| Summary: | 博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 100 === In this dissertation, the characteristics of efficiency droop under various current densities have been widely investigated. In the chapter one, we briefly discussed the leading causes of efficiency droop from literatures. In chapter two, the primary using machine–MOCVD (Metal-Organic Chemical Vapor Deposition) system was also being introduced. Subsequently, according to our structures design of epi-layers, the domination and subordination effects on efficiency droop were thoroughly studied and divided into four parts (chapter three to six).
In the chapter three, the wavelength-dependent InGaN-based light emitting diodes (LEDs) with peak emissions ranging from 400 to 445 nm, and investigated their efficiency-droop characteristics at injection currents of up to 1 A. It was found that the emissions of the wavelength-dependent InGaN LEDs underwent blue shifts at elevated currents. In addition, although the external quantum efficiencies (EQEs) changed dramatically when the critical current was less than 350 mA, the efficiency droop of each device exhibited a similar negative slope upon increasing the current from 350 mA to 1 A. Whereas the effects of piezoelectric polarization and different localized states in the active layer of the near-UV–to–blue LEDs influenced the peak EQEs and the dramatic decays of the EQE droops at lower injection currents, they were not responsible for the EQE droops at higher current levels. In addition, the piezoelectric effect and Auger non-radiative recombination were not dominating influences determining the efficiency droops of the wavelength-dependent LEDs at higher carrier densities.
In the chapter four, the EQE characteristics of InGaN/GaN light emitting diodes (LEDs) incorporating three-stepped AlGaN electron blocking layers (EBLs) were investigated. The LED featuring the three-stepped p-AlxGa1-xN EBL (x: 0.21, 0.14, 0.07) exhibited the highest EQE under low currents, but severe efficiency droop occurred upon increasing the current, relative to the performance of the reference LED incorporating a normal EBL (p-Al0.21GaN). In contrast, the LED with the three-stepped p-AlxGaN EBL (x: 0.07, 0.14, 0.21) displayed a notable improvement in its saturated peak efficiency at high current densities and mitigated efficiency droop upon elevating the injection current. The significant improvement in efficiency resulted from (i) an increase in the rate of hole injection upon decreasing the AlGaN barrier height of the valence band and (ii) the diminished built-in electric field after band-engineering with the three-stepped EBLs.
In the chapter five, we minimized efficiency droop by varying barrier thickness for InGaN/GaN multiple quantum wells (MWQs) featuring narrow quantum barriers (NQBs). The EQE for a light-emitting diode (LED) possessing NQBs improved by 18% at a current density of 200 A cm–2, compared to that of a conventional LED incorporating a 12-nm-thick barrier. The enhanced carrier distribution resulting from the presence of NQBs was practically approved from another experimental design in this study. We suggest that the NQBs displayed uniform carrier distribution in active layer and decreased the carrier density in the active layer at a critical current density.
In the chapter six, we observed a dramatic decrease in the efficiency droop of InGaN/GaN light-emitting diodes (LEDs) after positioning a p-InGaN insertion layer before the p-AlGaN electron-blocking layer (EBL). The saturated EQE of this device extended to 316 mA, with an efficiency droop of only 7% upon increasing the operating current to 1 A; in contrast, the corresponding conventional LED suffered a severe efficiency droop of 42%. We suspect that the asymmetric carrier distribution was effectively mitigated as a result of an improvement in the hole injection rate and a suppression of electron overflow.
Finally, we are going to conclude the leading effects on featuring EQEs in each stage, i.e. stage Ⅰ(under peak EQE), stage Ⅱ (peak EQE to 35 A cm-2) and stage Ⅲ (over 35 A cm-2). Furthermore, the state-of-the-art InGaN-based LED epi-structure is also demonstrated in chapter future work for the application of solid-state lighting.
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