利用山形空穴存储层结构对深紫外激光二极管的性能优化

为了有效提高深紫外激光二极管的空穴注入效率和减少电子泄露,优化其性能,设计出了在基础矩形空穴存储层结构上改进后的山形空穴存储层和倒山形空穴存储层。使用Crosslight软件模拟仿真倒山形和山形空穴存储层结构的电子浓度、电子电流密度、能带图以及P-I特性曲线。结果显示山形空穴存储层激光器的光学和电学性能优于矩形和倒山形激光器,因此山形空穴存储层激光器能有效地增加有源区空穴注入和减少电子泄露,提高有源区载流子浓度和辐射复合速率,实现了激光器优越的光电性能。     

Optimization towards reduction of efficiency droop in blue GaN/InGaN based light emitting diodes

Light emitting diodes (LEDs) based on GaN/InGaN material suffer from efficiency droop at high current injection levels. We propose multiple quantum well (MQW) GaN/InGaN LEDs by optimizing the barrier thickness and high–low–high indium composition to reduce the efficiency droop. The simulation results reflect a significant improvement in the efficiency droop by using barrier width of 10 nm and high–low–high indium composition in MQW LED.     

The enhanced performance of organic light-emitting diodes by using PCBM as the anode modification layer

[6,6]-Phenyl C₆₁ butyric acid methyl ester (PCBM) is used to modify an indium tin oxide (ITO)-coated substrate. Organic light-emitting diodes (OLEDs) using PCBM as the anode modification layer are fabricated. The dependence of performance on different PCBM thicknesses is also investigated. When the thickness of the PCBM film is appropriate, the brightness and efficiency of OLEDs are enhanced, which is attributed to an enhanced hole injection and an improved carrier balance. The enhancement of hole injection was ascribed to the formation of a dipole layer at the anode/organic interface.     

Performance characteristics of deep violet InGaN DQW laser diodes with InGaN/GaN superlattice waveguide layers

The effect of the indium (In) composition of In_xGa_1−xN (GaN) waveguide layers on the performance of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) emitting at 390 nm output emission wavelength has been numerically investigated. Simulation results indicated that by increasing In composition of the In_xGa_1−xN waveguide layers, the threshold current decreases, the slope efficiency, and differential quantum efficiency (DQE) increase, whereas the output power decreases. The increase in the In composition of the InGaN waveguide layers increases the refractive index and consequently increases the optical confinement factor (OCF) which result in the increase in the slope efficiency and DQE and the decrease in the threshold current. The decreasing movement of electron and hole carriers from the bulk waveguide layers to the active regions also causes to decrease the output power. A new LD structure with InGaN/GaN superlattice (SL) waveguide layers has been proposed to exploit the increased OCF of InGaN waveguide structures, and the enhanced electron and hole mobilities and the tunneling effect of the periodic structure of the SL structures. The results also showed that the use of InGaN/GaN SL waveguide structures effectively improves the output power, slope efficiency and DQE and decreases the threshold current of the LD compared with (In)GaN bulk waveguide structure.     

MoO_3作空穴注入层的有机电致发光器件(英文)

研究了三氧化钼(MoO3)薄层作为有机电致发光器件空穴注入层的器件性能和注入机制。发现1nm厚度下发光器件性能最佳,器件的最大电流效率比对比发光器件的最大电流效率提高1.6倍。器件的电容曲线表明MoO3薄层能有效提高空穴载流子的注入,多数载流子开始注入的拐点大约降低了9V。单空穴载流子电流曲线说明MoO3器件的电流注入是空间电荷受限电流注入机制,MoO3使阳极界面处形成欧姆接触,而对比器件的电流注入是陷阱电荷受限电流注入机制。器件的光伏曲线进一步说明器件性能的提高是由于MoO3层能使阳极界面能级分布发生改变,1nmMoO3厚度下器件的内建电势从对比器件的0.25V提高到了0.8V,有效降低了空穴注入势垒,提高了器件性能,但过厚的MoO3层由于增加了器件的串联内阻,会导致器件性能降低。     

