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In this study, we have fabricated a 368 nm LED with an epitaxial Indium Tin Oxide (ITO) contact layer. We analyze the thermal performance of the flip-chip LED with a symmetric electrode and metal reflective layer, applying ANSYS to build a coupled electro-thermal finite element model (FEM) of the temperature distribution in the multiple quantum wells (MQWs). We compare our system with the traditional Au-bump flip-chip LED and a flip-chip LED with a Distributed Bragg Reflector (DBR) layer. The simulation results have shown that the flip-chip LED with a metal reflective layer and symmetric electrode exhibits better heat dissipation performance, particularly at high input power. The influence of the insulating layer on the LED chip junction temperature is also examined. The simulation data establish an effect due to the thermal conductivity of the insulating layer in terms of heat dissipation, but this effect is negligible at an insulation layer thickness ≤1 µm. [ABSTRACT FROM AUTHOR]

Most plastic processing set points are variables that need to be defined for each sample point of the cycle. However, in the absence of on-line measures of part quality, the set points cannot be defined by feedback and need to be prescribed a priori for the entire cycle. In conventional practice, the shape of each set-point profile is defined either heuristically, according to qualitative experience, or mechanistically, to enforce a predefined trajectory for a simulated internal process state that is used as a surrogate measure of part quality (e.g., the velocity profile defined to dictate a constant melt front velocity). The purpose of this study is twofold: (i) to evaluate the efficacy of using a single internal state as the surrogate of part quality, and (ii) to explore the feasibility of devising a multivariate profiling approach, where indices of multiple process states act as surrogates of part quality. For this study; an injection-compression molding process used for production of digital video disks was considered as the development domain, and a pseudo-optimal cycle of the process was found by reinforcement learning to provide a basis for evaluating the ideal behavior of the process states. Compared to conventional molding, the results indicate that the asymmetric process capability index, CPK, was increased by ~50% with velocity profile optimization and to 120% with both velocity profile and pressure profile optimization. Two general conclusions result. First, velocity and pressure profiling provide important degrees of freedom for optimizing process control and maximizing part quality. Second, estimators for unobservable process states, in this case birefringence and warpage, can be used to develop different machine profiles to selectively trade off multiple quality attributes according to user preferences. [DOI: 10.1115/1.2738094]

A non-Newtonian, non-isothermal flow analysis has been developed to assist the design of a self-compensating polymer melt regulator, which is a device capable of regulating the melt pressure in polymer processing via an open loop control architecture. The governing mass and momentum equations for the two-dimensional, axisymmetric flow field are solved by a mixed finite element method, in which the velocity components are interpolated by quadratic functions, and the pressure is interpolated by a linear function. The temperature field is solved by the finite difference method. Results of the outlet pressure, valve pin position, bulk temperature rise, and flow rate as functions of the control force for Newtonian isothermal analyses and non-Newtonian non-isothermal analyses are provided. The simulation demonstrates the behavior of candidate regulator designs and provides the performance attributes such as outlet pressure, flow rate, temperature rise, etc., given the decision variables, such as valve parameters, process conditions, and polymer melt rheology. The results indicate that for a regulator design on the order of 20 mm diameter, the regulator operates in a mostly closed condition with an aperture opening varying between 0.1 and 1 mm. The results suggest that the bulk temperature increases with control force and flow rate and is largely attributable to the increases in viscous heating of the melt through the flow channels, rather than the pinch off between the value pin and the valve body. [DOI: 10.1115/1.2193545]

