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.     

Influence of light absorption on the metallic nanotextured reflectors of GaN-based light emitting diodes

Metallic nanotextured reflectors have been widely used in light emitting diodes (LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however, only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference time-domain (FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon (SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.     

Efficiency analysis of a—SiC：H thin film light emitting diodes

The hole injection ,the radiative recombination and the device lumi-nescent efficiencies of amorphous silicon carbide thin film p-I-n junction light emit-ting diodes are quantitatively calculated,and the effect of the carrier (especially the hole)injection and recombination processes on the device luminescent characteris-tics are revealed.Without considering the device junction temperature ,it is round that the device luminescent efficiency mainly depends on the hole injection efficiency at low field and the hole radiative recombination efficiency at high field respective-ly.The theoretical analyses are in well agreement with the experimental results.     

Efficiency analysis of a-SiC:H thin film light emitting diodes

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Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes

We investigated the electrical and structural qualities of Mg-doped p-type GaN layers grown under different growth conditions by metalorganic chemical vapor deposition (MOCVD). Lower 300 K free-hole concentrations and rough surfaces were observed by reducing the growth temperature from 1,040°C to 930°C. The hole concentration, mobility, and electrical resistivity were improved slightly for Mg-doped GaN layers grown at 930°C with a lower growth rate, and also an improved surface morphology was observed. In_0.25Ga_0.75N/GaN multiple-quantum-well light emitting diodes (LEDs) with p-GaN layers grown under different conditions were also studied. It was found from photoluminescence studies that the optical and structural properties of the multiple quantum wells in the LED structure were improved by reducing the growth temperature of the p-layer due to a reduced detrimental thermal annealing effect of the active region during the GaN:Mg p-layer growth. No significant difference in the photoluminescence intensity depending on the growth time of the p-GaN layer was observed. However, it was also found that the electroluminescence (EL) intensity was higher for LEDs having p-GaN layers with a lower growth rate. Further improvement of the p-GaN layer crystalline and structural quality may be required for the optimization of the EL properties of long-wavelength (∼540 nm) green LEDs.     

Highly efficient white transparent organic light emitting diodes with nano-structured substrate

Enhancing outcoupling efficiency and stabilizing emission spectra are of high technical importance in realizing high quality white transparent organic light emitting diodes (TOLEDs). In this work, we demonstrate a random nano-scattering layer (RSL) as a structure which can effectively address those tasks. The RSL contributes to bottom and top emissions by scattering and reflection, respectively. With the use of RSL, we achieved remarkable total efficiency enhancement of 101%. Also, a viewing angle independent stable white spectrum with a color rendering index of 79 was achieved. With its straight forward processing, our RSL can be readily applied to deal with various photonic applications to enhance both efficiency and emission spectra.     

Fabrication and characterization of single ZnO microwire Schottky light emitting diodes

ZnO microwires were grown using noncatalytic chemical vapor deposition method. The average diameter of the ZnO microwires were about 30 μm with length of up to 1–1.5 cm. Single ZnO microwire Schottky light emitting diode was fabricated using Au as Schottky contact electrode and using Al as ohmic contact electrode. The current–voltage (I–V) characteristics of Schottky diodes reveal good rectifying behavior. The Schottky barrier height and ideality factor were calculated to be 0.78 eV and 4.3, respectively. Furthermore, distinct electroluminescence with ultraviolet and visible emissions was detected from this device at room temperature.     

Degradation of AlGaN-based ultraviolet light emitting diodes

Aging the LEDs by driving at high current, results in the decrease of optical power proportional to the reciprocal square route of stress time. With aging time, change in the current–voltage characteristics indicates decrease of the current at low voltage below the light emission threshold, decrease of the forward voltage drop at high currents and usually no change in the series resistance. No change in the peak wavelength and half bandwidth were found with aging. Low frequency noise measured at low and high currents either did not depend on aging time or decreased. No correlation between noise, the device power, and the rate of the power degradation were found. These results are in strong contrast to previous studies of longer wavelength GaN-based LEDs. The possible degradation mechanism is the diffusion of the Al atoms out from the p-type cladding layer and lowering of the cladding layer potential barrier as a result.     