Investigation of InGaN green light-emitting diodes with chirped multiple quantum well structures

The effect of using chirped multiple quantum-well (MQW) structures in InGaN green light-emitting diodes (LEDs) is numerically investigated. An active structure, which is with both thick QWs with low indium composition on the p-side and thin QWs with high indium composition next to the n-region, is presented in this study. The thickness and indium composition in each single QW is specifically tuned to emit the same green emission spectrum. Comparing with conventional active structure design of green LEDs, which is using uniform MQWs, the output power is increased by 27% at 20 mA, and by 15% at 100 mA current injections. This improvement is mainly attributed to the enhanced efficiency of carrier injection into QWs and the improved capability of carrier transport.     

Simulation study of blue InGaN multiple quantum well light-emitting diodes with different hole injection layers

InGaN-based light-emitting diodes with p-GaN and p-AlGaN hole injection layers are numerically studied using the APSYS simulation software.The simulation results indicate that light-emitting diodes with p-AlGaN hole injection layers show superior optical and electrical performance,such as an increase in light output power,a reduction in current leakage and alleviation of efficiency droop.These improvements can be attributed to the p-AlGaN serving as hole injection layers,which can alleviate the band bending induced by the polarization field,thereby improving both the hole injection efficiency and the electron blocking efficiency.     

Towards focusing using photonic crystal flat lens

We report on the numerical simulation and fabrication of a two-dimensional flat lens based on negative refraction in photonic crystals. The slab acting as a lens is made of an hole array (operating at the wavelength of 1.5 μm) etched in a InP/InGaAsP/InP semiconductor layer. We first study the key issues for the achievement of a negative refractive index taking advantage of folding of dispersion branches with main emphasis in dispersion properties rather than the opening of forbidden gaps. The diffraction and refraction regimes are analysed according to the comparison of the wave-vector with respect to the relevant dimensions of the hole array. In the second stage, we illustrate technological challenges in terms of e-beam lithography on a sub-micron scale and deep reactive ion etching for an indium phosphide based technology.     

Improved efficiency droop characteristics in an InGaN/GaN light-emitting diode with a novel designed last barrier structure

<正>In this study,the characteristics of nitride-based light-emitting diodes with different last barrier structures are analysed numerically.The energy band diagrams,electrostatic field near the last quantum barrier,carrier concentration in the quantum well,internal quantum efficiency,and light output power are systematically investigated.The simulation results show that the efficiency droop is markedly improved and the output power is greatly enhanced when the conventional GaN last barrier is replaced by an AlGaN barrier with Al composition graded linearly from 0 to 15% in the growth direction.These improvements are attributed to enhanced efficiencies of electron confinement and hole injection caused by the lower polarization effect at the last-barrier/electron blocking layer interface when the graded Al composition last barrier is used.     

Two-layer light emitting diodes prepared by the sol–gel route

We have elaborated organic–inorganic hybrid light-emitting diodes (HLED). These devices emitting in the green are formed of two hybrid thin layers, exhibiting different functionalities, which are sandwiched between indium–tin oxide (ITO) and metallic electrodes. These layers have been prepared from silane precursors modified with hole transporting units and light-emitting naphthalimide moieties by the sol–gel technique. The hole transporting sol–gel layers exhibit about the same charge mobility as organic polymers having equivalent active units. The maximum external quantum efficiency of the best diode using LiF/Al cathode is about 1% and the luminance reaches 4000 cd · m ⁻² .     