Polymer process control is limited by a lack of observability of the distributed and transient polymer states. Three simulations of varying complexity are validated for on-line simulation of an injection molding process with a two drop hot runner system to predict the state of the polymer melt in real time and thereby improve product quality in situ. The simplest simulation is a Newtonian model, which predicts flow rates given the inlet and outlet pressures. An intermediate non-Newtonian and nonisothermal simulation utilizes a modified Ellis model that expresses the viscosity as a function of the shear stress in which the modeling of the heat transfer utilizes a Bessel series expansion to include effects of heat conduction, heat convection, and internal shear heating. A numerical simulation was also developed that utilizes a hybrid finite difference and finite element scheme to simultaneously solve the mass, momentum, and heat equations. Numerical verification indicates that the flow rate predictions of the described simulations compare well with the results from a commercial mold filling simulation. However, empirical validation utilizing a design of experiments indicates that the described analyses are qualitatively useful, but do not possess sufficient accuracy for quantitative process and quality control. Specifically, off-line validation using optimal transducer calibration with well characterized materials provided a coefficient of regression, [R.sup.2], of ~0.8. However, blind validation with previously untested materials and no transducer recalibration provided a regression coefficient of ~0.4. While the direction of the main effects was usually correct, the magnitudes of the effects were frequently outside the confidence interval of the observed behavior. Several sources of variance are discussed, including sensor calibration, constitutive modeling of the polymer melt, and numerical analysis. POLYM. ENG. SCI., 46:274-288, 2006. © 2006 Society of Plastics Engineers, INTRODUCTION A fundamental difficulty in injection molding and other dynamic polymer processing operations is the lack of observability and controllability of the polymer melt. Because of the presence of an [...]

The flow and thermally induced birefringence of injection-compression molded optical media such as compact discs and digital video discs is predicted by applying a stress-optical rule to the flow and thermally induced stresses, which are estimated with a viscoelastic material model integrated into a non-isothermal compressible flow simulation. The resulting model considers flow and cooling induced molecular orientation, and the transient effect of thermal stress and pressure. Contrary to previous research for polystyrene, the validated results indicate that, for polycarbonate, the magnitude of the thermally induced birefringence is comparable to the flow induced birefringence. Simulation results of the flow and thermally induced in-plane birefringence for compact-disc-recordable moldings with an optical grade of polycarbonate compared well with experimental observations at different mold and melt temperatures. Both simulation and experiments indicate that mold and melt temperatures have a significant effect on the level of birefringence; increasing mold or melt temperature significantly reduces the birefringence. Polym. Eng. Sci. 44:814-824, 2004., INTRODUCTION Optical media such as compact discs (CDs) and digital video discs (DVDs) are typically injection-compression molded with an optical grade of polycarbonate (PC). One critical quality of the mass [...]

A numerical simulation of a coining type of injection-compression molding is developed. A hybrid finite element/finite difference method is employed to model the temperature and pressure fields of the process using a non-isothermal compressible flow model. Simulation results for CD-R molding with respect to injection pressure and mold displacement are compared with experimental observations using an optical grade of polycarbonate. The simulation shows similar trends as experimental observations on the dependence of various processing parameters such as melt temperature, mold temperature, and packing pressure. However, the mold displacement measurement does not show the effect of punch delay time as does the simulation, and needs further investigation., INTRODUCTION Injection-compression molding is widely used in manufacturing of optical media, such as CD's and DVD's, using engineering thermoplastics. There are basically two types of injection-compression molding. One type is [...]

In this work, metal halide perovskite quantum dots (QDs) with Formamidinium (FA) and Cs mixed cations were fabricated using a solution-processed method at room temperature. By controlling Cs doping ratios in a precursor, the optical properties of mixed-cation perovskite QDs were systematically studied. With the increase in Cs ion doping, the photoluminescence (PL) spectra of perovskite QDs were blueshifted, which was mainly due to the smaller radius of Cs ions than those of FA. Temperature-dependent PL spectra were conducted on mixed-cation perovskite QDs. As the temperature gradually increased from 4 K to 300 K, PL peaks were blue shifted, and full-width at half maximum (FWHM) was widened, which was directly related to lattice thermal expansion and the carrier-photon coupling effect under temperature variation. At the same time, excess Cs ion doping had a prominent influence on optical properties at low temperatures, which was mainly due to the introduction of detrimental defects in perovskite crystals. Therefore, it is particularly important to control doping concentration in the preparation of high-quality perovskite QDs and efficient photoelectric devices. [ABSTRACT FROM AUTHOR]

