Abstract
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.