具有三角形InGaN/GaN多量子阱的高内量子效率的蓝光LED(英文)

对InGaN量子阱LED的内量子效率进行了优化研究。分别对发光光谱、量子阱中的载流子浓度、能带分布、静电场和内量子效应进行了理论分析。对具有不同量子阱数量的InGaN/GaN LED进行了理论数值比对研究。研究结果表明,对于传统结构的LED而言,2个量子阱的结构相对于5个和7个量子阱具有更好的光学性能。同时还研究了具有三角形量子阱结构的LED,研究结果显示,三角形多量子阱结构具有较高的电致发光强度、更高的内量子效率和更好的发光效率,所有的优点都归因于较高的电子-空穴波函数重叠率和低的Stark效应所产生的较高的载流子输入效率和复合发光效率。     

The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 °C to 100 °C. The current-dependent electroluminescence (EL) spectra, current–voltage (I–V) curves and luminescence intensity–current (L–I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of ?3.7 mV/K and the emission peak wavelength increased with a slope of +0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 °C and from 22.4 to 35.6 °C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 °C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of ?0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs.     

Two dimensional electron gas in a hybrid GaN/InGaN/ZnO heterostructure with ultrathin InGaN channel layer

We investigated the influence of an ultrathin InGaN channel layer on two-dimensional electron gas (2DEG) properties in a newly proposed hybrid GaN/In_xGa_1−xN/ZnO heterostructure using numerical methods. We found that 2DEG carriers were confined at InGaN/ZnO and GaN/InGaN interfaces. Our calculations show that the probability densities of 2DEG carriers at these interfaces are highly influenced by the In mole fraction of the InGaN channel layer. Therefore, 2DEG carrier confinement can be adjustable by using the In mole fraction of the InGaN channel layer. The influence of an ultrathin InGaN channel layer on 2DEG carrier mobility is also discussed. Usage of an ultrathin InGaN channel layer with a low indium mole fraction in these heterostructures can help to reduce the short-channel effects by improvements such as providing 2DEG with higher sheet carrier density which is close to the surface and has better carrier confinement.     

Admittance spectroscopy as a method for investigating relaxation processes in quantum-sized heterostructures

The wide possibilities of admittance spectroscopy as a tool for the effective study of relaxation processes in quantum-sized heterostructures are demonstrated by the example of comprehensive investigations of InGaN/GaN semiconductor heterostructures with multiple quantum wells. The nature of emitting centers in InGaN/GaN nanoheterostructures was determined from the character of charge carrier relaxation, as revealed from the temperature spectra of conductance.     

Quantum confined carrier transition in a GaN/InGaN/GaN single quantum well bounded by AlGaN barriers

We investigated the carrier transition properties of the GaN/InGaN/GaN single quantum well bounded by AlGaN barriers. In order to confirm the carrier transition coming from the single quantum well, the single quantum well layer was etched by reactive ion etching method. The structural property of the samples was characterized by high resolution X-ray diffraction measurements. In micro-photoluminescence measurements, it is clearly shown that the donor bound exciton transition of the single quantum well sample was redshifted compared to the etched one due to strain. Moreover, a lot of peaks were observed below the GaN band gap energy due to carrier localization in the InGaN/GaN single quantum well, including carrier localization center and quantum confined states. The excitation power dependence and time resolved photoluminescence spectra were investigated to characterize the optical transition of the single quantum well.     

Influence of growth parameters on the properties of InGaN/GaN multiple quantum well grown by metalorganic chemical vapor deposition

The effects of growth parameters such as barrier growth time, growth pressure and indium flow rate on the properties of InGaN/GaN multiple quantum wells (MQWs) were investigated by using photoluminescence (PL), high resolution X-ray diffraction (HRXRD), and atomic force microscope (AFM). The InGaN/GaN MQW structures were grown on c-plane sapphire substrate by using metalorganic chemical vapor deposition. With increasing barrier growth time, the PL peak energy is blue-shifted by 18 nm. For InGaN/GaN MQW structures grown at different growth pressures, the PL intensity is maximized in the 300 Torr – grown structure, which could be attributed to the improved structural quality confirmed by HRXRD and AFM results. Also, the optical properties of InGaN/GaN MQW are strongly affected by the indium flow rate.     

