Modeling and simulation of efficiency droop in GaN-based blue light-emitting diodes incorporating the effect of reduced active volume of InGaN quantum wells

The efficiency droop of InGaN-based blue light-emitting diodes (LEDs) is analyzed using numerical simulations with a modified ABC carrier recombination model. The ABC model is modified to include the effect of reduced effective active volume of InGaN quantum wells (QWs) and incorporated into the numerical simulation program. It is found that the droop of internal quantum efficiency (IQE) can be well explained by the effect of reduced light-emitting active volume without assuming a large Auger recombination coefficient. A simulated IQE curve with the modified ABC model is found to fit quite well with a measured efficiency curve of an InGaN LED sample when the effective active volume takes only 2.5% of the physical volume of QWs. The proposed numerical simulation model incorporating the reduced effective active volume can be advantageous for use in the modeling and simulation of InGaN LEDs for higher efficiency.     

Effect of carrier spillover and Auger recombination on the efficiency droop in InGaN-based blue LEDs

We have investigated efficiency droop in InGaN-based blue LEDs by considering radiative, nonradiative, and carrier spillover processes in the context of internal quantum efficiency (IQE) vs. injection current. If relied on fitting only, both the Auger recombination and an empirical formula for carrier spillover are consistent with experiments. However, the dependence of IQE on quantum well parameters and lack of droop in optical pumping experiments support the notion that carrier spillover is the main mechanism in play.     

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

Auger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III‐nitride quantum‐well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill‐out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum‐well LED are taken. Direct leakage due to non‐ideal carrier capture and re‐emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Efficiency droop suppression in GaN-based light-emitting diodes by chirped multiple quantum well structure at high current injection

Gallium nitride(Ga N) based light-emitting diodes(LEDs) with chirped multiple quantum well(MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells(QWs), LEDs with chirped MQW structures have better internal quantum efficiency(IQE) and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 m A/mm2,reduced down from 28.6%(conventional uniform LEDs) to 23.7%(chirped MQWs-a) and 18.6%(chirped MQWs-b),respectively. Meanwhile, the peak IQE increases from 76.9%(uniform LEDs) to 83.7%(chirped MQWs-a) and 88.6%(chirped MQWs-b). The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop.     

Investigation of wavelength-dependent efficiency droop in InGaN light-emitting diodes

The physical mechanisms leading to the efficiency droop of InGaN/GaN light-emitting diodes (LEDs) are theoretically investigated. We first discuss the effect of Auger recombination loss on efficiency droop by taking different Auger coefficients into account. It is found that the Auger recombination process plays a significant nonradiative part for carriers at typical LED operation currents when the Auger coefficient is on the order of 10⁻³⁰ cm⁶ s⁻¹. Furthermore, the InGaN/GaN multiple-quantum-well (MQW) LEDs with varied indium compositions in InGaN quantum wells are studied to analyze the wavelength-dependent efficiency droop. The simulation results show that the wavelength-dependent efficiency droop is caused by several different effects including non-uniform carrier distribution, electron overflow, built-in electrostatic field induced by spontaneous and piezoelectric polarization, and Auger recombination loss. These internal physical mechanisms are the critical factors resulting in the wavelength-dependent efficiency droop in InGaN/GaN MQW LEDs.     

量子阱数量变化对InGaN/AlGaN LED的影响

采用软件理论分析的方法分析了InGaN/AlGaN量子阱数量变化对发光二极管内量子效率、电子空穴浓度分布、载流子溢出产生的影响。分析结果表明:量子阱的个数不是越多越好,LED的光学性质和量子阱的个数并不成线性关系。量子阱个数太少时,电流溢出现象较明显;而当量子阱个数太多时,极化现象明显,且会造成材料浪费。因此应根据工作电流选择合适的量子阱个数。     

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.     

Theoretical investigation of Auger recombination on internal quantum efficiency of blue light-emitting diodes

The Auger recombination is recently proposed as one of the possible origins for the deteriorated internal quantum efficiency of InGaN light-emitting diodes. The Auger recombination behavior is quite different under widely varied Auger coefficients. The effect of Auger coefficient on the efficiency and output power is investigated numerically. The simulation results indicate that the Auger recombination with large Auger coefficient greatly decreases the efficiency in the whole current range under study. It is found that the electron current leakage and nonuniform hole distribution are the possible mechanisms responsible for the efficiency droop at high injection current.     

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。     

Current diffusion and efficiency droop in vertical light emitting diodes

Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes(LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect(CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency(IQE), light extraction efficiency(LEE), and external quantum efficiency(EQE) droop of the lateral LEDs.However, questions still exist for the vertical LEDs(V-LEDs). Here firstly the current diffusion length L_s(I) and L_s(II) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L_s(III) was developed by combining L_s(I) and L_s(II), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency(WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.     

不规则H形量子势垒增强AlGaN基深紫外发光二极管性能

针对AlGaN基多量子阱中有效的平衡载流子注入问题,研究了有源区势垒层中Al组分调制形成的非规则H形量子势垒对AlGaN基深紫外发光二极管(LED)器件性能的影响及载流子的输运行为。研究发现,与多量子阱中常用的单Al组分势垒相比,加入Al组分较高的双尖峰势垒可以有效地提高内量子效率和光输出功率。进一步研究表明,电子在有源区因凸起的尖峰势垒而得到了有效的阻挡,减少了电子的泄露,而空穴获得更多的动能从而穿过较高的势垒进入有源区。因此,采用非对称H形量子势垒的深紫外LED器件中载流子输运实现了较好的平衡,量子阱中的载流子复合速率远高于普通的深紫外发光二极管。     

Influence of polarization-matched AlGaInN barriers in blue InGaN light-emitting diodes

The advantages of blue InGaN light-emitting diodes with low bandgap energy and polarization-matched AlGaInN barriers are demonstrated numerically. Simulation results show that, besides the common benefit of enhanced electron-hole spatial overlap in the quantum well from the polarization-matched condition, the lower bandgap energy barriers can have additional advantages of more uniform carrier distribution among quantum wells while maintaining sufficient electron confinement. The internal quantum efficiencies of all the polarization-matched structures under study exhibit less severe efficiency droop, which is presumably attributed to the suppression of Auger recombination.     