We fabricated mesoporous perovskite nanocrystal for the first time, and investigated its optical properties and application in light-emitting diodes (LEDs). The transformation of mesoporous structure can be ascribed to the decomposition of nanocrystals under dilution condition, which results in the blueshift of luminescence. The mesoporous nanocrystals under proper dilution may achieve improved perovskite LEDs, with maximum luminance and current efficiency of 23370 cd m−2 and 6.7 cd A−1, respectively. This work provide an avenue to the optical engineering of perovskite nanocrystals, and demonstrate that perovskite concentration is one of key factors for realizing efficient LEDs. [ABSTRACT FROM AUTHOR]

The luminescence properties of InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with different quantum-well (QW) thicknesses were investigated. It is found that with decreasing the QW thicknesses, the integrated intensities of the photoluminescence (PL) and electroluminescence (EL) peaks demonstrate a contrary changing trend. The PL results show that the luminescence efficiency is improved by using thinner QWs. However, in the EL process, such a positive effect is counteracted by the low carrier injection efficiency in the thin QW LEDs, and consequently leads to a lower light output. Based on our experimental results, it is inferred that the tunneling leakage current associated with dislocations should be responsible for the low carrier-injection efficiency and the observed weaker EL integrated intensity of the LEDs with thin QWs. [ABSTRACT FROM AUTHOR]

An advanced three-dimensional electrothermal-coupled simulation model basing on finite-element method numerical simulation is developed to study the electrical and thermal properties of chip-level high-power GaN-based light-emitting diodes (LEDs). The current spreading, heat generation, and transfer in the device are comprehensively considered in this model. The current-spreading effect of the transparent current-spreading layer and the thermal performance of LEDs with interdigitated-electrodes are investigated. The simulation results prove that the temperature distribution in the active layer is strongly affected by the electrode pattern. The obvious heat accumulation in LEDs with conventional interdigitated-electrode patterns can be seen both in the simulated results and the infrared measured results. The heat transfer efficiency can be improved by using a symmetry electrode pattern design. The thermal management of the bump configurations in flip-chip LEDs is also studied. A more reasonable and thermal effective bump configuration is presented, and the simulated results show that a lower average temperature and more uniform heat distribution in the chips can be obtained. [ABSTRACT FROM AUTHOR]

Due to the high refractive index of nitrides, encapsulation is necessary to improve the light extraction efficiency for GaN-based light-emitting diodes (LEDs). Traditional thermal-curing encapsulant needs to be heated for several minutes to form a dome-shaped lens, which is time-consuming. In this paper, we have developed a novel kind of silicon-based ultraviolet (UV)-curing encapsulant, for which only 20 s of near-UV (NUV) light radiation at room temperature is needed. The composition of the encapsulants is optimized to enhance light extraction ability. For 392-nm NUV LEDs packaged by this UV-curing encapsulant, the light output power is enhanced by 10.0%. Moreover, the LEDs packaged by UV-curing and thermal-curing encapsulant presented similar stability and reliability after 480 h of the aging test under a high driving current of 200 mA. This paper demonstrates the potential of silicon-based UV-curing encapsulants for the package of NUV, blue and white LEDs, which is time- and cost-effective for the LED industry. [ABSTRACT FROM AUTHOR]

We designed and fabricated AlGaN-based near-ultraviolet light-emitting diodes (LEDs) operating at 368 nm with high output power using Ta2O5/SiO2 hybrid distributed Bragg reflectors (DBRs). The output powers of LEDs with eight-pair DBR and hybrid-DBR configurations were increased by 18% and 25% when compared with that of an LED without any DBR. The reflectance of the eight-pair DBR and hybrid-DBR were as high as 99%. In addition, the reflectance of the fabricated hybrid-DBR showed a slight dependence on the incident light wavelength and the angle of incidence. In comparison to the eight-pair DBR, the bandwidth of the hybrid-DBR’s reflectance band was broadened by up to 74 nm. Under current injection at 350 mA, output powers of 88, 104, and 110 mW were measured for LEDs without any DBR reflector, with the eight-pair DBR, and with the hybrid-DBR, respectively. [ABSTRACT FROM PUBLISHER]

