氮化銦鎵/氮化鎵多重量子井發光二極體之光學特性與內部量子效率研究

Abstract

本論文中，我們研究氮化銦鎵/氮化鎵多重量子井發光二極體的光學特性與內部量子效率，共包含兩個主題: (1) 成長在不同角度基板之氮化銦鎵/氮化鎵多重量子井發光二極體之光學特性 我們藉由變溫光激發螢光 (PL)、變溫時間解析光激發螢光 (TRPL)、變功率光激發螢光和微米光激發螢光 (μ-PL) 等光譜技術去研究成長在不同角度基板上之氮化銦鎵/氮化鎵多重量子井發光二極體的光學特性。在室溫PL實驗中，我們觀察到當基板角度從0度增加到1度時，光譜半高寬有先減少再增加之趨勢。μ-PL掃描影像顯示氮化銦鎵量子井發光能量之均勻性會受到基板角度之影響，而在變溫PL實驗中，我們觀察到明顯的載子侷限效應 (carrier localization effect)，光譜隨溫度增加而呈現紅移-藍移-紅移之變化，並隨著基板角度的改變而呈現不同趨勢，透過定量分析，我們得知載子侷限程度隨基板角度之變化情形，而成長在0.2度基板之樣品具有最微弱的量子侷限效應和最淺的侷限深度 (effective barrier of localized states)，此外，變溫TRPL實驗結果顯示0.2度樣品之氮化銦鎵/氮化鎵量子井具有最大之奈米結構量子侷限維度 (dimensionality of nanostructure)，再者，由變功率PL也觀察到其呈現最弱的能帶填滿效應 (Band filling effect)。 另一方面，我們藉由原子力顯微鏡 (AFM)、穿透式顯微鏡 (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，結果顯示在0.2度的基板上成長樣品，可以有效減少樣品內的差排密度 (dislocation density)，進而改善氮化銦鎵/氮化鎵多重量子井之均勻性，呼應光學特性之量測結果，元件特性量測結果也顯示成長在0.2度基板之元件，其輸出功率增加了30 %。 (2) 氮化銦鎵/氮化鎵多重量子井發光二極體之內部量子效率隨雷射激發強度變化之物理機制探討 我們藉由變功率光激發螢光和變功率時間解析光激發螢光等光譜技術去探討15 K和300 K時，影響氮化銦鎵/氮化鎵多重量子井發光二極體內部量子效率 (internal quantum efficiency, IQE) 之物理機制，藉由分析光譜能量分佈和載子生命期 (carrier lifetime) 隨雷射激發強度之變化情形，發現氮化銦鎵/氮化鎵多重量子井發光二極體之內部量子效率會隨著雷射激發強度而變化，主要受到量子井內之量子侷限史塔克效應 (quantum confined Stark effect, QCSE) 與能帶填滿效應 (Band filling effect) 所影響，而在300 K時，非輻射複合 (nonradiative recombination) 之抑制需加入考慮。

In this thesis, the optical characteristics and internal quantum efficiency (IQE) of InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) has been studied. And the study consists of two topics as following: (i) Optical properties of InGaN/GaN multiple quantum well light emitting diodes grown on sapphire substrate with different misorientation angle The optical properties of InGaN/GaN MQW LEDs grown on sapphire substrate with different misorientation angle toward [11-20] direction have been studied by temperature dependent PL and time resolved photoluminescence (TRPL), μ-PL and excitation power dependent PL. From room termperature PL results, we observed the full width at half maximum (FWHM) of spectra decreases as misorientation increases from 0o to 0.2o, but it increases as misorientation angle increases above 0.2o. The emission energy mapping images of μ-PL indicate that the fluctuation of emission energy in InGaN QW is influenced by the misorientation angle of sapphire substrate. The temperature dependent PL results shows the strong carrier localization effect, i.e., s-shaped temperature dependence of the PL emission energy, and it is strong dependence on sapphire substrate misorientation angle. And the sample grown on 0.2o sapphire substrate exhibits the weakest carrier localization effect and has smallest effective barrier of localized states. Moreover, the recombination dynamic of carriers and the dimensionality of nanostructure in InGaN/GaN QW have also been studied by temperature dependent TRPL measurement. The power dependent PL was performed to analyze the degree of band filling. All results indicate that the degree of potential fluctuation in InGaN/GaN MQWs can be decreased by using sapphire substrate with misorientation angle of 0.2o. On the other hand, the material properties of samples have been investigated by atomic force microscope (AFM), high resolution transmission electron microscopy (HRTEM), high resolution X-ray Diffraction (HRXRD) and Raman spectroscopy. The results show the dislocation density in LED can be decreased by using sapphire substrate of 0.2o and thus improves the homogeneity of InGaN/GaN MQWs. Moreover, the output power of LED is increased by approximately 30 % by using sapphire substrate with misorientation of 0.2o.

(ii) Physical mechanisms of excitation power dependent internal quantum efficiency in InGaN/GaN multiple quantum well light emitting diodes This research intends to investigate the physical mechanisms of excitation power dependent internal quantum efficiency (IQE) in InGaN/GaN MQW LEDs at temperature of 15 K and 300 K. The dependence of the IQE on excitation power density has been observed. From a detailed analysis of excitation power dependent emission energy, FWHM of spectra and carrier recombination dynamic by time-resolved PL (TRPL), we confirm the variation of IQE with increasing excitation power is due to coulomb screening of quantum confined Stark effect (QCSE) and band-filling of localized states in InGaN/GaN QW. Moreover, at temperature of 300 K and low excitation power density, the nonradiative recombination has to be taken into account.