具漸變折射率之光子晶體用於固態照明元件出光增益之研究

Abstract

為了增進發光二極體(light-emitting diode)的光萃取效率(extraction efficiency)，最常見的做法是在其主要材料氮化鎵(GaN)表面製作各種光子晶體(photonic crystals)結構(次波長週期性結構)。其主要目的有兩個：一為利用週期性結構的繞射光柵特性讓光能夠經由繞射穿透表面，以提升其直接出光效率；另一個目的則是利用此結構改變光的波向量，將原來因全反射而被限制在氮化鎵內部的導模(guided mode)耦合至逸漏模態(leaky mode)進而提升出光。 於本研究中我們首先利用嚴格耦合波分析(Rigorous Coupled Wave Analysis, RCWA)針對不同週期性結構對直接出光效率的影響進行分析，並探究其原因。由研究中可以發現具有漸變折射率性質的反角錐奈米週期性結構，藉由壓低(0,0)階反射，能夠將直接出光效率由原來的4%提升至超過11%，為表現最好的結構。若能於背面加入反射面和減少氮化鎵的厚度，能使出光效率進一步提升。 而在破壞波導效應的表現方面，我們發現若波導光在氮化鎵內部行進路徑太長，會因材料吸收而損失極大的能量。由於具有漸變折射率之週期性奈米結構的氮化鎵相較於單一折射率奈米週期性結構的氮化鎵而言是一個限制光能力較差的波導，使得導模逸漏的機率較大，進而提升出光效率。 最後我們想要實現角錐狀及反角錐週期性結構，所採取的方式有二：在高密度電漿反應式離子蝕刻設備中運用奈米球微影術及以奈米壓印在二氧化鈦薄膜表面製作此結構。在高密度電漿反應式離子蝕刻設備中我們成功的以二階段的方式製作了次波長角錐週期性結構，而在奈米壓印方式中，也藉由PDMS模板的轉印成功地從矽模板複製出較淺的角錐與反角錐結構至二氧化鈦薄膜上。實驗參數尚需進一步最佳化以成功運用在固態照明元件上。

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.