GaN基激光器多量子阱垒材料的研究

在(0001)蓝宝石衬底上分别用金属有机化学气相沉积技术外延生长了InGaN/GaN, InGaN/InGaN, InGaN/AlInGaN多量子阱激光器结构, 并分别制作了脊形波导GaN基激光器。同步辐射X射线衍射,电注入受激发射光谱测试及光功率-电流(L-I)测试证明,相对于GaN垒材料,InGaN垒材料,AlInGaN四元合金垒材料更能改善多量子阱的晶体质量,提高量子阱的量子效率及降低激光器阈值电流。相关的机制为：组分调节合适的四元合金垒层中Al的掺入使得量子阱势垒高度增加,阱区收集载流子的能力增强;In的掺入能更多地补偿应力,减少了由于缺陷和位错所产生的非辐射复合中心密度;In的掺入还减小了量子阱中应力引致的压电场,电子空穴波函数空间交叠得以加强,使得辐射复合增加。     

Simulation of InGaN/GaN light-emitting diodes with a non-local quantum well transport model

Blue InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) are simulated by the APSYS software with a non-local quantum well transport model which is used to describe the phenomenon that carriers can fly over the quantum wells directly. The simulation results based on this model are in good agreement with the experiment and show its significant influence on the output power, carrier transport, peak wavelength and current crowding effect of the InGaN/GaN MQW LEDs, indicating that the non-local quantum well transport plays an important role in these devices.     

具有超晶格应力调制结构的绿光InGaN/GaN多量子阱的发光特性

研究了具有InGaN/GaN超晶格(SL)插入结构的绿光InGaN/GaN多量子阱(MQW)的发光特性。结构测试表明,SL插入结构并没有引起MQW中平均In组份的增加,而是改变了In组份的分布,形成了高In组份的量子点和低In组份量子阱。其电致发光(EL)谱和光致发光(PL)谱均出现了双发光峰。我们认为这两个 峰分别来自于量子点和量子阱,且存在着载流子从阱向点转移的输运机制。最后变温PL积分强度的Arrhenius 拟合表明,SL插入结构并没有在MQW中引入新的缺陷,使其发光效率下降。     

Carrier transport via V-shaped pits in InGaN/GaN MQW solar cells

Carrier transport via the V-shaped pits(V-pits) in InGaN/GaN multiple-quantum-well(MQW) solar cells is numerically investigated. By simulations, it is found that the V-pits can act as effective escape paths for the photo-generated carriers. Due to the thin barrier thickness and low indium composition of the MQW on V-pit sidewall, the carriers entered the sidewall QWs can easily escape and contribute to the photocurrent. This forms a parallel escape route for the carries generated in the flat quantum wells. As the barrier thickness of the flat MQW increases, more carriers would transport via the V-pits. Furthermore, it is found that the V-pits may reduce the recombination losses of carriers due to their screening effect to the dislocations. These discoveries are not only helpful for understanding the carrier transport mechanism in the InGaN/GaN MQW, but also important in design of the structure of solar cells.     

Efficiency enhancement of InGaN based blue light emitting diodes with InGaN/GaN multilayer barriers

The advantages of InGaN based light-emitting diodes with InGaN/GaN multilayer barriers are studied.It is found that the structure with InGaN/GaN multilayer barriers shows improved light output power,lower current leakage,and less efficiency droop over its conventional InGaN/GaN counterparts.Based on the numerical simulation and analysis,these improvements on the electrical and the optical characteristics are mainly attributed to the alleviation of the electrostatic field in the quantum wells(QWs) when the InGaN/GaN multilayer barriers are used.     

