在本研究論文中，為了改善氮化鎵系發光二極體的電流散佈特性與光取出效率(light-extraction efficiency)，吾人研製一系列具有特定結構之高品質氮化鎵系發光二極體。分別提出了新穎的奈米材料與元件製程技術，其中包含鋁金屬反射層與二氧化矽絕緣層複合結構、奈米級粗糙化背鍍反射鏡以及奈米級圖案化藍寶石基板，有效提升氮化鎵系發光二極體的光電轉換效率，優化氮化鎵系發光二極體之性能與可靠度。本文對氮化鎵系發光二極體元件之光電特性與磊晶品質，以及單層二氧化矽奈米球結構與陽極氧化鋁薄膜兩奈米結構之製程與成長機制皆有深入且詳細的研究與探討。
首先，吾人研製具有鋁金屬反射層與二氧化矽絕緣層複合結構沉積於自然粗糙化p型氮化鎵層表面之氮化鎵系發光二極體中，以同時改善電流擁擠效應(current-crowding effect)與電極下方之光吸收。二氧化矽絕緣層可有效改善電流擁擠效應，增加電流散佈效率；鋁金屬反射層沉積於p型電極下方，可大幅減少電極之光吸收；自然粗糙化p型氮化鎵層表面則可有效降低內部全反射與增加光散射之機會。文中亦證實鋁金屬反射層之反射能力並不會因為沉積於自然粗糙化p型氮化鎵層表面而受限。雖然相較於傳統氮化鎵系發光二極體，此研發元件之功率消耗會略為增加，但光輸出功率(light output power)與光通量(luminous flux)分別可提升56%以及95%，可有效改善光電轉換效率。
其次，藉由塗佈一具有規則排列之單層二氧化矽奈米球結構於發光二極體藍寶石基板背面後，再利用電子束蒸鍍依序鍍上背鍍複合式反射鏡，以形成具有三維光子晶體背鍍式反射鏡結構，藉此類光子晶體反射鏡結構，減少元件背部封裝金屬之光吸收並增加光散射之機會，有效增加光取出效率。相較於傳統高功率氮化鎵系發光二極體，在不影響元件之電性特性下，此研發元件可提升光輸出功率與光通量高達136%以及165%，有效改善光電轉換效率。此外，吾人亦研究藉由塗佈單層二氧化矽奈米球結構於藍寶石基板背面當作蝕刻硬遮罩，再利用感應耦合電漿離子蝕刻(inductively coupled plasma, ICP)來轉印出三維結構，再依序鍍上背鍍複合式反射鏡，以形成具有三維結構之背鍍複合式反射鏡。此部分研究採用乾蝕刻轉印三維結構，而非選擇前者嵌入式單層二氧化矽奈米球結構，相較於傳統高功率氮化鎵系發光二極體，在不影響元件之電性特性下，仍可提升高功率氮化鎵系發光二極體之光輸出功率與光通量達118%以及142%，有效改善背鍍反射鏡之附著力，大幅增加元件製程之良率。
最後，探討藉由氧化還原方式成長出具有規律性蜂巢狀且高長寬比的陽極氧化鋁(anodic aluminum oxide, AAO)式奈米孔洞薄膜於藍寶石基板上，以其當作蝕刻硬遮罩，再利用感應耦合電漿離子蝕刻來轉印出奈米級圖案化藍寶石基板，以改善磊晶品質，有效降低氮化鎵系發光二極體元件之線差排(threading dislocation)缺陷密度，降低逆向漏電流改善元件電性特性，並減少主動區中非輻射復合(non-radiative recombination)的產生。此陽極氧化鋁式奈米孔洞圖案化藍寶石基板除了可提升內部量子效率，奈米孔洞圖案化藍寶石基板與氮化鎵磊晶層界面形成之類光子晶體空氣孔洞陣列，可反射由主動區發散之背向光，減少元件背部封裝金屬之光吸收，並增加光散射之機會，提升光輸出功率與外部量子效率達54%以及44%。本研究論文中所研製的高品質氮化鎵系發光二極體，皆可有效提升光電轉換效率，在商業化上具有相當之潛力。

In this dissertation, for purposes of enhancing the current spreading performance and light extraction efficiency (LEE), a series of high-performance GaN-based light-emitting diodes (LEDs) with specific approaches are fabricated and studied. Novel nanomaterials and device fabrication processes, including an aluminum reflecting layer and an SiO2 insulating layer hybrid structure deposited on a naturally textured p-GaN surface, nanoscale textured backside reflectors, and nanoscale patterned sapphire substrates, are proposed to improve wall-plug efficiency (WPE) of GaN-based LEDs. Thus, enhanced performance and reliability of GaN-based LEDs could be obtained. Optical and electrical properties and the epitaxial quality of GaN-based LEDs are studied and discussed. In addition, the fabrication processes and growth mechanism of a self-assembled SiO2 nanosphere monolayer and an anodized aluminum oxide (AAO) thin film are addressed and discussed in detail.
