光子晶體於氮化鎵發光二極體元件之研究

Abstract

隨著固態照明技術的演進，近年來發光二極體(light-emitting diodes, LEDs)的發光效率已大幅度提升，使得其應用範圍從早期的訊號指示燈到現在廣泛地應用於平面顯示器背光源、汽車頭燈、投影機光源乃至一般照明等，並且有逐漸取代傳統照明燈泡的趨勢。由於發光二極體的光源特性除了會直接影響應用端的設計外，各種用途都有不同的光源特性需求。故本論文主要研究目的係利用光子晶體(Photonic crystals)奈米結構特性來提升氮化鎵(GaN)發光二極體元件各種光源特性，包括：萃取光效率的提昇、光場場形的準直性和偏極化光的取出。其中，分別研究分析藍寶石基板(sapphire substrate)和磊晶薄膜轉移技術(epifilm-transferred)的氮化鎵光子晶體發光二極體之光源特性。 首先研究探討光子晶體奈米結構在氮化鎵發光二極體元件上的特性，我们主要將二維光子晶體製作於氮化鎵發光二極體為藍寶石基板(GaN sapphire-based LEDs)上，並特別設計環狀區域的波導結構，研究光經由光子晶體繞射出光的平面角度分佈特性。從實驗結果中發現了光子晶體具有非等向性萃取光之現象，並且隨著光子晶體週期增長，對稱光芒數目會隨之變化，且變化關係與光子晶體之對稱性有關。此現象可用Ewald construction法則來解釋與探討，文中也探討萃取光特性與光子晶體週期及晶格排列方向的相依關係。接著，在偏極化特性的探討上，由於氮化鎵光子晶體發光二極體具有非等向性的萃取光特性及InGaN系列材料出光均以TE模態為主，所以光線的極化方向會垂直於光線的傳播方向，當主動發光層發光後，進入同一波導方向之所有光線會具有相同的極化方向，並搭配光子晶體進行取光，使得此元件出光具有偏極化特性。從實驗上驗證，藉由旋轉偏極片觀測到樣品確實具有偏極化現象，且此元件的極化消光比(P/S ratio)經由量測結果大約為5.5，接近85%的偏極化光源特性。另外，我們也探討此元件的遠場場形特性，從實驗結果得到在此元件的半功率角度為發散的大角度分佈，並且利用角度頻譜解析得到此元件皆為高階傳導模態與光子晶體相互作用。 從上面的研究結果已知光子晶體於藍寶石基板的氮化鎵發光二極體元件上並無法與低階模態相互作用進而得到準直光場場形，所以我們進一步將光子晶體奈米結構製作在磊晶薄膜轉移的氮化鎵微共振腔發光二極體元件(GaN micro-cavity LEDs)上，也同樣進行光萃取效率、遠場準直性和偏極化特性的研究探討。首先在光萃取效率和遠場準直特性上的研究，依據實驗分析結果得知，磊晶薄膜轉移後的氮化鎵厚度愈薄，則微共振腔和光子晶體相互作用效應愈大，而在萃取光效率及光場場形準直度上的特性愈好。另外，有無製作光子晶體的氮化鎵微共振腔發光二極體元件在不同收光角度內的收光量增益比，在±20度收光角內的增益可以達2.4倍。進一步利用角度頻譜解析實驗得到此元件皆為低階傳導模態與光子晶體相互作用，並且較容易從理論模型上針對光子晶體作設計而得到準直遠場場形。而此元件的偏極化特性探討上，也從實驗結果觀測到經由光子晶體萃取出的傳導模態極化特性，分別為線性和非線性耦合模態的組合，並且能和二維自由光子能帶圖互相匹配。故此研究結果表明高萃取光效率的準直光源可應用於一般照明、顯示產業、投影照明、特殊照明等產業上，具有系統整合及高效率、省能等優勢，減低成本並增加產品競爭力。

The next-generation of light-emitting diodes (LEDs), for applications such as projector displays, backlight displays, and automobile headlights, requires further improved light source properties. Within a few years, solid-state LEDs will be in a position to replace conventional lighting sources. Optimizing LED performance for a specific system requires detailed knowledge of light extraction, far-field emission distribution, and polarized characteristics. This dissertation studies the sapphire-based and micro-cavity (MC) types of GaN LEDs, using photonic crystal (PhC) nanostructures to improve light extraction efficiency, directional far-field emission, and polarized light emission. First, anisotropic light extraction distribution in the azimuthal plane has been investigated through the optical images taken from GaN sapphire-based LEDs with a special annular PhC region in which the guided electroluminescence (EL) light generated at the center. With increasing PhC lattice constant, symmetric patterns with varying number of petals according to PhCs were observed and analyzed. In addition, a map of the anisotropic for various PhC lattice constants and numerical apertures was constructed. Then, polarized characteristics of the GaN sapphire-based PhC LEDs have been experimentally and theoretically studied. Strong polarization dependence on the lattice constant and lattice orientation of the PhC was observed. It is found that the PhC can be as a polarizer to improve the P/S ratio of the extracted EL emission. The results of the P/S ratio for light propagating in different lattice orientation were found to be consistent with the results obtained using the PhC Bloch mode coupling theory. This polarization behavior suggests an efficient means to design and control the GaN sapphire-based PhC LEDs for polarized light emission. Next, directional far-field emission measurements show a divergent angle distribution on GaN sapphire-based PhC LEDs. This is due to the PhC interacting with higher-order guided modes. These results show that a surface PhC will not closely interact with the lower-order guided modes of GaN sapphire-based PhC LEDs because the GaN thickness is too thick. The solution to this problem is to make a GaN PhC MCLED to allow the lower-order guided modes to interact with the PhCs. This study examines the light extraction efficiency, directional far-field patterns, and polarization properties of GaN PhC MCLEDs. Angular-spectra-resolved electroluminescence (EL) measurements revealed directional far-field emission distribution from the GaN PhC MCLEDs based on the lower-order guided modes extraction. In addition, the light enhancement in GaN PhC MCLEDs compared to GaN non-PhC MCLEDs within a ±20° collection cone angle was enhanced by a factor of ~2.4. Additionally, angular-spectra-resolved EL measurements were also made in the polarized-resolved manner that has been used to analyze reveal guided modes polarization in the GaN PhC MCLEDs. The measurement results revealed the evolution of the collinearly and non-collinearly coupled modes. These results good agreement with the two-dimensional free-photon band structure is obtained. The present results indicate that the high directionality light extraction enhancement could contribute to many applications, especially for etendue-limited application such as pico-projector.