| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 97 === Improving the extraction efficiency of light-emitting diodes (LED) is one of the most critical issues in the approaching energy crisis. Techniques for fabricating a variety of photonic crystal structures (periodic sub-wavelength structures) were employed to address this issue because the extraction efficiency can be enhanced due to the ability of photonic crystals to diffract lights and couple the guided mode light confined in the Gallium Nitirde (GaN) into the leaky mode.
In the first part of this thesis, the extraction efficiency improvement between various periodic nano-scale structures was compared through the Rigorous Coupled Wave Analysis (RCWA) method. We found the optimal structures in cone photonic crystals, which can rise the efficiency from 4% to over 11% because zero order reflection is eliminated by the graded refractive index property of this structure. The extraction efficiency can be further promoted by inserting a backside reflective layer or placing the light source properly.
In the second part of this study, it can also be observed that the GaN slab with cone photonic crystals could be an inferior waveguide since the difference of the refractive index between the core and the cladding is slight in this structure. The lights travelling in such waveguide could more easily escape from the core of GaN and to enhance light emission.
Finally, periodic cone and pyramid structures were realized with the application of colloidal lithography (CL) and nanoimprinting technique. The sub-wavelength periodic cone structures were successfully fabricated in inductively coupled plasma reactive ion etching (ICP-RIE) system with a two-step process, and shallow periodic cones and pyramids were replicated from silicon molds onto titanium oxide thin films by soft lithography. The parameters of manufacturing these structures need to be optimized for its future applications on solid state lighting devices.
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