Organic light-emitting diode performance enhancement via indium tin oxide glass surface modification

A self-assembly monolayer (SAM), with phenyl-triethoxysilane (PTES), was used to modify the indium tin oxide anode for an organic light-emitting diode (OLED) in order to improve the OLED performance. The enhancement of OLED performance is attributed to the blockage of excessive hole injection by PTES, balancing the hole and electron injection numbers. The result indicates that the SAM process reaction time duration greatly affects the OLED performance outcomes. If the reaction time is too long, it will impact on the optical efficiency due to molecular aggregates accumulated on the SAM layers, thus reducing the performance of the OLEDs. The electrical and optical characteristics of the OLEDs are modeled by using the modified Shockley equation. Modified Shockley parameters are extracted to interpret the experimental data with excellent accuracy. Those parameters, both electrical and optical, can be used as the DC level modeling parameters for OLED product design simulations.     

Electrical and optical characteristics of GaN-based light-emitting diodes fabricated with emission wavelengths of 429-467 nm

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with emission wavelengths of 429-467 nm were investigated. The optical output increased with increasing emission wavelength, which is attributed to the enhanced quantum confinement effect as a result of indium composition fluctuation. With higher indium content, the LEDs exhibited unfavorable performance, including a larger efficiency droop, spectral blueshift, and spectral broadening, due to indium-induced strains. The effect of heterointerfaces associated with the indium content of the active region on the device resistance was negligible.     

Increased and balanced light emission of transparent organic light-emitting diodes by enhanced microcavity effects

We report on improved and controlled light outcoupling of transparent organic light-emitting diodes (TOLEDs) by inserting thin silver layers between the indium tin oxide anode and the hole transporting layer. The introduction of Ag layers influences both the bottom and top emission of the TOLEDs, and it results in dramatic changes in the electroluminescence spectra and angular distribution. We find that the overall external quantum efficiency can be increased up to 18.8%, and the ratio of bottom and top emission can be almost identical.     

Photoemission cross section:A critical parameter in the impurity photovoltaic effect

A numerical study has been conducted to explore the role of photoemission cross sections in the impurity photovoltaic(IPV) effect for silicon solar cells doped with indium. The photovoltaic parameters(short-circuit current density, opencircuit voltage, and conversion efficiency) of the IPV solar cell were calculated as functions of variable electron and hole photoemission cross sections. The presented results show that the electron and hole photoemission cross sections play critical roles in the IPV effect. When the electron photoemission cross section is 10-20cm~2, the conversion efficiencyη of the IPV cell always has a negative gain(?η 0) if the IPV impurity is introduced. A large hole photoemission cross section can adversely impact IPV solar cell performance. The combination of a small hole photoemission cross section and a large electron photoemission cross section can achieve higher conversion efficiency for the IPV solar cell since a large electron photoemission cross section can enhance the necessary electron transition from the impurity level to the conduction band and a small hole photoemission cross section can reduce the needless sub-bandgap absorption. It is concluded that those impurities with small(large) hole photoemission cross section and large(small) electron photoemission cross section,whose energy levels are near the valence(or conduction) band edge, may be suitable for use in IPV solar cells. These results may help in judging whether or not an impurity is appropriate for use in IPV solar cells according to its electron and hole photoemission cross sections.     

On the improvement of the electroluminescent signal of organic light emitting diodes by the presence of an ultra‐thin metal layer at the interface organic/ITO

The electroluminescent (EL) signal of organic light emitting diodes (OLEDs) based on simple “hole transporting layer/electron transporting layer” (HTL/ETL) structures has been studied as a function of the anode/HTL interface, the anode being an indium tin oxide (ITO) film. It is shown that the electroluminescent (EL) signal increases when a metal ultra‐thin layer is introduced between the anode and the HTL. Experimental results show that the work function value of the metal is only one of the factors which allow improving the EL signal via better hole injection efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth dynamics and optimization of Ga(In)AsN/GaAs towards 1.3 μm and 1.55 μm