A nonvolatile memory (NVM) based on an amorphous-indium–gallium–zinc oxide (a-IGZO) thin-film transistor (TFT) with defect-engineered gate insulator was demonstrated. The gate insulator was a blocking alumina/storage alumina/tunneling alumina stack structure, which was simply assembled in a single atomic layer deposition step. The memory device showed a positive shift of threshold voltage as large as 14.4 V after +20 V, 1 s programming. In contrast, the memory erasing was not sensitive to negative gate voltage in the dark. Once programmed, the memory can only be light erased. Furthermore, the light combined with a negative bias improved the erasing speed effectively. In addition, a 10-year memory window as large as 7.5 V was extrapolated at room temperature with a charge loss of 34.7%. Based on the observation of blisters in the storage alumina layer after high temperature annealing, Fourier transform infrared spectroscopy measurement and first-principles calculations, the high electron storage capacity can be attributed to the deep defect levels in the storage alumina layer, which were originated from hydrogen impurity. This a-IGZO TFT charge trapping NVM with high performance and simple process is a candidate device for the application of fully functional transparent system on panel. [ABSTRACT FROM AUTHOR]

Hole transport control and carrier injection improvement have been demonstrated in the InGaN/GaN light-emitting diodes (LEDs) with step-stage multiple-quantum-well (MQW) structure and Si-doped hole-blocking barriers. Single-wavelength emission was obtained under electrical pumping in these LEDs by utilizing hole-blocking effect. The light emission around 450 nm showed a substantial increase compared with the reference sample with single or step-stage indium-content MQWs. The droop behavior and wavelength stability were also improved significantly. These improvements were attributed to the enhanced carrier injection to the active region due to the alleviation of the quantum-confined Stark effect and the effective hole-blocking effect of the Si-doped barriers. [ABSTRACT FROM AUTHOR]

In this study, we have systematically investigated the effect of Si doping level in the n-cladding layer on the performance of InGaN/GaN-based light-emitting diodes (LEDs). Detailed structural, optical and electrical properties of the sample with different Si doping level were studied. Based on these investigations, it was found that with a low level of Si doping (0.738 nmol/min, \sim \5.4\times \10^17\ \cm^3), the subsequent multiple-quantum-well (MQW) structure showed superior crystalline quality with low-density threading dislocations (TDs); however, the operation voltage of the LED chips became unbearable. On the other hand, the highly-doped (14.40 nmol/min, \sim \1.1\times \10^19\ \cm^3) samples showed a substantially lower operation voltage, but the MQW quality deteriorated with much higher TD density. Finally, the effect of inserting a 100-nm-thick undoped spacer GaN layer between the n-cladding layer and active layers was also investigated. The electrical and emission properties both deteriorated while the crystalline quality improved for LEDs with this structure, proving the importance of electron injection on the performance of LEDs. [ABSTRACT FROM PUBLISHER]

We report the observation of electroluminescence from the first to fourth quantum wells (QWs) from the p-GaN layer in InGaN/GaN multiple-QW light-emitting diodes (LEDs) with various indium contents (4%–16%) in each QW. The investigated LED sample showed a lower turn-on voltage and ideality factor as well as a reduction of etching pit density compared with the reference sample. Also, the X-ray reciprocal space maps revealed a partial strain relaxation in the active region. The enhanced hole injection efficiency was attributed to the weakening of strain-induced polarization field in the QWs and the good crystalline quality. [ABSTRACT FROM PUBLISHER]

The lateral current spreading (CS) effect on the efficiency droop in GaN-based LEDs has been studied in terms of the CS distance (LCS) using a designed pattern with the 2-D current spreading profile. The correlations of CS effect with the electrical, luminescent and electric-thermal properties of the LEDs have been discussed. LEDs with the LCS longer than the theoretically calculated effective CS length (Leff) suffer from more serious efficiency droop and the degradation of luminescent properties. However, the influence of CS effect on the ideality factors of LEDs is not obvious. Higher chip temperature caused by poor CS was observed and may further enlarge the efficiency droop. [ABSTRACT FROM PUBLISHER]

