利用奈米壓印技術提升氮化鎵發光二極體之元件效率

Abstract

因為氮化鎵其材料及發光二極體元件的優點，所以被廣泛的研究及應用在各種產品上面，包含螢幕背光源、指示光源、顯示器、以及引人注意的白光固態照明。為了製作出更好效率的氮化鎵發光二極體元件，必須使用各種不同方法去改善發光二極體的磊晶品質及光萃取效率。本論文中，考慮到簡化製程及製程穩定性，我們使用單純快速的奈米壓印技術所製的均勻奈米圖形於發光二極體元件上，來改善磊晶品質與光萃取效率。 一般來說，氮化鎵系列發光二極體的元件製造過程，包含晶粒製程、二極體結構磊晶及基板設計三個部分。本研究中，我們使用奈米壓印技術設計出奈米級圖形結構在這三個部分。首先是提升光萃取效率的實驗。在晶粒製程中，於二極體p型氮化鎵表面及n型氮化鎵區域分別製作規則的低深度與高深度奈米柱陣列。這種表面結構的設計，能夠在元件表面達到最大範圍粗糙化效果。此外，奈米柱的幾何圖形能有效增加元件與空氣之間的面積，大幅度增加光子被萃取出來的機會。此外，我們利用奈米壓印技術與黃光微影技術，設計出在晶粒MESA區域內，高深度奈米柱混合微米洞陣列的結構。這種混合式奈米柱結構可以讓粗糙結構的深度不受限於p型氮化鎵的厚度。在不影響電激發特性下，利用高深度混合式奈米柱形成導光柱的特性，有效將光子從元件萃取出來。 除了光萃取效率，增強內部量子轉換效率也是改善發光二極體效率的重要因素。我們藉由在n型氮化鎵層中加入二氧化矽奈米圖形，再進行二次磊晶成長成完全的發光二極體結構。實驗結果也顯示出加入二氧化矽奈米圖形在n型氮化鎵層中可以提高量子轉換效率，進而增加發光效率。為了能達到元件更好效能，我們進一步製作雙重奈米圖形於發光二極體元件，其中包含奈米級圖形藍寶石基板及n型氮化鎵層中的二氧化矽奈米圖形。結果展示出使用雙重奈米圖形的發光二極體元件，可以得到比使用單一圖形;如奈米圖形化藍寶石基板或n型氮化鎵層中的二氧化矽奈米圖形的發光二極體元件更好的發光效率，磊晶差排缺陷密度也遠遠小於沒有任何結構的發光二極體元件。 由於使用奈米圖形藍寶石基板之發光二極體提升亮度的效果有限，考慮到與使用微米級圖形化藍寶石基板之高效率發光二極體的差距，我們改變奈米壓印製程製作高深寬比奈米圖形化藍寶石基板，用來達到更好的磊晶品質。實驗結果而言，我們成功開發出具有競爭能力及潛力的奈米級圖形化基板。

Due to the advantages of GaN material and light emitting diodes (LEDs) devices, GaN based LEDs have been widely studied and developed for various of applications including backlight of monitor, traffic lighting, LEDs display and the very interesting one of them, white solid state lighting. However, LEDs devices poor efficiency became a barrier to those applications delivering. In order to complete high performance GaN based LEDs for supporting the developments, it is necessary to further improve the epitaxy quality and light extraction efficiency (LEE) of GaN based LEDs through several kinds of methods. In the thesis, considering to process simplification and stability, a rapid high qualitied nano-imprint lithography (NIL) is utilized to fabricate uniform regular nano-patterns for improvement of GaN thin film epitaxial growth and LEE. In General, devices structure of GaN based LEDs includes chip formation, epitaxial thin film and template. In this study, the NIL was employed to map nano-patterns on the three sections respectively for the purpose of devices optimizing. For LEE, we fabricated a regular nano-rods array on p-GaN surface and n-side region during the proceeding of chip formation. The nano-rods array was respectively with lower depth on p-GaN surface and higher depth in n-side region over most chip area. The nano-rods array could be a roughness structure as an extendable interface of GaN/Air to increase possibility of light extracting. Moreover, we utilized nano-imprint lithography and photolithography in a meantime to fabricate hybrid nano-rods in the chip mesa region. The hybrid nano-rods which designed inside micro-holes allowed the roughness depth over the thickness of p-GaN layer in the mesa region without damages to electric characteristic. High depth hybrid nano-rods could act as light guiding pillars to extract most light trapped in the devices. By the excellent enhancement of LEE, LEDs devices were further improved. Besides LEE improving, to enhance internal quantum efficiency (IQE) was also an important factor for devices efficiency. We embedded a SiO2 nano-pattern in n-GaN layer through a regrowth process. It was demonstrated a better IQE of LEDs with an embedded SiO2 nano-pattern. In addition, we designed double nano-patterns including an embedded SiO2 nano-pattern in n-GaN layer and a nano-patterned sapphire substrate (NPSS) in LED devices to achieve a higher performance. Experiments demonstrated the best electroluminescent characteristic of LEDs by using double nano-patterns than LEDs with single nano-pattern. The use of double nano-patterns could significantly reduce the treading dislocations density of GaN based LEDs. Considering the enhanced efficiency of NPSS method was much lower than the common patterned sapphire substrate (PSS) in micro-level as well known for LEDs，we modified NIL process to fabricate a high aspect ratio nano-cone patterned sapphire substrate (HAR-NPSS) as a growth template for pursuit of good epitaxial quality. In results, it was believed that we successfully demonstrated HAR-NPSS as a promising template to growth GaN based LEDs.