| dc.description.abstract | 本論文利用有機金屬氣相沉積法(Metal-organic Chemical Vapor Deposition)成長氮化物材料，並針對氮化物光電半導體材料特性與元件特性進行分析與探討，研究內容主要以氮化鎵薄膜與氮化物藍光發光二極體(Nitride-based Blue LED)為主，包含開發新式緩衝層結構、奈米尺寸基板之懸空磊晶技術(pendeo-expitaxy)研究與改變氮化物藍光發光二極體晶粒幾何結構之製程開發。
首先在緩衝層研究上，我們開發出新式緩衝層－氮化鎂/氮化鋁 (MgxNy/AlN)雙層緩衝層(buffer layer)結構，並藉由此緩衝層結構特性來降低錯位(dislocation)的產生，由X-ray分析及霍爾量測結果得知使用氮化鎂/氮化鋁雙層緩衝層確實可提升氮化鎵薄膜品質，利用計算腐蝕坑密度 (etching pits density, EPD)，可知其錯位缺陷密度可有效地由傳統使用低溫氮化鋁緩衝層~3.7×108 cm-2降低至9.0×107 cm-2。
懸空磊晶技術方面，打破一般傳統懸空磊晶技術需要曝光顯影(lithography)製程，利用鎳(Ni)當金屬蝕刻遮罩(metal hard mask)，製作出具高深寬比之氮化鎵奈米柱基板(nano-rod GaN template, NR GaN template)，藉由調整磊晶成長條件，直接將不具緩衝層結構之氮化鎵薄膜成長於此氮化鎵奈米柱基板，由霍爾量測與X-ray分析結果判斷，相較於傳統利用緩衝層成長氮化鎵薄膜，此懸空磊晶法成長氮化鎵薄膜於氮化鎵奈米柱基板確實可獲得較佳品質，計算其腐蝕坑密度(etching pits density, EPD)，發現錯位密度可減少至1.9 × 108 cm-2，接續此實驗，更進一步將n-GaN/MQW/p-GaN LED結構直接成長於氮化鎵奈米柱基板，節省了緩衝層與無摻雜氮化鎵層磊晶時間，比較傳統發光二極體(conventional LED, C LED)與成長於氮化鎵奈米柱基板之發光二極體( NR LED)，由其光電特性量測結果來看，NR LED在電性上與C LED無太大差異，在20 mA注入電流下之驅動電壓約3.30 V，同時，NR LED光輸出功率比C LED提升約39.8％，其中光輸出功率提升的主因，大致分成內部量子效率與光萃取能力兩方面探討。以光萃取能力而言，因懸空磊晶技術成長易形成空隙(void)，此空隙影響的折射率變化增強了光散射效果，進而提升了光萃取率；另一方面，透過變溫光致螢光(Photoluminescence : PL)量測結果，發現NR LED內部量子效率16.9％高於傳統LED14.9％，此為另一影響光輸出功率的因素。
最後針對提升LED光萃取效率，著力於改變LED晶粒幾何結構，有效運用了氮化鎵N-face高化學活性特性，開發不需曝光顯影製程的濕式酸蝕刻方式，過程中將晶粒製程後的LED鍍上SiO2當蝕刻遮罩層，接續浸泡於攝氏250度硫酸：磷酸=3：2混合酸液中4分鐘，此時LED晶粒四周被蝕刻出倒金字塔結構，最後利用氫氟酸與氟化銨(HF/NF4H)混合成之緩衝溶液(Buffered Oxide Etchant, BOE)除去殘餘SiO2，完成具有倒金字塔結構LED晶粒製程 (inverted pyramid sidewalls LED, IPS LED)，由光特性量測結果發現，相較於傳統LED而言，IPS LED光輸出功率上提升了約27％，進一步探究其原因，透過中央大學武茂仁老師實驗室提供協助，利用finite-difference time-domain (FDTD)模擬此倒金字塔結構對光輸出功率的影響，發現倒金字塔結構具有較佳的光子侷促能力，其模擬結果說明光輸出功率可增強24％，此模擬數據與實際結果27％也十分相近；另一方面，利用光功率密度量測，發現IPS LED晶粒四周圍繞著一圈光環，此光圈強度明顯高於傳統LED晶粒四周的強度，此結果也與模擬的結果相呼應，充分說明倒金字塔結構的確能提升光輸出功率。
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| dc.description.abstract | Through metal-organic chemical vapor deposition (MOCVD) to grow nitride-based materials, this dissertation mainly analyzes the characteristics of the nitride-based materials and optoelectronic semiconductor. The research focuses on GaN film and nitride-based blue LED, including advanced buffer layers, pendeo-expitaxy on nano-scaled template, and the development of nitride-based blue LED with the geometric sidewalls.