It is demonstrated that a nitrogen supply, manifested as both a direct nitrogen beam from a rf-activated plasma source and dispersive nitrogen radicals from the growth-chamber background, is appropriate for the growth of GaAs-based nitrides. The nitrogen incorporation efficiency shows a linear relationship with increasing rf power when the nitrogen is supplied dispersively, and a saturation behavior when the nitrogen is supplied as a direct nitrogen beam. The indium composition causes a decrease in the nitrogen incorporation efficiency, which is suspected to be brought about by the increase of the growth rate due to the indium flux, because our experiments have proven that the nitrogen composition of GaAsN epilayers apparently decreases as the growth rate increases. It is more controllable to fix the indium flux first than to fix the nitrogen source condition, so as to facilitate wavelength control. The temperature window for the growth of quality nitride is concluded to be between 440 °C and 500 °C. For a quantum well with a certain well width, it is better to add more indium and less nitrogen to reach the desired wavelength in order that the quality of the InGaAs material is kept acceptable. PACS 81.15.Hi; 81.05.Ea; 42.70.-a     

Performance improvement of blue InGaN light-emitting diodes with a specially designed n-AlGaN hole blocking layer

Blue InGaN light-emitting diodes (LEDs) with a conventional electron blocking layer (EBL), a common n-AlGaN hole blocking layer (HBL), and an n-AlGaN HBL with gradual Al composition are investigated numerically, which involves analyses of the carrier concentration in the active region, energy band diagram, electrostatic field, and internal quantum efficiency (IQE). The results indicate that LEDs with an n-AlGaN HBL with gradual Al composition exhibit better hole injection efficiency, lower electron leakage, and a smaller electrostatic field in the active region than LEDs with a conventional p-AlGaN EBL or a common n-AlGaN HBL. Meanwhile, the efficiency droop is alleviated when an n-AlGaN HBL with gradual Al composition is used.     

Numerical study of optical properties of InGaN multi-quantum-well laser diodes with polarization-matched AlInGaN barrier layers

The optical properties of InGaN multi-quantum-well laser diodes with different polarization-matched AlInGaN barrier layers have been investigated numerically by employing an advanced device simulation program. The use of quaternary polarization-matched AlInGaN barrier layers enhances the electron–hole wave function overlap due to the compensation of polarization charges between InGaN quantum well and AlInGaN barrier layer. According to the simulation results, it is found that, among the polarization-matched quantum-well structures under study, lower threshold current and higher slope efficiency can be achieved simultaneously when the aluminum composition in AlInGaN barrier layers is about 10–15%. The optimal polarization-matched InGaN/AlInGaN laser diode shows lower threshold current and higher slope efficiency compared to conventional InGaN/InGaN laser diodes.     

The influence of AlGaN/GaN superlattices as electron blocking layers on the performance of blue InGaN light-emitting diodes

P-AlGaN/P-GaN superlattices are investigated in blue InGaN light-emitting diodes as electron blocking layers.The simulation results show that efficiency droop is markedly improved due to two reasons:(i) enhanced hole concentration and hole carrier transport efficiency in AlGaN/GaN superlattices,and(ii) enhanced blocking of electron overflow between multiple quantum-wells and AlGaN/GaN superlattices.     

Numerical study of performance characteristics of deep violet InGaN DQW laser diodes with AlInGaN quaternary multi quantum barrier electron blocking layer

The performance characteristics of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) with different electron blocking layer (EBL) including a ternary AlGaN bulk EBL, a quaternary AlInGaN bulk EBL and ternary AlGaN multi quantum barrier (MQB) EBL has been numerically investigated. Inspired by the abovementioned structures, a new LD structure with a quaternary AlInGaN MQB EBL has been proposed to improve the performance characteristics of the deep violet InGaN DQW LDs. Simulation results indicated that the LD structure with the quaternary AlInGaN MQB EBL present the highest output power, slope efficiency and differential quantum efficiency (DQE) and lowest threshold current compared with the above mentioned structures. They also indicated that choosing an appropriate aluminum (Al) and indium (In) composition in the quaternary AlInGaN MQB layers could control both piezoelectric and spontaneous polarizations. It will decrease the electron overflow from the active region to p-side and increased the contribution of electron and hole carriers to the radiative recombination effectively. Enhancing radiative recombination in the well using the quaternary AlInGaN MQB EBL also increased the optical output power and optical intensity.