基于非等温模型的LED效率衰落及芯片结构优化研究
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摘要
GaN基发光二极管(LED)具有节能、高效、体积小、寿命长等优点,已经被广泛用于全彩色显示、固体照明、光存储、通讯等领域,具有很大的商机潜力。目前正朝着大功率、高性能的方向发展。然而,随着注入电流的升高LED的内量子效率会逐渐下降,称为效率衰落效应,导致LED在大电流情况下性能显著降低,严重阻碍了大功率LED的发展与应用。因此,挖掘效率衰落效应的物理机制,改善效率衰落效应,提高大功率LED的发光效率,是研究者面临的重大课题。为深入理解效率衰落效应的物理机制,试图提高LED发光效率,本文揭示了LED性能与芯片非等温效应之间的依变关系,并在此指导下提出了几种新型LED芯片结构,在一定程度上改善了LED的效率衰落效应,提高了LED的发光效率。主要包括以下内容：
首先建立了LED的非等温多物理场耦合模型,精细刻画了芯片的内热源和温度场分布,发现内热源分布是不均匀的,并且以焦耳热和非辐射复合热为主,汤姆逊热和帕尔帖热的贡献很小可忽略。内热源主要集中在量子阱内,产生电流拥塞效应。分析了LED芯片内部热源、温度场与其性能之间的依变关系,指出了等温模型在预测芯片性能时的局限性。俄偈复合所产生的复合热不是内热源的主要贡献,基本可忽略不计。电子漏电流和俄偈复合是导致效率衰落效应的主要原因。为提高LED的量子效率,改善效率衰落效应,本文在非等温模型的基础上提出了几种LED新结构,主要包括：多势垒结构电子阻挡层、锯齿形电子阻挡层、梯形电子阻挡层、耦合插入层电子阻挡层、In组分梯度渐变活性区、渐变尾垒结构。
多势垒结构电子阻挡层的引入显著提高了芯片的内量子效率,使效率衰落度显著降低了12.85%,并在提高发光复合率的同时,保证了芯片的热稳定性与发光稳定性。通过优化电子阻挡层中的各项参数大大提高了LED内量子效率。提出的AlGaInN电子阻挡层耦合插入InGaN层结构,以及锯齿形电子阻挡层结构,可显著改善LED发光效率。原因是此两种结构的引入,不仅增强了对电子的限制能力,还增加了空穴的注入率,也使活性区中的极化电场减小,减弱了量子限制斯塔克效应。还提出了In浓度梯度升高InxGa1-xN势垒活性区及A1组分渐变AlxGa1-xN尾垒结构。研究发现,In浓度梯度升高的InxGa1-xN势垒改善了活性区最后两个量子阱附近的能带结构,增强了电子阻挡层抑制电子漏电流的有效势垒高度,增强了对电子的限制能力,也提高了空穴从p型区注入活性区的注入率。Al组分渐变AlxGa1-xN尾垒结构使最后一个势垒与电子阻挡层界面处,极化效应诱导的能带弯曲得到显著改善,因此提高了电子阻挡层的有效势垒高度,增强了对电子从活性区溢出到p型区的限制能力,使内量子效率和发光功率都得到提高。
GaN-based light-emitting diode (LED) has attracted much attention in recent years owing to their low energy consumption, high efficiency, compact size, and long life time. Their applications include full-color display, solid-state lighting, optical storage, and mobile platform etc. The injection current of LED increases with illumination intensity increased. However, when injection current increases in GaN-based LED, there exists a phenomenon called "efficiency droop" that is the internal quantum efficiency is reduced with the injection current increased, especially at high injection current density. It forms the obstacle to develop high power and high performance LED. Though a few explanations have been proposed, the mechanism of efficiency droop is still under debate now. Consequently, to clarify the origin mechanism of efficiency droop and improve the efficiency of high power LED becomes a significant issue for researchers. In this paper, the relationship between the self-heating effect and the LED's performance was investigated to understand the mechanism of efficiency droop better, then based on the results several new LED structures were proposed to improve the efficiency droop. The main contents are as follows:
A non-isothermal multi-physics coupling model for LED was proposed, the temperature field and internal heat source are elaborately described. It is found that, the Joule heat and recombination heat contribute the major part of the whole heat generation, the Thomson heat and Peltier heat can be neglected. The internal heat source is accumulated in the quantum wells and the last quantum well has the highest heat source intensity, which causes the current crowding effect. The relationship between the self-heating effect and the performance of LED was analyzed, then the limitation of the isothermal model for predicting LED's performance was proposed. Auger recombination heat is not the major contributor for internal heat source and it can be neglected. Increasing Auger recombination rate causes little chip temperature change. Electron leakage and Auger recombination are the main responsible mechanisms for efficiency droop. Based on the above, multi-quantum barrier electron blocking layer, sawtooth shaped electron blocking layer, trapezoidal electron blocking layer, AlGalnN electron blocking layer coupled with inserting InGaN layer, gradually increased In-composition InxGa1-xN barriers, and last AlGaN barrier with graded Al composition are proposed to improve the LED efficiency.
