Anomalous disorder-related phenomena in InGaN/GaN multiple quantum well heterosystems

The influences of InGaN/GaN multiple quantum well (MQW) heterostructures with InGaN/GaN and GaN barriers on carrier confinement were investigated. The degree of disordering over a broad range of temperatures from 20 to 300 K was considered. The optical and electrical properties were strongly influenced by structural and compositional disordering of the InGaN/GaN MQW heterostructures. To compare the degree of disordering we examined the temperature dependence of the luminescence spectra and electrical conductance contingent on the Berthelot-type mechanisms in the InGaN/GaN MQW heterostructures. We further considered carrier transport in the InGaN/GaN disordered systems, probability of carrier tunneling, and activation energy of the transport mechanism for devices with InGaN/GaN and GaN barriers. The optical properties of InGaN/GaN disordered heterosystems can be interpreted from the features of the absorption spectra. The anomalous temperature-dependent characteristics of the disordered InGaN/GaN MQW structures were attributable to the enhancement of the exciton confinement.     

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths

Four blue-violet light emitting InGaN/GaN multiple quantum well(MQW) structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.     

硅衬底InGaN/GaN基蓝光发光二极管droop效应的研究

InGaN/GaN基阱垒结构LED当注入的电流密度较大时, LED的量子效率随注入电流密度增大而下降, 即droop效应.本文在Si (111)衬底上生长了 InGaN/GaN 基蓝光多量子阱结构的LED,通过将实验测量的光电性能曲线与利用ABC模型模拟的结果进行对比, 探讨了droop效应的成因.结果显示:温度下降会阻碍电流扩展和降低空穴浓度, 电子在阱中分布会越来越不平衡,阱中局部区域中因填充了势能越来越高的电子而溢出阱外, 从而使droop效应随着温度的降低在更小的电流密度下出现且更为严重, 不同温度下实验值与俄歇复合模型模拟的结果在高注入时趋势相反.这此结果表明,引起 droop效应的主因不是俄歇非辐射复合而是电子溢出,电子溢出的本质原因是载流子在阱中分布不均衡.     

垒层厚度对InGaN/GaN多量子阱电注入发光性能的影响及机理

研究了不同垒厚对InGaN/GaN多量子阱电注入发光性能的影响及机理。实验发现,当GaN垒层的厚度从6 nm增大到24 nm时,垒厚的样品发光强度更强,而且当注入电流增加时,适当增加垒厚,可以更显著增加发光强度。进一步结合发光峰位和光谱宽度的研究表明,由于应力和极化效应的存在,当垒层厚度在6~24 nm范围内时,适当增加垒层厚度不仅会使得能带的倾斜加剧,减少电子泄露,而且也会增加InGaN阱层的局域态深度,从而改善量子阱的发光性能。     

Excitonic optical absorption in wurtzite InGaN/GaN quantum wells

Within the framework of the effective-mass and envelope function theory, exciton states and optical properties in wurtzite (WZ) InGaN/GaN quantum wells (QWs) are investigated theoretically considering the built-in electric field effects. Numerical results show that the built-in electric field, well width and in composition have obvious influences on exciton states and optical properties in WZ InGaN/GaN QWs. The built-in electric field caused by polarizations leads to a remarkable reduction of the ground-state exciton binding energy, the interband transition energy and the integrated absorption probability in WZ InGaN/GaN QWs with any well width and In composition. In particular, the integrated absorption probability is zero in WZ InGaN/GaN QWs with any In composition and well width L > 4 nm. In addition, the competition effects between quantum confinement and the built-in electric field (between quantum size and the built-in electric field) on exciton states and optical properties have also been investigated.     

InGaN/GaN laser diode characterization and quantum well number effect

The effect of quantum well number on the quantum efficiency and temperature characteristics of In- GaN/GaN laser diodes （LDs） is determined and investigated. The 3-nm-thick In0.13Ca0.87N wells and two 6-am-thick GaN barriers are selected as an active region for Fabry-Perot （FP） cavity waveguide edge emitting LD. The internal quantum efficiency and internal optical loss coefficient are extracted through the simulation software for single, double, and triple InGaN/GaN quantum wells. The effects of device temperature on the laser threshold current, external differential quantum efficiency （DQE）, and output wavelength are also investigated. The external quantum efficiency and characteristic temperature are improved significantly when the quantum well number is two. It is indicated that the laser structures with many quantum wells will suffer from the inhomogeneity of the carrier density within the quantum well itself which affects the LD performance.     

Enhancement of light-emission efficiency of ultraviolet InGaN/GaN multiple quantum well light emitting diode with InGaN underlying layer

Two ultraviolet InGaN/GaN light emitting diodes (LEDs) with and without InGaN underlying layer beneath the multiple quantum wells (MQWs) were grown by metal-organic vapor phase epitaxy. Based on the photoluminescence excitation measurements, it was found that the Stokes shift of the sample with a 10-nm-thick In_0.1Ga_0.9N underlying layer was about 64 meV, which was smaller than that of the reference sample without InGaN underlying layer, indicating a reduced quantum-confined Stark effect (QCSE) due to the decrease of the piezoelectric polarization field in the MQWs. In addition, by fitting the photon energy dependence of carrier lifetime values, the radiative recombination lifetime of the sample with and without InGaN underlying layer were obtained about 1.22 and 1.58 ns at 10?K, respectively. The shorter carrier lifetime also confirmed that the QCSE in the MQWs was weakened after inserting the InGaN underlying layer. In addition, although the depth of carrier localization in the sample with InGaN underlying layer became smaller, the nonradiative recombination centers (NRCs) inside it decreased, and thus suppressed the nonradiative recombination process significantly according to the electroluminescence measurement results. Compared to the reference sample, the efficiency droop behavior was delayed in the sample with InGaN underlying layer and the droop effect was also effectively alleviated. Therefore, the enhanced light-emission efficiency of ultraviolet InGaN/GaN MQW LEDs could be attributed to the decrease of QCSE and NRCs.     

