Investigation of stress on GaN epitaxial layer prepared on various Patterned Sapphire Substrates and its application
|關鍵字:||氮化鎵;發光二極體;光激發螢光光譜;拉曼光譜;圖案化藍寶石基板;Gallium Nitride;Light Emitting Diode;Photoluminescence;Raman Spectroscopy;Patterned Sapphire Substrate|
首先進行氮化鎵磊層成長於四種不同尺寸的圓錐狀圖案化藍寶石基板之磊晶製程，而後分別對氮化鎵磊晶層進行光激發螢光光譜及拉曼光譜儀(Raman Spectroscopy) 量測。光激發螢光光譜量測的部分顯示，其量測光譜中會檢測出雙波峰現象，並藉由一系列實驗證實了雙波鋒現象分別為氮化鎵磊晶層成長於圖案化藍寶石基板的c-plane區域及圖案化區域所產生之螢光光譜波峰。氮化鎵磊晶層的拉曼光譜儀(Raman Spectroscopy)量測部分顯示，磊晶層所受之應力大小與圖案化藍寶石基板c-plane區域占表面積之比例呈正相關，c-plane區域占表面積之比例越低，則氮化鎵磊晶層所承受之應力越小。最後歸納出將雙波峰現象應用於檢測氮化鎵磊晶層應力的方法，由觀察氮化鎵磊晶層之背向光激發螢光光譜量測的長波長波峰判斷，其波長藍移即代表氮化鎵磊晶層應力降低，反之亦然。
關於應用四種不同尺寸的圓錐狀圖案化藍寶石基板的氮化鎵發光二極體元件的光電特性，其中c-plane區域占圖案化藍寶石基板表面積最小之樣品，在20mA操做電流下之光輸出功率為最佳的11.1mW，比起光輸出功率最低之樣品提高了14.0%，此差異之主因為c-plane區域占圖案化藍寶石基板表面積越少，代表圖案化區域增加，增加主動層發出的光離開氮化鎵二極體薄膜的機率，使得其光萃取效率提高，故c-plane區域占圖案化藍寶石基板表面積比例最低之樣品，其相對光萃取效率為實驗樣品中最高的37.8%，比起c-plane區域占圖案化藍寶石基板表面積比例最高者提高了5.5%; 同時c-plane區域占圖案化藍寶石基板表面比例最低之樣品，其薄膜承受應力最小，量測所得之相對內部量子效率為56.5%，比起c-plane區域占圖案化藍寶石基板表面積比例最高者提高了2.3%，因其薄膜承受應力較小，氮化鎵發光二極體主動區內之壓電場效應也隨之減小，能帶傾斜的現象獲得減緩，使得樣品之相對內部量子效率提高，同時也使得氮化鎵發光二極體元件具有較低的效率下滑(Efficiency droop)。|
In this dissertation we investigated properties of GaN epitaxial layer prep- ared on four kinds of different pattern size cone-shaped patterned sapphire su- bstrate (PSS). First, we discuss the stress in GaN epitaxial layer and film quali- ty of GaN epitaxial layer. Moreover, we applied the PSS mentioned above ser- ve as template for GaN-based LED structure growth. Furthermore we discuss the optical and electrical properties of light emitting diodes (LEDs) device aff- ected by GaN epitaxial layer stress. First, we obtain GaN epitaxial layer prepared on four kinds of different pattern size cone-shaped PSS samples through epitaxial process. The samples were measured by photoluminescence (PL) and Raman spectroscopy measure- ment system afterward. A two peak phenomenon was detected in the PL mea- surement spectrum result. The origination of two peak phenomenon was inves- tigated through designing various PL measurement experiments. It was fou nd that the PL two peak phenomenon is a combination of PL spectrum of GaN epitaxial layer grown on c-plane and patterned area of PSS. It was also found that the PL two peak phenomenon would be more significant when GaN epi- taxial layer was prepared on PSS with smaller pattern spacing. The Raman sp- ectroscopy results indicate that the stress in GaN epitaxial layer is related to the c-plane surface area ratio of PSS. GaN epitaxial layer prepared on PSS with lower c-plane surface area ratio has lower stress, and vice versa. At last, we suggested a method for estimating stress in GaN epitaxial layer by obser- ving the peak wavelength of the PL two peak phenomenon. The optical and electrical properties of LEDs device prepared on four kinds of different pattern size cone-shaped PSS were also been discussed. The LED prepared on PSS with lower c-plane surface area ratio has higher light output power. The light output power of the sample at 20mA operation current is 11.1mW, which is 14.0% higher than the sample with lowest light output power. It was found that relative light extraction efficiency and relative inner quantum efficiency of LED samples is related to the c-plane surface area ratio of PSS. The relative light extraction efficiency of LED sample with lower c-plane surface area ratio is 37.8%, which is 5.5% higher than the sample with the lowest relative light extraction efficiency. Furthermore the relative internal quantum efficiency (IQE) of LED sample with lower c-plane surface area ratio is 56.5%, which is 2.3% higher than the sample with the lowest relative IQE. The cause of the relative IQE difference could be attributed to the different stress in the epitaxial layers. With lesser stress in the GaN-based LED film, the film was revealed less piezoelectric effect therefore reduce the band bending in active region of LEDs. The result of this phenomenon is higher IQE, meanwhile causing the LED to achieve lower efficiency droop.