Self-Consistent Analysis of Strain-Compensated InGaN–AlGaN Quantum Wells for Lasers and Light-Emitting Diodes

Strain-compensated InGaN–AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band $kcdot p$ formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN–AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN–AlGaN QW as active regions for diode lasers.      

A Reliability Study on Green InGaN–GaN Light-Emitting Diodes

In this letter, the reliability of green InGaN-GaN light-emitting diodes (LEDs) has been analyzed by correlating the defect density of wafers with various device parameters, including leakage current, 1/f noise, and degradation rate. It was found that as the wavelength of green LEDs increases from 520 to 550 nm by increasing the indium content in the quantum wells, the defect density also increases, thus leading to larger leakage current, enhanced noise magnitude, and shortened device lifetime.     

Gradient Doping of Mg in p-Type GaN for High Efficiency InGaN–GaN Ultraviolet Light-Emitting Diode

The performance of a InGaN-GaN multiple quantum-well (MQW) ultraviolet (UV) light-emitting diode (LED) with an emission of 385 nm was enhanced by a gradient doping of Mg in the p-GaN layer. The optical output power was enhanced by 21% at an input current of 20 mA compared to that of a UV LED with a uniformly doped p-GaN layer. The improved performance of the UV LED could be attributed to the decrease in diffusion of Mg into MQW and the suppression of electron transport from the conduction band of the MQW to the acceptor level of the deep donor acceptor pair bands in the p-GaN layer by a gradient doping of Mg in p-GaN layer.     

Improvement of the Efficiency of InGaN–GaN Quantum-Well Light-Emitting Diodes Grown With a Pulsed-Trimethylindium Flow Process

This study demonstrated the enhancement of the light output power of InGaN–GaN multiple quantum-well light-emitting diodes (LEDs) that are grown with a pulsed-trimethylindium (pulsed-TMIn) flow process by metal–organic vapor-phase epitaxy. At an injection current of 20 mA, the output power of the pulsed-TMIn treated LEDs was improved by 16% as compared to that of the conventional LEDs. In addition, a minor droop (versus injection current) in terms of external quantum efficiency was also observed in the pulsed-TMIn treated LEDs as compared to conventional LEDs. This improvement could be attributed to the fact that the significant carrier localization effect in the pulsed-TMIn treated LEDs can lead to higher recombination efficiency. This contention is perhaps tentatively evidenced by the temperature-dependent photoluminescence results in which the activation energy of the pulsed-TMIn treated LEDs was increased by 21.8% as compared to that of conventional LEDs.      

Ultraviolet ZnO Nanorod/P-GaN-Heterostructured Light-Emitting Diodes

Both i-ZnO and n-ZnO : In nanorod arrays were grown on a p-GaN layer with an anodic alumina membrane template using a vapor cooling condensation method. Electroluminescence emissions were observed from the resulting p-n (p-GaN/n-ZnO : In nanorod array) and p-i-n (p-GaN/i-ZnO nanorod array/n-ZnO : In nanorod array) heterostructured light-emitting diodes (LEDs). The ultraviolet emission peak at 386 nm observed in the p-i-n heterostructured LEDs was attributed to radiative recombination of the near-band edge in the i-ZnO nanorods. Using power-law fitted current–voltage relationships, it was shown that a space-charge-limited current and associated effects occurred in the p-n and p-i-n nanorod heterostructured LEDs.      

Improvement of Electrostatic Discharge Characteristics and Optical Properties of GaN-Based Light-Emitting Diodes

To improve the positive- and negative-voltage electrostatic discharge (ESD) characteristics of GaN light-emitting diodes (LEDs), an air gap was introduced as an ESD protection structure in an Al film on the bottom side of a sapphire substrate. The negative-voltage ESD characteristic of GaN LEDs with an air gap was remarkably improved from $-$0.3 to $-$ 4 kV. The degradation of electroluminescent intensity of GaN LEDs, which was caused by ESD stress, was also suppressed by an air gap in GaN LEDs. An ESD-stress-induced current is believed to flow in an air gap to protect the multiquantum well of GaN LEDs.      

