| Summary: | 博士 === 國立成功大學 === 微電子工程研究所 === 103 === 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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