利用圖型化藍寶石基板改變磊晶面向與改善氮化鎵磊晶品質

Abstract

高亮度發光二極體 (Light Emitting 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。但功率與亮度都需進一步提升才得以符合目前固態照明的需求。圖型化藍寶石基板(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。 在研究的第一部分， 利用溼式蝕刻法製作出具有週期性排列的金字塔型圖形化藍寶石基板，且側壁蝕刻裸露出了許多不同傾斜角度的面向，分別為{34 ̅17}、{41 ̅3 ̅18}和{123 ̅5}，接著利用穿透式電子顯微鏡 (Transmission Electron Microscopy, TEM)以及掃描式電子顯微鏡(Scanning Electron Microscopy, SEM)來發現，由金屬有機化學沉積法(Metal Organic Chemical Vapor Deposition, MOCVD)所磊晶出來的氮化鎵具有半極性的面向產生，並且成長在藍寶石基板{123 ̅5}面上的磊晶關係為(011 ̅4 ̅)GaN // (33 ̅06 ̅)sapphire and [022 ̅1]GaN // [112 ̅0]sapphire，在氮化鎵內部的缺陷密度會隨著磊晶厚度的增加而逐漸減少。此外，由光子激發放光光譜儀(Photoluminescence, PL)與陰極射線激發放光光譜儀(Cathodoluminescence, CL)來研究GaN磊晶層的光學特性，可以發現成長在藍寶石基板{123 ̅5}面上的半極性氮化鎵具有很強的near-band-edge emission。 在研究的第二部分，一種較為簡單並且便宜的液相沉積法(Liquid Phase Deposition, LPD)將作為光罩用的半球形二氧化矽沉積在藍寶石基板上。此實驗使用了兩種不同的試片來比較，第一種為MNPSS (modified nano-patterned sapphire substrate)，此種試片是經過溼蝕刻但保留原有的二氧化矽，第二種為FLAT，此種試片則是不經過溼蝕刻和液相沉積沉積二氧化矽，目的是為了當做對照組。經過實驗可以發現MNPSS具有較佳的磊晶品質，原因則是透過穿透式電子顯微鏡可發現差排 (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。

High brightness light emitting 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 the first part, Periodic pyramidal array patterned sapphire substrate (PSS) with various inclined facets was fabricated by wet etching. It was found that {34 ̅17}, {41 ̅3 ̅18}, and {123 ̅5} facets are exposed on the PSS structure after etching. GaN grown on the PSS by metal-organic chemical vapor deposition was found to have a semi-polar orientation by structural characterization with scanning and transmission electron microscopy. Also, the results show that GaN is mainly grown on the {123 ̅5} sapphire facet with the orientation relationship between GaN and sapphire as (011 ̅4 ̅)GaN // (33 ̅06 ̅)sapphire and [022 ̅1]GaN // [112 ̅0]sapphire, and the dislocation density in the grown GaN decreased with thickness. In addition, photoluminescence and catholuminescence measurements show only strong near-band-edge emission form the semi-polar GaN on the {123 ̅5} sapphire facet. In the second 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).Two kinds of nanoscale PSS were used to grow GaN, namely “MNPSS” which is modified nano-patterned sapphire substrate and “FLAT” which is no-patterned c-plane 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.