On the metallic bonding of GaN-based vertical light-emitting diode

In this work, metallic bonding in GaN-based vertical light-emitting diode (VLED) is systematically characterized by using combined methodology of transmission Kikuchi diffraction (TKD) and energy dispersive X-ray spectroscopy (EDS) in a scanning electron microscope (SEM). SEM-based TKD with EDS identifies chemical composition, grain morphology, orientation, and phases at metallic bonding, while transmission electron microscopy (TEM) provides nanoscale characteristics of metallic diffusion bonding, and its interface-related defects and nano-twinned boundaries. Our results from SEM-TKD and TEM techniques provide unparalleled insight into the metallic bonding, and its future optimization.     

Improving performance of high-power indium gallium nitride/gallium nitride-based vertical light-emitting diodes by employing simple n-type electrode pattern

In this work, simple n-type electrode structures were used to enhance the electrical and optical performances of fully packaged commercially mass-produced vertical-geometry light-emitting diodes (VLEDs). The forward bias voltage of the VLED with a Y-pattern electrode increased less rapidly than that of VLEDs with a reference electrode. The VLEDs with the reference and Y-pattern electrodes exhibited forward voltages of 2.93±0.015 and 2.89±0.015 V at 350 mA and 3.77±0.015 and 3.53±0.015 V at 2000 mA, respectively. The VLEDs with the Y-pattern electrode resulted in a higher light output than the VLEDs with the reference electrode with increase in the drive current to 2000 mA. The emission images showed that the VLEDs with the Y-pattern electrode exhibited better current spreading behavior and lower junction temperatures than the VLEDs with the reference electrode. With increase in the current from 350 to 2000 mA, the VLEDs with the Y-pattern electrode experienced a 39.4% reduction in the wall plug efficiency, whereas the VLEDs with the reference electrode suffered from a 43.3% reduction.     

Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

Investigation of GaN-based light-emitting diodes using double photonic crystal patterns

GaN-based LEDs with photonic crystal (PhC) patterns on an n- and a p-GaN layer by nano-imprint lithography (NIL) are fabricated and investigated. At a driving current of 20 mA on Transistor Outline (TO)-can package, the light output power of the GaN-based LED with PhC patterns on an n- and a p-GaN layer is enhanced by a factor of 1.30, and the wall-plug efficiency is increased by 24%. In addition, the higher output power of the LED with PhC patterns on the n- and p-GaN layer is due to better crystal quality on n-GaN and higher scattering effect on p-GaN surface using PhC pattern structure.     

具有背面反射镜的GaN基高压LED

High-voltage light-emitting diodes （HV-LED） with backside reflector, including Ti305/SiO2 distributed Bragg reflector （DBR） or hybrid reflector combining DBR and Al or Ag metal layer, are investigated using Monte Carlo ray tracing method. The hybrid reflector leads to more enhancement of light-extraction efficiency （LEE）. Moreover, the LEE can also be improved by redesigning the thicknesses of DBR. HV-LED with four redesigned DBR pairs （4-MDBR）, and those with a hybrid reflector combining 4-MDBR and Al metal layer （4-MDBR-Al）, are fabricated. Compared to 4-MDBR, the enhancement of light-output power induced by 4-MDBR-A1 is 4.6%, which is consistent with the simulated value of 4.9%.     

具有微米/纳米复合粗化ITO的GaN基LED

Three types of textured indium-tin-oxide （ITO） surface, including nano-texturing and hybrid micro/nano-texturing with micro-holes （concave-hybrid-pattem） or micro-pillars （convex-hybrid-pattern）, were applied to GaN-based light-emitting diodes （LEDs）. The nano-texturing was realized by maskless wet-etching, and the micro-texturing was achieved by standard photolithography and wet-etching. Compared to LED chips with flat ITO surface, those with nano-pattern, concave-hybrid-pattern, and convex-hybrid-pattern exhibit enhancement of 11.3%, 15.8%, and 17.9%, respectively, for the light-output powers at 20 mA. The electrical performance has no degradation. Moreover, the convex-hybrid-pattern show higher light-output efficiency under small injection current, while the concave-hybrid-pattern exhibit better light-output efficiency at large injection current. The light- extraction efficiency is simulated by use of two-dimensional finite difference time domain method, and the numer- ical results are consistent with the experiments.     

Temperature-dependent electroluminescence in InGaN/GaN multiple-quantum-well light-emitting diodes

We present a comparative study on temperature dependence of electroluminescence (EL) of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with identical structure but different indium contents in the active region. For the ultraviolet (UV) and blue LEDs, the EL intensity decreases dramatically with decreasing temperature after reaching a maximum at 150 K. The peak energy exhibits a large redshift in the range of 20–50 meV with a decrease of temperature from 200 K to 70 K, accompanying the appearance of longitudinal-optical (LO) phonon replicas broadening the low energy side of the EL spectra. This redshift is explained by carrier relaxation into lower energy states, leading to dominant radiative recombination at localized states. In contrast, the peak energy of the green LED exhibits a minimal temperature-induced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. We attribute the different temperature dependences of the EL to different degrees of the localization effects in the MQW regions of the LEDs.     