The performance of InGaN/GaN multiple quantum wells (MQWs) blue light-emitting diodes (LEDs) was improved by inserting a thin Mg-delta-doped hole injection layer at the end of the MQWs. The forward- and reverse-leakage currents were significantly reduced compared with those of the LEDs without the inserting layer. The light output power was enhanced by 13% at a 350 mA injection current. The improved performance could be ascribed to the dislocation suppression and hole concentration enhancement in the p-type GaN by inserting the Mg-delta-doped structure. [ABSTRACT FROM PUBLISHER]

In this letter, InGaN/GaN multiquantum well LEDs were fabricated with aluminum-doped zinc oxide (AZO) transparent conductive layer (TCL) grown by metal organic chemical vapor deposition (MOCVD) on n+-InGaN contact layer. Ultralow forward voltage ( V\!f ) of 2.86 V at 20 mA was obtained, attributing to the epitaxial-like excellent interface between AZO/n+-InGaN contact layer confirmed by high-resolution transmission electron microscopy. The most worthy is that the V\!f uniformity can be demonstrated on a 2-in wafer with a standard deviation of 0.02 V. Combined with the excellent light extraction, the MOCVD grown AZO-TCL is expected to be an alternative of tin-doped indium oxide in GaN-based LEDs for mass production. [ABSTRACT FROM AUTHOR]

We report significantly improved electrostatic discharge (ESD) properties of InGaN/GaN-based light-emitting diodes (LEDs) with inserting Si-delta-doped layers between multiple quantum wells and n-cladding layer. The ESD endurance voltage increased from -1200 V to -4000 V with the insertion of delta-doped layers. The mechanism of the enhanced ESD properties was then investigated. According to capacitance-voltage results, the factor of capacitance modulation was ruled out. However, infrared microscopy image proved better current spreading in the LEDs with delta-doped layers. In addition, current-voltage, photoluminescence, and atomic force microscope measurements demonstrated substantial quality improvements. These two reasons were considered as the dominant mechanisms of the enhanced ESD properties. [ABSTRACT FROM AUTHOR]

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure. [ABSTRACT FROM AUTHOR]

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading. [ABSTRACT FROM AUTHOR]

We report the anomalous mobility properties of Si-delta-doped GaN with periodically doping profile. Samples with different delta-doping periods or with varied Si source flow were investigated. It is found that for the short delta-doping-period (<26.5 nm) samples, the Hall mobility increases with decreasing electron concentration; while for the longer-doping-period samples, the situation is just the opposite. To interpret this observation, a two-layer model has been built up for long-period samples based on secondary ion mass spectroscopy measurements. The fitting results using this model are well consistent with the experimental data. [ABSTRACT FROM AUTHOR]

In the paper, a fast coalescence growth is introduced to the epitaxial growth of GaN on silicon substrate. With the fast coalescence growth method, a thin low pressure GaN (LP-GaN) layer used as a function layer, the GaN film could coalesce quickly within a thin thickness, additionally, a smooth surface and high crystal quality could be achieved. With further investigation, it was found that the general GaN coalescence thickness was mainly influenced by the thickness and the growth pressure of the LP-GaN interlayer. And the LP-GaN interlayer has a critical thickness, if over the critical thickness, the crystal quality would degrade. At the same time, it is found that the GaN quality was not affected by the coalescence thickness with a thin LP-GaN interlayer under critical thickness. [ABSTRACT FROM AUTHOR]

