Influences of Silicon-doping and Interfacial Layers on the Optical Characteristics and Nanostructures of InGaN/GaN Quantum Wells

本論文第一部份中，我們有系統的研究不同銦組成及矽摻雜條件下氮化銦鎵/氮化鎵多重量子井結構內光學及材料微結構特性，並提出一個位能變化的模型來說明矽摻雜效應。其光學特性的量測包括光激發螢光光譜、激發螢光吸收光譜, 陰極激發螢光光譜，時間解析之激發螢光光譜, 以及自發放大放射譜。其結果顯示矽摻雜於位障層之氮化銦鎵/氮化鎵量子井樣品內，有較弱的壓電場，因此量子侷限史塔克效應較小，然而其載子侷限最強，產生近似量子點特性，因此，其發光強度與增益特性大為提升。在相同矽摻雜條件下，高銦濃度者有較強的位能變化與載子侷限效果。 材料的奈米微結構之應變分析上進一步證實在具有矽摻雜的樣品中會形成較多的銦聚集結構，此現象在位障層摻雜的樣品更為明顯。同時，在相同矽摻雜條件下，高銦濃度有較低銦濃度樣品更明顯的銦聚集特性。而在量子井界面特性上，高銦濃度樣品銦擴散出量子井至位障層也因矽摻雜而明顯增強。 第二部份的研究中，我們探討在有位障層摻雜的氮化銦鎵/氮化鎵多重量子井結構中具有介於量子井及位障層之間的四種薄界面層(約一奈米厚) 樣品，這四種界面層分別是氮化銦、矽摻雜的氮化銦、矽掺雜的氮化銦鎵以及矽掺雜氮化鎵。研究結果顯示在兩個具有氮化銦界面層的樣品中，光激發螢光及電激發螢光光譜的發光效率都大幅提升，特別是在具無矽摻雜氮化銦界面層的樣品中效果特別明顯。由激發螢光吸收光譜及奈米微結構之應變分析可說明量子井界面品質的改善，也就是降低了銦由量子井至位障層的擴散乃是促使發光效率提升的主要因素。另外，具有矽掺雜的氮化銦鎵界面層的樣品中，其發光效率還是比一般樣品高，我們認為此樣品在該成長條件下呈現較一般樣品有較強的載子侷限特性。總之，加入氮化銦或氮化銦鎵界面層在該成長條件下的確能改善光學特性。In this dissertation, we systematically study the optical characteristics and associated nanostructure of InGaN/GaN multipl-quantum-well samples of different average indium contents and different silicon doping layers. A nanostructure model is built for describing the potential fluctuation differences between the samples. From the results of strain state analysis, we find that indium-rich clustering is the strongest in the barrier-doped sample, followed by the well-doped sample and then un-doped sample among the samples of the same average indium content. Among the samples of the same doping condition, the higher indium content leads to stronger clustering behavior. Based on the model, the dominance of either carrier localization or quantum-confined Stark effect (QCSE) determines the optical behaviors. Generally speaking, QCSE results in the S-shape of photoluminescence (PL) peak position, strong photoluminescence excitation (PLE) intensity, large Stokes shift, and long PL decay time. On the other hand, carrier localization leads to the blue shift of PL peak position, high radiative efficiency (also due to weaker QCSE), PL decay time enhancement, and the behavior of 3-D-like confinement in radiative decay time. Meanwhile, the amplified spontaneous emission (ASE) behaviors showed that stronger carrier localization in the cases of stronger clustering behaviors leads to higher ASE intensities or gains. In temperature-dependent variations, the weaker carrier localization results in the evolution of a two-peak spectral ASE feature at low temperatures into a broad spectrum with Fabry-Perot resonance. In a strongly clustering sample, particularly in the sample of high-indium-content and barrier doping, the ASE spectrum evolves from a broad spectrum with Fabry-Perot resonance at low temperatures into a single major peak feature at room temperature. Such evolution is attributed to carrier liquidation when carriers gain thermal energy. We also compare four InGaN/GaN multiple quantum-well (QW) samples of different interfacial layers in optical property and nano-structure. In two of the samples, InN interfacial layers are placed between wells and barriers for improving the QW interface quality. Compared with a standard barrier-doped QW sample, the addition of the InN interfacial layers does improve the QW interface quality and hence the photon emission efficiency. The insertion of intrinsic InN layers in a sample leads to a reasonably good QW structure and the highest PL and electro-luminescence (EL) efficiencies. However, clustering structures are observed, resulting in carrier localization for a strong S-shape variation in PL spectral peak, a relatively strong PLE intensity, and a sharp PL decay time variation beyond its peak in temperature dependence. With silicon-doped InN interfacial layers, another sample shows the highest QW quality and relatively higher PL and EL emission efficiencies. It is speculated that