Electrical characteristics of In/ITO p-type ohmic contacts for high-performance GaN-based light-emitting diodes

We have investigated the annealing-induced improved electrical properties of In(10 nm)/ITO(200 nm) contacts with p-type GaN. The contacts become ohmic with a specific contact resistance of 2.75×10^–3 Ω cm² upon annealing at 650 °C in air. X-ray photoemission spectroscopy (XPS) Ga 2p core levels obtained from the interface regions before and after annealing indicate a large band-bending of p-GaN, resulting in an increase in the Schottky barrier height. STEM/energy dispersive X-ray (EDX) profiling results exhibit the formation of interfacial In-Ga-Sn-oxide. Based on the STEM and XPS results, the ohmic formation mechanisms are described and discussed. It is also shown that patterning by nano-imprint lithography improves the light output power of blue LEDs by 18–28% as compared to that of LEDs fabricated with unpatterned In/ITO contacts.     

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.     

Investigation of voltage reduction in nanostructure-embedded organic light-emitting diodes

We investigated the reduction of the operating voltage in organic light-emitting diodes containing WO₃ nanoislands. The thickness of the organic layer and the periodicity of the nanoislands were varied in order to quantitatively analyze the electrical changes. The thickness of the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) layer was varied from 150 nm to 600 nm, and various periodic nanoislands of 300 nm, 330 nm, and 370 nm were fabricated. Two geometric factors, which are the effective length and effective area, influence the operating voltage. The effective length is determined by the relative thickness of the nanoislands compared with the organic thickness, and the reduction of the operating voltage is linearly proportional to the relative thickness. The effective area is a nonlinear function of periodicity, and the voltage is reduced as the periodicity decreases.     

AlGaN/GaN层叠结构插入层改善氮化镓基发光二极管抗静电性能研究

在GaN基发光二极管的uGaN与nGaN之间插入AlGaN/GaN层叠结构,增大了外延层的张应力,降低了外延层中的穿透位错密度,改善了外延材料的质量。对比了AlGaN/GaN层叠结构中不同Al组分对LED的抗静电能力的影响,含6.8%铝组分AlGaN/GaN层叠结构的LED人体模式抗静电能力提高到了6000V,合格率超过了95%。     

图形化蓝宝石衬底结构对GaN基LED发光性能的影响

为了研究图形化蓝宝石衬底(PSS)的结构和形貌对GaN基发光二极管(LED)光学性能的影响,对PSS的制备工艺和参数进 行了调控,从而 形成具有不同填充因子的蒙古包形PSS(HPSS)和金字塔形PSS(TPSS)两种衬底,用于生长 和制备蓝光LED 芯片。通过对TPSS-LED的光学性能测试和分析得到,随着PSS填充因子的增大, LED的 光输出功 率也增大;进而比较具有相同填充因子的HPSS和TPSS的光学性能表明,HPSS明显优于TPSS。 因此, PSS填充因子的增大,能够提高LED的光输出功率;优化PSS的结构可以改善LED中光出射途径 ,从而更有效提高LED的光发射效率。     

GaN 基蓝光发光二极管分布布拉格反射器的设计

采用传输矩阵法对GaN基蓝光发光二极管分布布拉格反射器(DBR)反射光谱进行研究.计算发现正入射时S偏振(TE模)与P偏振(TM模)反射带是一致的; S偏振和P偏振反射带随着入射角的增大都向高频(短波)方向移动,且两者之间的差别也随之增大,DBR反射带蓝移快慢与入射介质相关;低折射率入射介质时DBR具有更宽角度响应.通过修改结构参数多次计算表明;入射角修正的方法能较快地找到提高全方向反射的结构.复合DBR以降低反射率或者成倍增加膜层厚度为代价实现大角度范围的反射,复合DBR比传统DBR有更好的光谱特性,这对提高发光二极管的出光效率有现实意义.     

Enhancement of light output power of GaN-based vertical light emitting diodes by optimizing n-GaN thickness

The light output and electrical characteristics of GaN-based vertical light emitting diodes were investigated as a function of n-GaN thickness. The forward voltage increases from 3.34 to 3.42 V at an injection current of 350 mA as the n-GaN thickness decreases from 5.0 to 2.0 μm. Even at a high injection current of 2.0 A, LEDs with 2.0 μm-thick n-GaN reveal stable forward characteristics which are comparable to those of LEDs with 5.0 μm-thick n-GaN. All the samples exhibit almost the same reverse current up to approximately −8 V. The output power increases with decreasing n-GaN layer thickness. For example, LEDs with 2.0 μm-thick n-GaN yield about 12% higher light output power as compared to LEDs with 5.0 μm-thick n-GaN. Their light output power continuously increases without saturation as the injection current increases up to 1 A. The n-GaN thickness dependence of the electrical characteristics is described and discussed.     