A design strategy is proposed to optimize the structure of photonic crystal (PC) slabs in order to achieve high reflection on its interface. A broad band (Δλ/λ > 19%) and ultrahigh (>95%) reflection spectrum can be realized for one-dimensional (1D) and two-dimensional (2D) PC slabs. Tolerance analysis of design parameters reveals distinct merits for different PC structures: as to maintain high reflection, 1D PC slabs show better structural tolerance, while 2D PC ones have a much greater tolerance for the material parameter. Such a difference may provide a criterion for the design of PCs-assist mirror structure in optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this work, the influence of the V/III ratios of the low temperature (LT) AlN interlayer on GaN grown on Si<111> substrate have been investigated. It was found that V/III ratio of LT-AlN interlayer is another important growth parameter, which the crystalline quality of GaN was strongly dependent on. By optimizing the V/III ratio of LT-AlN, the high quality crack-free GaN epilayer with lower dislocation density on Si substrate have been obtained. We attribute it to the different LT-AlN surface morphology originated from optimizing the V/III ratio. Different V/III ratios could result in different GaN growth rates in vertical and lateral direction, which could effectively prevent the threading dislocation from penetrating through the LT-AlN interlayer. [ABSTRACT FROM AUTHOR]

A novel vertical GaN-based light-emitting diode (LED) structure on a Si(111) substrate with through-holes was reported in this letter. The through-holes were formed by dry etching from the n-GaN layer to the Si substrate. Metals connecting the n-GaN layer and Si substrate were used to fill the holes. The series resistances induced by the AlN buffer layer and other interlayers were shorted by the metals filling the holes. Compared with those of the conventional LED structure, the series resistance and operating voltage at 20 mA were reduced from 26 to 22.5 Ω, and from 4.4 to 4.0 V, respectively. Light output intensity shows an increase of 29%. [ABSTRACT FROM AUTHOR]

Carbazole oxime ester exhibited good absorption in visible light region, all of co-initiator had absorption below 350 nm. Multi-component OXE/ION/co-initiator photoinitiator systems exhibited excellent polymerization efficiency, and double-bond conversions in this type of system were higher than those in two-component systems. Synergism between OXEs and co-initiator were experimentally studied. • Two new arylaminocarbazole oxime esters (OXE1 and OXE2) with visible absorption were successfully synthesized. • Obtaining a series of multi-component oxime esters photoinitiating systems with high performance. • Synergism between OXEs and co-initiators were experimentally studied. • The thermodynamic feasibility and the photopolymerization kinetics further proved the synergistic effect of multi-component initiators. To broaden the application of oxime ester and obtain a photoinitiating system with high performance in visible light photopolymerization, we prepared the two arylaminocarbazole oxime ester (CZ–OXE), OXE1 and OXE2, with visible absorption. The multicomponent initiation systems of OXE as one of the components were evaluated in visible photopolymerization. Synergism between OXEs and diaryliodonium salt (ION), N-vinylcarbazole (NVK), ethyl 4-dimethylamino benzoate (EDB), and triphenylphosphine (TPP) were experimentally studied via steady state photolysis and fluorescence quenching. The thermodynamic feasibility for electron transfer reaction in photoinitiating systems was confirmed by cyclic voltammetry experiments. Electron spin resonance techniques proved the production of active species in the systems. Polymerization experiments under a 405 nm light source showed that the two-component initiation system OXEs/ION was more efficient than the one-component initiator OXEs. Similarly, three-component OXEs/ION/co-initiator photoinitiating systems are more efficient than OXEs/ION systems. [ABSTRACT FROM AUTHOR]

Abstract: We report the effect of periodic Si-delta-doping on the yellow luminescence (YL) and stress evolution in n-GaN epilayers. Photoluminescence measurements indicated that the YL component could be effectively suppressed with increasing dopant flow rate or decreasing period length. X-ray diffraction, transmission electron microscopy and secondary ion mass spectroscopy measurements revealed that the variations of dislocation density and carbon concentration are not the responsible mechanisms for the of YL emissions. The reduction of Ga vacancy related defects seems to be the probable reason for the YL suppression by Si-delta-doping. Moreover, Raman spectroscopy showed no obvious stress variation among these samples with different equivalent electron concentration. These observed features are entirely different from those reported for uniformly Si-doped GaN. [Copyright &y& Elsevier]