Introducing an AlxGa1-xN/GaN multi-quantum barrier electron blocking layer structure can increase the internal quantum efficiency markedly. The degree of efficiency droop is significantly decreased, ensuring the light output stability and thermal stability of LED simultaneously. The performance of LED was improved significantly by optimizing the structure parameters of electron blocking layer. It is due to the modified energy band diagrams which are responsible for the enhanced carrier concentration in the active region. The proposed sawtooth shaped electron blocking layer and AlGaInN electron blocking layer coupled with inserting InGaN layer can improve the output power performance of LED significantly, which can be explained by the reduced electron leakage and enhanced hole injection efficiency, as well as alleviated electrostatic fields in the quantum wells. The gradually increased In-composition InxGa1-xN barriers and last AlGaN barrier with graded Al composition were also proposed. It is found that, the output power was increased by28%for the LED with gradually increased In-composition InxGa1-xN barriers when compared with the conventional GaN barrier LED at180mA. The improved performance is caused by the enhanced electron confinement and increased hole injection efficiency. The efficiency droop is markedly improved and the output power is greatly enhanced when the conventional GaN last barrier is replaced by AlGaN barrier with Al composition graded linearly from0to15%in the growth direction. These improvements are attributed to enhanced efficiency of electron confining and hole injection caused by the less polarization effect at the last-barrier/electron blocking layer interface when the graded Al composition last barrier is used.
首先建立了LED的非等温多物理场耦合模型,精细刻画了芯片的内热源和温度场分布,发现内热源分布是不均匀的,并且以焦耳热和非辐射复合热为主,汤姆逊热和帕尔帖热的贡献很小可忽略。内热源主要集中在量子阱内,产生电流拥塞效应。分析了LED芯片内部热源、温度场与其性能之间的依变关系,指出了等温模型在预测芯片性能时的局限性。俄偈复合所产生的复合热不是内热源的主要贡献,基本可忽略不计。电子漏电流和俄偈复合是导致效率衰落效应的主要原因。为提高LED的量子效率,改善效率衰落效应,本文在非等温模型的基础上提出了几种LED新结构,主要包括：多势垒结构电子阻挡层、锯齿形电子阻挡层、梯形电子阻挡层、耦合插入层电子阻挡层、In组分梯度渐变活性区、渐变尾垒结构。
多势垒结构电子阻挡层的引入显著提高了芯片的内量子效率,使效率衰落度显著降低了12.85%,并在提高发光复合率的同时,保证了芯片的热稳定性与发光稳定性。通过优化电子阻挡层中的各项参数大大提高了LED内量子效率。提出的AlGaInN电子阻挡层耦合插入InGaN层结构,以及锯齿形电子阻挡层结构,可显著改善LED发光效率。原因是此两种结构的引入,不仅增强了对电子的限制能力,还增加了空穴的注入率,也使活性区中的极化电场减小,减弱了量子限制斯塔克效应。还提出了In浓度梯度升高InxGa1-xN势垒活性区及A1组分渐变AlxGa1-xN尾垒结构。研究发现,In浓度梯度升高的InxGa1-xN势垒改善了活性区最后两个量子阱附近的能带结构,增强了电子阻挡层抑制电子漏电流的有效势垒高度,增强了对电子的限制能力,也提高了空穴从p型区注入活性区的注入率。Al组分渐变AlxGa1-xN尾垒结构使最后一个势垒与电子阻挡层界面处,极化效应诱导的能带弯曲得到显著改善,因此提高了电子阻挡层的有效势垒高度,增强了对电子从活性区溢出到p型区的限制能力,使内量子效率和发光功率都得到提高。
GaN-based light-emitting diode (LED) has attracted much attention in recent years owing to their low energy consumption, high efficiency, compact size, and long life time. Their applications include full-color display, solid-state lighting, optical storage, and mobile platform etc. The injection current of LED increases with illumination intensity increased. However, when injection current increases in GaN-based LED, there exists a phenomenon called "efficiency droop" that is the internal quantum efficiency is reduced with the injection current increased, especially at high injection current density. It forms the obstacle to develop high power and high performance LED. Though a few explanations have been proposed, the mechanism of efficiency droop is still under debate now. Consequently, to clarify the origin mechanism of efficiency droop and improve the efficiency of high power LED becomes a significant issue for researchers. In this paper, the relationship between the self-heating effect and the LED's performance was investigated to understand the mechanism of efficiency droop better, then based on the results several new LED structures were proposed to improve the efficiency droop. The main contents are as follows:
