氮化銦鎵發光二極體成長於氮化鎵及藍寶石基板之特性研究與比較

Abstract

在本論文中，我們利用有機金屬氣相沉積法 (MOCVD) 將高效率氮化銦鎵/氮化鎵量子井結構發光二極體成長於同質磊晶材料之氮化鎵基板上，並同時與成長於藍寶石基板和圖形化藍寶石基板之發光二極體進行分析和比較，分別針對其材料特性和光學特性進行探討。 首先，藉由穿透式電子顯微鏡 (TEM)、拉曼光譜(Raman)、X射線繞射 (XRD)等材料分析技術去觀察樣品之材料特性，結果顯示出將氮化銦鎵/氮化鎵量子井發光二極體成長在氮化鎵基板，可有效減少應力、改善磊晶品質並減少樣品內的差排密度 (dislocation density)，進而減少磊晶層中之非幅射復合中心以增加發光二極體元件之發光效率。 此外，我們藉由變功率光激發螢光光譜(PL) 、變溫光激發螢光光譜、變波長時間解析光激發螢光光譜 (TRPL)和微米光激發螢光光譜 (µ-PL) 等量測方法去研究成長在不同基板上之氮化銦鎵/氮化鎵多重量子井發光二極體之光學特性。我們利用變功率光激發螢光分析在低溫及室溫的光譜表現去研究氮化銦鎵/氮化鎵多重量子井發光二極體成長在不同基板上之載子復合機制，其波長及半高寬之差異也反映了量子井內之非輻射複合中心的多寡和所受到的量子侷限史塔克效應(Quantum confined Stark effect, QCSE)與能帶填滿效應(Band filling effect)的大小。我們也利用變溫光激發螢光光譜和變波長時間解析光激發螢光光譜來分析成長在不同基板之氮化銦鎵/氮化鎵多重量子井之載子侷限程度和侷限深度，結果發現成長於氮化鎵基板之多重量子井由於所受應力較小，內部銦叢集也較少，因此等效載子侷限程度和侷限深度也較小，但由於其較佳之磊晶品質和較陡峭之多重量子井結構，整體之內部量子效率仍較成長於藍寶石系列基板之發光二極體為高，在高載子注入下差異更為顯著。另外，透過微米光激發螢光光譜 (µ-PL) 之量測，我們也發現成長於氮化鎵基板之多重量子井在空間上有較好的波長穩定性。 最後，元件特性量測結果也顯示成長於氮化鎵基板之元件其發光效率已和成長於圖形化藍寶石基板相當，前者具有非常高的內部量子效率而後者在光萃取效率上具有優勢，因此氮化鎵基板磊晶技術未來在固態照明的發展扮演重要的角色。

In this thesis, the high performance InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were grown on a homoepitaxial GaN substrate by using metal-organic chemical vapor deposition (MOCVD). The same LED structures were also grown on sapphire substrate and patterned sapphire substrate for comparison. The material and optical properties were investigated in detail. First, the material properties were investigated by high resolution transmission electron microscopy (HRTEM), high resolution X-ray Diffraction (HRXRD) and Raman spectroscopy. The results show that by using GaN substrate, the compressive strain, crystalline quality and dislocation density can be effectively improved, which result in the reduction of nonradiative centers in epilayer and the increase of LEDs light output efficiency. In addition, the excitation power dependent PL, the temperature dependent PL, wavelength dependent TRPL and microscale PL measurements were employed to investigate the optical properties of InGaN/GaN MQWs grown on various substrates. We use the excitation power dependent PL at low temperature and room temperature to investigate the carrier recombination mechanism of InGaN/GaN MQWs LEDs grown on various substrates. The variations of wavelength and FWHM reflect the quantity of nonradiative centers, the Quantum confined Stark effect (QCSE) and band filling effect in MQWs. The temperature dependent PL and the wavelength dependent TRPL were also used to analyze the degree of localized states and the depth of localized states in InGaN/GaN MQWs on various substrates. The results indicate that fewer indium clusters in the InGaN/GaN MQWs on GaN substrate due to it suffers less compressive strain. As a result, we observed less degree of localized states and the depth of localized states in InGaN/GaN MQWs on GaN substrate. However, because of the superior crystalline quality and abrupt MQWs strctures for LEDs on GaN substrate, the overall internal quantum efficiency (IQE) is also higher than LEDs on PSS and sapphire substrate, especially for high carrier injection. Moreover, by using microscale PL measurement, we observed that the MQWs on GaN substrate had better spatial wavelength stability. Finally, the EL measurements show that the light output efficiency of LEDs grown on GaN substrate is comparable to LEDs grown on PSS. The former has a superior IQE; while the latter has an advantage in light extraction efficiency (LEE). Consequently, the homoepitaxial technology for LEDs grown on GaN substrate plays an important role in solid-state lighting.