载流子复合机制对InGaN多量子阱蓝光LED调制带宽的影响

通过设计InGaN多量子阱LED有源区的不同结构,研究了载流子复合机制对LED调制速度的影响。结果显示,由于窄量子阱LED的载流子空间波函数重叠几率更高,且电子泄露效应更显著,所以复合速率更快,调制带宽更高。In组分为1%的InGaN量子垒LED可提高辐射复合的权重,使得调制带宽高于GaN量子垒LED;In组分为5%时,电子泄露和俄歇复合占据主导地位,且由于这两种复合机制复合速率很快,所以调制带宽显著提高。     

Internal quantum efficiency of GaN-based light-emitting diodes grown on silicon substrates determined from rate equation analyses

We present a convenient and reliable method for determining the internal quantum efficiency (IQE) in GaN-based blue light-emitting diodes (LEDs) grown on Si(111) substrates based on the carrier rate equation model. By using the peak point of the efficiency curve in photoluminescence (PL) measurements as the parameter of the rate equation analysis, the IQE can be unambiguously determined without any pre-assumed parameters. The theoretical IQE model is used to fit the measured PL efficiency curves and the IQE of LED samples are determined. The maximum IQE of the LED sample grown on the Si substrate was obtained to be 0.74, which is found to agree well with the results obtained by conventional temperature-dependent PL measurements.     

Efficiency enhancement of an InGaN light-emitting diode with a p-AlGaN/GaN superlattice last quantum barrier

In this study,the efficiency droop of an InGaN light-emitting diode(LED)is reduced significantly by using a pAlGaN/GaN superlattice last quantum barrier.The reduction in efficiency droop is mainly caused by the decrease of electron current leakage and the increase of hole injection efficiency,which is revealed by investigating the light currents,internal quantum efficiencies,energy band diagrams,carrier concentrations,carrier current densities,and radiative recombination efficiencies of three LED structures with the advanced physical model of semiconductor device(APSYS).     

Determination of carrier density dependent lifetime and quantum efficiency in semiconductors with a photoluminescence method (application to InGaAsP/InP heterostructures)

The light beam of a laser is focussed near the surface of a semiconductor sample. Thereby the excitation rate can be controlled precisely assuming a Gaussian intensity distribution of the beam. Measuring the recombination light intensity yields the quantum efficiency of the sample. By sinusoidal modulation of the excitation light and measurement of the resulting phase shift of the recombination light, the carrier density dependent lifetime is obtained. By evaluation of measured internal quantum efficiency and phase shift, Auger and radiative recombination coefficients are determined. The analysis takes into account the carrier density dependence of the radiative coefficient and shows that for most experimental conditions carrier diffusion can be neglected. In this case the analysis can be performed without numerical integration. Application of the method to quaternary InGaAsP material yielded values for Auger coefficient and radiative coefficients in accordance with published results.     

Improvement of characteristics of an InGaN light-emitting diode by using a staggered AlGaN electron-blocking layer

The optical and physical properties of an InGaN light-emitting diode (LED) with a specific design of a staggered AlGaN electron-blocking layer (EBL) are investigated numerically in detail. The electrostatic field distribution, energy band, carrier concentration, electroluminescence (EL) intensity, internal quantum efficiency (IQE), and the output power are simulated. The results reveal that this specific design has a remarkable improvement in optical performance compared with the design of a conventional LED. The lower electron leakage current, higher hole injection efficiency, and consequently mitigated efficiency droop are achieved. The significant decrease of electrostatic field at the interface between the last barrier and the EBL of the LED could be one of the main reasons for these improvements.     

Improvement of characteristics of InGaN light-emitting diode by using a staggered AlGaN electron-blocking layer

The optical and physical properties of InGaN light-emitting diode (LED) with a specific design of staggered AlGaN electron-blocking layer (EBL) are investigated numerically in detail. The electrostatic field distribution, energy band, carrier concentration, electroluminescence (EL) intensity, internal quantum efficiency (IQE), and the output power are simulated. The results reveal that this specific design has a remarkable improvement of optical performance compared with the design of conventional LED. The lower electron leakage current, higher hole injection efficiency, and consequently mitigated efficiency droop are achieved. The significant decrease of electrostatic field at the interface between the last barrier and the EBL of LED could be one of the main reasons for these improvements.     

Effect of Droop Phenomenon in InGaN/GaN Blue Laser Diodes on Threshold Current

Electroluminescence(EL) and temperature-dependent photoluminescence measurements are performed to study the internal quantum efficiency droop phenomenon of blue laser diodes(LDs) before lasing. Based on the ABC mode, the EL result demonstrates that non-radiative recombination rates of LDs with threshold current densities of 4 and 6 kA/cm~2 are similar, while LD with threshold current density of 4 kA/cm~2 exhibits a smaller augerlike recombination rate compared with the one of 6 kA/cm~2. The internal quantum efficiency droop is more serious for LD with higher threshold current density. The internal quantum efficiency value estimated from temperature-dependent photoluminescence is consistent with EL measurements.