Research On Efficiency Droop And Structure Optimization Of Light-emitting Diodes With Non-isothermal Model

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.