高亮度發光二極體效能之提昇 -針對雷射剝離損傷機制與圖形化藍寶石基板之研究

Abstract

目前高亮度發光二極體（light emission diodes, LED）的發展主要受限於熱和發光效率的不良影響而導致效率不彰，近年來熱效應可藉由雷射剝離（laser lift-off, LLO）的技術來改善，而發光效率則可藉由圖形化藍寶石基版（patterned sapphire substrate, PSS）來增進。本論文係針對雷射剝離的損傷機制與濕式蝕刻藍寶石圖形化基板做一詳細的研究。 在LLO損傷機制的研究中，本實驗使用KrF excimer laser與Nd:YAG laser兩種不同光源的雷射對GaN LED進行雷射剝離。結果發現經由Nd:YAG雷射處理的YAG-LED會有明顯的漏電流現象，TEM的影像也發現YAG-LED差排密度明顯比較多。其原因是由於GaN對355 nm Nd:YAG laser的吸收係數較小，所以雷射的穿透深度會比較深，導致材料內部（bulk region）產生較多的差排密度，甚至微裂縫的出現，進而導致YAG-LED有較大的漏電流情形出現。而GaN對248 nm KrF excimer laser的吸收係數較高，其雷射能量大多在GaN/sapphire界面附近就已被吸收完畢。故，雷射所造成的界面熱分解現象會較為劇烈；同時，又因為雷射穿透的深度較淺，所以KrF excimer laser較不會在材料內部造成傷害。由於KrF excimer laser較長的脈衝時間會導致塑形波與振動波的產生，進而在剝離界面約200nm處形成非常密集的缺陷。所以KrF excimer laser在剝離的過程所形成的缺陷大多只局限於剝離界面的表面區域（superficial region）。另一方面，同時Nd:YAG laser較短的脈衝時間，並不會導致振動波的形成，也就不會造成GaN/sapphire近界面處的缺陷產生。

在PSS的研究中，本實驗成功的利用混酸溶液（硫酸：磷酸＝3:1）與純磷酸溶液的對sapphire不同反應機制的特點，成功的利用二次濕式蝕刻方式的製作出各種不同幾何形狀的PSS基板。其電性量測的結果顯示，濕式蝕刻的PSS-LED與傳統平面型LED的結果無異，代表濕式蝕刻並不會影響磊晶品質。由XRD、TEM與EPD的結果顯示，利用二次蝕刻所製作的PSS基板可以大幅的減少材料內部的缺陷密度。發光強度與輸出功率的量測結果顯示，不管一次或二次濕式蝕刻的PSS-LED都有提昇亮度的功能，但是二次蝕刻PSS-LED的提昇效果更為顯著。而且PSS-LED的發光效率很明顯會隨著側向成長的面積成正比。利用二次蝕刻的特點來控制PSS基板上的角錐面積、寬度、斜面角度與f面形貌。不僅可以達到減少c-sapphire的面積與增加GaN側向成長面積的要求，更可以提昇LED的光取出率。成功的製做出可已達到大幅提昇內部量子效率與光取出率的PSS基板。

The primary objective of this dissertation is provided some detail research of laser lift-off damage mechamism and two-step wet etching patterned sapphire substrate. The effects of the frequency-tripled Nd:YAG laser (355 nm) and the KrF pulsed excimer laser (248 nm) on the structural damage mechanisms and reverse-bias leakages of GaN-based LEDs were investigated. The absorption depth of the YAG laser was much thicker than that of the KrF excimer laser because the absorption coefficient of GaN at 248 nm KrF excimer laser is 3.33 times higher than that at 355 nm Nd:YAG laser. As a result, the MQW screw dislocation density and the reverse-bias leakage current of YAG-LED were much higher than those of KrF-LED. Most KrF laser energy was absorbed around the GaN/sapphire interface. Consequently, the LLO surface dislocation density of KrF-LED was higher than that of YAG-LED. Moreover, the longer pulse time of the KrF laser (35 ns) brought about plastic and shock waves, which caused dense dislocation and deformed the superficial structure. By using the characteristic of etching mechamism of mixed acid solution and pure phosphoric acid, periodic triangle pyramidal array patterned sapphire substrates (PSSs) with various morphology were fabricated successfully by two-step wet etching method. The width, area, slant angle and morphology of pyramidal protrusion are controllable during the etching process. Forward voltage and luminance wavelength are similar in conventional LED and PSS-LED. According to the XRD, TEM and EPD measurement, the crystal quality and performance of GaN PSS-LEDs improved with decrease the c-sapphire area. This is because most of the growth of GaN was initiated from c-planes. As the growth time increased, GaN epilayers on the bottom c-plane covered these pyramids by lateral growth causing the threading dislocation to bend toward the pyramids. Which result in reduce the defect density. Beside, the wider pyramid protrusion area may further increase the photons ecscape efficiency. Therefore, the performance of output power, luminance intensity, internal quamtum efficiency and light extraction efficiency are improved by using the two step wet etching patterned sapphire substrate.