Droop improvement in blue InGaN light-emitting diodes with GaN/InGaN superlattice barriers

GaN/InGaN superlattice barriers are used in InGaN-based light-emitting diodes （LEDs）. The electrostatic field in the quantum wells, electron hole wavefunction overlap, carrier concentration, spontaneous emission spectrum, light-current performance curve, and internal quantum efficiency are numerically investigated using the APSYS simulation software. It is found that the structure with GaN/InGaN superlattice barriers shows improved light output power, and lower current leakage and efficiency droop. According to our numerical simulation and analysis, these improvements in the electrical and optical characteristics are mainly attributed to the alleviation of the electrostatic field in the active region.     

Parameters-dependent nonlinear absorptions in InGaN/GaN MQW and GaN film

The optical nonlinearities of an InGaN/GaN multiple quantum well (MQW) and a GaN film were experimentally investigated by using femtosecond Z-scan method in this paper. It was observed that the InGaN/GaN MQW displays a nonlinear saturable absorption (SA) effect and the nonlinear absorption coefficient β was determined to be ; and the GaN film shows a reverse saturable absorption (RSA) effect and the β is . It is also found that the absorption cross sections and quantum confinement effect (QCE) give an influence on the SA of the InGaN/GaN MQW structure.     

Evaluation of polarization field in InGaN/GaN multiple quantum well structures by using electroluminescence spectra shift

In order to investigate the inherent polarization intensity in InGaN/GaN multiple quantum well(MQW) structures,the electroluminescence(EL) spectra of three samples with different GaN barrier thicknesses of 21.3 nm, 11.4 nm, and 6.5 nm are experimentally studied. All of the EL spectra present a similar blue-shift under the low-level current injection,and then turns to a red-shift tendency when the current increases to a specific value, which is defined as the turning point.The value of this turning point differs from one another for the three InGaN/GaN MQW samples. Sample A, which has the GaN barrier thickness of 21.3 nm, shows the highest current injection level at the turning point as well as the largest value of blue-shift. It indicates that sample A has the maximum intensity of the polarization field. The red-shift of the EL spectra results from the vertical electron leakage in InGaN/GaN MQWs and the corresponding self-heating effect under the high-level current injection. As a result, it is an effective approach to evaluate the polarization field in the InGaN/GaN MQW structures by using the injection current level at the turning point and the blue-shift of the EL spectra profiles.     

InGaN/GaN多量子阱结构发光二极管发光机理转变的低频电流噪声表征

本文对InGaN/GaN多量子阱结构发光二极管开启后的电流噪声进行了测试, 结合低频电流噪声的特点和载流子之间的复合机理, 研究了低频电流噪声功率谱密度与发光二极管发光转变机理之间的关系. 结论表明, 当电流从0.1 mA到10 mA逐渐增大的过程中, InGaN/GaN发光二极管的电流噪声行为从产生-复合噪声逐渐接近于低频1/f噪声, 载流子的复合机理从非辐射复合过渡为电子与空穴之间载流子数的辐射复合, 并具有标准1/f噪声的趋势, 此时多量子阱中的电子和空穴之间的复合趋向于稳定. 本文的结论提供了一种表征InGaN/GaN多量子阱发光二极管发光机理转变的有效方法, 为进一步研究发光二极管中载流子的复合机理、优化和设计发光二极管、提高其发光量子效率提供理论依据.     

Fang–Howard wave function modelling of electron mobility in AlInGaN/AlN/InGaN/GaN double heterostructures

