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.     

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.     

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)     

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.     

Three-center Auger effect and the quantum yield of the luminescence of ZnS-based phosphors

In this communication, the luminescence properties of ZnS luminophors are characterized. The effect of concentration quenching of the photo- and cathodoluminescence (PL, CL) is discussed. An appropriate mechanism of nonradiative recombination is proposed, and its efficiency is estimated. It is shown that the three-center Auger effect can be responsible for the cathodoluminescence saturation observed under increased current density.     

Modeling and analysis of the effects of inhomogeneous carrier distributions in InGaN multiple quantum wells

It is well known that carrier distribution in InGaN multiple quantum wells (MQWs) can be significantly inhomogeneous. However, the conventional ABC recombination model assumes that carriers are uniformly distributed throughout the MQW. In this paper, a modified ABC model that considers the unequal carrier density in the QWs was developed. From the analysis of the developed ABC model, the effective recombination coefficients and modified internal quantum efficiency (IQE) were obtained for an arbitrary carrier distribution in MQWs. The efficiency droop was found to be aggravated as the carrier distribution was increasingly inhomogeneous. However, it was also found that the effect of inhomogeneous carrier distribution alone was not sufficient to explain the IQE droop with the theoretical Auger recombination coefficient based on indirect Auger processes. The developed ABC model is expected to provide insight into the influence of inhomogeneous carrier distributions in MQWs on the efficiency droop in GaN-based light-emitting diodes.     

Mechanisms of Auger recombination in semiconducting quantum dots

Microscopic calculation of the probability of Auger recombination of charge carriers localized in a semiconducting quantum dot (QD) is carried out. It is shown that two mechanism of Auger recombination (nonthreshold and quasi-threshold) operate in the QD. The nonthreshold Auger recombination mechanism is associated with scattering of a quasimomentum from a heterobarrier, while the quasi-threshold mechanism is connected with spatial confinement of the wave functions of charge carriers to the QD region; scattering of carriers occurs at the short-range Coulomb potential. Both mechanisms lead to a substantial enhancement of Auger recombination at the QD as compared to a homogeneous semiconductor. A detailed analysis of the dependence of Auger recombination coefficient on the temperature and QD parameters is carried out. It is shown that the nonthreshold Auger recombination process dominates at low temperatures, while the quasi-threshold mechanism prevails at high temperatures. The dependence of the Auger recombination coefficient on the QD radius experiences noticeable changes as compared to quantum wells and quantum filaments.     

Optical excitation study on the efficiency droop behaviors of InGaN/GaN multiple-quantum-well structures

Efficiency droop is generally observed in electroluminescence under high current injection. Optical characterization on efficiency droop in InGaN/GaN multiple-quantum-well structures has been conducted at 12 K. Clear droop behaviors were observed for the sample excited by above-bandgap excitation of GaN with pulse laser. The results show that dislocation is not the crucial factor to droop under high carrier density injection, and Auger recombination just slightly affects the efficiency. The radiative recombination may be mainly affected by a multi-carrier-related process (diffusion and drift with a factor of n ^3.5 and n ^5.5) at the interface between GaN barrier and InGaN well.     

Analytical Simulation of an InAsSb Photovoltaic Detector for Mid-Infrared Applications

A generic model of a mid-infrared photodetector based on a narrow bandgap semiconductor has been developed. The model has been applied for analysis and simulation of an InAs_0.89Sb_0.11 photovoltaic detector for operation at room temperature in 2–5 μm wavelength region. The model takes into account the effect of tunneling and other components of dark current on the detectivity of the device by considering all the three dominant recombination mechanisms e.g., radiative, Shockley-Read-Hall and Auger recombination. The study revealed that the dark current of the photodetector under reverse bias is dominated by trap-assisted tunneling component of current and this causes the detectivity of the device to decrease at high reverse bias. It is further concluded that by operating the device at a suitable low reverse bias it is possible to improve the room-temperature detectivity significantly as compared to its value at zero bias.     

Influence of Auger recombination on the lifetime of nonequilibrium carriers in InGaAsSb/AlGaAsSb quantum well structures

Carrier recombination processes, including the Auger recombination, are studied in InGaAsSb/AlGaAsSb quantum well nanostructures. Based on the dynamics of photoluminescence, we estimate the emission time of an optical phonon, determined the recombination rate as a function of optical-excitation intensity, and estimated the coefficient characterizing the rate of the resonance Auger recombination.     

Extended temperature range of absorptive optical bistability in semiconductors

Optical bistability based on the temperature dependence of the absorption coefficient in a semiconductor is considered. It is theoretically demonstrated that the upper bound of the thermostat temperatures at which the bistability is possible can be raised by a factor of three as against its conventional value reported in the literature. This can be achieved in the case of the Auger recombination of free electrons in semiconductors.     

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.     

