| Summary: | 博士 === 國立中興大學 === 材料科學與工程學系所 === 100 === In this dissertation, atmospheric pressure metal-organic chemical vapor deposition system is used for fabricating ultraviolet light emitting diode (UV LEDs) epitaxial wafers in the wavelength range of 365 to 400-nm. So far when the light-emitting wavelength greater than 395-nm, the crystal defects do not affect luminescence properties seriously. For the UV LEDs in longer wavelength (>395-nm), I focus on seeking an epitaxial method to enhance the light extraction efficiency of the components. In this study, three different epitaxial surface roughening techniques were used to achieve the enhancement of light extraction efficiency. The first method is using a heavily Mg-doped treating layer capped with a p-type GaN capping layer to form the micro-island type texturing surface. The second method is using a lower temperature (775~875℃) growth of p-type GaN layer to form the nano-hole type texturing surface. Finally, a coral-like texturing surface formed by combining the micro-island type and nano-hole type surfaces is the third method. The enhancements of light extraction efficiencies using micro-island, nano-hole, and coral-like texturing surface are 14%, 6%, and 32%, respectively.
With the light-emitting wavelength are getting shorter, the crystal defects affect the luminescence properties of UV LEDs become serious. Along with the increase of dislocation density, internal quantum efficiency (IQE) of UV LEDs are worse significantly. Therefore, for the UV LEDs with emitting wavelength shorter than 385-nm, how to reduce the dislocation densities in the materials and improve the internal quantum efficiencies of UV LEDs were focused. In this study, the growth mode of GaN films was transited from two-dimension (2D) growth to three-dimension (3D) growth by using a heavily Mg-doped GaN growth mode transition layer (GMTL). Threading dislocations are bended or terminated during the 3D growth process. The dislocation density in the GaN film can be reduced from 2.5x108cm-2 to 3.5x107cm-2. Due to the improvement of epitaxial quality, the internal quantum efficiency of UV LEDs is also improved significantly. After fabricating the epitaxial wafer with GMTL into vertical chip (size: 1mmx1mm), a 28% enhancement of output power can be achieved. Although this GMTL method also can be used to fabricate UV LEDs with emitting wavelength shorter than 380-nm, but the enhancement of IQE will be offset by the self-absorption effect from heavily Mg-doped GMTL. Therefore, I use a heavily Si-doped insertion layer to prevent the self-absorption issue and increase the IQE of UV LEDs in the emitting wavelength of 370~375-nm. By using the Si-doped insertion layer, the dislocation density in the n-type AlGaN layer can be reduced from 9x108cm-2 to 8x107cm-2. After vertical type chip fabricated, a 40% enhancement of output power can be achieved.
Generaly, AlGaN is commonly used for the materials of quantum barrier layers to enhance the quantum confinement effect in UV LEDs. However GaN-based materials grown at low temperature always demonstrate pool crystal quality especially when the materials contain aluminum (Al) components. In this study, we use AlInGaN quaternary materials instead of AlGaN ternary materials to growing quantum barrier layers. Both of the crystal quality and the interface between well and barrier layers are improved. From APSYS simulation results, the carrier distribution in light emitting region becomes more uniform. The enhancement of output power for 380 and 365-nm UV LEDs are 25% and 62%, respectively.
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