利用二維光子晶體提升氮化鎵發光二極體效率

Abstract

近年來利用光子晶體當作繞射光柵解決氮化鎵發光二極體萃取效率不佳和發光場型的問題相當普遍，但是多數的研究為了避免破壞多層量子井，都是將光子晶體做在氮化鎵的表面，這樣的結構只能萃取出材料內的高階模態，而具有高能量的低階模態則因為和光子晶體作用的範圍很小則無法有效被萃取出來。這樣的結構使得利用光子晶體來增加發光二極體的效率非常有限。 在本篇論文中，我們利用電子束微影將二維光子晶體做在發光二極體發光區的周圍，並蝕刻到多層量子井形成奈米洞陣列。這樣的結構有效的將材料內部往側向的低階模態與光子晶體作用因而被耦合到空氣中增加萃取效率。隨著奈米洞陣列越大，傳導模態與光子晶體作用的越好，萃取效率越佳，直到40 μm的奈米洞陣列讓萃取效率發生飽和。藉由光子晶體的參數設計(不同週期與直徑)還可以達到不同的萃取效率以及發光場型的變化。週期/直徑為400/320 nm的元件在垂直於元件表面(90度)的方向中有很高的指向性可以達到31.5 %的光強度增加；而在週期/直徑為400/280 nm的元件中在75度的方向上有很高的指向性可以達到21.7 %的光強度增加。 延續上個實驗，接下來這個結構除了在二極體發光區周圍有奈米洞陣列把低階模態耦合出來，我們在氮化鎵表面也做了光子晶體破壞全反射，讓原本在材料理面的傳導模態全都有效的被耦合出來。我們將這個新穎的結構和一般使用光子晶體的發光二極體(只有表面做光子晶體)做比較，可以發現這個結構可以達到56 %的光強度增加，而一般光子晶體的發光二極體則是只有40%的光強度增加。從量測與模擬的結果印證了我們的想法，多了奈米洞的陣列可以萃取出來往側向行進的低階模態，而不再僅限於只有高階模態的取出，這是一般使用光子晶體的發光二極體所無法達到。

In the recent years, PhCs as diffraction grating have been widely explored to improve light extraction and to modify radiation profiles of LEDs. But most reports in the related field utilized shallow PhC structure on the device surface in order not to damage the multiple quantum wells (MQWs). As a result, the shallow patterns are only effective on higher order modes while a large portion of the optical energy of low order modes is poorly extracted due to less overlap with the PhCs. Therefore, this kind of structure limits the potential of light extraction efficiency by utilizing PhC structure. In this thesis, we define two dimensional PhC pattern by e-beam lithography at the periphery of the light-emitting mesa area and etch them through MQWs as nanohole array. The structure can effectively diffract out the low order modes of laterally propagated light by interacting with PhCs and thus reaches high output power. With larger width of nanohole arrays, the light output power enhancement is enlarger due to the better coupling effect between PhCs and guided modes, then saturate till the width of 40 μm. The different extraction efficiency and the emission pattern can be achieved by designing parameters of photonic crystal (pitch and diameter). The light intensity enhancement factor of the device with a/d=400/320 and a/d=400/280 are 31.5% at the vertical direction (90°) and 21.7% at the 75°, respectively. Continuing from the previous experiment, we not only fabricate the nanohole reflectors surrounding the light emitting mesa but also the PhC structure on the mesa surface to diminish the total internal reflection. In that way, all the guided modes in the material can be effectively diffracted by PhCs of this structure. We compare the novel structure with the general PhCLED(LED only with surface PhC), and the light intensity enhancement factor are 56% and 40%, respectively. The measurement and the simulation results consist with our idea. With the nanohole arrays applied at the periphery of the mesa, a higher output power can be achieved due to the enhanced collection and diffraction of low order modes of laterally propagated light interacted with nanoholes. The interaction of in-plane optical wave with the nanoholes is much stronger than that with surface PhC, suggesting an efficient light diffraction to the surface normal by nanoholes. keywords：photonic crystal、GaN、nanohole array、light-emitting diode