Research for the Fabrication of Light Emitting Diodes with Ultra-high Efficiency

本项目的目标是找到实现<ext>覆盖紫外（UV）、蓝色、绿色和白色光谱范围的超高效率（η_ &gt;40%）发光二极管（LED）的关键。2000年至2001年期间取得的成果可概述如下。(1)在脉冲电流注入下，对基于In_xGa_ N单量子阱结构的绿色、蓝色和近紫外发光二极管进行了时间分辨电致发光（TREL）研究<1-x>，并在实际器件运行中对发光机理进行了评估。结果发现，EL线宽以及斯托克斯位移的增加，由于激子（和/或载流子）本地化的增加，在有源层中的平均成分。(2)利用紫外光学元件，在光学显微镜下聚焦紫外脉冲激光（波长266 nm，脉宽1.5 ps），建立了时空分辨光致发光（TSRPL）光谱。通过使用该系统，评估了外延横向过生长（ELO）GaN，并且对具有10^8 cm^的穿透位错密度（TDD）的4 μ m宽的窗口区域<-3>或具有10^6 cm^的TDD的8 μ m宽的翼区域进行位置选择性光激发<-3>。结果表明，翼区的PL寿命略大于窗区，表明穿透位错起非辐射中心的作用。然而，考虑到TDD中的两个阶差，寿命的差异很小。因此，非辐射复合的途径是由其他中心的起源可能是点缺陷的限制。(3)对空气桥横向外延生长（ABLEG）InGaN层的TSRPL研究表明，在有源层中加入少量In，由于减少了源于点缺陷的非辐射复合中心，导致了PL效率的提高

The objective of this project is to find the key to achieve ultra-highly efficient (η_<ext>>40%) light emitting diodes (LEDs) covering ultraviolet (UV), blue, green and white spectral range. The results obtained in the period of 2000 to 2001 can be summarized as follows.(1) Time-resolved electroluminescence (TREL) was performed in green, blue and near-UV LEDs based on In_xGa_<1-x>N single quantum well structures under pulsed current injection, by which emission mechanism could be assessed in the actual device operation. It was found that EL linewidth as well as Stokes shift was enhanced with increasing mean In composition in active layers due to the increase of exciton (and/or carrier) localization.(2) Time-space resolved photoluminescence (TSRPL) spectroscopy was set up by focusing UV-pulsed laser (wavelength : 266 nm, pulse width : 1.5 ps) in an optical microscope using UV optical components. By using this system, epitaxially lateral overgrown (ELO) GaN was assessed, and site selective photo-excitation was made to either 4 μm-wide window region with threading dislocation density (TDD) of 10^8 cm^<-3>, or 8 μm-wide wing region with TDD of 10^6 cm^<-3>. As a result, PL lifetime in the wing region was slightly larger than that in the window region, indicating that threading dislocations act as nonradiative centers. However, the difference of lifetime was very small considering two-orders difference in TDD.Therefore, it is suggested that pathway to nonradiative recombination is limited by other centers whose origin is probably point defects.(3) TSRPL in air-bridged laterally epitaxial grown (ABLEG) InGaN layers revealed that the addition of small amount of In to active layers results in the enhancement of PL efficiency because of the reduction of nonradiative recombination centers originating from point defects