高效率近紫外光發光二極體元件製作

Abstract

近年來，由於氮化鎵近紫外光發光二極體(near-UV LED)具有非常大的發展潛力及廣泛應用，如驗鈔、光觸媒除臭、空氣淨化器等紫外光光源的使用，因此越來越多的團隊開始研究高效率元件的製作及使用近紫外光於固態照明與螢光粉轉換光源之應用。 然而，低銦含量下的發光二極體其內部量子效率(IQE)驟降迅速，晶格品質與氮化鎵自我吸收的問題更導致製作高效率近紫外光發光二極體是相當困難的。文獻指出低銦含量氮化銦鎵(InGaN)量子井設計中，使用氮化鋁鎵(AlGaN)的位障來提高載子局限能力是必須的；但由於此兩材料氮化銦鎵(InGaN)及氮化鋁鎵(AlGaN)最佳化的成長溫度大不相同，因此深深的影響了兩者的晶格品質及其元件效率的表現。 此外，由於相對低價格及高品質的優點，藍寶石(Sapphire)是目前最廣泛使用於氮化鎵材料的磊晶基板，但由於其導電性及導熱性差的特性致使元件的表現及設計處處受限。近年來，許多研究團隊致力於發展雷射剝離laser lift-off (LLO)技術，及使用氮化鉻(CrN)、氧化鋅(ZnO)及矽摻雜氮化鎵(Si:GaN)等犧牲層來實現化學剝離chemical lift-off (CLO)技術，來實現獨立的氮化鎵薄膜及垂直式發光二極體元件製作，以提升更高的光電元件品質與效率。然而，高溫的雷射剝離製程將導致氮化鎵/藍寶石(GaN/sapphire)界面及保護層因熱衝擊而受損，進而造成元件漏電及低良率等問題；化學剝離製程雖然能避免雷射瞬間高溫加熱元件造成傷害，卻也造成後續蝕刻傷害及使用犧牲層造成晶格品質的問題。 本論文中，我們利用氮化鋁鎵銦(InAlGaN)來取代原本的氮化鋁鎵(AlGaN)的位障以實現高效率的近紫外光發光二極體，並開發一種使用倒六角錐結構Hexagonal Inversed Pyramid (HIP)來實現機械式剝離mechanical lift-off (MLO)製程以製作高品質氮化鎵垂直式發光二極體(Vertical-LED)，此外效率驟降的物理意義、剝離的機制及其光電特性分析也將與本論文中有詳細的量測及調查。 首先，我們調查量子井中以氮化鋁鎵(AlGaN)及氮化鋁鎵銦(InAlGaN)為位障之氮化銦鎵(InGaN)近紫外光發光二極體效率驟降的表現，電致發光(Electroluminescence)結果顯示使用四元化合物材料(quaternary)的LED發光強度於350 mA及1000 mA電流操作下分別提升了25 %及55 %，更進一步的物理模擬顯示使用四元化合物材料的LED於高電流注入下提升了62 %的輻射再結合率及13 %較少的效率驟降，推測這些改善來自於載仔濃度的提升及載仔更均勻的重新分佈。 再者，我們使用倒六角錐結構來實現機械式剝離製程以製作高品質氮化鎵垂直式發光二極體，倒六角錐結構的製作採用高溫氫氧化鉀(KOH)濕蝕刻其氮化鎵/藍寶石基板的界面來完成，TEM分析結果顯示平均的線缺陷密度估計從2×10^9 降低至 1×10^8 cm−2，拉曼(Raman)光譜也揭露出使用倒六角錐結構的氮化鎵磊晶層之壓應力能有效的被釋放，最後我們成功的使用倒六角錐結構為犧牲層並於晶片鍵合降溫過程中實現了機械式剝離製程，其中剝離的物理機制、材料及元件的光電特性也將於本論文中詳細探討。

GaN-based near-ultraviolet light emitting devices (LEDs) have attracted great attention in last few years due to its potential applications in photo-catalytic deodorizing such as air conditioner, and there have been interests in solid-state lighting by using near-UV LEDs light for the phosphor-converting source. However, it is difficult to fabricate near-UV LEDs with high efficiency, because the internal quantum efficiency (IQE) decreases drastically under the low indium composition. Moreover, crystalline quality and light absorption of GaN are significant for short wavelength near-UV LEDs. It’s well known that in low indium content InGaN QWs, AlGaN barrier is necessary for carrier confinement. But the two materials of AlGaN and InGaN are very different in growth temperature which affects strongly on the quality of material and device performances. In addition, the sapphire is the most commonly used substrate because of its relative low cost, but it also limits the devices performance due to its poor electrical and thermal conductivity. During the last decade of years, the techniques of laser lift-off (LLO) and chemical lift-off (CLO) which use CrN layer, ZnO layer, and Si-doped n-GaN layer as the sacrificial layer have been adopted to fabricate the freestanding GaN membrane and the vertical LEDs (V-LEDs) for the purpose of high performance optoelectronic devices. However, the LLO process may induce some possible damages under high temperature in the GaN/sapphire interface. And even though the CLO can prevent the GaN layer from the laser damage during the laser lift-off process, but it also makes another chemical etching damages and reduce the crystal quality. Hence, we study the fabrication of mechanical lift-off (MLO) for high quality GaN-based V-LEDs with Hexagonal Inversed Pyramid (HIP) structures. In this study, we demonstrate high efficient near-UV LEDs by replacing AlGaN by InAlGaN barrier in active region and mechanical lift-off technology. Furthermore, the efficiency droop characteristics, theoretical analysis of lift-off process, and optoelectronic properties of high efficient near-UV LEDs have been measured and investigated. First, the efficiency droop in InGaN-based near-UV LED with AlGaN and InAlGaN barrier is investigated. Electroluminescence results indicate that the light performance of quaternary LEDs can be enhanced by 25 % and 55 % at 350 mA and 1000mA, respectively. Furthermore, simulations show that quaternary LEDs exhibit 62 % higher radiative recombination rate and low efficiency degradation of 13 % at a high injection current. We attribute this improvement to increasing of carrier concentration and more uniform redistribution of carriers. Second, we report the fabrication of mechanical lift-off high quality GaN-based V-LED with HIP structures. The HIP GaN/air/Sapphire structures were formed at the GaN/sapphire substrate interface under high temperature during KOH wet etching process. The average threading dislocation density (TDD) was estimated by transmission electron microscopy (TEM) and found the reduction about one order. Raman spectroscopy revealed that the compressive stress of GaN epilayer was effectively relieved in the GaN-based LED with HIP structures. Finally, the mechanical lift-off process is claimed to be successful by using the HIP structures as a sacrificial layer during wafer bonding process. Furthermore, theoretical analysis of lift-off process, and optoelectronic properties of devices have been investigated.