氮化鎵發光二極體之發光場形暨出光偏振比例之特性探討

Abstract

目前為止，發光二極體許許多多的應用使用了諸如奈米小球微影與光子晶體繞射的方式來做為操縱發光場形與增益光萃取效率的方法。然而，在這些應用背後仍有些議題與特性尚未獲得充分的探討與解釋。例如，多層表面結構發光二極體之結構形狀與放射特性的依存性探究。另外還有，非極性發光二極體之各向異性極化光放射與光子晶體繞射的相關性探討。這些尚未被充分探究的議題將會是本論文的主軸。在第一段，我們運用單層表面結構化的研究與模擬去進行於垂直式發光二極體上的混和式結構製備。實驗結果顯示發光角度與不同單一表面結構的依存性，至於混和式結構，有兩組傳導光模會分別傾向由奈米柱或微米圓頂結構耦合出來。此種混合式結構的光輸出增益正好會幾乎同等於具單一奈米柱結構以及單一微米圓頂結構的發光二極體之光輸出增益於一定角度範圍內的線性疊加。藉由調變這些單一表面結構的結構參數，本研究成果提供了一個操控光於不同方向上的增益強度與不同形態的發光場形之方法。於第二段，我們在a-晶面發光二極體上鋪排了長方形光子晶體陣列。我們根據a-晶面發光二極體之各向異性的極化光放射特性來排列光子晶體陣列的方向，以利於在不同軸向上增益偏振比例值與繞射程度。我們利用透光子晶體放射的輪廓圖判斷出E//m極化光模得到比E//c極化光模更多的繞射。繞射的方向亦可以從輪廓圖中判斷，其主要繞射方向就如同發光場形所顯示的峰值所在方向。我們更進一步地將輪廓圖中的光繞射強度替換成偏振比例值，並發現它們的強烈訊號所在處跟E//m極化光模的繞射方向幾乎重疊。研究結果顯示只要能夠以E//m極化光模為目標去繞射，偏振比例值與光萃取效率可以被有效的提升，同時，繞射光特性也可以藉由調整光子晶體週期來進行微調。

Various applications of surface structure patterning, such as nano-sphere lithography and photonic crystal planting are regarded to be obvious ways of manipulating emission profile and improving light extraction. However, there are still some issues and characteristics beyond through investigation. Such characteristics are morphology-related radiation properties of multi-structure LEDs, and interactions of anisotropic polarized emission and photonic crystals on non-polar LEDs. These are the focuses of our researches. In the first part, for vertical LED, a two-step surface patterning hybrid structure is applied based on investigations on single-structure LED and simulations. The results suggest that light will emit to various angles while interacting with distinct single-structure LEDs. As for the hybrid structure, two fractions of guiding modes are diffracted out by either nanorods or truncated microdomes. The percentage of emission enhancement of hybrid structure happens to be the superposition of enhancements of both single-structure LEDs at a certain range in angle. The result illustrates a way of manipulating emission directions and the way of adjusting radiation profile by variations of parameters of single-structure morphology. In the second part, for a-plane LED, an array of rectangular photonic crystals is planted. According to the intrinsic property of anisotropic polarized emission of a-plane LEDs, we align the photonic crystals in specific way to enhance the degree of polarization and the diffractions on different axes. Emission contours are mapped to verify the massive diffraction on E//m modes and rather inferior diffraction on E//c modes. The emission directions of diffraction are also implied by the contours and show agreement to the radiation profile measurements. Furthermore, we replace intensity by polarization ratio in the mapped contours. Their stronger ratio locations show overlap with the E//m mode diffraction in the emission contours. The result show that with E//m modes being the dominant diffraction by photonic crystals, the polarization ratio and light extraction of a-plane LED can be improved significantly as well as the diffraction characteristics can be adjusted by varying the period of photonic crystals.