| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 98 === The primary objective of this dissertation is provided some detail research of laser lift-off damage mechamism and two-step wet etching patterned sapphire substrate.
The effects of the frequency-tripled Nd:YAG laser (355 nm) and the KrF pulsed excimer laser (248 nm) on the structural damage mechanisms and reverse-bias leakages of GaN-based LEDs were investigated. The absorption depth of the YAG laser was much thicker than that of the KrF excimer laser because the absorption coefficient of GaN at 248 nm KrF excimer laser is 3.33 times higher than that at 355 nm Nd:YAG laser. As a result, the MQW screw dislocation density and the reverse-bias leakage current of YAG-LED were much higher than those of KrF-LED. Most KrF laser energy was absorbed around the GaN/sapphire interface. Consequently, the LLO surface dislocation density of KrF-LED was higher than that of YAG-LED. Moreover, the longer pulse time of the KrF laser (35 ns) brought about plastic and shock waves, which caused dense dislocation and deformed the superficial structure.
By using the characteristic of etching mechamism of mixed acid solution and pure phosphoric acid, periodic triangle pyramidal array patterned sapphire substrates (PSSs) with various morphology were fabricated successfully by two-step wet etching method. The width, area, slant angle and morphology of pyramidal protrusion are controllable during the etching process. Forward voltage and luminance wavelength are similar in conventional LED and PSS-LED. According to the XRD, TEM and EPD measurement, the crystal quality and performance of GaN PSS-LEDs improved with decrease the c-sapphire area. This is because most of the growth of GaN was initiated from c-planes. As the growth time increased, GaN epilayers on the bottom c-plane covered these pyramids by lateral growth causing the threading dislocation to bend toward the pyramids. Which result in reduce the defect density. Beside, the wider pyramid protrusion area may further increase the photons ecscape efficiency. Therefore, the performance of output power, luminance intensity, internal quamtum efficiency and light extraction efficiency are improved by using the two step wet etching patterned sapphire substrate.
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