To study the electron transport properties in InGaN channel-based heterostructures,a revised Fang-Howard wave function is proposed by combining the effect of GaN back barrier.Various scattering mechanisms,such as dislocation impurity(DIS) scattering,polar optical phonon(POP) scattering,piezoelectric field(PE) scattering,interface roughness(IFR) scattering,deformation potential(DP) scattering,alloy disorder(ADO) scattering from InGaN channel layer,and temperature-dependent energy bandgaps are considered in the calculation model.A contrast of AlInGaN/AlN/InGaN/GaN double heterostructure(DH) to the theoretical AlInGaN/AlN/InGaN single heterostructure(SH) is made and analyzed with a full range of barrier alloy composition.The effect of channel alloy composition on InGaN channel-based DH with technologically important Al(In,Ga)N barrier is estimated and optimal indium mole fraction is 0.04 for higher mobility in DH with Al_(0.4)In_(0.07)Ga_(0.53)N barrier.Finally,the temperature-dependent two-dimensional electron gas(2 DEG) density and mobility in InGaN channel-based DH with Al_(0.83)In_(0.13)Ga_(0.0)4 N and Al_(0.4)In_(0.07)Ga_(0.53)N barrier are investigated.Our results are expected to conduce to the practical application of InGaN channel-based heterostructures.     

Surface optical phonon modes in ternary aligned crystalline InGaN/GaN multi‐quantum‐well nanopillar arrays

The optical properties of focused ion beam‐engraved, perfectly aligned and spatially controlled multi‐quantum‐well (MQW) InGaN/GaN nanopillars were investigated. Crystalline MQW nanopillars with a diameter of 30–95 nm and high aspect ratios (7:1–16:1) showed a maximum of three‐fold enhancement in emission intensity per unit active area. A light‐emitting contour map of Taiwan is drawn with a nanopillar structure to demonstrate the site control of the technique adopted in the present study. Raman scattering studies were used to characterize the newly created surfaces. Unknown peaks in GaN and InGaN nanostructures are identified for surface optical (SO) phonon modes with proper assignments of wave vectors using multiple excitations, and the SO mode for the ternary phase is reported for the first time. Copyright © 2009 John Wiley & Sons, Ltd.     

Superior material qualities and transport properties of InGaN channel heterostructure grown by pulsed metal organic chemical vapor deposition

Pulsed metal organic chemical vapor deposition is introduced into the growth of In Ga N channel heterostructure for improving material qualities and transport properties. High-resolution transmission electron microscopy imaging shows the phase separation free In Ga N channel with smooth and abrupt interface. A very high two-dimensional electron gas density of approximately 1.85 × 1013cm-2is obtained due to the superior carrier confinement. In addition, the Hall mobility reaches 967 cm2/V·s, owing to the suppression of interface roughness scattering. Furthermore, temperature-dependent Hall measurement results show that In Ga N channel heterostructure possesses a steady two-dimensional electron gas density over the tested temperature range, and has superior transport properties at elevated temperatures compared with the traditional Ga N channel heterostructure. The gratifying results imply that In Ga N channel heterostructure grown by pulsed metal organic chemical vapor deposition is a promising candidate for microwave power devices.     

Simulation of InGaN/GaN multiple quantum well light-emitting diodes with quantum dot model for electrical and optical effects

We report a 2D simulation of electrical and optical characteristics of green color InGaN/GaN multiple quantum well light-emitting diodes by APSYS software with a dot-in-well model. The In-rich quantum dot-like structure in InGaN/GaN multiple quantum wells has been considered in the LED experimental data analysis. Simulation results based on the quantum dot model are in better agreement with the experimental data than those based on the purely quantum well model, indicating that the quantum dot spontaneous emission and the non-equilibrium quantum transport play important roles in the InGaN/GaN multiple quantum well light-emitting diodes.     

Improved carrier injection and confinement in InGaN light-emitting diodes containing GaN/AlGaN/GaN triangular barriers

In this study, an InGaN lighting-emitting diode(LED) containing GaN/AlGaN/GaN triangular barriers is proposed and investigated numerically. The simulation results of output performance, carrier concentration, and radiative recombination rate indicate that the proposed LED has a higher output power and an internal quantum efficiency, and a lower efficiency droop than the LED containing conventional GaN or AlGaN barriers. These improvements mainly arise from the modified energy bands, which is evidenced by analyzing the LED energy band diagram and electrostatic field near the active region.The modified energy bands effectively improve carrier injection and confinement, which significantly reduces electron leakage and increases the rate of radiative recombination in the quantum wells.