both carrier localization and QCSE are relatively weaker in this sample. Then, the broadening of InGaN well layer in another sample by inserting silicon-doped InGaN interfacial layers leads to quantum dot-like structures and the strongest carrier localization. Therefore, in this sample we observe quite high PL and EL efficiencies, increasing EL spectral peak position with temperature, strong PLE intensity, and sharp PL decay time variation beyond its peak in temperature dependence. Compared with the aforementioned samples, the normally used QW sample (a reference sample) shows the lowest PL and EL emission efficiencies, the lowest PL and EL emission photon energies, the weakest PLE intensity, and generally longest PL decay times. It is speculated that the quantum-confined Stark effect is strongest in this sample.中文摘要……………………………………………………………….i Abstract………………………………………………………………iii Contents……………………………………………………………...vi Chapter 1 Overviews of Nitrides Compounds 1.1 Applications of Nitride-Based Materials………………2 1.2 Crystal Structure of Nitride…………………………3 1.3 Substrates for Nitride Compounds…………………….4 1.4 P-type GaN…………………………………………….5 1.5 Defects in GaN………………………………………6 1.6 Characteristics of InGaN/GaN MQWs………………...8 1.6.1 Spontaneous Polarization and Stress/Strain Effect………8 1.6.2 Spinodal Decomposition and Phase Separation…….……11 1.6.3 Kinetic Growth Effects…………………………………13 1.6.4 Carrier Localization and Quantum-confined Optical Stark Effect in InGaN/GaN QWs……………………………14 1.6.5 Silicon Doping Effect on InGaN/GaN Quantum Well…16 1.7 Strain-State Analysis (SSA)…………………………..17 1.8 Motivations of This Study……………………………20 1.9 Organization of This Dissertation……………………23 References…………………………………………………24 Chapter 2 Silicon Doping Effects on Green-luminescence InGaN/GaN Multiple-quantum-well Structures 2.1 Introduction……………………………………………39 2.2 Sample Preparation and Measurement Conditions……40 2.3 Photoluminescence and Photoluminescence Excitation.... .......................................................................................42 2.4 Cathodoluminescence (CL)..........................................46 2.5 Strain State Analysis.....................................................48 2.6 Discussions...................................................................50 2.7 Summary.......................................................................56 References………………………………………………58 Chapter 3 Effects of Silicon Doping Condition on the Optical Properties of InGaN/GaN Multiple-quantum well Structures with Different Average Indium Contents 3.1 Introduction……………………………………………77 3.2 Sample Description and Experimental Procedures…77 3.3 Photoluminescence and Photoluminescence Excitation Studies…………………………………………………78 3.4 Time-resolved Photoluminescence Study……………82 3.5 Strain State Analysis Images……………………….85 3.6 Discussions……………………………………………86 3.7 Summary………………………………………………89 References…………………………………………………91 Chapter 4 Amplified Spontaneous Emission Behaviors in InGaN/GaN Multiple-quantum-well Structures of Various Silicon-doping Conditions 4.1 Introduction…………………………………………105 4.2 Experimental Procedures………………………….105 4.3 Results of Amplified Spontaneous Emission………106 4.4 Discussions…………………………………………114 4.5 Summary……………………………………………120 Reference………………………………………………122 Chapter 5 Effects of Interfacial Layers in InGaN/GaN Quantum Well Structures on Their Optical and Nanostructure Properties 5.1 Introduction…………………………………………135 5.2 Sample Preparation and Measurement Methods…….137 5.3 Results of Optical Characterization………………….139 5.4 Cathodoluminescence Study…………………………145 5.5 Strain State Analysis…………………………………146 5.6 Discussions…………………………………………..148 5.7 Summary……………………………………………..152 References………………………………………………..154 Chapter 6 Conclusions……………………………………………...170 Publication List……………………………………….17