Population inversion and IR amplification induced by intersubband electron transitions and resonant Auger processes in quantum wells

A mechanism for electron population inversion and mid-IR amplification is proposed for the case of the current or optical injection of electron-hole pairs into the undoped region of a heterojunction with quantum wells. The presence of an upper long-lived size-quantization level and resonant Auger recombination in the well are crucial features of the mechanism. A long electron lifetime at the upper level or a relatively low probability of electron scattering to the other subbands is achieved by choosing the shape of the well and its parameters in such a way as to provide weak overlap between the upper level and two lower level electronic wave functions. Resonant Auger recombination plays a positive role. It stabilizes the electron and hole concentrations at lower levels and makes a substantial contribution to the excitation of the upper electronic level and the population inversion. The degree of population inversion and the gain coefficient are estimated.     

Pivotal role of ballistic and quasi-ballistic electrons on LED efficiency

Significant progress in the power conversion efficiency and brightness of InGaN-based light emitting diodes (LEDs) has paved the way for these devices to be considered for LED lighting. In this realm, however, the efficiency must be retained at high injection levels in order to generate the lumens required. Unfortunately, LEDs undergo a monotonic efficiency degradation starting at current densities even lower than 50 A/cm² which would hinder LED insertion into the general lighting market. The physical origins for the loss of efficiency retention are at present a topic of intense debate given its enormous implications. This paper reviews the current status of the field regarding the mechanisms that have been put forward as being responsible for the loss of efficiency, such as Auger recombination, electron overflow (spillover), current crowding, asymmetric injection of electrons and holes, and poor transport of holes through the active region, the last one being applicable to multiple quantum well designs. While the Auger recombination received early attention, increasing number of researchers seem to think otherwise at the moment in that it alone (if any) cannot explain the progressively worsening loss of efficiency reduction as the InN mole fraction is increased. Increasing number of reports seems to suggest that the electron overflow is one of the major causes of efficiency degradation. The physical driving force for this is likely to be the relatively poor hole concentration and transport, and skewed injection favoring electrons owing to their relatively high concentration. Most intriguingly there is recent experimental convincing evidence to suggest that quasi-ballistic electrons in the active region, which are not able to thermalize within the residence time and possibly longitudinal optical phonon lifetime, contribute to the carrier overflow which would require an entirely new thought process in the realm of LEDs.     

N型掺杂应变Ge发光性质

应变锗材料具有准直接带特性,而且与标准硅工艺兼容,成为实现硅基发光器件重要的候选材料之一.本文基于vandeWalle形变势理论,计算了应变情况下半导体Ge材料的能带结构以及载流子在导带中的分布;通过分析载流子直接带和间接带间的辐射复合以及俄歇复合、位错等引起的非辐射复合的竞争,计算了N型掺杂张应变Ge材料直接带跃迁的内量子效率和光增益等发光性质.结果表明,张应变可有效增强Ge材料直接带隙跃迁发光.在1.5%张应变条件下,N型掺杂Ge的最大内量子效率可以达到74.6%,光增益可以与III-V族材料相比拟.     

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.     

Internal power loss in GaN-based lasers: mechanisms and remedies

GaN-based laser diodes transform only a relatively small fraction of the electrical input power into laser light. The inherently large series resistance of these devices causes significant self-heating that leads to the typical power roll-off at high current. We analyze recently reported measurements using advanced numerical laser simulation and investigate the physical mechanisms that limit the lasing power in continuous-wave operation. Contrary to common expectations, our analysis reveals a strong influence of Auger recombination since the self-heating leads to a rising quantum well carrier density above the lasing threshold. As possible remedy, we investigate the effect of a tunnel-junction contact and predict a significant enhancement of lasing power and efficiency.     

Anomalous PL brightening caused by partial Auger recombination of a (D, X) complex during an impact ionization avalanche in n-GaAs

Partial Auger recombination of the (D⁰, X) bound-exciton in n-GaAs at 4.2 K during an impact ionization avalanche under an applied pulse voltage has been investigated. The bright photoluminescence (PL)-pattern observed by applying the pulse voltage, characterizes well the formation of a current density filament. The observation of the bright PL spectra inside the current density filament gives a quite new result concerning the partial Auger recombination process of the (D⁰, X) complex, leaving the neutral donor in the excited states.     

Electroluminescence explored internal behavior of carriers in InGaAsP single-junction solar cell

The internal behaviors of carriers in InGaAsP single-junction solar cell are investigated by using electroluminescence (EL) measurements. Two emission peaks can be observed in current-dependent electroluminescence spectra at low temperatures, and carrier localization exists for both peaks under low excitation. The trends of power index α extracted from excitation-dependent EL spectra at different temperatures imply that there exists a competition between Shockley-Read-Hall recombination and Auger recombination. Auger recombination becomes dominant at high temperatures, which is probably responsible for the lower current density of InGaAsP solar cell. Besides, the anomalous "S-shape" tendency with the temperature of band-edge peak position can be attributed to potential fluctuation and carrier redistribution, demonstrating delocalization, transfer, and redistribution of carriers in the continuum band-edge. Furthermore, the strong reduction of activation energy at high excitations indicates that electrons and holes escaped independently, and the faster-escaping carriers are holes.