藉著電漿處理濕式蝕刻圖案化藍寶石基板提升氮化鎵發光二極體之效能

Abstract

製造圖案化藍寶石基板( patterned sapphire substrate，PSS )的方式中，由於設備價格較低以及單次產量較高等優點，一般多選擇以黃光微影搭配濕式蝕刻的方式製造。然而濕式蝕刻會形成含有特定斜面的角錐，這些特定斜面會影響到後續的氮化鎵磊晶層，使得由斜面處成長的氮化鎵為閃鋅礦結構( zinblende structure )，這種結構與一般的氮化鎵結構 - 纖鋅礦結構( wurtzite )大相逕庭；同時也會在磊晶層產生不必要的孔洞，進而影響到其上成長的元件效率。 本實驗利用以BCl3為主的電漿處理已經由濕式蝕刻所形成的圖案化藍寶石基板，藉由自由基 - BCl與藍寶石基板表面反應，降低斜面磊晶的機會，並觀察於其上成長發光二極體( light emitting diode，LED )元件後的效率改善。LED電性結果顯示，經由電漿處理後的圖案化藍寶石基板 – LED-DWPSS I 、 LED-DWPSS II具有較佳的光輸出效率：在固定電流350mA下，LED-WPSS、LED-DWPSS I ( 電漿處理300秒 )、LED-DWPSS II ( 電漿處理600秒 )的輸出功率分別為425.5 mW、437.6 mW 以及453.8 mW。相較於LED-WPSS、LED-DWPSS I，較長電漿處理時間的LED-DWPSS II 可分別提升6.7% 與3.7%的光輸出功率。 此外，由PL、XRD、EPD等分析可得知，藉由電漿處理於PSS基板上，可有效改善其上的發光主動層、氮化鎵層的磊晶品質，減少穿隧性差排的數量。藉由TEM觀察於圖案化藍寶石基板上的磊晶層，可看出DWPSS II上的差排數量變少；藉由STEM觀察於圖案化藍寶石基板中的角錐斜面，可看出DWPSS II上的氮化鎵晶粒變小、數量變少。顯示利用電漿處理由濕式蝕刻所形成的圖案化藍寶石基板，可有效降低其斜面磊晶的機會，改善元件效率。

In choosing the way to fabricate patterned sapphire substrate ( PSS ), standard photolithography and wet-etching are preferred process is because of their lower cost, higher throughput than dry-etching process. However, several facets ( sidewalls ) on PSS formed by wet-etching are observed, and these facets will influence the structure of the subsequent GaN epitaxy layer. Instead of normal wurtzite GaN, a disparate zincblende GaN has been found on these facets. Meanwhile, irregular voids have been found in the GaN epitaxy film during the GaN coalescence process. All of the above situations will affect the efficiency and performance of LED. In this study, wet-etched patterned sapphire substrates were treated by BCl3-based plasma. Through the reaction between sapphire surface and BCl radicals, the opportunity of growth of GaN on sapphire sidewalls was changed. Also, the effect of BCl3-based plasma on the performance of LEDs was investigated. The results of device measurement showed that the PSS with plasma treatment–LED-DWPSS owned improved output power. At 350mA, the output power of LED-WPSS、LED-DWPSS I ( plasma treatment time : 300 sec )、LED-DWPSS II ( plasma treatment time : 600 sec ) were 425.5 mW、437.6 mW and 453.8 mW, respectively. The output power of LED-DWPSS II was 6.7% larger than LED-WPSS, and 3.7% larger than LED-DWPSS I. Besides, from the results of PL, XRD and EPD, we conclude that plasma treatment could improve the quality of MQW and GaN epi-layer, and reduce the number of threading dislocations as well. From TEM and STEM cross-section images of GaN film on substrate facets, we found that the number of dislocations on DWPSS II was reduced and the size of GaN grains decreased as plasma treatment time increased. All of experimental results revealed that the additional plasma treatment could restrain the growth of GaN grains on facets on PSS and thus improved the performance of device.