光子晶體奈米洞陣列對發光二極體出光特性之影響

Abstract

在氮化鎵發光二極體中，利用二維光子晶體於作圍繞射光柵以增加出光效率，近年來已經是相當普遍的做法，多數研究為了避免破壞多層量子井，光子晶體僅製作於表面，因此無法將具有高能量的低階模態與光子晶體耦合而萃取出半導體表面。在過去的研究裡，我們曾利用電子束微影技術將光子晶體奈米洞陣列製作於發光二極體發光區外圍，由於光和光子晶體的交互作用與表面光子晶體相比較為強烈，因此出光效率得以提升。在此同時，由於光子晶體週期性的結構，發光場型亦能得到改變。 在本篇論文中，延續過去的實驗，我們在同樣在發光二極體發光區外圍製作週期與奈米洞半徑各為500奈米與200奈米的奈米洞陣列。實驗結果顯示，在我們製作的元件中，在可見光波段具有負折射的現象，這種情況會使得光根據不同偏振方向在不同角度匯聚。對於TE模態來說，當光子與光子晶體交互作用時，同時產生正向和負向的群速度，使得出光方向匯聚在10度的地方。另一方面，光子晶體內空氣柱對於TM模態的光子影響較小，因此對於出光角度的改變並不明顯。在本實驗中，我們利用三維遠場分析，以便更加了解能量的分布，同時也利用等頻率曲線分析，以得到理論上和實驗上的印證。 在接下來的實驗中（第四章），我們在整個元件的表面都製作了奈米洞陣列（週期與半徑分別為400及140奈米），以得到更強的光子晶體與不同模態的交互作用。不同於以往的實驗結果，在這樣的結構中，TM模態相較於TE模態，和光子晶體的繞射作用更為明顯，使得TM模態的出光角度在14度的地方有最大值，而另一個極值則位於32.5度。反觀TE模態，除了位於0度的地方，並沒有特別的峰值，但對比沒有任和結構的元件，在具有奈米洞光子晶體陣列的元件中，發光場型較為發散。在本章中，我們利用模態萃取出光，以及光子晶體造成的繞射，解釋不同模態發光場型的不同。

In recent years, using photonic crystals (PhCs) in GaN light-emitting diodes (LEDs) as optical diffraction grating is a reliable way for improved light extraction. Generally, most researches in related field utilized shallow PhCs fabricated on either a p-type of n-type semiconductor layer in order not to damage the multiple quantum wells (MQWs). Despite reducing the surface defects, the low-order modes, which carry large portion of optical energy, could not couple with PhCs effectively and be extracted. In our previous work, the LEDs with surrounded two-dimensional PhC nanohole arrays were demonstrated. Due to the stronger interaction between emission light and PhCs, the extraction efficiency and directionality are improved. In this work, the PhC array was fabricated at the mesa edge, with lattice constant of 500 nm and radius of 200 nm. The results show that the electroluminescent (EL) devices with negative refraction in the visible wavelength range. This approach produces polarization-dependent collimation behavior. For TE modes, when photons interact with the PhC, changes between positive and negative group velocities result in the confinement of radiation light to 10°. On the other hand, TM-mode photons are less perturbed by the air hole column, bringing about less effect on extracting angle. This study uses three-dimensional far-field analysis and equi-frequency contour to obtain theoretical and experimental results. Next, in the second part of this article, we fabricated the LED, of which the entire V mesa covered by PhC nanohole array with lattice constant of 400 nm and radius of 140 nm, to obtain the stronger interaction of light with PhC. In this structure, the diffraction by PhCs in TM modes is effective than in TE modes, causing TM polarized light to congregate at 14° (another peak intensity is at 32.5°). Conversely, there is no obvious peak intensity at all angle except 0°. Compared with conventional planar LED, the radiation profile is wider. In this chapter, we introduce two mechanisms, vertically guided mode extraction and laterally propagating light diffraction by PhCs, to explain the difference of radiation with polarization.