Study of dual-blue light-emitting diodes with asymmetric AlGaN graded barriers

A dual-blue light-emitting diode （LED） with asymmetric A1GaN composition-graded barriers but without an AlGaN electron blocking layer （EBL） is analyzed numerically. Its spectral stability and efficiency droop are improved compared with those of the conventional InGaN/GaN quantum well （QW） dual-blue LEDs based on stacking structure of two In0.18Ga0.szN/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate. The improvement can be attributed to the markedly enhanced injection of holes into the dual-blue active regions and effective reduction of leakage current.     

Improvement of the efficiency droop of GaN-LEDs using an AlGaN/GaN superlattice insertion layer

With an n-AlGaN(4 nm)/GaN(4 nm) superlattice(SL) inserted between an n-GaN and an InGaN/GaN multiquantum well active layer,the efficiency droop of GaN-based LEDs has been improved.When the injection current is lower than 100 mA,the lumen efficiency of the LED with an n-AlGaN/GaN SL is relatively small compared to that without an n-AlGaN/GaN SL.However,as the injection current increases more than 100 mA,the lumen efficiency of the LED with an n-AlGaN/GaN SL surpasses that of an LED without an n-AlGaN/GaN SL. The wall plug efficiency of an LED has the same trend as lumen efficiency.The improvement of the efficiency droop of LEDs with n-AlGaN/GaN SLs can be attributed to a decrease in electron leakage due to the enhanced current spreading ability and electron blocking effect at high current densities.The reverse current of LEDs at -5 V reverse voltage decreases from 0.2568029 to 0.0070543μA,and the electro-static discharge(ESD) pass yield of an LED at human body mode(HBM)-ESD impulses of 2000 V increases from 60%to 90%.     

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.     

基于纳米球镜光刻的选区生长的光子晶体提高InGaN LED光提取效率的研究

We report a new method for the fabrication of two-dimensional photonic crystal(PhC) hole arrays to improve the light extraction of GaN-based light-emitting diodes(LEDs).The PhC structures were realized using nanospherical-lens photolithography and the selective-area epitaxy method,which ensured the electrical properties of the LEDs through leaving the p-GaN damage-free.At a current of 350 mA,the light output power of LEDs with PhC hole arrays of 450 nm and 600 nm in diameter with the same lattice period of 900 nm were enhanced by 49.3% and 72.2%,respectively,compared to LEDs without a PhC.Furthermore,the LEDs with PhC hole structures showed an obviously smaller divergent angle compared with conventional LEDs,which is consistent with the results of finite-difference time-domain simulation.     

表面增透结构AlGaInP LED的研究

A kind of AlGaInP light emitting diode （LED） with surface anti-reflecting structure has been introduced to solve the problems of low light efficiency and restricted luminous intensity. The new structure can be demonstrated theoretically and experimentally, and LEDs with the new structure have higher on-axis luminous intensity and larger saturation current than conventional LEDs and LEDs with ITO film only, which is caused by higher external quantum efficiency and also higher internal quantum efficiency. The new LEDs are especially suitable for working at large injected currents.     

Influence of lattice symmetry and hole shape on the light enhanced transmission through the subwavelength hole arrays

The golden films with various subwavelength hole arrays on the film surface are designed and fabricated on glass substrate by electron beam lithograply （EBL）, focused ion beam （FIB）, and reactive ion etching （RIE）, respectively. The influences of the hole array symmetry and the hole shape on the light-enhanced transmission through the films are observed and simulated. The experimental results show that when the array lattice constant and the hole diameter are the same in the different array structures which are 1 μm and 350 nm respectively, the square hole arrays exhibit two transmission peaks at 1170 nm and 1580 nm with the transmissivities of 3% and 6%, respectively, while the hexagonal hole arrays exhibit an enhanced peak of 14% at 1340 nm; when the lattice constant and the duty cycle are the same for different array stucture,.the transmission peaks are different for different hole shapes, which are at 763 nm with transmissivity of 12% for rectangular holes and at 703 nm with the one of 9%, respectively The numerical simulation results by using the transfer matrix method （TMM） are consistent with the observed results.     

Design of violet InGaN light-emitting diode with staggered quantum well structure

The staggered InGaN quantum well （QW） structure and the conventional InGaN QW structure for the emission at a particular wavelength of 400 nm are designed and theoretically investigated, including the distribution of the carders＇ concentration, the radiative recombination rate, the Shockley-Read-Hall （SRH） recombination rate as well as the output performance and the internal quantum efficiency. The theoretical result indicates that the staggered QW structure offers significant improve- ment of carriers＇ concentration in the QW, especially the hole concentration. The output power and the internal quantum efficiency also show 32.6 % and 32.5 % enhancement, respectively, in comparison with that of the conventional InGaN QW structure. The reduction of the electron overflow can be the main factor for the improvement of the optical performance for novel staggered InGaN QW structure.     

