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Sapphire substrates were patterned by a chemical wet etching technique in the micro- and nanoscale to enhance the light output power of InGaN/GaN light-emitting diodes (LEDs). InGaN/GaN LEDs on a pyramidal patterned sapphire substrate in the microscale (MPSS) and pyramidal patterned sapphire substrate in the nanoscale (NPSS) were grown by metalorganic chemical vapor deposition. The characteristics of the LEDs fabricated on the MPSS and NPSS prepared by wet etching were studied and the light output powers of the LEDs fabricated on the MPSS and NPSS increased compared with that of the conventional LEDs fabricated on planar sapphire substrates. In comparison with the planar sapphire substrate, an enhancement in output power of about 29% and 48% is achieved with the MPSS and NPSS at an injection current of 20 mA, respectively. This significant enhancement is attributable to the improvement of the epitaxial quality of GaN-based epilayers and the improvement of the light extraction efficiency by patterned sapphire substrates. Additionally, the NPSS is more effective to enhance the light output power than the MPSS. [ABSTRACT FROM AUTHOR]

InGaN light-emitting diodes (LEDs) with InGaN-GaN quantum barriers of gradually decreasing indium concentration along the growth direction are proposed to be used. In the proposed structure, simulation results show that the holes are widely distributed in the active region due to the larger hole injection/transportation efficiency by reducing the effective potential height for holes and the radiative recombination rate is efficiently improved, which leads to the significant enhancement of light-output power and internal quantum efficiency (IQE) over the conventional LED with GaN barriers. Moreover, the optical performance is further improved when the graded InGaN/GaN supperlattice (SL) is introduced between the multiple quantum wells (MQWs) and p-AlGaN electron blocking layer (EBL) in our designed LED structure, permitting more freedom for the optimization. [ABSTRACT FROM AUTHOR]

InGaN light-emitting diodes (LEDs) with the alternating quantum barriers (AQB) of AlGaN and InGaN is proposed for LED applications. With this design, simulation results show that the carrier concentration and transport in the multi-quantum well (MQW) active region are improved and accordingly, the radiative recombination rate is enhanced and the electron leakage is suppressed, due to the appropriate band engineering. Thus, the proposed structure shows an efficient improvement in the internal quantum efficiency (IQE) and efficiency droop, compared to the original structures with GaN barriers, or AlGaN barriers, or InGaN barriers. Our studies suggest that relatively simple engineering of the MQW region may still have notable positive effects on LED performance. [ABSTRACT FROM PUBLISHER]

In InGaN-based light-emitting diodes (LEDs), some specific designs on the quantum barrier layers by alternating AlGaN barriers with GaN barriers are proposed. Compared with the original structures with GaN barriers or AlGaN barriers, the proposed structure shows significant improvement in internal quantum efficiency (IQE) and efficiency droop, due to the better hole injection/transportation and electron confinement of the multiquantum well active region, and accordingly suppressed electron leakage, which provides us a simple but highly effective approach for high-performance LED applications. Moreover, the simulation suggests that the IQE and power can be further enhanced without efficiency droop when our designed structure is with increasing Al composition because of the uniformly dispersed carriers, permitting more freedom for the optimization. [ABSTRACT FROM PUBLISHER]

Nitride-based light-emitting diodes (LEDs) have been developed significantly in terms of both fundamental understanding as well as application, but there are still many new challenges, such as the droop effects and reliability issues, which hinder further progress. SIMS is an important technique and has been widely used to monitor the doping concentrations in the LED multilayers and characterize the layer interface qualities. However, most of them are performed by magnetic sector-based instruments. In this study, we measured layer structures for nitride-based LED epilayers using time-of-flight SIMS. By changing different surface qualities, the primary ion raster sizes, the oxygen partial pressures, and the optimized conditions to achieve better LED depth profiles were obtained, under which the super lattice structures of AlGaN/GaN and the modulation of Mg atom concentrations can also be clearly distinguished. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The advantages of an InGaN light-emitting diode (LED) structure with Mg–Si-codoped barriers are studied numerically in this paper. Energy band diagrams, carrier concentrations in the active region, radiative recombination rates in the quantum wells (QWs), light-current performances, and internal quantum efficiency (IQE) are investigated. Simulation results suggest that by employing the Mg–Si-codoped barriers, the light-output power and IQE are more significantly improved than LEDs with step-Si-doped barriers when compared with the conventional LEDs with wholly Si-doped barriers, due to modified band diagrams, which are favorable for improvement of hole injection efficiency. Moreover, simulation also indicates that the optical performance can be further improved if the Si doping concentration of quantum barriers (QBs) is decreased gradually along the growth direction for the LED with Mg–Si-codoped barriers. [Copyright &y& Elsevier]

The advantages of InGaN-based light-emitting diodes (LEDs) with InGaN/GaN superlattice (SL) and graded-composition InGaN/GaN SL interlayers in the last-barrier/EBL space are studied numerically. The electrostatic field, energy band diagrams, carrier concentrations, radiative recombination rate, light-current-voltage (L-I-V) performances, and internal quantum efficiency (IQE) are investigated. Simulation results show that the light-output power and IQE are both enhanced compared to the conventional LED due to the appropriately modified energy band diagrams, which are favorable for the improvement in hole injection efficiency and electron blocking capability, especially for the LED with graded-composition InGaN/GaN SL interlayer. [ABSTRACT FROM AUTHOR]