A non-isothermal multi-physics coupling model for LED was proposed, the temperature field and internal heat source are elaborately described. It is found that, the Joule heat and recombination heat contribute the major part of the whole heat generation, the Thomson heat and Peltier heat can be neglected. The internal heat source is accumulated in the quantum wells and the last quantum well has the highest heat source intensity, which causes the current crowding effect. The relationship between the self-heating effect and the performance of LED was analyzed, then the limitation of the isothermal model for predicting LED's performance was proposed. Auger recombination heat is not the major contributor for internal heat source and it can be neglected. Increasing Auger recombination rate causes little chip temperature change. Electron leakage and Auger recombination are the main responsible mechanisms for efficiency droop. Based on the above, multi-quantum barrier electron blocking layer, sawtooth shaped electron blocking layer, trapezoidal electron blocking layer, AlGalnN electron blocking layer coupled with inserting InGaN layer, gradually increased In-composition InxGa1-xN barriers, and last AlGaN barrier with graded Al composition are proposed to improve the LED efficiency.
Introducing an AlxGa1-xN/GaN multi-quantum barrier electron blocking layer structure can increase the internal quantum efficiency markedly. The degree of efficiency droop is significantly decreased, ensuring the light output stability and thermal stability of LED simultaneously. The performance of LED was improved significantly by optimizing the structure parameters of electron blocking layer. It is due to the modified energy band diagrams which are responsible for the enhanced carrier concentration in the active region. The proposed sawtooth shaped electron blocking layer and AlGaInN electron blocking layer coupled with inserting InGaN layer can improve the output power performance of LED significantly, which can be explained by the reduced electron leakage and enhanced hole injection efficiency, as well as alleviated electrostatic fields in the quantum wells. The gradually increased In-composition InxGa1-xN barriers and last AlGaN barrier with graded Al composition were also proposed. It is found that, the output power was increased by28%for the LED with gradually increased In-composition InxGa1-xN barriers when compared with the conventional GaN barrier LED at180mA. The improved performance is caused by the enhanced electron confinement and increased hole injection efficiency. The efficiency droop is markedly improved and the output power is greatly enhanced when the conventional GaN last barrier is replaced by AlGaN barrier with Al composition graded linearly from0to15%in the growth direction. These improvements are attributed to enhanced efficiency of electron confining and hole injection caused by the less polarization effect at the last-barrier/electron blocking layer interface when the graded Al composition last barrier is used.
引文
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