LED在生物医学方面的应用和前景

本文综述了近年来发光二极管（light emitting diode,LED)在生物医学方面的研究与应用,内容包括：LED作用于动物细胞的研究;LED作用于动物模型的研究;LED在临床实验方面的研究;几种用于生物医学的LED新器件;LED在生物医学方面的前景展望。     

Recent development of polyfluorene-based RGB materials for light emitting diodes

As a continuation of our presentation at IUMRS-2000 in Hong Kong, we report the latest development of polyfluorene-based Red, Green and Blue (RGB) materials for light emitting diodes at The Dow Chemical Company. A modified Suzuki coupling process is used to synthesize RGB fluorene-based homopolymers and copolymers. Optimization of reaction conditions has led to a highly efficient procedure to generate polymers with controlled molecular weight (Mw), ranging from 10,000 to above 500,000 depending on the requirements of the desired applications. The optical and electronic properties of the polymers are tailored through selective incorporation of different aromatic units into the fluorene copolymer systems. By using this methodology, a portfolio of fluorene-based polymers has been designed and synthesized, achieving emissive colors that cover the entire visible spectrum.The performance of fluorene-based polymers in light emitting devices has been optimized by modifying the polymer compositions so as to increase charge mobilities and to improve the carrier injection balance. As a result of these compositional changes, devices based on Dow's green emitters, using bi-layer structures on indium tin oxide substrates and evaporated metal cathodes, have demonstrated unprecedented high efficiencies at high brightness levels and long lifetime performance. A device comprised of a Dow green emitting polymer has a low turn-on voltage of 2.25 V and exhibits a peak efficiency of 10.5 Cd/A at 6600 Cd/m² at 4.85 V. These devices maintain an efficiency of greater than 10.0 Cd/A up to 50,000 Cd/m²and demonstrate very good stability as exemplified by a device half-life of greater than 1500-h starting from 1100 Cd/m². Similar outstanding progress with red and blue emitters has also been made and will be discussed.     

Magnetic resonance studies of GaN based light emitting diodes

We report the application of electrical detection of magnetic resonance (EDMR) and electroluminescence detection of magnetic resonance (ELDMR) to study the recombination processes in InGaN/AlGaN double heterostructure p-n junctions. These techniques are especially well suited to the problems of defects in device structures in that they are much more sensitive than conventional paramagnetic resonance and are responsive to only those defects involved in the electrooptical properties of the structure. One resonance is observed at g≈2.00 and is identified as a Zn-related acceptor trap in the InGaN layer. A second resonance with g≈1.99 is identified as a deep donor.     

High efficiency and non-color-changing orange organic light emitting diodes with red and green emitting layers

We report a high performance orange organic light-emitting diode (OLED) where red and green phosphorescent dyes are doped in an exciplex forming co-host as separate red and green emitting layers (EMLs). The OLED shows a maximum external quantum efficiency (EQE) of 22.8%, a low roll-off of efficiency with an EQE of 19.6% at 10,000 cd/m², and good orange color with a CIE coordinate of (0.442, 0.529) and no color change from 1000 to 10,000 cd/m². The exciplex forming co-host system distributes the recombination zone all over the EMLs and reduces the triplet exciton quenching processes.     

Temperature-dependent minority-carrier lifetime measurements of red AlGaAs light emitting diodes

Electroluminescent decay and internal quantum efficiency measurements are made as a function of temperature on two double heterostructure AIGaAs light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. The electroluminescent lifetimes increase by more than a factor of ten and the internal quantum efficiency falls by a factor of three as the temperature is raised from 90 to 400K. By analyzing the data with a model that accounts for the transfer with increasing temperature of the minority-carrier electrons from the direct-gap to the indirect-gap minima in the p-type active layer of these near-crossover LEDs, values for the radiative and nonradiative lifetimes as a function of temperature are obtained. A fit to the radiative-lifetime data results in an estimate of 1.3 × 10⁻¹⁰ cm³s⁻¹ for the room-temperature radiative recombination coefficient of Al_0.39Ga_0.61As, which is very similar to values reported for GaAs. The nonradiative lifetimes are found to be nearly independent of temperature from 220 to 400K and provide upper limits of 940 and 1250 cms⁻¹ for the interface recombination velocities of the two samples. These values are roughly an order of magnitude lower than any previously reported values for high-Al-content (x > 0.3) Al_xGa_1−xAs heterostructures.     

