光子晶體陣列與氮化鋁奈米柱層於發光二極體之研究

Abstract

論文包含兩個部分，首先是光子晶體奈米洞陣列對發光二極體出光特性影響的研究，第二部分則是利用高溫爐管在藍寶石基板上成長氮化鋁奈米柱層作為氮化鎵磊晶的緩衝層，使磊晶過程中有側向磊晶生長作用。 在氮化鎵發光二極體中，利用二維光子晶體於作為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。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），並蝕刻穿過多重量子井。因此可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。反而TE模態，其發光場型較為發散。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。 另一方面，使用有機金屬化學氣相沉積在藍寶石基板上做氮化鎵發光二極體 磊晶是當今主流的做法。但是因為兩種材料間晶格常數(13%)以及熱膨脹係數(62%)的差異，使得氮化鎵磊晶層有大量的貫穿式差排。為了改善氮化鎵的磊晶品質，我們利用氣-液-固機制在藍寶石基板表面成長氮化鋁奈米柱層，使氮化鎵有側向磊晶生長的效果。從(002)搖擺曲線半高寬可知成長於奈米柱層的氮化鎵磊晶品質均較一般成長於藍寶石基板的為佳。拉曼光譜則可發現氮化鎵磊晶層相較於成長在藍寶石基板，成長於氮化鋁奈米柱層的應力從1.25 GPa下降至0.78 GPa。從穿透式電子顯微鏡影像發現貫穿式差排的密度確實減少，並於奈米柱層上方產生疊層缺陷可用於阻擋貫穿式差排。最後量測變溫光致螢光發現成長於氮化鋁奈米柱上的多重量子井內部量子效率提升了12.2%。

The thesis consist of two parts. First of all, we studied in the effects of photonic crystal nanohole arrays on light output properties of LEDs. Second, we used the high temperature tube furnace to grow AlN nanorod template. It acts as buffer layers between GaN epilayers and sapphire substrate and comes out with the epitaxial lateral overgrowth process. In recent years, using photonic crystals (PhCs) in GaN LEDs as optical diffraction grating is a reliable way for improving light extraction. Generally, most researches utilized shallow PhCs in order not to damage the MQWs. Despite reducing the surface defects, the low-order modes could not couple with PhCs effectively and be extracted. We fabricated the LED, of which the entire mesa covered by PhC nanohole array with lattice constant of 400 nm, radius of 140 nm and etching through the MQWs layer. 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°). Compared with conventional planar LED, the radiation profile of TE mode is much 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. Besides, typically, GaN epilayers are grown on a planar sapphire substrate by metal-organic chemical vapor deposition (MOCVD). However, GaN epilayers suffer from a high threading dislocation density due to their large lattice mismatch (13%) and thermal expansion coefficient misfit (62%). The AlN nanorod templates were grown on sapphire substrate by VLS mechanism and have the effect of lateral epitaxial to improve the crystalline quality of GaN epilayers. The FWHM of (002) XRD rocking curve represent that the samples with the templates have higher crystal quality. It is found that the residual stress was reduced from 1.25 to 0.78 GPa in the GaN epitaxial layer by inserting the AlN nanorod templates. The TEM images reveal lower dislocation density in GaN epilayers on AlN nanorod template and the stacking fault on the templates can block the threading dislocation. Moreover, the internal quantum efficiency of MQWs is enhanced by 12.2% compare to reference sample.