本論文提出具有不同表面形貌設計之奈米圖案化c面(0001)藍寶石基板(nanoscale patterned c-plane sapphire substrates, c-NPSS)，由其光學特性找出在寬頻譜、高入射角度下，具有高穿透率之最佳化設計。c-NPSS於可見光到近紅外光波長範圍能擁有95%的高穿透率，且在45度入射角，還能夠維持90%的高穿透率。在這樣的奈米結構設計下，可將其利用於增加LED的光萃取率以改善LED的發光效率。同時，本論文並討論利用c-NPSS成功地製作出高品質氮化鎵(gallium nitride, GaN)磊晶薄膜，該奈米結構製程為一不需要半導體玻璃光罩技術之方法，其乃利用自然顯影法(natural lithography)和感應偶合式電漿反應離子蝕刻技術(inductively coupled plasma–reactive ion etching, ICP-RIE)來定義出設計之圖案，並具有均勻性佳、結構密集度高和可大面積製作之優點。此奈米結構分為兩種設計，其一為具有c面(0001)支配的藍寶石奈米結構(c-NPSS-c)，另一種為n面(11-23)支配且沒有c面裸露的奈米結構(c-NPSS-n)，此兩種設計成功地將不同極性的GaN磊晶層成長於其上，分別為高品質(0001) GaN成長於c-NPSS-c，其X-ray 搖擺曲線 (X-ray rocking curve, XRC)的半高寬(full width at half maximum, FWHM)為211 arcsec；另外高品質非極性(10-10) m-plane GaN成長於c-NPSS-n，其XRC的FWHM為316 arcsec。由AFM分析此兩種磊晶薄膜，其缺陷密度與表面平整度分別為10^7~10^8 cm-2、0.30 nm，由此可證明c-NPSS不只改善了GaN磊晶薄膜也成功地磊晶成長出非極性GaN薄膜。另外，發光二極體(light-emitting diode, LED)成功地製作於c-NPSS-c，利用奈米結構改善發光二極體的光輸出特性，進而增加發光二極體的發光效率，因GaN橫向磊晶於c面藍寶石，因此將形成許多孔洞埋藏於奈米結構中，其不只是改善GaN的磊晶品質更可因為折射率的不同而增加光萃取率。此具有奈米結構之發光二極體與一般發光二極體在操作電流為20 mA相比較下，可提升 2.38倍之光輸出功率。不過，由於c面GaN為具有極性之材料，容易因為產生量子侷限史塔克效應(quantum confined Stark effects, QCSE)而影響內部量子效率，為了改善此一問題，非極性氮化鎵的磊晶成長於是被提出可以增加內部量子效率和避免此效應產生的發光光譜紅移現象，因此，本研究利用目前商用c面藍寶石晶片，成功地製作出高品質非極性氮化鎵材料，將有助於未來發光二極體的發展。

Recently, GaN-based wide bandgap semiconductors are very important material system for fabrication of light emitting devices (LEDs) in a wide range of wavelength. The GaN-based laser diodes (LDs) and LEDs have been widely used in many areas, such as optical storage, backlight in liquid crystal displays, traffic signal and solid state lighting. High external quantum efficiency (EQE) of LEDs with high crystalline quality are urgently demanded for achieving high performance LEDs on nanoscale patterned c-plane sapphire substrates (c-NPSS) technologies. This work is carried out not only to realize high performance c-plane GaN-based LEDs but also to project the high quality of nonpolar GaN epitaxy on c-NPSS.
This dissertation explores the fabrication, structural properties, and physical features, optical and electronic properties of polar and nonpolar GaN epilayers grown on c-NPSS. The main focus of this dissertation can be divided into four parts. First part is the fabrication of c-NPSS by inductively coupled plasma-reactive ion etching (ICP-RIE) with different materials as nano-masks for enhancing the light extraction and improving crystalline quality. Second part is the optical characterization of c-NPSS with different feature sizes. The formation of the c-NPSS is employed by ICP-RIE with self-assemble nickel silicides as the nano-masks. The optimal feature size was presented with high transmittance over broadband spectra. The transmittance of visible to near-IR spectra was found to be 95% at normal incidence and over 90% at an incident angle of 45 degree. In the mid-IR spectrum, the transmittance exceeds 88% until the reflection is no longer suppressed by nanostructures. The polarization properties have also been investigated. The nanostructures can enhance the reflectivity ratio 90% for wavelengths shorter than 400 nm. As the amplitude ratio, enhanced from 50% to 80% over the whole visible spectrum. Next part is the morphology of c-NPSS has two designs, one is c-facet sapphire dominated (c-NPSS-c) and the other is n-facet sapphire dominated (c-NPSS-n). The formation of the nanostructures is employed by ICP-RIE with polysilicon as the nano-masks formed by low pressure chemical vapor deposition and wet-etching methods. The polar and nonpolar GaN epilayers was grown on c-NPSS using MOCVD. This part focus on the fabrication and optical properties are investigated using AFM for surface morphology, high resolution XRD for crystalline orientation, TRPL for crystalline quality, and SEM and TEM for growth/structure analysis. The GaN epilayers were characterized by XRD and AFM, which reveals the GaN epilayer can be c-plane (0001) or m-plane (10-10) orientation depending on the surface morphology of nanopatterned sapphire substrate. The corresponding full width at half maximum (FWHM) of the X-ray rocking curves (XRC) for c- and m-plane GaN are 211 and 316 arcsec, respectively. The root-mean-square (rms) surface roughness was measured to be 0.30 nm by AFM. The pits density on GaN surface improved two orders of magnitude than that on convenitional sapphire substrate (CSS) by AFM. The last part presents the comparison of GaN-based LEDs grown on c-NPSS-c and CSS. The fabrication, electronic properties and light output performance of GaN-based LEDs are analyzed in detail. The optical and structural properties are investigated by using XRD, electroluminescence (EL) measurement, SEM, and high resolution TEM. The LEDs fabricated on c-NPSS-c exhibit an output power of 33.10 mW at a driving current of 20 mA, which is 2.38-fold higher than that on CSS. In addition, the corresponding EQE are 58.30% and 24.50% for LED on c-NPSS-c and CSS, respectively.

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