臺灣博碩士論文加值系統

氮化鎵材料具有許多優點，因此被廣泛應用在光電與電子元件上。氮化鎵的能隙大小為3.4eV，發光波長涵蓋了可見光、紅外光與紫外光波段，因此適合發展光電元件。寬能隙的特性，讓元件操作於更高的電壓，因此氮化鎵材料非常適合應用於高壓電子元件上。然而，氮化鎵之材料缺少氮化鎵基板，因此造成晶體品質不佳。如何提高氮化鎵之品質是個很重要的課題。於本論文中，使用嵌入式之週期性結構來提升氮化鎵的品質，進一步提升光電與電子元件之效率。
第二章節將介紹此技術應用於發光二極體並探討其物理意義。使用嵌入式氧化矽於火山口圖案型藍寶石上，能夠提升發光二極體的內部量子效率與光萃取率。提升內部量子效率之原因主要來自於：(1) 改善了氮化鎵之晶體品質，由於氮化鎵於氧化矽上之側向成長；(2) 抑制了量子侷限史塔克效應，由於應力之釋放。另外此方法也提升了光萃取效率，提升之原因是由於角錐型形狀的空氣與嵌入式的氧化矽增加了光逃逸出發光二極體之機率。藉由製作嵌入式氧化矽於火山口圖案型藍寶石上，於電流操作350mA，提升了72%之發光效率。
第三章節將介紹此技術應用於高電子遷移率電晶體。藉由週期性氧化矽圖形結構設計於氮化鎵/藍寶石基板上，進而降低線缺陷之數目，降低線缺陷貢獻來自於氧化矽上之側向成長。我們使用黃光二次對準之技術，將高電子遷移率電晶體元件對準於圖案化之氧化矽上，並且探討高電子遷移率電晶體之直流特性。從實驗結果，藉由設計週期行氧化矽圖形，提升了飽和電流與降低導通電阻。主要原因來自於提升晶體之品質，降低線缺陷散射機率，進而提高二維電子氣之遷移率。另外，藉由控制氮化矽於一個週期內之比例，來控制蝕刻孔洞密度數量，此蝕刻孔洞密度代表品質之好壞。於實驗結果驗證，隨時蝕刻洞密度之降低，飽和汲極電流隨之提高。和傳統之高電子遷移率電晶體比較，飽和電流由原本的380mA/mm提升到410mA/mm。

GaN-based materials have drawn much attention for application to optoelectronic devices owing to their nature of wide bandgap. GaN has wide bandgap of 3.4eV, which provides special properties for applications in light-emitting diode (LED) and high-electron-mobility transistor (HEMT). For LED, the bandgap of GaN-based materials can be tuned by the alloy composition such that the emission wavelength covers the entire visible spectrum, while for HEMT, the wide bandgap material of GaN permits device to be operated at higher voltages, frequencies and temperatures than conventional silicon material. However, GaN-based materials suffer from poor crystal quality due to the lack of native substrate. For GaN-based materials, heteroepitaxial process was employed to reduce the high expense of substrate. Unfortunately, in the heteropitaxial process, high defect density is caused by the large lattice mismatch and thermal expansion between epitaxial GaN layer and foreign substrates. Consequently, this adversely degrades device performance and raises the issue about the long-term reliability of device. In this dissertation, we introduce the embedded periodic SiO2 structure to the GaN-based LED and HEMT to effectively improve the quality of epitaxial GaN layer, and therefore the device performance is also enhanced.
More specifically, the external quantum efficiency (EQE) of the GaN-based LED is determined by the internal quantum efficiency (IQE) and the light-extraction efficiency (LEE). A low IQE can be caused by low quality of epitaxial GaN layer. This phenomenon is because of the non-radiative recombination resulting from the defect level. In this study, highly efficient GaN-based LEDs were grown on volcano-shaped patterned sapphire substrates with embedded SiO2 (SVPSS). Raman spectroscopy and transmission electron microscopy revealed that the LEDs grown on the SVPSS had high internal quantum efficiency which is the outcome of the more relaxed compressive strain and the fewer threading dislocations in the GaN epitaxial layers. Experimentally measured data and ray-tracing simulations suggested that the enhancement in the light extraction efficiency was due to the light scattering effect and the gradual refractive index matching. The former arises from the conical air voids and the latter results from the embedded SiO2. Compared with a conventional LED operated at an injection current of 350 mA, the light output power from the LED grown on the SVPSS was increased by 72%.
On the other hand, the buffer leakage current of the GaN-based HEMT is caused by higher background carrier density in GaN buffers. A high background carrier density can arise from low quality of epitaxial GaN layer due to high concentrations of deep center. Here, we proposed a method to grow the high-performance GaN-based HEMT on SiO2 patterned GaN template (SGT). The proposed method combines the advantages of epitaxial lateral overgrowth method and selective-area high-quality GaN epitaxial layer approach by designing patterns of alignment. From the experiments, the measured data showed that the saturation current density and ON-resistance can both be improved, since the quality of the GaN epitaxial layer for active region was enhanced by ELOG technology. Moreover, the experimental trend showed that as the etching pit density (EPD) was reduced, the saturation current density and ON-resistance would be improved. The saturation current density for HEMT grown on SGT was increased from 380mA/mm to 410mA/mm as compared to the one grown on conventional GaN template (CST).

審定書 I
英文審定書 II
致謝 III
中文摘要 VI
Abstract VIII
Contents XI
List of Figures XIII
List of Table XVI
Chapter 1 Introduction 17
1.1 Why GaN-based Materials? 17
1.2 The Issue of GaN-based Materials 18
1.3 Literature Review 20
1.4 The organization of this doctoral dissertation 22
Chapter 2 Enhanced performance of InGaN-based light-emitting diodes grown on volcano-shaped patterned sapphire substrates with embedded SiO2 28
2.1 Introduction 28
2.2 Experimental 30
2.3 Results and Discussion 32
2.4 Conclusions 40
Chapter 3 Effects of dislocation reduction on AlGaN/GaN HEMTs using ELOG technology 49
3.1 Introduction 49
3.2 Experimental 51
3.3 Results and Discussion 53
3.4 Conclusions 57
Chapter 4 Summary and Future Work 70
4.1 Summary 70
4.1 Future Work 71
References 72
Appendix : Publication List 81

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