石墨烯及氧化鋅/氮化鎵材料之成長及其元件應用

Abstract

在本研究計畫中，本實驗室將進行氧化鋅(ZnO)/氮化鎵(GaN)異質結構與石墨烯 (Graphene)材料之成長特性以及其應用元件之材料物理特性研究。 最近幾年，氧化鋅材料吸引了非常多國內外的研究團隊投入研究。因為氧化鋅具有 很高的熱穩定性和化學穩定性，它和氮化鎵一樣是屬於直接能隙半導體，在常溫下能帶 寬度約3.37eV，而且氧化鋅的自由激子的束縛能為60meV，比氮化鎵的25meV還要來的 大，因此在室溫下氧化鋅中的激子對依然可以存在，因此理論上將氧化鋅做為發光二極 體材料其發光效率會比氮化鎵還要來的好。 對於將氧化鋅應用於發光二極體材料，在目前而言，有一個共通的問題。即為摻雜 為p型的氧化鋅材料一直沒有辦法穩定的形成。目前各研究團隊也對於p型的氧化鋅之成 長機制解釋也是眾說紛紜。因此，將氧化鋅材料成長在不同材料之p型半導體上以研究 氧化鋅材料發光二極體發光機制及其物理特性的第一課題。在成長方面我們將利用化學 氣相成長(CVD)法，在改變不同成長條件下，例如成長溫度以及氣體流量，以不同方向 性之氮化鎵基板成長氧化鋅奈米線以及薄膜，並且研究成長條件與極化與非極化基板對 於氧化鋅材料的成長機制，缺陷成因及物理性質做深入探討。由於半導體之摻雜與材料 缺陷特性息息相關，故我們也將利用在不同方向性基板上以不同成長條件之氧化鋅材料 作進一步摻雜的研究，以了解氧化鋅材料摻雜的物理特性。材料應用方面將分成兩大部 分，第一將利用氧化鋅/氮化鎵異質結構為主體結構，以研究氧化鋅材料發光二極體的 發光特性。其中包含了研究極化與非極化之氧化鋅/氮化鎵異質結構發光二極體，氧化 鋅同質結構發光二極體。另一方面將應用氧化鋅的表面化學吸附特性，進一步的研究以 氧化鋅作為改質閘極之氮化鎵高速電晶體的感應器元件特性，例如一氧化碳偵測器。 石墨烯也是最近引起廣泛研究之半導體材料。利用化學氣相成長法可以改變成長 之環境，例如成長壓力，氣體流量，以及成長溫度等，以進一步的研究其成長機制以及 摻雜特性。利用超高真空高能電子繞射可以更進一步的了解成長條件對於缺陷的影響。 目的在於成長出低缺陷的薄膜。在應用元件方面，石墨烯是單層碳原子結構的二維空 間排列且其電子遷移率當其載子濃度為1012cm-2時高達106 cm2/V．S。且其透明度 高達97.98%。，故石墨烯成為最具潛力的太陽能光電池之透明電極材料之一。另 一方面，石墨烯材料之導熱性極佳。其導熱係數高達5300 W/m‧K，比奈米碳管以 及金剛石更佳，且其會隨著溫度升高而下降。故石墨烯材料也是新一代的光電元件散熱 材料。 故本計畫將把石墨烯應用於太陽能電池上，當作透明導電薄膜以及散熱層，希望可 以改善太陽能電池之外部效率。另一方面，石墨烯表面因為是碳的sp2的鍵結，我們可以 合成不同的短鏈ＤＮＡ(aptamer)在其表面上做改質的實驗，藉由其高速電子遷移率， 若有吸附到欲感應物會造成其電性的改變。我們希望可以藉由此特性發展出極高靈敏度 以及高專一性之生物感應器。

Recently, Zinc oxide (ZnO) has attracted a lot of attention because its high thermal and chemical stability. ZnO is an attractive candidate for ultraviolet light emission because of its wide bandgap of 3.37 eV at room temperature (RT) and its large exciton binding energy, which is approximately 60 meV, significantly larger than that of ZnSe (22meV) and GaN (25 meV). Such a high binding energy ensures the survival of excitons even at RT, as evidenced by optically pumped RT exciton lasing and high-temperature excitonic stimulated emission. The problem that why ZnO material can’t replace GaN material as a commercial optoelectric devices is that p-ZnO is not stable enough and well understood to be ready to use. In this project, we will study the defect in the ZnO material via grow ZnO material on GaN templates with different orientation. Since the GaN templates with different orientation will have different lattice constant, so there should be different lattice mismatch between GaN templates and ZnO grown on it. On the other hand, the GaN templates with different orientation will have different polarization, the ZnO grown on the templates will show different polarization, too. This will help us to study the relations between growth conditions (like growth temperature, orientation of GaN templates, gas ratio, etc.) and defect inside. By understanding the mechanisms of defect, we will further trying to dope n-ZnO material on different oriented GaN templates with dopants, like N, As, etc., to study the doping mechanism of ZnO material. For the applications, in order to understand the luminescence mechanism of ZnO material, we will study heterostructure of ZnO grown on p-GaN template light emitting diode first. Since the GaN templates with different orientation will have different polarization, the ZnO grown on it should have different orientation, too. We will study the luminescence mechanism of polar and nonpolar ZnO LED through the heterostructure of ZnO grown on GaN templates. On the other hand, since the ZnO grown on GaN template with different orientation will have different surface properties, like O-face and Zn-face, etc, so the ability of chemical adsorption will certainly different. We will use this property to study the sensitivity on ZnO gated GaN/AlGaN HEMT bio-sensor, such as CO sensor. Graphene is a promising material for transparent contact layer and heat spreading layer since its high transparency, high electron mobility under room temperature, and high heat conductance. There are lots of methods to grow graphene on different substrate. The most promising way is chemical vapor depositon because of the higher film quality and low cost. In this project, we will grow graphene using CVD with different growth conditions (like growth temperature, gas ratio, substrates, etc) to study the defect mechanism of graphene. Since graphene is such a thin material that only a few layer, the surface property will dominated. So we will study the thin film quality using high energy electron beam diffraction under ultra high vacuum in order to understand the defect inside and to improve the quality of the thin film. For the applications, because of the high transparency and high electron mobility of graphene, it is a very promising material for optoelectrical devices as transparency contact layer. We will compare the output efficiency of solar cells using graphene and ITO as transparent contact layer. In the mean time, since it has very good thermal conductivity, so, we will study how to apply graphene as a heat spreading layer for optoelectrical devices, like solar cell and LEDs. On the other hand, since it is possible to be modified if we put different aptamers on the graphene surface as a gate of GaN/AlGaN HEMTs. So, we will study the modified gated GaN/AlGaN HEMTs sensor using different aptamer attached graphene as gate material in order to develop a new bio-sensor with high sensitivity and high specificity.