A new blue light emitting and electrochromic polyfluorene derivative for display applications

A novel blue emitting and electro-chromic conjugated copolymer based on 9,9′dioctylfluorene (F8) and spiro(fluorene-9,9′-xanthene) (SFX) has been prepared. Optical, photophysical and electrochemical characterizations are given for the synthesized polymer; poly[spirofluorene-co-9,9′dioctylfluorene] P(F8-SFX). Switching of the corresponding polymer between yellow and purple states is demonstrated, and blue emission with Commission Internationale de L’Eclairage (CIE) coordinate at (0.19, 0.15) is obtained from PLED device. The addition of a thin polymer interlayer ((F8)_0.5–(TFB)_0.5) is further investigated to maintain color purity. Furthermore, all polymer electrochromic device (ECD) based on P(F8-SFX) and poly(ethylene dioxythiophene) (PEDOT) was constructed. Spectroelectrochemistry and switching ability of the device were also investigated by UV–vis spectroscopy.     

硅衬底GaN基LED的研究进展

与蓝宝石衬底相比，硅衬底具有低成本、大面积、高质量、导电导热性能好等优点，普遍认为使用Si片作GaN薄膜衬底有可能实现光电子和微电子的集成，因此Si基GaN的研究受到了广泛关注。本文回顾了Si衬底GaN基LED的研究进展，同时简要介绍了在Si衬底上制备GaN基LED的实验结果，及研制出工作电压为3．6V、串联电阻为31Ω、输出功率近1mW的Si衬底GaN基蓝光LED。     

基于深隔离槽刻蚀的高压发光二极管制造

在制作高压发光二极管(HV LED)时,为了将数个独立的LED 串联起来,需要将GaN进行电感耦合等离子(ICP)深隔离刻蚀。本文制作了隔离槽深度为5um,台面侧壁为79.2?的GaN基HV LED。刻蚀表面和结构侧面的形貌通过激光显微镜和扫描电子显微镜进行观察分析。在形成金属接触并退火之后,测量HV LED输入电流-正向电压曲线,分析了其电学特性。与传统LED相比,I-V曲线趋势一致;相同尺寸和发光面积下,接触电阻下降了4.6Ω,而输出功率提高了5W。结果表明,可以应用于实际生产     

基于BAlq的有机电致发光器件的磁效应

研究了ITO/NPB(40nm)/BAlq(60nm)/BCP(5nm)/LiF(0.8nm)/AI有机电致发光器件(OLED)的磁效应.实验结果表明,磁场在10mT时,器件的效率最大增加了34%,这一结果是由于三线态激子与三线态激子间的相互淬灭产生激发单线态激子从而使单线态激子比率增加,致使电致发光(EL)增强.当磁...     

Light extraction efficiency enhancement of GaN-based light emitting diodes by a ZnO current spreading layer

preading layers are analyzed.     

使用ZnO电流扩展层提高GaN基LED提取效率

Gallium nitride （GaN） based light emitting diodes （LEDs） with a thick and high quality ZnO film as a current spreading layer grown by metal-source vapor phase epitaxy （MVPE） are fabricated successfully. Compared with GaN-based LEDs employing a Ni/Au or an indium tin oxide transparent current spreading layer, these LEDs show an enhancement of the external quantum efficiency of 93% and 35% at a forward current of 20 mA, respectively. The full width at half maximum of the ZnO （002） ω-scan rocking curve is 93 arcsec, which corresponds to a high crystal quality of the ZnO film. Optical microscopy and atomic force microscopy are used to observe the surface morphology of the ZnO film, and many regular hexagonal features are found. A spectrophotometer is used to study the different absorption properties between the ZnO film and the indium tin oxide film of the GaN LED. The mechanisms of the extraction quantum efficiency increase and the series resistance change of the GaN-based LEDs with ZnO transparent current spreading layers are analyzed.     

MOCVD生长双波长发光二极管

根据白光照明和可变换波长的光通信中对单芯片双波长发光二极管(LED)要求,在分析了反向偏置隧道结性质的基础上,设计了用反向偏置隧道结连接两个有源区的单芯片双波长LED,用金属有机化学气相沉积技术(MOCVD)在GaAs衬底上一次外延生长了同时发射两种波长的LED,其包含一个AlGaInP量子阱有源区和一个GaInP量子阱有源区,两个有源区由隧道结连接;通过后工艺制备了双波长LED器件,在20mA电流注入下,可以同时发射626nm和639nm两种波长,光强是127mcd,正向电压是4.17V。与传统的单有源区LED进行对比表明,双波长LED有较强的光强;对比单有源区LED的2.08V正向电压,考虑到双波长LED包含隧道结和两个有源区,隧道结上的压降很小。