利用濕蝕刻基板提高氮化鎵發光二極體外部量子效應

Abstract

在本篇論文中，主要是利用圖樣化的藍寶石基板，製作氮化鎵發光二極體元件，並且利用蒙地卡羅光束追跡法，設計基板表面的圖樣。由於我們利用濕式蝕刻的方式來製作圖樣化基板，會在表面蝕刻出R-plane的晶面，由於R-plane和基板原本的表面(C-plane)具有57.6度的夾角，這樣的結構可以大幅提高元件的光取出效率。 利用圖樣化的藍寶石基板，我們製作了兩種不同結構的發光二極體，其一利用兩次基板轉移以及雷射剝離技術，將基板作為一個底部的反射鏡，在這一部份之中，模擬和量測的結果皆顯示，元件最高可以提高約40％的發光效率，兩者互相吻合。 其二是利用圖樣化藍寶石基板作為發光二極體磊晶層的基板，此舉可以同時提高磊晶層的品質並且改善元件的光取出效率。由量測中我們發現，這樣的結構在20mA的電流之下，可以提高40％的發光效率，和模擬的結果相為呼應。並且，在信賴度測試之中，這樣的結構有較好的表現。

In this thesis, we employed patterned sapphire substrate (PSS) to fabricate two kinds of light emitting diodes (LEDs), V-shape sapphire facet reflector LEDs and chemical wet-etched patterned sapphire substrate(CWE-PSS) LEDs. In the first part, GaN-based LEDs with V-shape sapphire facet reflector was fabricated using the double transferred scheme and sapphire chemical wet etching. The {1-102} R-plane V-shape facet reflector with a 57o against {0001} C-axis has the superior capability for enhancing the light extraction efficiency. The light output power of the V-shape sapphire facet reflector LED was 1.4 times higher than that of a flat reflector LED at an injection current of 20 mA. The significant improvement is attributable to the geometrical shape of sapphire facet reflector that efficiently redirects the guided light inside the chip toward to the top escape-cone of the LED surface. In the second part, characterization of GaN-based LEDs grown on the CWE-PSS with different evolved crystallography-etched facets was investigated. According to high-resolution X-ray rocking curves (HR-XRDs) and reliability test results, the CWE-PSS LEDs exhibited a better epitaxical film quality comparing to conventional LEDs. In addition, CWE-PSS LEDs also demonstrated significant increase on light extraction efficiency due to the contribution of high-slope inclined crystallography-etched facets. An impressive improvement of 40 % on the overall external quantum efficiency was achieved by adopting this novel CWE-PSS scheme. A Monte-Carlo ray-tracing method was also employed to derive the optimized condition of sapphire etching time, and the calculated result was consistent with the real device measurement. Therefore, the achieving improvement by CWE-PSS was not only due to the improvement of the internal quantum efficiency upon the good epitaxial quality, but also contributed to increase of the extraction quantum efficiency since crystallography-etched facets efficiently scattered the guided light to enter the escape cone on the top of device surface.