Effect of different processing methods for the hole transporting layer on the performance of double layer organic light-emitting devices

The hole transporting layer （HTL） of organic light-emitting device （OLED） was processed by vacuum deposition and spin coating method, respectively, where N,N＇-biphenyl-N, N＇-bis（3-methylphenyl）- 1, l＇-biphenyl-4,4＇ -diamine （TPD） and poly （vinylcarbazole） （PVK） acted as the hole-transport materials. Tris-（8-hydroxyquinoline）- aluminum （Alq3） was utilized as both the light-emitting layer and the electron transporting layer. The basic structure of the device cell was： indium-tin-oxide （1TO）/PVK ： TPD/Alq3/Mg：Ag. The electroluminescent （EL） characteristics of devices were characterized. The results showed that the peak of EL spectra was located at 530 nm, which conformed to the characterizing spectrum of Alq3. Compared with using vacuum deposition method, the green emission with a maximum luminance up to 26135 cd/m2 could be achieved at a drive voltage of 15 V by selecting proper solvent using spin-coating technique, and its maximum lumi nance efficiency was 2.56 lm/W at a drive voltage of 5.5 V.     

Efficient Blue Organic Light-emitting Diodes with Simple Structure Based on N,N′-bis （1-naphthyl）-N,N′-biphenyl-1, 1′-biphenyl-4,4′-diamine

Small molecular Organic light-emitting diodes （OLEDs） with high efficiency composed of 2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline （BCP） and N, N%bis-（1-naphthyl）-N, N′,bipheny1-l,1′-biphenyl-4,4′- diamine （NPB） as an electron transporting layer and an emitting layers respectively were studied. Two devices of indium-tin-oxide （ITO）/NPB/BCP/8-hydroxyquinoline aluminum （Alq3）/Mg：Ag and ITO/NPB/BCP/ Mg：Ag with unique simple structure were fabricated. The luminance-voltage and current density-voltage characteristics of devices were investigated. The results demonstrated that the maximum luminance of the double and triple-layer devices is 11500cd/m^2 and 5000 cd/m2 at 15V, respectively. The maximum luminous power efficiency is 1.21m/W at the luminance of 300cd/m^2 and the luminous efficiency of 2.7cd/A. The peak of Electroluminescence （EL） spectrum locates at 433nm and the Commissions Internationale d＇Eclairage （CIE） coordinates are （0.18, 0.17）, which is consistent with the Photoluminescent （PL） spectrum of NPB. The bright blue light emission is independent on the variation of bias voltage. The diversity of device performance with two different structures was discussed.     

Laser lift-off technique and the re-utilization of GaN-based LED films grown on sapphire substrate

A thin GaN LED film, grown on 2-inch-diameter sapphire substrates, is separated by laser lift-off. Atom force microscopy （AFM） and the double-crystal X-ray diffraction （XRD） have been employed to characterize the performance of Gan before and after the lift-off process. It is demonstrated that the separation and transfer processes do not alter the crystal quality of the GaN films obviously. InGaN/GaN multi-quantum-wells （MQW＇s） structure is grown on the separated sapphire substrate later and is compared with that grown on the conventional substmte under the same condition by using PL and XRD spectrum.     

A tunable lighting system integrated by inorganic and transparent organic light-emitting diodes

A tunable surface-emitting integrated lighting system is constructed using a combination of inorganic light-emitting diodes （LEDs） and transparent organic LEDs （OLEDs）. An RB two-color LED is used to supply red and blue light emission, and a green organic LED is used to supply green light emission. Currents of the LED and OLED are tuned to produce a white color, showing different Commission Internationale d＇Eclairage （CIE） chromaticity coordinates and correlated color temperatures with a wide adjustable range. Such an integration can compensate for the lack of the LED＇s luminance uniformity and the transparent OLED＇s luminance intensity.     

Influence of PVK ： NPB hole transporting layer on the characteristics of organic light-emitting devices

A doping system consisting of NPB and PVK is employed as a composite hole transporting layer （CHTL）. By adjusting the component ratio of the doping system, a series of devices with different concentration proportion of PVK ： NPB are constracted. The result shows that doping concentration of NPB enhances the competence of hole transporting ability, and modifies the recombination region of charge as well as affects the surface morphology of doped film. Optimum device with a maximum brightness of 7852 cd/m^2 and a power efficiency of 1.75 lm/W has been obtained by choosing a concentration proportion of PVK ： NPB at 1：3.     

Computer Modeling and Simulation Evaluation of High Power LED Sources for Secondary Optical Design

Proposed and demonstrated is a novel computer modeling method for high power light emitting diodes（LEDs）. It contains geometrical structure and optical property of high power LED as well as LED dies definition with its spatial and angular distribution. Merits and non-merits of traditional modeling methods when applied to high power LEDs based on secondary optical design are discussed. Two commercial high power LEDs are simulated using the proposed computer modeling method. Correlation coefficient is proposed to compare and analyze the simulation results and manufacturing specifications. The source model is precisely demonstrated by obtaining above 99% in correlation coefficient with different surface incident angle intervals.     