On one hand, we developed a double buffer layers- MgxNy/AlN. Judging from the result of X-ray analysis and Hall measurement, the high quality GaN could be obtained by the features of these buffer layers to reduce dislocation. Compared with GaN on conventional AlN buffer layer and MgxNy/AlN double buffer layers, the dislocation density of GaN film can be effectively reduced from 3.7×108 cm-2 to 9.0×107 cm-2 from the result of etching pits density (EPD).
On the other hand, we broke fresh ground in pendeo-expitaxy without lithographic process. We produced nano-rod GaN template (NR GaN template), with high aspect ratio by Ni as metal hard mask. By adjusting the growing condition, GaN was directly grown on NR GaN template without buffer layers. Judging from the result of Hall measurement and X-ray analysis, implementing pendeo-expitaxy to grow GaN film on the NR GaN template can acquire better quality. From result of EPD, the dislocation density of GaN film could be reduced to 1.9×108 cm-2 by using NR GaN template. In addition, the following experiment makes further research on n-GaN/MQW/p-GaN LED structure directly grown on the NR GaN template. This has been profited by saving growth time of buffer layer and unintentionally doped GaN layer when LED structure was grown on nano-rod GaN template. In terms of the optoelectronic characteristics, there are no obvious differences in electronic properties between NR LED and C LED. The forward voltage is around 3.30V under the electronic current at a 20mA injection; in the meanwhile, the output power of NR LED promotes about 39.8% more than the C LED. The main factor in enhancing the light output power is focused on light extraction and internal quantum efficiency. Not only does the growth of pendeo-expitaxy form the void between the GaN/sapphire interface which increases the effects upon light scattering for light extration, but also the efficiency of internal quantum in NR LED up to 16.9% higher than in C LED which is only about 14.9%.
Last but not least, we enhance the light extraction efficiency of LED by making changes in geometric structure of LED chip die. We applied the feature of high chemical activity of GaN N-face and developed wet etching without lithographic process to change the sidewall of LED chip die. After the chip process, LED was deposited with SiO2 as hard mask, and then soaked in the mixed acid H2SO4 : H2PO4 = 3 : 2 at 250 oC for four minutes. After this wet etching process, the edge of LED chip die was etched as inverted pyramid structure. Finally, the residue of SiO2 was removed by utilizing buffered oxide etchant (BOE) to complete the fabrication of inverted pyramid sidewalls LED (IPS LED). Compared to conventional LED, it was found that the light output power of IPS LED has enhanced about 27% through the measurement of light characteristics. To go a step further, with the help of Prof. Wu in National Central University, it was found that inverted pyramid structure performs better on confined photons judging from the simulation of by finite-difference time-domain (FDTD). This statistics 24 % is similar to the practical result which reaches to 27%. Further, by taking advantage of the measurement of light power density, a light rectangle was discovered around the edge of IPS LED chip die. The intensity of the light rectangle is obviously higher than the conventional LED. This result responds to the stimulated statistics as well as interprets that inverted pyramid structure can indeed enhance light output power.
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