Single-Layered Hybrid DBPPV-CdSe–ZnS Quantum-Dot Light-Emitting Diodes

We have demonstrated the fabrication and characterization of single-layered hybrid polymer-quantum-dot light-emitting diodes (PQD-LEDs) with the emissive composite film of 2,3-dibutoxy-1,4-poly(phenylene vinylene) (DBPPV) and inorganic CdSe-ZnS core/shell quantum dots (QDs). It is observed that both the electrical and optical characteristics are significantly improved by adjusting the thickness of the emissive layer. For the device with composite film thickness of 103 nm, the turn-on voltage is 4.1 V, and the maximum luminance of 4100 cd/m as well as maximum luminous efficiency of 1.35 cd/A are achieved at 9.6 and 7.6 V, respectively. The optimum emission contribution of luminescence from QDs to the whole luminance is 38%. However, the QD luminescence is mainly limited by the Foumlrster energy transfer mechanism and no obvious QD-related injected carrier trapping is observed. To our knowledge, this is the first demonstration of PQD-LEDs that consist of DBPPV and CdSe-ZnS QDs.     

Capacitance–Voltage and Current–Voltage Measurements of Nitride Light-Emitting Diodes

Capacitance-voltage (C-V ) and current-voltage (I-V) characteristics of nitride light-emitting diodes were measured. The apparent carrier distributions obtained from the C-V curves yielded much information about the samples, including information about the presence of acceptor-like defects in the active layer and the problem of electron overflow. The inconsistency between the experimental and simulated I-V curves also supported the presence of the defects. After compensating the acceptor-like defects by Si dopants and adjusting the overlap between the depletion region and the light-emitting structure, device performance was improved.     

Influence of Interference on Extraction Efficiency of Ultraviolet Vertical Light-Emitting Diodes

We report on enhanced efficiency of ultraviolet vertical light-emitting diodes (VLEDs) with interference between the reflective mirror and the multiple quantum well. The dimensions of the cavity are fixed at 30 nm for the p-AlGaN layer, while various thicknesses of p-GaN from 60 nm to 140 nm were used. The light output power of the VLED in constructive compared with destructive interference condition increased by 23.9% at 350 mA. These improvements could be attributed to the predominant constructive interference of vertical radiation due to an optical cavity with optimal p-GaN thickness.     

ZnO-Based Fairly Pure Ultraviolet Light-Emitting Diodes With a Low Operation Voltage

A ZnO-based metal-insulator (HfO₂) -semiconductor diode was synthesized on a commercially available n⁺-GaN/sapphire substrate using a radio-frequency magnetron sputtering system. Electroluminescence measurements revealed that the diode exhibited fairly pure ultraviolet (UV) emission peaking at ~ 370 nm with a line width of less than 8 nm. By choosing a proper thickness of the insulator HfO₂ layer, the threshold voltage of the emission could be reduced to 2 V, demonstrating that this ZnO-based fairly pure UV light-emitting diode can be driven by two ordinary dry batteries. The reason for low threshold voltage is proposed in terms of the n⁺-GaN/sapphire substrate and the high-k insulator HfO₂ layer.     

Minimizing Optical Energy Losses for Long-Lifetime Perovskite Light-Emitting Diodes

Regardless of the rapid advance on perovskite light-emitting diodes (PeLEDs), the lack of long-term operational stability hinders the practicality of this technology. Particularly, thermal management is indispensable to control the Joule heating induced by charge transport and parasitic re-absorption of internally confined photons. Herein, a synergetic device architecture is proposed for minimizing the optical energy losses in PeLEDs toward high efficiency and long lifetime. By adopting a carefully modified perovskite emitter in combination with an improved light outcoupling structure, red PeLEDs emitting at 666 nm achieve a peak external quantum efficiency of 21.2% and an operational half-lifetime of 4806.7 h for an initial luminance of 100 cd m^-2. The enhanced light extraction from trapped modes can efficiently reduce the driving current and suppress optical energy losses in PeLEDs, which in turn ameliorate the heat-induced device degradation during operation. This work paves the way toward high-performance PeLEDs for display and lighting applications in the future.     

垂直结构LED和倒装结构LED的发光特性研究

研究了1.16 mm GaN基蓝光芯片的垂直封装结构LED和倒装封装结构LED在驱动电流达到和超过工作电流350mA的发光特性和变化趋势.随着驱动电流的逐渐增大,与垂直结构LED相比,倒装结构LED光通量的饱和电流值增加350mA,在1 200 mA电流时的光通量高出25.9％,色温的异常电流值增加了400 mA,发光效率平均提高81 m/W.实验结果表明,倒装结构LED具有更高的抗大电流冲击稳定性和光输出性能,可有效提高LED在实际应用中使用寿命.     

High-Brightness InGaN–GaN Power Flip-Chip LEDs

We report the fabrication of InGaN–GaN power flip-chip (FC) light-emitting diodes (LEDs) with a roughened sapphire backside surface prepared by grinding. It was found that we can increase output power of the FC LED by about 35% by roughening the backside surface of the sapphire substrate. The reliability of the proposed device was also better, as compared to power FC LEDs with a conventional flat sapphire backside surface.      

