Enhanced light extraction from GaN-based LEDs with a bottom-up assembled photonic crystal

Photonic crystal (PhC) structure is an efficient tool for light extraction from light-emitting diodes (LEDs). The fabrication of a large area PhC structure on the light output surface of LEDs often involves sophisticated equipments such as nanoimprint lithography machine. In this study a monolayer of polystyrene (PS) microspheres was employed as a template to fabricate a noninvasive photonic crystal of indium tin oxide (ITO) on the surface of GaN-based LED. PS spheres can help to form periodic arrangement of bowl-like holes, a photonic crystal with gradually changed fill factors. Importantly, the electroluminescence intensity of LED with a photonic crystal was significantly enhanced by 1.5 times compared to that of the conventional one under various forward injection currents.     

High performance of GaN-based flip-chip light-emitting diodes with hole-shape patterned ITO ohmic contact layer and AgIn reflector

The enhancement of light extraction efficiency is observed when the hole-shape patterned ITO ohmic contact layer and AgIn reflector is adopted in GaN-based flip-chip (FC) light emitting diodes (LEDs). ITO layer (140 nm) and AgIn (200 nm) was deposited on the top of p-GaN by in-line DC sputtering and electron beam evaporating system, respectively. The ITO ohmic contact layer showed a low specific contact resistance of 2.66 x 10(-5) Omega cm(-2) and high transmittance of >85% at visible spectral regions. The AgIn reflector exhibited a low specific contact resistance of 1.90 x 10(-5) Omega cm(-2) and high reflectance of approximately 84% at visible spectral regions. Comparing with unpatterned ITO/AgIn layer, the optical output power of GaN-based FC LEDs improves approximately 30% by the adoption of micro size hole-shape patterned ITO ohmic contact layer and AgIn reflector.     

Enhanced performance of GaN-based light-emitting diodes by using a p-InAlGaN/GaN superlattice as electron blocking layer

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) with various kinds of electron blocking layers (EBLs) are analyzed numerically. The results indicate that an enhanced hole injection efficiency and a reduced electron leakage could be achieved with the GaN-based LED where a p-InAlGaN/GaN superlattice (SL) was employed as EBL as compared with the conventional GaN-based LEDs using rectangular p-AlGaN EBL or p-AlGaN/GaN SL EBL. Moreover, it was found that the efficiency droop could be significantly improved at high injection current density for GaN-based LEDs with p-InAlGaN/GaN SL EBL.     

Improved performance of nanocrystal quantum dots-based LEDs by modifying hole transport layer

Reported herein are the effects of the fabrication variables and surface capping of nanocrystal quantum dots (NQDs) on the characteristics of NQDs-based light-emitting diodes (LEDs). The molecular weight of the hole transport layer (HTL) material and the annealing conditions of the NQDs layer were chosen as fabrication process variables. Their effects on the layer characteristics and device efficiency were characterized. The maximum brightness varied over 50% according to the molecular weight of the HTL material. The optimized annealing temperature was shown to improve the maximum brightness by 20%. The surface-capping molecules of the NQDs were changed from conventional trioctyl phosphine/trioctyl phosphine oxide (TOPO/TOP) to pyridine and were incorporated into LEDs, and its effects on the device characteristics were discussed.     

Efficient LEDs with a conjugated co-polymer as the emissive layer

We report on the use of a new highly luminescent conjugated polymer as an emissive layer in single and multi-layer electroluminescence devices. Poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) [PmPV-co-DOctOPV] was prepared via a Wittig synthesis reaction. The resulting polymer has a high photoluminescence quantum efficiency in the solid state with an emission spectrum peaking at 506 nm (2.45 eV) in the green. Three different electroluminescence devices were fabricated: (i) Single layer devices containing only PmPV-co-DOctOPV; (ii) Double layer devices with PmPV-co-DOctOPV and an evaporated film of 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl)phenylene [OXD-7] as an electron transport layer; (iii) Triple layer devices containing PmPV-co-DOctOPV, OXD-7 and in addition a polyvinylcarbazole [PVK] hole transport layer. All the devices utilised an ITO anode and a MgAg cathode. Electroluminescence external quantum efficiencies for these devices were found to be respectively up to 0.08%, 0.55% and 1% respectively, corresponding to luminous efficiencies OF = 0.5, ≈ 3 and ≈ 6 lm/W and power efficiencies of 8.5 × 10⁻⁵, 5.9 × 10⁻⁴ and 6.0 × 10⁻⁴ W/W.     

