Numerical Simulation on the Design of Quantum Well Active Region in Green InGaN Light-Emitting Diodes

• Main Authors: Yih-Ting Kuo, 郭毅廷
• Other Authors: Yen-Kuang Kuo
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/65270956470883655776

碩士 === 國立彰化師範大學 === 光電科技研究所 === 100 === Since light-emitting diodes (LEDs) poscess the advantages such as low power consumption, no toxic and friendly to the environment, long lifetime, compact size, and high hardness, they are widespreadly utilized in many applications, in which the solid-state lighting is the most attracting market when LEDs are considered to be the best candicate to replace the conventional light bulbs. Up to date, the white LED is mostly composed by a blue LED chip with an yttrium aluminum garnet phosphor, and the combination of red, green, and blue chips to form white emission is also one of the possible solution. However, the development of such technology is restricted because the output power and luminescence efficiency of green LEDs is not so high when compared to those of the red and blue LEDs. Therefore, the white LEDs made by this solution are not so popular. In green InGaN LEDs, the In composition in InGaN quantum wells (QWs) is normally higher than 30% and hence a large lattice mismatch at the InGaN/GaN interface is resulted, which leads to strong polarization field that in turn induces the band bending of the energy bands. Besides, the QW depth of the green LEDs becomes deeper when compared to that of blue LEDs; therefore, the efficiency for carrier transport is lower and the output efficiency is limited. In this study, some new design of the QW active region by using asymmetric InGaN QWs and barriers is presented with an aim to improve the output performance of green LEDs. In chapter 1, the development and other applications for the green VII InGaN LEDs are described, and the methods that how to enhance the output efficiency in green InGaN LEDs by some researchers are also reviewed. In chapter 2, the structures of the green InGaN LEDs under study are shown. The physical parameters such as the bandgap energies, bowing parameters, band-offset ratios, polarization effects, and Shockley-Read-Hall (SRH) lifetime of the III-nitride alloys used in the APSYS simulation program are listed. In chapter 3, a green InGaN LED with asymmetric AlGaN composition-graded barriers and without the use of AlGaN (EBL) were presented that possess markedly enhanced optical and electrical output characteristics. The optical and electrical properties, band diagrams, electron and hole concentrations distributed in the active region and radiative recombination rate are analyzed and compared. In chapter 4, the effect of using chirped multiple-QW structures in green InGaN LEDs is numerically investigated. Comparing to conventional active structure design of green LEDs, which uses uniform MQWs, the output performance is enhanced. The physical origins for the enhancement in optical and electrical properties are figured out. Finally, a summary of the aforementioned studies is given in chapter 5.