| Summary: | 碩士 === 國立彰化師範大學 === 光電科技研究所 === 100 === Light-emitting diodes (LEDs) have been unmitigatedly developed since they can be applied in the market of solid-state lighting. To the present, the conventional white LEDs are composed by the InGaN LEDs with blue emission and the YAG phosphors that are pumped by the blue light to generate amber emission. However, the white LEDs that can also be fabricated by using the near-ultraviolet (NUV) LEDs coupling with red, green, and blue phosphors have attracted much attention due to the advantages of excellent color rendering and color stability. Therefore, the NUV LEDs still have great potentials in the application of solid-state lighting.
Unfortunately, the output power of NUV InGaN multiple-quantum well (MQW) LEDs is often restricted by the poor carrier confinement in the active region, which is resulted from the low indium composition in the InGaN wells and consequently the small band-offset between the GaN barriers and InGaN wells. Also, the large polarization field existed in GaN material system tilts the energy band, which in turn results in the poor overlap of electron-hole wavefunctions and the decrease in radiative recombination. In this thesis, the effect of polarization on the NUV LEDs has been numerically investigated. In addition, some optimized device structures are proposed with an aim of enhancing the optical characteristics of the NUV LEDs.
In chapter 1, the historical review and the development of solid-state lighting and other applications for the NUV LEDs are investigated. The effects of polarizations and the calculation method are also described in detail. In addition, prior studies regarding the methods to improve the efficiency of the NUV InGaN LEDs are introduced.
In chapter 2, the structure of the NUV LED used under study and the physical parameters used in APSYS simulation program are presented.
In chapter 3, the influences of bandgap energy and polarization of the electron blocking layer (EBL) in NUV LEDs are systematically investigated. When the bandgap of the EBL increases, the polarization and polarization-induced charge increase accordingly; however, it is a trade-off between these two mechanisms when confining electrons by the EBL is of major concern. The simulation results show that, the NUV LEDs with an EBL of large bandgap or small polarization have improved performance due to the enhanced efficiency of electron confining and hole injection.
In chapter 4, advantages of NUV LEDs with polarization-matched InGaN/AlGaInN MQWs are investigated numerically. Simulation results show that, the polarization-matched structure possesses improved efficiency droop and higher output power under high current injection due to better overlap of electron-hole wavefunctions and reduced Auger recombination.
Finally, a summary to the aforementioned studies is given in chapter 5.
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