Enhanced performance of printed organic diodes using a thin interfacial barrier layer

Printed, organic diodes with a thin organic interfacial layer forming a Schottky barrier were fabricated and characterized. Experiments indicated that the thickness of the barrier layer is <10 nm. The interfacial layer reduces the reverse current of the diode by 2 orders of magnitude without significantly affecting the forward characteristics above 1 V. As a result, printed organic diodes with a rectification ratio of 5 orders of magnitude were fabricated. The diodes enable applications where low reverse currents are needed.     

Fabrication of a nano-cone array on a p-GaN surface for enhanced light extraction efficiency from GaN-based tunable wavelength LEDs

We report on the fabrication of a nano-cone structured p-GaN surface for enhanced light extraction from tunable wavelength light emitting diodes (LEDs). Prior to p-contact metallization, self-assembled colloidal particles are deposited and used as a mask for plasma etching to create nano-cone structures on the p-GaN layer of LEDs. A well-defined periodic nano-cone array, with an average cone diameter of 300?nm and height of 150?nm, is generated on the p-GaN surface. The photoluminescence emission intensity recorded from the regions with the nano-cone array is increased by two times as compared to LEDs without surface patterning. The light output power from the LEDs with surface nano-cones shows significantly higher electroluminescence intensity at an injection current of 70?mA. This is due to the internal multiple scattering of light from the nano-cone sidewalls. Furthermore, we have shown that with an incorporation of InGaN nanostructures in the quantum well, the wavelength of these surface-patterned LEDs can be tuned from 517 to 488?nm with an increase in the injection current. This methodology may serve as a practical approach to increase the light extraction efficiency from wavelength tunable LEDs.     

高效大功率LED用蓝宝石图形衬底制备的研究进展

近年来,图形化的蓝宝石衬底在改善GaN晶体外延生长质量以及提升LED器件发光提取效率方面作用显著,引起了广泛的研究兴趣。综述了蓝宝石图形衬底的制备方法(干法刻蚀、湿法刻蚀、外延生长法),并较系统地介绍了蓝宝石图形衬底表面周期性图形参数(图形形貌、图案尺寸、图形占位比及其深度)对LED发光薄膜及器件的影响及其机理,最后对蓝宝石图形衬底的发展趋势进行了展望。     

Pulse electroplating of copper film: a study of process and microstructure

Copper films with high density of twin boundaries are known for high mechanical strength with little tradeoff in electrical conductivity. To achieve such a high density, twin lamellae and spacing will be on the nanoscale. In the current study, 10 microm copper films were prepared by pulse electrodeposition with different applied pulse peak current densities and pulse on-times. It was found that the deposits microstructure was dependent on the parameters of pulse plating. Higher energy pulses caused stronger self-annealing effect on grain recrystallization and growth, thus leading to enhanced fiber textures, while lower energy pulses gave rise to more random microstructure in the deposits and rougher surface topography. However in the extremes of pulse currents we applied, the twin densities were not as high as those resulted from the medium or relatively high pulse currents. The highest amount of nanoscale twinning was found to form from a proper degree of self-annealing induced grain structure evolution. The driving force behind the self-annealing is discussed.     

Enhanced performance of organic light-emitting diodes by inserting wide-energy-gap interlayer between hole-transport layer and light-emitting layer

We demonstrated that driving voltages, external quantum efficiencies, and power conversion efficiencies of organic light-emitting diodes (OLEDs) are improved by inserting a wide-energy-gap interlayer of (4,4′-N,N′-dicarbazole)biphenyl (CBP) between a hole-transport layer of N,N-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (α-NPD) and a light-emitting layer of tris(8-hydroxyquinoline)aluminum. By optimization of CBP thicknesses, the device with a 3-nm-thick CBP layer had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. We attributed these improvements to enhancement of a carrier recombination efficiency and suppression of exciton–polaron annihilation. Moreover, we found that the degradation of the OLEDs is caused by decomposition of CBP molecules and excited-state α-NPD molecules.     

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.     

Influence of molybdenum layer on the laser plasma generated from interfacing copper layer

The results for laser-induced breakdown spectroscopy (LIBS) measurement of thin Cu films (1 μm) on soda-lime glass (SLG) substrates with and without a supporting thin Mo layer (1 μm) are reported. The ablation was carried out using a nanosecond Q-switched Nd:YAG laser (λ=1064 nm, τ=4 ns, spot diameter=50 μm, top-hat profile) in the laser fluence range of 19.16-65.97 J/cm(2). It was found that, under the same laser irradiance conditions, the depth and morphology of ablation craters produced with and without the Mo layer were completely different. The electron number densities of the plasma from the two samples calculated from the measured LIBS spectra differed by a factor of 4 as 4.1×10(17) cm(-3) (Cu/Mo/SLG) and 17.7×10(17) cm(-3) (Cu/SLG), which was attributed to the matrix effects resulting from ionization of Na atoms diffused into the Mo layer. It is demonstrated that a nanosecond-laser-based LIBS is applicable for the characterization and composition analysis of thin film layers of a few micrometer thickness on glass substrates, especially for the measurement of Na contents of each layer.     

