利用奈米圖案化藍寶石基板改善氮化鎵品質以及成長半極性氮化鎵於a面圖案化藍寶石基板

Abstract

高亮度發光二極體 (light emission diodes, LED) 近年來蓬勃發展並且廣泛使用在路燈、汽車頭燈、螢幕背光板以及投影機等常見日常生活用品上。但亮度與功率都需進一步提升才得以符合固態照明的需求。圖案化藍寶石基板(patterned sapphire substrate, PSS) 近年來常被用來改善磊晶品質以增加內部量子效率以及光取出效率。 本研究的第一部分則是使用方法較為簡單的液相沉積法 (liquid-phase deposition, LPD) 搭配溼式蝕刻來製作圖案化藍寶石基板。另外，將氮化鎵 (GaN)成長於常見的c-面藍寶石基板上則會因內建極化效應的影響使得主動層內復合效率下降以及發光波長紅移，此部分則希望藉由a-面圖案化藍寶石基板以改善此項問題。 在研究的第一部分，一種較為簡單並且便宜的液相沉積法將作為光罩用的半球形二氧化矽沉積在藍寶石基板上。此實驗使用了三種不同的試片來比較，第一種為NPOS (nano-patterned oxide sapphire substrate)，圖案為剛沉積完的半球型二氧化矽。第二種為NPSS (nano-patterned sapphire substrate)，係將前一種試片透過溼蝕刻製作出來的基板，最後將原有的二氧化矽移除。第三種為 MNPSS (modified nano-patterned sapphire substrate)，此種試片則是同樣經過溼蝕刻但保留原有的二氧化矽。經過陰極發光實驗(cathodoluminescence, CL) 可以發現MNPSS具有最佳的磊晶品質，原因則是透過穿透式電子顯微鏡 (transmission electron microscopy, TEM) 可發現差排 (dislocation) 在磊晶初期會有較明顯的彎曲現象並且部分會終止在底層的孔洞上，因而使MNPSS 磊晶品質有所改善。 在研究的第二部分，不同於以往使用的c-plane sapphire，此次使用a-plane sapphire作為基板。透過黃光微影以及蝕刻過程製作出與sapphire c-axis 夾60度的線狀圖案。此實驗的GaN磊晶層由側壁所成長出來並透過TEM的擇區繞射可以得出以下關係: (0 0 0 2)GaN // (2 1-1-3)sapphire and [21-1-0]GaN // [2 35-2]sapphire 。再透過X-ray 繞射搭配TEM的結果可以定出(101-4) 半極性的氮化鎵成長在這種基板上。並且透過 X-ray rocking curve 以及 CL對此磊晶層的性質做分析。最後推論缺陷在磊晶初期以彎曲並終止在縫合的介面，因此上方區域的則呈現較低缺陷聚集的狀態。

High brightness emissing diodes (LEDs) have been highly demand in various fields like using on road light, car headlight, backlight and projector. For the purpose of next-generation application of solid-state lighting, LEDs with higher internal and external quantum efficiency are required. Among many methods, lots of efforts have been invested on patterned sapphire substrates (PSS) to improve the quality of GaN film and light extraction efficiency. In first part of this study, a relatively simple, easy and inexpensive liquid-phase deposition (LPD) method was used to fabricate PSS. Besides, if GaN is grown on c-plane sapphire, a built-in polar will decrease the recombination efficiency in active layers and also cause red-shift in wave length. In second part of this study, a-plane PSS is fabricated and expect to reduce this effect. In the first part, a simple, easy and relatively inexpensive liquid phase deposition (LPD) method was used to introduce nano SiO2 on sapphire substrates to fabricate nanoscale patterned sapphire substrates (PSS). Three kinds of nanoscale PSS were used to grow GaN, namely “NPOS” which is nano-pattern oxide on sapphire substrate and “NPSS” which is nano-patterned sapphire substrate and “MNPSS” which is modified nano-patterned sapphire substrate. It was found that upper region of MNPSS-GaN had the best quality. This is because as the growth time increased, laterally-grown GaN caused the threading dislocations to bend toward the patterns. Besides, voids formed on the MNPSS pattern sidewalls caused more threading dislocation bending toward these voids. In the second part, a (1 0 1-4) semipolar GaN layer was grown on periodic stripe patterned a-plane sapphire substrate fabricated by using two-steps etching process. The stripes in prism shape were inclined with sapphire c-axis in 60 degrees. The orientation relationship between semipolar GaN and sapphire is (0 0 0 2)GaN // (2 1-1-3)sapphire and [21-10]GaN // [235-2]sapphire as defined by selected area diffraction in transmission electron microscopy (TEM). Growth of semipolar GaN on the sidewalls of the prism stripes is evidenced from TEM and x-ray φ scan. The quality of semipolar GaN film is reasonably good as examined with x-ray rocking curves. In addition, TEM and cathodoluminescence results show that the dislocation density in the semipolar GaN is significantly reduced near the film surface.