雷射剝離對發光二極體中圖形化藍寶石基板的影響

Abstract

目前在改良發光二極體(Light-emitting diode, LED)元件效能及應用的技術包括了圖形化藍寶石基板(Patterned sapphire substrate, PSS)、雷射剝離(Laser lift-off)等。結合上述兩種技術不但可以增加LED的光取出率、提升LED的內部量子效率，也能降低熱效應對元件所造成的影響。然而當結合PSS及LLO兩種技術時，LED的漏電流往往會有些許的提升；另外，我們發現PSS的表面形貌在經過LLO之後與原PSS的形貌有明顯的不同，且此現象原因尚未被討論。本論文係對於LLO後的PSS做研究並且討論其產生變化的原因。 本實驗利用鹽酸(HCl)、磷酸(H3PO4)以及氫氧化鉀(KOH)在不同的溫度下，依步驟清洗雷射玻璃後的圖形化藍寶石基板用以區別其表面的殘留物，並且利用SEM觀察其表面，藉此評估圖形化藍寶石基板重複使用的可行性。在經過雷射剝離的製程後，從剝離能量E1以及Eb可觀察到，圖形化藍寶石基板的高度有明顯的下降，從原本的1.5μm分別降至1.43以及1.29μm，將試片進一步的利用鹽酸及磷酸清洗過後更降至1.16μm及1.04μm。 此外，由TEM、EDX分析可以得知，圖形化藍寶石基板頂部部分的單晶氧化鋁因為在短時間內快速的升、降溫，在過程中轉變成為非晶質的結構。此現象可由簡單的司乃爾定律解釋：頂部能量密度原本就較大，再加上部分光線在到達氮化鎵層之前就在圖形化藍寶石基板內發生全反射，導致PSS頂部溫度急遽上升，使得不但氮化鎵分解，氧化鋁也達到軟化甚至融化的溫度。顯示雷射剝離後藍寶石基板若要回收再利用，必須在圖形化方式進一步作改良。

Current LED manufacture technology including pattern sapphire substrate (PSS) and laser lift-off (LLO). When we combine this two technology not only can increase light extraction efficiency and internal quantum efficiency, but also can decrease the effect of joule heat effect. However when we come by PSS and LLO two technology the leakage current might increase several times. We also found that the PSS morphology after LLO is different from the original PSS, and this phenomenon hasn't been discussed yet. In this experiment we use hydrochloric acid , phosphoric acid and potassium hydroxide to clean PSS at different temperature step by step. We use SEM analysis to evaluate PSS is reusable or not. After the laser lift off ,the height of PSS on E1 and Eb was decrease to 1.43 µm and 1.29 µm from 1.5 µm, and further decrease to 1.16 µm and 1.04 µm after hydrochloric acid and phosphoric acid cleaning. By TEM and EDX analysis we know that the sapphire on the top of PSS change to amorphus aluminum oxide from crystal structure because of the temperature increase in a very short time. We can explain this via Snall's law. The origenal energy density on top of PSS is larger than the energy density on buttom of PSS. Too much energy may cause large density of defect and increase the leakage current. Moreover, because of the nitrigen gap between PSS and GaN layer, the internal reflection will happen inside the PSS ,and cause more energy density on top of PSS. As a result, the sapphire on top of PSS may melt and turn to liquid phase, and finally formed amorphous aluminum oxide. The experiment shows that if we want to reuse the PSS after LLO, we must change the shape of PSS.