近年來，由於發光二極體LED(Light Emitting Diode)的蓬勃發展，因此具有高亮度的 GaN based 發光二極體受到各界重視。目前覆晶式(flip -chip)和垂直式(thin-GaN)兩種LED結構被提出來製作超高亮度LED。一個具有高反射性與低接觸電阻之P型GaN金屬接觸層是重要的關鍵。銀在可見光範圍中有最高的反射性以及經由退火後可以與p型氮化鎵形成良好的歐姆接觸，然而在熱退火處理過程中，銀薄膜會發生聚集的現象，而使得接觸電阻增加和反射性降低。在本研究中，我們利用 AgLa 合金材料製作電極，此種接觸結構在300 ℃快速熱退火後，會具有比銀接觸結構具有更低的特徵接觸電阻以及更高的反射率，分別為5.874×10-5 Ω cm2和93％。它能具備如此優良的表現是因為鑭在銀裡之溶解度非常低，且在空氣氣氛下退火時，在晶界上可能有析出物產生，而析出物的出現減緩了銀的聚集以及遷移現象。此外為了解其在高功率發光二極體元件晶圓鍵合時之穩定度，藉由長時間熱處理來測定銀鑭合金結構之熱穩定性，實驗結果顯示此結構在長時間熱處理後，仍然具有良好的電性質，因此銀鑭合金結構對於當作p型氮化鎵的反射性歐姆接觸是非常合適的選擇。Due to Light-emitting diode has great progress in recent years, high power GaN-based Light-emitting diode has been emphasized. But conventional LED could not satisfy this demand, two types of LED device structure, which are vertical structure LED and flip-chip structure LED, have been proposed to fabricate high power LED. Because p-type GaN has high resistivity, it is critical to search for a metal which has high reflectance and good ohmic contact to reduce its contact resistivity. silver has the highest reflectivity in visible spectrum and forms a good contact to p-GaN by annealing. But Ag film would agglomerate rapidly when it was rapid-thermal-annealed for obtaining low contact resistance or bonded with another substrate. The phenomena described above resulted in the increase of low contact resistance and the decrease of light reflectance. Therefore, it is important to avoid the agglomeration of silver thin film.In this study, we fabricate a alloy structure p-GaN / AgLa . After rapid-thermal-annealing at 300 ℃, this structure exhibited the lower specific contact resistivity of 5.874×10-5 Ω cm2 and higher reflectivity of 93％ than single Ag contact. The reason why it has better performance is that solubility of lanthanum in silver is considerablely low and may form precipitate in grain boundary. The presence of precipitate in grain boundary suppress Ag film from the phenomenon of agglomeration and migration when it was annealed in air ambience. Besides, we tested the thermal stability of AgLa alloy structure for investigating the stability of this structure during the wafer bonding process of two LEDs. Experimental results showed that it still had the good property after long-time heat treatment. Therefore, AgLa (180 nm) alloy structure is a excellent candidate for the reflective ohmic contact of p-GaN.