Impact of layer thickness and light transmission of ZnO nanomaterials on GaN-based light emitting diodes

Highly transparent ZnO nanomaterials have been successfully dispersed in the form of nanoparticles and nanorods on InGaN/GaN-based surface mounted light emitting diodes (SM-LEDs). An effortless spin-coating technique is employed to disperse the ZnO nanoparticle layers, and a well-known hydrothermal technique is used for growing the ZnO nanorods. The layer thickness and the light transmission at a specific wavelength are the major factors in improving the light output power of the devices. Field emission scanning electron microscope (FESEM) images are used to confirm the uniform dispersion of the ZnO nanostructures on the top of the SM-LEDs. The layer thickness and the level of light transmission at 460 nm are examined from the cross-sectional FESEM images and UV absorption spectra, respectively.     

Enhanced mechanical performance of grain boundary precipitation-hardened high-entropy alloys via a phase transformation at grain boundaries

Grain-boundary(GB)precipitation has a significant adverse effect on plasticity of alloys,which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading.Herein,we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs.This in situ deformation-induced phase transformation at GBs introduces a well-known effect:transformation-induced plasticity(TRIP),which enables an exceptional elongation to fracture(above 38％)at a high strength(above 1.5 GPa)in a GB precipitation-hardened high-entropy alloy(HEA).The present strategy in terms of"local stress concentration-induced phase transformations at GBs"may provide a fundamental approach by taking advantage of(rather than avoiding)the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.     

Direct-on-barrier copper electroplating on ruthenium from the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide

The "direct-on-barrier" electroplating of copper on ruthenium from a 1 mol dm⁻³ solution of CuCl in the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide, [C₂mim][N(CN)₂], is reported. Continuous layers of copper with a preferential Cu(111) orientation were obtained from this electrolyte. The copper coatings were investigated by top view scanning electron microscopy (SEM), X-ray diffraction (XRD), and focused ion beam transmission electron microscopy (FIB-TEM). The nucleation density was both theoretically and experimentally evaluated by the Scharifker-Hills model and transmission electron microscopy, respectively. The direct plating of copper on resistive substrates for advanced interconnects and package is a promising new application of ionic liquids.     

Light extraction efficiency enhancement of GaN-based light emitting diodes on n-GaN layer using a SiO2 photonic quasi-crystal overgrowth

In this paper, GaN-based LEDs with a SiO2 photonic quasi-crystal (PQC) pattern on an n-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 better light output power of LED III (d = 1.2 microm) was enhanced by a factor of 1.20. After 1000 h life test (55 degrees C/50 mA) condition, Normalized output power of LED with a SiO2 PQC pattern (LED III (d = 1.2 microm)) on an n-GaN layer only decreased by 5%. This results offer promising potential to enhance the light output power of commercial light-emitting devices using the technique of nano-imprint lithography.     

Enhanced sensing performance toward alcohols using copper oxide based on exposed crystal facet driven catalytic oxidation

Herein, we have reported the CuO micro-polyhedrons with special exposed crystal facet, showing enhanced gas sensing performance based on improved catalytic activation of oxygen. The as-prepared CuO showed the uniform shapes of polyhedrons with the size of several micrometers and can be facilely controlled by simply adjusting the precipitants and heating steps. Then, the gas sensitive performance of CuO polyhedrons was evaluated toward typical alcohols at a working temperature of 235 °C. It showed a good sensitivity of n-butanol among several typical alcohols. Moreover, the response value of CuO-m3 (2.43) is significantly higher than that of CuO-m1 (1.92) and CuO-m2 (1.83), which is directly proportional to the exposure degree of (110) crystal facet of CuO. Analysis of mechanism showed that this crystal facet may have the best adsorption capacity of oxygen, thus more reactive oxygen can be generated via a catalytic oxidation process. Our work offered a kind of cheap oxide with exposed facet toward typical alcohols, which is very potential for the development of related industrial sensors.

     

Effect of the functional group of polyethylene glycol on the characteristics of copper pillars obtained by electroplating

Journal of Materials Science: Materials in Electronics - Five polyethylene glycol (PEG)-based compounds with varying terminal functional groups were investigated as suppressors to investigate the...