This paper is focused on the temperature-dependence of photoluminescence (PL) internal quantum efficiency (IQE) and defect-recombination in InGaN light-emitting diodes. It is found low-temperature PL-IQE is remarkably lower than 100% for some samples, suggesting the existence of residual defect-recombination under low temperature. Theoretical analysis deduces that defect-recombination cannot be frozen out at low temperature, because it follows T1/2 temperature-dependent defect-capture coefficient without defect-activation energy. The empirical formulas describing temperature-dependence of defect-recombination lifetime and PL-IQE are correspondingly revised. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

InGaN/GaN multi-quantum-well blue (461±4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 °C in O2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg–H complex by forming H2O, annealing in O2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated MgGa–ON complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen. [ABSTRACT FROM AUTHOR]

We report a low-cost and high-throughput process for the fabrication of two-dimensional SiO2 photonic crystal (PhC) by nanospherical-lens photolithography method to improve the light extraction of GaN-based light-emitting diodes (LEDs). The PhC structures were realized by the selective area growth of p-GaN using SiO2 nanodisks, which were patterned utilizing a self-assembled nanosphere as an optical lens. Without prejudice to the electrical properties of LEDs, the light output power (at 350 mA) of LEDs with the SiO2 and corresponding air-hole PhC was enhanced by 71.3% and 49.3%, respectively, compared to that without PhC. The LEDs with selectively grown PhC structures were found to exhibit partial compression strain release and reduced emission divergence. The finite-difference time-domain simulation was also performed to further reveal the emission characteristics of PhC LEDs. [ABSTRACT FROM AUTHOR]

Through designing the quantum well (QW) structure by introducing the InGaN interlayers into the GaN barriers, the compressive strain magnitude is significantly decreased. Two distinct emission peaks corresponding to In-rich localized state and quantum well ground state emissions observed from the electroluminescence (EL) spectra demonstrate that the phase separation is enhanced to form In-rich localized states in the original structure with larger strain, but no significant phase separation is observed in the designed structures with less strain. This abnormal phenomenon is contrary to the previous reports that phase separation in the InGaN layer can be suppressed by compressive strain. Therefore, it is suggested that the strain-driven kinetic phase separation should be taken into account in our case to more fully and precisely understand the role played by strain on phase separation that strain can hinder the phase separation thermodynamically but may drive phase separation kinetically. [ABSTRACT FROM AUTHOR]

Abstract: Effects of doping molybdenum oxide (MoO3) in copper phthalocyanine (CuPc) as hole injection layer in OLEDs are studied. A green OLED with structure of ITO/MoO3-doped CuPc/NPB/10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1)-benzopyropyrano(6,7,8-i,j) quinolizin-11-one (C545T): tris(8-hydroxyquinoline) aluminum (Alq3)/Alq3/LiF/Al shows the driving voltage of 4.4V, and power efficiency of 4.3lm/W at luminance of 100cd/m2. The charge transfer complex between CuPc and MoO3 plays a decisive role in improving the performance of OLEDs. The AFM characterization shows that the doped film owns a better smooth surface, which is also in good agreement with the electrical performance of OLEDs. [Copyright &y& Elsevier]

Abstract: We report a hybrid up-conversion device integrating a In0.2Ga0.8As/GaAs MQWs photodetector with an organic light emitting diode (OLED), that converts input 980nm infrared light to output 520nm green light. Devices with different interface layer, used as the hole injection layer (HIL) in OLEDs were fabricated and tested. It was found that the device with an HIL of MoO3-doped CuPc exhibited a lowest turn-on voltage of 2.6V. The maximum external up conversion efficiency of 0.81W/W% is achieved at the bias of 20V under an input 980nm NIR power density of 1mW/mm2. [Copyright &y& Elsevier]

Abstract: Sapphire substrates patterned by a selective chemical wet and an inductively coupled plasma (ICP) etching technique was proposed to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs were fabricated on sapphire substrates through metal organic chemical vapor deposition (MOCVD). The LEDs fabricated on the patterned substrates exhibit improved device performance compared with the conventional LED fabricated on planar substrates when growth and device fabricating conditions were the same. The light output powers of the LEDs fabricated on wet-patterned and ICP-patterned substrates were about 37% and 17% higher than that of LEDs on planar substrates at an injection current of 20mA, respectively. The enhancement is attributable to the combination of the improvement of GaN-based epilayers quality and the improvement of the light extraction efficiency. [Copyright &y& Elsevier]

Abstract: The tandem organic light-emitting diodes (OLEDs) with an effective charge-generation connection structure of Mg-doped tris(8-hydroxyquinoline) aluminum (Alq3)/Molybdenum oxide (MoO3)-doped 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) were presented. At a current density of 50 mA/cm2, the current efficiency of the tandem OLED with two standard NPB/Alq3 emitting units is 4.2 cd/A, which is 1.7 times greater than that of the single EL device. The tandem OLED with the similar connection structure of Mg-doped PTCDA/ MoO3-doped PTCDA was also fabricated and the influences of the different connection units on the current efficiency of the tandem OLED were discussed as well. [ABSTRACT FROM AUTHOR]

InGaN/GaN multi-quantum-well blue (461±4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 °C in O2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg–H complex by forming H2O, annealing in O2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated MgGa–ON complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen. [ABSTRACT FROM AUTHOR]