380‐nm Ultraviolet Light‐Emitting Diodes with InGaN/AlGaN MQW Structure

In this paper, we demonstrate the capabilities of 380‐nm ultraviolet (UV) light‐emitting diodes (LEDs) using metal organic chemical vapor deposition. The epi‐structure of these LEDs consists of InGaN/AlGaN multiple quantum wells on a patterned sapphire substrate, and the devices are fabricated using a conventional LED process. The LEDs are packaged with a type of surface mount device with Al‐metal. A UV LED can emit light at 383.3 nm, and its maximum output power is 118.4 mW at 350 mA.     

Electrostatic Reliability Characteristics of GaN Flip-Chip Power Light-Emitting Diodes With Metal–Oxide–Silicon Submount

The electrostatic reliability characteristics of gallium nitride flip-chip (FC) power light-emitting diodes (PLEDs) with metal-oxide-silicon (MOS) submount are investigated for the first time. The electrostatic damage reliability of the reported diode submount and that of our proposed simple structure MOS submount are fabricated and compared. Their corresponding electrostatic protection capabilities are increased from 200 V (conventional PLED) to 500 V (FC-PLED on diode submount), to 500 V (FC-PLED on MOS submount with a SiO₂ thickness of 297 A?), and even to a value as high as 1000 V (FC-PLED at a SiO₂ thickness of 167 A?), which are much higher than the PLED industrial test value of 150 V at -5 V/-10 ? A criterion and are also much more robust than the previous academic reports.     

Effect of N-Type AlGaN Layer on Carrier Transportation and Efficiency Droop of Blue InGaN Light-Emitting Diodes

The effect of an n-type AlGaN layer on the physical properties of blue InGaN light-emitting diodes (LEDs) is investigated numerically. The p-type AlGaN electron-blocking layer is usually used in blue LEDs to reduce the electron leakage current. However, the p-type AlGaN layer also retards the injection of holes, which leads to the degradation of efficiency at high current. To improve the efficiency droop of blue InGaN LEDs at high current, an n-type AlGaN layer below the active region is proposed to replace the traditional p-type AlGaN layer. The simulation results show that the improvement in efficiency droop is due mainly to the sufficiently reduced electron leakage current and more uniform distribution of holes in the quantum wells.      

High-Performance InGaN-Based Green Resonant-Cavity Light-Emitting Diodes for Plastic Optical Fiber Applications

High-performance InGaN-based green resonant- cavity light-emitting diodes (RCLEDs) with a plating Cu substrate for plastic optical fiber communication applications are reported. Good stability of emission wavelength was obtained at 0.016 nm/mA . The RCLEDs presents low temperature dependence, showing only a 3% drop in light output power as the temperature increasing from 25 to 85 $^{circ} $C. The superior performance can be attributed to the decreased dynamic series resistance and the enhanced thermal dissipation of the heat sink substrate.      

Effects of Defects on the Thermal and Optical Performance of High-Brightness Light-Emitting Diodes

Defects in terms of voids, cracks, and delaminations are often generated in light-emitting diodes (LEDs) devices and modules. During various manufacturing processes, accelerated testing, inappropriate handling, and field applications, defects are most frequently induced in the early stage of process development. One loading is due to the nonuniform loads caused by temperature, moisture, and their gradients. In this research, defects in various cases are modeled by a nonlinear finite-element method (FEM) to investigate the existence of interfaces, interfacial open and contacts in terms of thermal contact resistance, stress force nonlinearity, and optical discontinuity, in order to analyze their effects on the LED's thermal and optical performance. The simulation results show that voids and delaminations in the die attachment would enhance the thermal resistance greatly and decrease the LED's light extraction efficiency, depending on the defects' sizes and locations generated in packaging.     

Effect of Silicon Doping in the Quantum-Well Barriers on the Electrical and Optical Properties of Visible Green Light-Emitting Diodes

The effect of Si doping in the GaN quantum-well (QW) barriers of the InGaN–GaN multiple QW active region in visible green light-emitting diodes (LEDs) was studied. As the doping level of Si increases, the intensity of electroluminescence (EL) decreases, while the forward voltage of the diodes is improved. Degradation of EL is believed to be mainly due to the hole transport blocking effect caused by Si doping in the QW barriers resulting in increased potential barriers. This effect is believed to be more significant in green LEDs than in violet and blue LEDs.