Improved III-nitrides based light-emitting diodes anti-electrostatic discharge capacity with an AlGaN/GaN stack insert layer

Through insertion of an AlGaN/GaN stack between the u-GaN and n-GaN of GaN-based light-emitting diodes(LEDs),the strain in the epilayer was increased,the dislocation density was reduced.GaN-based LEDs with different Al compositions were compared.6.8%Al composition in the stacks showed the highest electrostatic discharge(ESD) endurance ability at the human body mode up to 6000 V and the pass yield exceeded 95%.     

Highly efficient and foldable top-emission organic light-emitting diodes based on Ag-nanoparticles modified graphite electrode

Top-emission flexible organic light-emitting devices (TE-FOLEDs) are highly suitable for next generation display due to their numerous assets including top-emitting configuration and mechanical flexibility. One major challenge in TE-FOLEDs is to prepare a deformable and reflective bottom electrode capable of effective carrier injection. In this paper, a new strategy for efficient and foldable TE-FOLEDs is demonstrated. It is based on a highly conductive Ag-nanoparticles (Ag-NPs) modified graphite that is used as a flexible bottom electrode. The good reflectance to full-color emission (>59% over the whole visible wavelength range), ultralow sheet resistance (<5 Ω/sq), and high tolerance to mechanical bending (almost unchanged in resistance after bending 1000 times with an angle of ±90°) of the modified graphite synergistically constitute a breakthrough in the domain of TE-FOLEDs. The maximum current efficiencies reach 15.0, 50.2, 16.8 cd/A for red, green, blue emissions, respectively. Colorimetric gamut increased by 99.6% compared to bottom emission structure with the corresponding Commission Internationale de L'Eclairage (CIE) coordinates of the red/green/blue (R/G/B) devices. In particular, the TE-FOLEDs incorporating highly flexible graphite electrodes offer great mechanical durability and the initial brightness of 5000 cd/m can be maintained over 90% after bending for 1000 bending cycles. This approach is expected to open a new avenue for developing foldable displays.     

具有AlGaN/GaN超晶格的GaN基LED的PL分析

主要分析了具有p-ALGaN/GaN超晶格的360 nmGaN基LED的光学性能。插入的p-ALGaN/GaN超晶格结构可以看做是提高空穴注入的空穴限制层。经过PL测试发现,PL谱出现了明显的双峰以及蓝带发光,前者是由插入的超晶格引起,而蓝带荧光则是由超晶格中深能级N空位与Mg浅受主能级之前的辐射跃迁引起的。     

Electrical and luminescent properties and the spectra of deep centers in GaMnN/InGaN light-emitting diodes

Electrical and electroluminescent properties were studied for GaN/InGaN light-emitting diodes (LEDs) with the n-GaN layer up and with the top portion of the n layer made of undoped GaMnN to allow polarization modulation by applying an external magnetic field (so-called “spin-LEDs”). The contact annealing temperature was kept to 750°C, which is the thermal stability limit for retaining room-temperature magnetic ordering in the GaMnN layer. Measurable electroluminescence (EL) was obtained in these structures at threshold voltages of ∼15 V, with a lower EL signal compared to control LEDs without Mn. This is related to the existence of two parasitic junctions between the metal and the lower contact p-type layer and between the GaMnN and the n-GaN in the top contact layer.     

GaN基蓝光发光二极管分布布拉格反射器设计研究

采用传输矩阵法对GaN基蓝光发光二极管分布布拉格反射器（DBR）反射光谱进行研究。计算发现正入射时S偏振(TE模)与P偏振(TM模)反射带是一致的;S偏振和P偏振反射带随着入射角的增大都向高频(短波)方向移动,且两者之间的差别也随之增大,DBR反射带蓝移快慢与入射介质相关;低折射率入射介质时DBR具有更宽角度响应。通过修改结构参数多次计算表明：入射角修正的方法能较快的找到提高全方向反射的结构。复合DBR以降低反射率或者成倍增加膜层厚度为代价实现大角度范围的反射。复合DBR比传统DBR有更好的光谱特性,这对提高发光二极管的出光效率有现实意义。     

Sputter-patterned ITO-based organic light-emitting diodes with leakage current cut-off layers

We demonstrate the cost-effective fabrication of organic light-emitting diodes (OLEDs) using a sputter-patterned indium–tin-oxide (ITO). This scheme brings in a leakage current on the slope of the sputter-patterned ITO edges due to spike-like surface. To suppress it, we place thermally evaporated organic insulating molecules right on the ITO edges for preventing hole leakage, just below the aluminum (Al) cathode for blocking electron leakage, or both on the ITO edges and below the Al cathode. It is demonstrated that blocking off both hole- and electron-leak pathways (via the spikes) is highly desired to enhance the current efficiency and lifetime of the sputter-patterned ITO-based OLEDs.