Synthesis of a New Optoelectronic Material Based on Oriented Adsorption of Dyes to Nanoparticles Surface

Synthesis of the optoelectronic storage material with structure for coating by nanosized metal and azo-dye was reported. The characterization of composites was made by using transmission electron microscope （TEM）, ultraviolet-visible spectrometer （UV-Vis） and thermogravity analyzer （TGA）. It is found that, due to the specific structure, in which azo-dye molecules are oriented and adsorbed on the spherical surface of nanosized metal, the absorption maximum of azo-dye methyl orange shift towards shorter wavelength band. The experimental results show that the proposed technique here wouM offer a promising way to synthesize short wavelength optoelectronic storage material by doping of metal nanoparticles coated with dyes in polymer. Furthermore, the composites based on the structure can present excellent thermal properties suitable for the requirements of optical storage. This new type of material is capable of matching semiconductor laser （GaN） in optoelectronic storage technology.     

High performance blue organic light-emitting devices fabricated by using the doped AIq3 in NPB hole transmistion layers

We have designed a new structure blue emission device with doped Alq3 of 3% in hole transmission layers of NPB. The CIE coordination of the devices is （0.17,0.19）. The maximum electroluminescence efficiency is 4.1 cd/A at 11 V, the brightness is 118.8 cd/m^2 at 7 V, and the maximum brightness is 10770 cd/m^2 at 13 V.     

Spectral Properties of Yellow Fluorescent Powder for White LEDs

Much attention has been paid to white LEDs because of their potential applications in the illumination.The doping of rate earth ions plays an important role in the optical properties of yellow fluorescent powder.And mainly aiming to raise the intensity and the luminous rate of the white LEDs and by photoluminescence and electroluminescence,the luminescence spectrum of yellow fluorescent powder bought from different places is measured.Furthermore,the luminous intensity in the normal direction and the angle distribution of half maximum power for the white LEDs packed with cylindrical φ5 epoxy on the same blue GaN chips are also measured under the same manufacture conditions.The results show that the yellow fluorescent powder bought from China mainland has higher optical output rate than that bought from China Taiwan and hence is more suitable to fabricate the white LEDs for practical use.     

RF-MBE Grown AlGaN/GaN HEMT Structure with High Al Content

A Si doped AlGaN/GaN HEMT structure with high Al content (x=43%) in the barrier layer is grown on sapphire substrate by RF-MBE.The structural and electrical properties of the heterostructure are investigated by the triple axis Xray diffraction and Van der PauwHall measurement,respectively.The observed prominent Bragg peaks of the GaN and AlGaN and the Hall results show that the structure is of high quality with smooth interface.The high 2DEG mobility in excess of 1260cm2／（V·s） is achieved with an electron density of 1.429e13cm－2 at 297K,corresponding to a sheet-densitymobility product of 1.8e16V－1·s－1.Devices based on the structure are fabricated and characterized.Better DC characteristics,maximum drain current of 1.0A/mm and extrinsic transconductance of 218mS/mm are obtained when compared with HEMTs fabricated using structures with lower Al mole fraction in the AlGaN barrier layer.The results suggest that the high Al content in the AlGaN barrier layer is promising in improving material electrical properties and device performance.     

GaN/AIN multiple quantum wells grown on GaN-AIN waveguide structure by metalorganic vapor-phase epitaxy

High-optical-confinement waveguide structure based on nitride semiconductors is proposed and demonstrated for the first time with metal organic vaporphase epitaxy. The waveguide structure composed of 1-μm-thick AIN cladding layer, 2-μm-thick GaN guiding layer, and 40 periods of GaN/AIN multiple quantum wells (MQWs) was grown using optimized growth conditions for each layer. For improved material quality, the two-step growth technique using low-temperature AIN and GaN nucleation layers was utilized to reduce the stress induced by lattice mismatch between each layer. The high-optical-confinement structure could therefore be grown with high quality, leading to a successful observation of inter-sub-band absorption in GaN/AIN MQWs. The inter-sub-band absorption wavelength observed in such structure is in good agreement with that of MQWs grown on GaN layer, showing that the proposed waveguide structure can be used as a standard structure for optical devices based on inter-sub-band absorption.     

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%.     

Formation of GaN Nanowires by Ammoniating Ga2O3 Films Deposited on Oxidized Al Layers on Si Substrate

Large quantities of gallium nitride（GaN） nanowires have been prepared via ammoniating the Ga2O3 films deposited on the oxidized aluminum layer at 950℃ in a quartz tube. The nanowires have been confirmed as crystalline wurtzite GaN by X-ray diffraction, X-ray photoelectron spectrometry scanning electron microscope and selected-area electron diffraction. Transmission electron microscope （TEM） and scanning electron microscopy（SEM） reveal that the nanowires are amorphous and irregular, with diameters ranging from 30 nm to 80 nm and lengths up to tens of microns. Selected-area electron diffraction indicates that the nanowire with the hexagonal wurtzite structure is the single crystalline. The growth mechanism is discussed briefly.