Study of the Internal Quantum Efficiency in InGaN Light Emitting Diodes

本論文中，我們使用不同的雷射功率激發方式，探討藍光發光二極體的內部量子效率。並且探討當發光二極體元件表面具有透明導電膜時對量測內部量子效率的影響。 實驗第一部分，在發光二極體晶片上鍍上一層厚度250nm的銦錫氧化物(ITO)，定義出不同面積的正方形元件，探討10K與300K下不同雷射功率激發元件的情形。當在10K下時我們可以觀察到隨著激發功率提升，螢光強度遞增，並且發現沒有ITO以及不同面積的ITO其光激螢光強度在同功率下激發的螢光強度相近，這是由於10K下雷射激發產生的載子被p-GaN且ITO侷限無法擴散。300K下，光激螢光強度隨著ITO面積變大而下降，由於受到熱激活化以及ITO阻值下降使得導電性增加的影響，載子被擴散到中心點(雷射激發點)以外的地方，造成中心點有效的複合載子數目下降。並且觀察到沒有ITO樣品的波長曲線隨著激發功率增加會有先紅移再藍移的現象；相較於最大面積ITO(寬為10000μm方形)樣品在高功率下波長皆為紅移，這代表著該面積下大功率激發是落在沒有ITO的低功率激發下的現象。由於ITO面積變大使得300K下螢光強度除以10K下的螢光強度計算的內部量子效率逐漸下降。最大功率下，沒有ITO為61.5%，最大面積ITO為14.5%。 實驗第二部分，將發光二極體結構磊晶層成長在具有平台區的藍寶石基板(FPSS)以及具有尖錐狀的藍寶石基板(PPSS)上，探討其內部量子效率、壓電場、電激發以及雷射激發的光伏特性。首先觀察反向偏壓光激螢光光譜，FPSS與PPSS分別在-12V以及-8V達到平坦能帶，經由計算FPSS壓電場大小為-1.12(MV/cm)而PPSS為-0.92(MV/cm)。在10K與300K下，不同功率的光激發螢光光譜中發現：隨功率增加，內部量子效率會先上升而後達到穩定值，FPSS與PPSS在最高激發功率下內部量子效率分別為76.1%與83.8%。在10K時發現FPSS以及PPSS隨功率增加波長藍移量分別為2.0nm以及0.7nm。在電激發螢光光譜，FPSS螢光強度在20mA下提升約23.6%；並且其波長在1～20mA下，FPSS與PPSS波長藍移量分別為1.7nm與1.3nm。由於FPSS較大的壓電場使得能帶傾斜，造成載子注入效應明顯，以及電子電洞波函數重疊機率降低，導致波長藍移量增加和螢光強度下降。觀察元件的太陽能電池特性發現：FPSS在300K下時，隨激發功率改變，有較大的短路電流以及太陽能效率。最後我們發現PPSS有較佳的發光二極體特性，較高的內部量子效率和螢光強度；另一方面具有較低的壓電場和波長藍移量以及較差太陽能電池特性。In this thesis, the internal quantum efficiencies of InGaN-based light-emitting diodes (LEDs) with and without ITO layers were analyzed through the power-dependent photoluminescence measurement. In the first part of this thesis, the different chip size of the InGaN LED structure with 250nm-thick ITO layer were fabricated through the laser scribing process. At 10K, the PL intensity of the LED structure with the flat ITO layer was increased by adding laser excitation power that was similar the LED structure without ITO layer. It's due to photo-induced carriers were frozen at the p-GaN and ITO layers at low temperature. At 300K, the PL intensity of the LED structure was decreased by increasing ITO conductive area. Because of the thermal activation in InGaN quantum well structure and the increasing conductivity of the ITO, the carriers diffused away from the laser excitation spot that the number of carriers' recombination were reduced. By increasing the laser excitation power, the PL peak wavelengths of LED structure without the ITO layer had the red-shift phenomenon at low excitation power and the blue-shit phenomenon at high excitation power. Nevertheless, LED devices with maximum ITO shows red-shift phenomenon at higher excitation power. This peak red-shift phenomenon of LED devices with maximum ITO layer at high excitation power was similar to the LED devices without ITO layer at low power. The internal quantum efficiencies (integrated PL intensity ratios, I300K/I10K) were calculated as 61.5% for devices without ITO and 14.5% for devices with maximum ITO. In the second part of this thesis, the LED structures were grown on the flat-form (FPSS) and the pyramid (PPSS) pattern sapphire substrate. In the reverse-bias PL spectra, the flat-band voltages were measured at -12V and -8V for FPSS and PPSS structures, respectively. The piezoelectric fields were calculated at -1.12 and -0.92 MV/cm for FPSS-LED and PPSS-LED, respectively. In the power dependent PL measurement, we find steady IQE value at high excitation power that the IQE values were measured at 76.1% and 83.8% for the FPSS-LED and the PPSS-LED, respectively. At 10K, the PL peak wavelength blue-shifted of FPSS-LED and PPSS-LED were measured as 2.0nm and 0.7nm. The light output power of PPSS-LED had a 23.6% enhancement at 20mA. The wavelength blue-shift of the EL spectra were measured as 1.7nm and 1.3nm for FPSS-LED and PPSS-LED, respectively. It could be caused by larger electric field induced band tilted of the InGaN well layer that induced the low electron-hole wave function overlap and large band filling effect. The larger Isc and the higher photovoltaic efficiency were observed in the FPSS-LED compared with the FPSS-LED structure.中文摘要 i Abstract ii 第一章 前言 1 1-1發光二極體發展歷史 1 1-2 Ⅲ-Ⅴ氮化鎵材料簡介 3 1-3研究動機 5 第二章 原理以及文獻回顧 6 2-1發光二級體之發光原理 6 2-2發光二極體之光取出效率 8 2-2-1內部量子效率 8 2-2-2外部量子效率 10 2 -3壓電場效應(piezoelectric field) 15 2-3-1應變(strain) 15 2-3-2壓電場效應 17 2-3-3 壓電場的量測 19 2-4 發光二極體的光伏打特性 22 第三章 實驗方法與步驟 24 3-1 試片製備 24 3-2 實驗流程 28 3-2-1 實驗(一)流程示意圖 28 3-2-2 實驗(二)流程示意圖 29 3-3 分析儀器 30 3-3-1 場發式掃描電子顯微鏡(FE-SEM) 30 3-3-2 變溫系統 30 3-3-3光激發螢光 30 3-3-4電激發螢光光譜 (EL) 32 第四張 結果與討論 34 4-1 不同面積透明導電層的發光二極體結構 34 4-1-1 PL螢光光譜隨雷射功率改變量測 34 4-1-2 近場PL線掃描(line-scan) 40 4-2 不同圖案化藍寶石基板發光二極體結構 44 4-2-1 場發掃描式電子顯微鏡 44 4-2-2 PL螢光光譜隨雷射功率改變 45 4-2-3 改變外加反向偏壓探討壓電場對能帶影響 54 4-2-4 不同雷射功率激發的光電特性 57 4-2-5 操作直流電下的電激發螢光特性 63 第五章 結論 66 5-1 實驗結論 66 5-2 未來展望 67 參考文獻 6