以新式磊晶側向成長法成長高品質氮化鎵薄膜之研究

Abstract

在本論文中，我們利用有機金屬化學氣相磊晶機台(Metal organic chemical vapor deposition，MOCVD)成長氮化鎵薄膜於圖案化二氧化矽/濺鍍氧化鋁緩衝層/藍寶石基板。首先本論文利用濺鍍氧化鋁緩衝層(Sputtered AlN buffer layer)於藍寶石基板上，並利用黃光製程在氮化鋁緩衝層上製作圖案化二氧化矽，如此一來，我們只需要利用MOCVD成長氮化鎵薄膜一次即可，我們稱之為新式磊晶側向成長技術(Novel epitaxial lateral overgrowth，Novel ELOG)，並搭配已開發成功的脈衝式磊晶成長法(Pulsed growth method)，利用不同的脈衝週期數可有效控制空氣間隙(Air-void)之形狀。 在圖案化二氧化矽/氧化鋁/藍寶石基板上成長具有扁平狀空氣間隙之氮化鎵薄膜Sample-1與具有柱狀空氣間隙之氮化鎵薄膜Sample-2，相較於成長於藍寶石基板上之氮化鎵薄膜Sample-C，其氮化鎵(002)面之半高寬可由485 arcsec減少至376 arcsec及416 arcsec，(102)面之半高寬可由600 arcsec減少至322 arcsec及360 arcsec。單位面積缺陷數亦可由5.32×108/cm2 降低至2.40×108/cm2及2.24×108/cm2，結果顯示利用Novel ELOG成長氮化鎵薄膜可明顯提升薄膜品質。 我們進一步成長氮化鎵發光二極體結構於三種實驗樣品上，從變溫(20K-300K)光激發螢光光譜(PL)量測顯示出LED-1與LED-2的相對內部量子效率相較於LED-C提升了20.0%與13.0%；而且從12 mil×12 mil尺寸的LED元件量測結果顯示，在操作電流為20 mA下，LED-C、LED-1、LED-2的光輸出功率分別為3.41 mW、5.52 mW、4.95 mW，LED-1與LED-2分別比LED-C提升了61.9 %與45.2 %的光輸出功率。並由計算結果得知，LED-1的光萃取效率比LED-2提升了5.6%，由光學顯微鏡觀察得知此結果乃歸因於光散射中心的增加。而從變電流對發光波長的量測中得知，LED-C、LED-1、LED-2的發光波長分別藍移8.2nm、7.4nm與5.9nm，具有空氣間隙結構之LED-1與LED-2相較於LED-C能釋放更多應力，這是因為空氣間隙之形成有助於應力之釋放。 利用新式磊晶側向成長技術結合脈衝式磊晶成長法，此技術不但具有簡化製程、縮短製程時間之優點，更可得到較好的薄膜品質，且其發光二極體元件之光萃取效率亦有明顯的提升，使其之光電特性有更好的表現。

In this study, GaN-based light-emitting diodes (LEDs) with embedded air voids grown on patterned SiO2/sputtered AlN buffer layer/sapphire templates were investigated. To reduce growth time, one-step GaN epitaxy was achieved using patterned SiO2/sputtered AlN buffer layer/sapphire templates, a process called "novel epitaxial lateral overgrowth" (novel ELOG). In addition, the void shape could be successfully controlled using a pulsed growth method with various pulse periods. X-ray diffraction (XRD) full-width at half maximum (FWHM) and etching pits density (EPD) measurements showed that the GaN film exhibited an improved crystal quality after the novel ELOG technique was used. The LED samples grown on SiO2 patterned AlN/sapphire templates with embedded flat-shaped and pillar-shaped voids were labeled LED-1 (60 periods) and LED-2 (360 periods), respectively. For comparison, a conventional LED (i.e., LED-C) grown on sapphire was also prepared. By employing a 20mA injection current, the output power of LED-1 and LED-2 were enhanced by 61.9% and 45.2%, respectively, compared with the conventional LED. The improvements could be attributed to enhanced light extraction efficiency (LEE) by using air voids and the improved internal quantum efficiency (IQE) by using novel ELOG technique. By increasing the injection current, a smaller EL wavelength blueshift was evident in the LED-1 and LED-2 structures, which could have been caused by partial release of the compressed strain from the air voids.