First, a GaN-based LED with aluminum reflecting and SiO2 insulating layers (RIL) deposited on a naturally textured p-GaN surface is fabricated and studied. The use of an RIL structure could enhance the current spreading performance and reduce the photon absorption by the p-pad metal. A textured surface is used to limit the total internal reflection and increase photon scattering. Experimentally, the reflectivities of a Cr/Pt/Au metal pad metal with an Al reflecting layer are always higher than those with only a metal pad, regardless of wavelength as well as whether a textured p-GaN surface is employed or not. Even though a GaN-based LED with naturally textured p-GaN surface exhibits remarkably higher light output power than dose the one with a planar p-GaN surface, the performance could still be improved by the use of an RIL structure. Effects of the use of an Al RL and/or an SiO2 insulating layer on the performance of GaN-based LEDs are systematically studied and compared in detail. As compared with a conventional GaN-based LED with a planar p-GaN surface at 20 mA, the studied device exhibits a 56% and 95% enhancement in light output power and luminous flux. Although power consumption is slightly increased because of the insertion of an RIL structure, this drawback could be surpassed by the mentioned optical improvements. Therefore, although a naturally-textured surface is utilized to enhance LEE, the performance of GaN-based LEDs with a naturally-textured p-GaN surface could be further enhanced by employing the RIL structure.
Second, enhanced LEE of high-power GaN-based LEDs is achieved by inserting a self-assembled SiO2 nanosphere monolayer between the substrate and a hybrid backside reflector (a distributed Bragg reflector and a metal mirror). A self-assembled 100 ± 5 nm SiO2 nanosphere monolayer arrangement is drop-coated on the backside of a sapphire substrate. A hybrid backside reflector is directly deposited on the SiO2 nanosphere monolayer by an electron beam evaporator. Due to the presence of concave surfaces and photonic crystal (PhC)-like air voids, downward photons emitted from the active region toward the 3-D textured hybrid backside reflector, could be reflected, scattered, and redirected in arbitrary directions for light extraction. As compared with a conventional high-power GaN-based LED without a backside reflector, at 350 mA, the studied device exhibits a 136% and 165% enhancement in light output power and luminous flux without the degradation of electrical properties. Therefore, performance could be significantly improved.
A high-power GaN-based LED with a nano-hemispherical hybrid backside reflector is also fabricated and studied. A self-assembled 100 ± 5 nm SiO2 nanosphere monolayer arrangement is drop-coated on the backside of a sapphire substrate. Then, the SiO2 nanosphere monolayer is utilized as a hard mask to transfer nano-hemispherical patterns onto the backside of the sapphire substrate by an inductively coupled plasma (ICP) etching process. Due to the presence of ICP-transferred nano-hemispherical patterns on the backside of the sapphire substrate, nano-hemispherical patterns could be transferred to the deposited hybrid backside reflector. Hence, reflected photons could be redirected and scattered into arbitrary directions for light extraction and create more opportunities to find escape cones. As compared with a conventional LED without a backside reflector, at 350 mA, the studied device exhibits a 118% and 142% enhancement in light output power and luminous flux without the degradation of electrical properties. Notably, the adhesion between an ICP-transferred sapphire substrate and hybrid backside reflector is better than that of directly inserting an SiO2 nanosphere monolayer in the device. Thus, the process yield could be enhanced for applying in solid-state lighting.
Finally, a GaN-based LED grown on an anodized aluminum oxide-nanoporous pattern sapphire substrate (AAO-NPSS) is fabricated and studied. Nanoporous patterns are transferred on a sapphire substrate by using a well-ordered AAO thin film as a mask for the ICP etching process. This well-ordered AAO thin film with a high aspect ratio is grown on a sapphire substrate using an oxalic acid-based electrochemical system and a three-step anodization. The strain state generated during epitaxial growth could be effectively alleviated by the use of AAO-NPSS. Thus, an enhanced crystalline quality could be obtained. The treading dislocation (TD) density could be reduced. The decrease in non-radiative recombination caused by the reduction of the TD density certainly leads to an increase in internal quantum efficiency (IQE). In addition, due to the presence of PhC-like air voids, part of the reflected photons upward the top side could be scattered by these air voids. Therefore, more photons could be extracted outside. Experimentally, at 20 mA, as compared with a conventional LED grown on a planar sapphire substrate, the studied LED exhibits a 54% and 44% enhancement in light output power and external quantum efficiency as well as a reduced leakage current.
All of these specific approaches, which are fabricated and studied in this dissertation, could significantly improve performance of GaN-based LEDs. To compete with traditional light sources in applications of solid-state lighting, high-performance GaN-based LEDs could be expected to have some success.

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