臺灣博碩士論文加值系統

本論文研究主題為N型氮化鎵磊晶薄膜在KOH蝕刻溶液中所形成的六角錐結構之成長機制。由於MOCVD磊晶成長之氮化鎵薄膜的線缺陷密度與分布可以藉由藍寶石基板的圖形來控制，透過SEM及TEM的觀察，可以發現經KOH蝕刻後的N型氮化鎵表面結構與線缺陷分布有很大的關連性，因此，在論文中提出由線缺陷主導的六角錐成長機制。一般來說，N型氮化鎵經由KOH蝕刻過後都是顯露出由(10-1-1)平面所構成的六角錐結構，但將圖形化藍寶石基板剝離後的N型氮化鎵表面，經由KOH蝕刻卻顯露出由(10-1-1)及(10-1-3)平面所構成的火山口結構，這兩個平面與(0001)平面的夾角分別為58度及32度。將此火山口結構應用到覆晶型薄膜LED上，相較於六角錐結構之表面，提升了27%的發光強度。
在第三章中，利用多光束KrF準分子雷射在N型氮化鎵表面蝕刻生成圓孔洞結構，當N型氮化鎵表面具有圓孔洞結構，會影響KOH溶液的蝕刻，並在孔洞中形成(10-1-3)平面，藉由不同傾斜度的(10-1-1)及(10-1-3)平面複合所構成的表面，可以有效提升垂直型氮化鎵LED的發光效率達20-30%，論文中我們提出了再捕捉效應來解釋其複合平面造成LED發光效率提升的原因。

In this thesis, GaN-based LED was fabricated to study the wet etching mechanism of GaN epitaxial thin film by hot KOH solution. The dislocation density and distribution were determined by patterned sapphire substrate epitaxy growth. Surface morphology evolution and dislocation distribution of GaN epilayers have been investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A correlation is observed between the threading dislocation distribution and the etching pattern on the N-GaN surface. It implies that the threading dislocation seemingly guides the etching on the N-GaN surface. A dislocation-orientated etching mechanism on the N-GaN surface is proposed in this study. With wet-etching process in KOH solution, the morphology of the etching N-GaN surface evolved from the inverted circular cavities to volcanic craters. Interestingly, the inner side-wall of volcanoes is composed by (10-1-1) and (10-1-3) planes, which intersects with the c-plane GaN about 58o and 32o, respectively. With forming the volcanic pattern on the N-GaN surface, the optical power TFFC-LED can be enhanced by 27 %.
In Chapter 3, concave downward cavities, which are created on the N-GaN surface by multi-beam KrF-laser ablation, are used to investigate the formation mechanism of the (10-1-3) planes. This study shows that KrF-laser-ablated cavities enhance the light-extraction efficiency of the KOH-etched pyramidal N-GaN surface by 25%. With further etching by KOH, the curved-surface sidewall of laser ablated cavities do not form pyramids; instead, relatively large inclined facet sidewalls are formed in the laser-ablated cavities. We believe that these inclined facet sidewalls in laser-ablated cavities further enhance the light-extraction efficiency of KOH-etched pyramidal N-GaN surfaces.

Abstract (Chinese) I
Abstract (English) II
Table of Contents III
List of figures IV
List of tables VI
Chapter 1 Background and Motivation 1
1.1 Introduction 1
1.1.1 The efficiency of the GaN-based LED 1
1.1.2 Total Internal Reflection Loss 2
1.1.3 Fresnel Reflection Loss 3
1.2 Fabrication process of vertical thin-GaN LED 5
Chapter 2 Light extraction improvement by forming volcanic crater on N-polar GaN emitting surface 7
2.1 Introduction 7
2.2 Experimental 8
2.3 Result and Discussion 9
2.3.1 N-GaN Etching morphology by alkali solution 9
2.3.2 N-GaN Etching mechanism in alkali solution 12
2.3.3 Optical efficiency of TFFC-LEDs 14
Chapter 3 Multi-beam KrF laser ablation on N-GaN surface 18
3.1 Introduction 18
3.2 Experimental 18
3.3 Result and Discussion 19
3.4 Light extraction enhancement by cavity-array textured N-polar GaN surface by KrF laser ablation 27
Chapter 4 Conclusion 37
References 38

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