氮化鎵族奈米結構之光學特性

Abstract

本論文中，我們研究兩種以化學氣相沉積法成長的奈米結構樣品，前者為氮化銦鎵/氮化鎵量子點，後者為以a軸成長的氮化銦鎵/氮化鎵的多重量子井，我們利用光激發螢光(PL) ，螢光激發光(PLE)等光譜技術配合理論模擬進行光學特性的分析。 在第一部分我們利用一種新的成長方法長出高密度氮化鎵量子點之樣品，透過原子力顯微鏡的量測得知量子點的大小會隨著SiNx處理時間的增加而變大，藉由光激發螢光光譜的分析顯示小量子點表現出較強的量子侷限效應，因此相較於大的量子點其波長較短，同時我們以模擬的方式計算量子侷限效應的理論值，和實驗大致吻合。再者我們將量子點的樣品做了熱退火的處理。我們觀察到隨著退火的溫度從650 ˚C 增加至 950 ˚C時，量子點的光譜表現出先紅移再藍移的趨勢，且小的量子點相較於大的量子點紅移較多。針對這個結果我們也建立了一個物理模型以理論計算解釋成因。 第二部分我們以相同的長晶條件製備了兩組樣品，為以a軸和c軸成長的氮化銦鎵/氮化鎵多重量子井，實驗的結果顯示a軸樣品之內建電場不影響量子井的位能，我們期待可藉由此特性提高內部量子效率。除此之外，我們發現在20k的低溫下a軸樣品的光譜包含了兩個波峰。高能量的波峰在變溫的過程中表現出快速的熱抑制(thermal quenching)效應，而低能量的波峰維持至室溫。在螢光激發光PLE的實驗中我們也觀察到a軸樣品的史托克位移(stoke’s shift)較大，藉由這兩個結果可得知銦含量在a軸樣品中較不均勻所以導致明顯的侷限效應(localization effect) 。在本論文的最後我們以側向成長的結構減少缺陷密度，改善了a軸樣品的品質。

In this thesis, two kinds of nanostructures grown by metalorganic chemical vapor deposition have been investigated. First, the characteristics of InGaN/GaN quantum dots in different sizes were demonstrated. Second, the comparison of a-plane InGaN/GaN multiquantum wells grown on r-plane sapphire and c-plane samples were studied. We utilized photoluminescence (PL), photoluminescence excitation (PLE) system and simulation to investigate the optical properties of our samples. In the first part, we investigate the optical and structural properties of ultra-high-density InGaN/GaN QDs with different sizes. Atomic force microscopy revealed the size of QDs witch was increased with the duration of SiNx. Small QDs with larger quantum confine exhibited blue-shift on PL peak than the larger QDs. The emission energies are in agreement with the calculated transition energies. On the other hand, the thermal anneal effect were studied. When annealing temperature increased from 650 ˚C to 950 ˚C, a red-shift followed by a blue-shift on PL peaks emitted from InGaN/GaN QDs was observed. And we observed small QD exhibited large red shift than large quantum dot. Finally, we develop a model to explain the mechanism. It is proposed that the red-shift was dominated by size enlargement of QD. As further increasing the annealing time, the blue-shift of PL emission peaks was attributed to decrease of the QD potential depth caused by out-diffusion of the indium composition in QDs. In the second part, a-plane and c-plane MQWs with the same growth condition were prepared to be compared in their optical characteristic. The presence of obvious power-depend blue-shift on PL peak was observed for c-plane MQWs, whereas less shift was observed for a-plane MQWs. The power dependent PL experiment indicated the polarization does not affect in the direction of well in a-plane InGaN/GaN MQWs. This characteristic could be introduced to enhance the internal quantum efficiency. Furthermore, we investigate the localization effect on a-plane InGaN MQW using temperature dependent PL and PLE Low temperature PL spectrum exhibited two dominant peaks in a-plane MQW. The apparent thermal quenching on high energy peak, whereas the luminance of low energy one maintain even until room temperature. And the lager stoke shift in a-plane MQW than c-plane MQW was addressed by PLE measurement. Those phenomenon demonstrated that the localization effect obviously appear in a-plane InGaN MQW. Finally, epitaxial lateral overgrowth (ELOG) method was used to reduce the defect density and also improve sample quality.