was 40% and 19% higher than those of the traditional LED and the new epitaxial LED without CBL. It was also found that the novel LED with ITO and CBL shows better thermal characteristics.