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。     

引入n型InGaN/GaN超晶格层提高量子阱特性研究

利用金属有机物化学气相淀积技术在蓝宝石衬底上生长了InGaN/GaN量子阱结构. 研究了引入n型InGaN薄层或InGaN/GaN超晶格层的量子阱特性,结果表明通过引入n型InGaN薄层或InGaN/GaN超晶格层缓解了量子阱有源区中的应力,改善了多量子阱表面形貌,减少了V型缺陷密度,而且提高了多量子阱的光致发光强度,从而也改进了LED的发光效率. 关键词： InGaN/GaN多量子阱 原子力显微镜 X射线双晶衍射 光致发光     

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.     

Reversible Carriers Tunnelling in Asymmetric Coupled InGaN/GaN Quantum Wells

Temperature-dependent photoluminescence （PL） and time resolved photoluminescence （TRPL） are performed to study the PL characteristics and carrier transfer mechanism in asymmetric coupled InGaN/GaN multiple quantum wells （AS-QWs）. Our results reveal that abnormal carrier tunnelling from the wide quantum well （WQW） to the narrow quantum well （NQW） is observed at temperature higher than about lOOK, while a normal carrier tunnelling from the NQW to the WQW is observed at temperature lower than 100 K. The reversible carrier tunnelling between the two Q Ws makes it possible to explore new types of temperature sensitive emission devices. It is shown that PL internal quantum efficiency （IQE） of the NQW is enhanced to about 46% due to the assistant of the abnormal carrier tunnelling.     

蓝光激光器结构中InGaN/GaN多量子阱界面效应的精细光致发光光谱研究

金属有机化学气相沉积(MOCVD)方法制备InGaN/GaN多量子阱结构时,在GaN势垒层生长的N2载气中引入适量H2,能够有效改善阱/垒界面质量从而提升发光效率.本工作利用光致发光(PL)光谱技术,对蓝光激光器结构中的InGaN/GaN多量子阱的发光性能进行了精细的光谱学测量与表征,研究了通H2生长对量子阱界面的调控...     

Efficiency droop alleviation in blue light emitting diodes using the InGaN/GaN triangular-shaped quantum well

The InGaN/GaN blue light emitting diode(LED) is numerically investigated using a triangular-shaped quantum well model,which involves analysis on its energy band,carrier concentration,overlap of electron and hole wave functions,radiative recombination rate,and internal quantum efficiency.The simulation results reveal that the InGaN/GaN blue light emitting diode with triangular quantum wells exhibits a higher radiative recombination rate than the conventional light emitting diode with rectangular quantum wells due to the enhanced overlap of electron and hole wave functions(above 90%) under the polarization field.Consequently,the efficiency droop is only 18% in the light emitting diode with triangular-shaped quantum wells,which is three times lower than that in a conventional LED.     

Exciton states in zinc-blende InGaN/GaN quantum dot

Within the framework of effective-mass approximation, exciton states confined in zinc-blende(ZB) InGaN/GaN quantum dot(QD) are investigated by means of a variational approach, considering finite band offsets. The ground-state exciton binding energy and the interband emission energy are investigated as functions of QD structural parameters in detail. Numerical results show clearly that both the QD size and In content of InGaN have a significant influence on the exciton states and interband optical transitions in the ZB InGaN/GaN QD.     

Growth and characterization of InGaN nanodots hybrid with InGaN/GaN quantum wells

Uniform InGaN nanodots were successfully grown on SiO₂ pretreated GaN surface. It was found that the InGaN nanodots were 20?nm in diameter and 5?nm in height, approximately. After the growth of two periods of InGaN/GaN quantum wells on the surface of InGaN nanodots, nanodot structure still formed in the InGaN well layer caused by the enhanced phase separation phenomenon. Dual-color emissions with different behavior were observed from photoluminescence (PL) spectrum of InGaN nanodots hybrid with InGaN/GaN quantum wells. A significant blueshift and a linewidth broadening were measured for the low-energy peak as the increase of PL excitation power, while a slight blueshift and a linewidth narrowing occurred for the high-energy peak. Accordingly, these two peaks were assigned to be from the In-rich nanodots and quantized state transition from the InGaN/GaN quantum wells with indium content, respectively.     

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。     

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.     

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.     

Electric field effects on optical properties in zinc-blende InGaN/GaN quantum dot

The effect of electric field on exciton states and optical properties in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated theoretically in the framework of effective-mass envelop function theory. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy, interband transition energy, oscillator strength and linear optical susceptibility in InGaN/GaN QD. It is also found that the electric field effects on exciton states and optical properties are much more obvious in QD with large size. Moreover, the ground-state exciton binding energy and oscillator strength are more sensitive to the variation of indium composition in InGaN/GaN QD with small indium composition. Some numerical results are in agreement with the experimental measurements.     

Effect of low-energy electron irradiation on the cathodoluminescence of multiple quantum well (MQW) InGaN/GaN structures

The effect of low-energy electron-beam (e-beam) irradiation on the InGaN-related cathodoluminescence in multiple quantum well (MQW) InGaN/GaN light-emitting diode (LED) structures has been studied. It is shown that the e-beam exposure leads to an increase of emission intensity and to a formation of new blue-shifted emission bands. The changes observed were explained by the enhancement of In diffusion stimulated by excess carrier recombination.