臺灣博碩士論文加值系統

在製作以藍寶石為基板的氮化鎵(GaN)類發光二極體(Light-Emitting Diodes, LEDs)時，由於磊晶層與基板間的晶格常數及熱膨脹係數不同，將使磊晶層受到壓縮應力(Compressive Strain)，並產生大量穿隧差排(Threading Dislocations, TDs)，導致降低磊晶層的品質及LED發光效率。透過圖案化藍寶石基板(Patterned Sapphire Substrates, PSSs)是一種可以有效減少壓縮應力和穿隧差排的方法，並且可以提高光萃取率和發光效率。
本篇論文將探討在PSS的設計中，如何調變表面結構來減少穿隧差排密度及壓縮應力。吾人利用電子束微影系統搭配我們的濕蝕刻技術，在C-Plane的藍寶石基板上製作一系列不同週期、不同結構大小、但深度同為1200奈米的表面微結構，並使用有機金屬化學氣象沉積系統來磊晶。透過PL光致激發系統及Raman光譜儀系統，我們發現在基板表面微結構相互夾止成特殊”鷹嘴狀”或是”三芒星狀”時，將達到主動層之穿隧差排密度的最低點，而其LED元件也擁有最佳的發光效率。在其中，我們證明效率衰減起因於主動層之穿隧差排，而且以四方(Cubic)排列之週期性結構相較於六方(Hexagonal)排列的結構，擁有較佳之發光效率。

In the light-emitting diodes (LEDs) technology, we usually use sapphire for the substrate. Due to the large lattice mismatch between GaN and Sapphire substrate, GaN will get compressive strain from substrate and exhibits lots of threading dislocations(TDs), decreasing the device efficiency. To solve these problems, patterned sapphire substrates (PSSs) is a common technology.

In this research, we fabricated a series of PSSs by our wet etching method and E-beam lithography system. After the epitaxy via MOCVD system, we use micro-PL, micro-Raman system and X-Ray diffraction system to measure the MQWs TDs density, Bulk TDs density, and Compressive stress. We find that the MQWs TDs can be reduced when the Bottom C-plane shape pinch off to special “Engel Beak” or “Benz mark” shapes. In the device level we use Electro Luminescence (EL) for measurement. Demonstrating that efficiency droop can be reduced when bottom shape pinch off as well. After our analysis, the decreasing of efficiency droop should be attributed to the decreasing of MQWs TDs Density. Besides, we proved that cubic arrangement is better than hexagonal arrangement in PSS design. A detailed research of epitaxial model will be discussed in this thesis.

致謝 I
中文摘要 II
AbstractIII
目錄 IV
圖目錄 VI
表目錄 VIII
一、 導論 1
1.1 前言 1
1.2 動機 3
二、 理論基礎與材料分析 6
2.1 藍寶石基板簡介 6
2.2 氮化鎵薄膜簡介 9
2.2.1 晶體結構 9
2.2.2 應力 11
2.2.3 錯位差排 14
2.2.4 晶體成長 17
2.3 氮化鎵類發光二極體 20
2.3.1 V字形凹孔(V-pit)與區域位能障礙 20
2.3.2 極化 21
2.3.3 氮化鎵類發光二極體之效率衰減 24
2.4 拉曼光譜(Raman Spectrum)的量測原理 26
2.5 拉曼光譜選擇律(Selection Rule) 28
2.6 蝕刻原理 29
2.6.1 蝕刻種類 29
2.6.2 濕式時刻原理 31
2.7 光萃取率LEE 33
2.7.1 模擬軟體介紹 33
2.7.2 模擬參數設定 33
三、 實驗儀器與樣品製備 35
3.1 實驗儀器 35
3.1.1 電子束微影系統(E-Beam Lithography) 35
3.1.2 電子槍蒸鍍系統(E-Gun) 37
3.1.3 感應式耦和電漿蝕刻(ICP-RIE) 38
3.1.4 掃描式電子顯微鏡(SEM) 39
3.1.5 有機金屬化學氣象沉積(MOCVD) 40
3.1.6 光致激發螢光量測系統(µ-PL) 41
3.1.7 拉曼光譜量測系統(µ-Raman) 44
3.1.8 X-Ray繞射儀(XRD) 48
3.1.9 電致激發螢光光譜(EL)與積分球(IS)量測 54
3.2 樣品製備 55
四、 實驗結果與分析 61
4.1 實驗設計 61
4.2 量測分析(LED磊晶層) 67
4.2.1 樣品 主動層缺陷密度 比較與分析 67
4.2.2 樣品 磊晶層缺陷密度 比較與分析 69
4.2.3 樣品 內部應力 比較與分析 73
4.2.4 樣品篩選 76
4.3 量測分析(LED發光元件) 78
4.3.1 效率衰減(Efficiency Droop)比較 78
五、 結論 80
六、 參考資料 81

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