Bing-Cheng Lin

Hole injection and electron overflow improvement in InGaN/GaN light-emitting diodes by a tapered AlGaN electron blocking layer.

A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design.

Efficiency and Droop Improvement in GaN-Based High-Voltage Flip Chip LEDs

The GaN-based high-voltage flip chip light-emitting diode (HVFC-LED) is designed and developed for the purpose of efficiency enhancement. In our design, the distributed Bragg reflector (DBR) is deposited at the bonded substrate to increase the light extraction. After the flip chip process, the general current-voltage characteristics between the flip chip sample and the traditional sample are essentially the same. With the help of great thermal conductive silicon substrate and the bottom DBR, the HVFC-LED is able to enhance the power by 37.1% when compared to the traditional high-voltage LEDs. The wall-plug efficiencies of the HVFC-LED also show good droop reduction as high as 9.9% compared to the traditional devices.

Improving the Angular Color Uniformity of Hybrid Phosphor Structures in White Light-Emitting Diodes

This letter examines a hybrid phosphor structure for white light-emitting diodes. The hybrid phosphor structure produces more efficient luminosity and more uniform angular correlated color temperature (CCT) than the conventional dispensing method. The experimental results show that the CCT deviation improved from 453 to 280 K between -70° and 70 °. This was likely caused by the large blue light divergent angle. The lumen output produced was higher than that produced by dispense and conformal phosphor structures. Therefore, the results show that the hybrid phosphor structure can be used in solid-state lighting.

Advantages of Blue LEDs With Graded-Composition AlGaN/GaN Superlattice EBL

InGaN/GaN light-emitting diodes (LEDs) with graded-composition AlGaN/GaN superlattice (SL) electron blocking layer (EBL) were designed and grown by metal-organic chemical vapor deposition. The simulation results demonstrated that the LED with a graded-composition AlGaN/GaN SL EBL have superior hole injection efficiency and lower electron leakage over the LED with a conventional AlGaN EBL or normal AlGaN/GaN SL EBL. Therefore, the efficiency droop can be alleviated to be ~ 20% from maximum at an injection current of 15-120 mA, which is smaller than that for conventional AlGaN EBL (30%). The corresponding experimental results also confirm that the use of a graded-composition AlGaN/GaN SL EBL can markedly enhance the light output power by 60%.

High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers

In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-mean-square value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (101̅2) reflections in the double crystal X-ray rocking curve were ~330 and 450 respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <; 6× 108 cm-2. From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaN-based LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of ~110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.

Luminous efficiency enhancement of white light-emitting diodes by using a hybrid phosphor structure

Abstract. This paper presents a “hybrid” structure for the coating of yellow YAG∶Ce3+ phosphor on blue GaN-based light-emitting diodes (LEDs). The luminous efficiency of the hybrid phosphor structure improved by 5.9% and 11.7%, compared with the conventional remote and conformal phosphor structures, respectively, because of the increased intensity of the yellow component. The hybrid structure also has an advantage in the phosphor usage reduction for the LEDs. Furthermore, the electric intensity of the hybrid phosphor structure was calculated for various thicknesses by conducting TFCalc32 simulation, and the enhanced utilization of blue rays was verified. Finally, the experimental results were consistent with the simulation results performed using the Monte-Carlo method.

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.

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.

Efficiency droop behavior improvement through barrier thickness modification for GaN-on-silicon light-emitting diodes

Abstract. Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an AlxGa1−xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to <7×108  cm−2, yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE.

Innovative Fabrication of Wafer-Level InGaN-Based Thin-Film Flip-Chip Light-Emitting Diodes

In this letter, an innovative fabrication method for InGaN-based light-emitting diode (LED) was proposed, and the produced optical device was called the wafer-level InGaN-based thin-film flip-chip LEDs (wTFFC-LEDs). Packaging technologies, such as wafer-to-wafer bonding, laser lift-off, textured surface, and interconnection techniques, were applied to complete the device. Through this architecture, the absorption caused by electrodes in traditional vertical injection LEDs (V-LEDs) can be minimized, as well as the light extraction efficiency can be improved. Light-output power of wTFFC-LEDs (350 mA) was increased by 36.5% and 17.2% compared with the V-LEDs and flip-chip LEDs. Furthermore, the external quantum efficiency was also relatively enhanced by 36.3% and 15.5% higher than those of reference devices.