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標題: | 大面積石墨烯薄膜應用於光電元件之特性研究 Investigation of optoelectronic devices made with large-area graphene films |
作者: | Che-Wei Chang 張哲瑋 |
指導教授: | 楊英杰(Ying-Jay Yang) |
共同指導教授: | 陳永芳(Yang-Fang Chen) |
關鍵字: | 石墨烯,氮化鎵,太陽能電池,電致發光,奈米粒子,?化合物,光偵測器, graphene,GaN,solar cell,electroluminescence,nanoparticles,zinc compounds,photodetectors, |
出版年 : | 2013 |
學位: | 博士 |
摘要: | 近年來由單層碳原子組成的石墨烯薄膜,由於其優異的電學和光學性質,引起重大關注。石墨烯具有高電子和電洞遷移率,高透明性和高穩固性。因此,與其他常用的導電材料相比,比如鎳金合金和銦錫氧化物,石墨烯同時提供了低成本和高透明度的優點,因此被認為是一種很有潛力的導電材料。在本論文中,結合跨領域的知識及新穎的石墨烯材料,我們探討石墨烯於光電元件之潛在應用,其科學成就的亮點,簡要介紹如下。
1. 石墨烯/二氧化矽/氮化鎵 金氧半結構電致調變光發射器 在此研究中,透過石墨烯優異的透明性和導電性,我們利用新設計的金屬/絕緣體/半導體結構,使得發光二極體具有電致調變發光性質。在實驗中,電子和電洞的穿隧現象可以分別解釋所觀察到的在正向和反向偏壓下的電致調變發光頻譜。此類新型金氧半結構展現出一種新概念的石墨烯薄膜應用,並有助於發展應用於光通訊系統的低成本發光元件。此外,使用石墨烯作為一種多用途的電極可以使得電晶體和記憶體與發光元件整合成一個全新的主動式元件,並開闢一個新的研究領域。 2. 金氧半結構雙穩態光記憶體 在傳統的記憶體中,讀取的程序只能以序列方式進行。在此研究中,我們發展一新型的雙穩態光記憶體。相對於利用電學方式,透過光學方式讀取記憶體資料,可以平行地提供資料讀出以及實現高速數據頻寬。雙穩態光記憶體的實現,是利用我們之前開發的金屬/絕緣體/半導體的發光二極體與以二氧化矽為基礎的記體體元件結合而成。當高於一定電壓時,雙穩態光記憶體呈現導通,從高電阻狀態切換到低電阻狀態,其切換機制是由於金屬絲於二氧化矽內的形成,改變元件電阻。同時,調整電阻狀態,可以簡單地控制發光強度,達到光記憶體的效果。在實驗中,電流的穿隧現象可以解釋所觀察的雙穩態電致發光頻譜。基於發光二極體和記憶體的融合,我們的成果,除了展現兩類型元件的整合之外,更有益於光通信,新型記憶體和顯示面板的發展,此類新開發的光記憶體,具有結構簡單容易的製造技術,更集成了多種複合材料的優點,相信在實際應用中是非常有用的。 3. 利用化學摻雜改善石墨烯導電度以增加太陽能電池的光電流 我們利用化學方式摻雜石墨烯形成p型石墨烯並轉印在單晶矽太陽能電池表面,發現太陽能電池的功率轉換效率顯著地提升。在本實驗中,我們利用恆定氯化金溶液濃度摻雜石墨烯薄膜,以不同的摻雜時間作為變數觀察太陽能電池的功率轉換效率變化,另外,透過太陽能電池的電流特性以及外部量子效率可以發現p型石墨烯可以幫助收集光生載子和提高短路電流,展現此類p型石墨烯的可靠度以及其應用價值。 4. 利用摻雜石墨烯產生載子反射以及背面電場 展示了石墨烯以及摻雜石墨烯做為電極的應用後,我們發現摻雜石墨烯可以減少太陽能電池的載子復合損失。在實驗中, 於p型半導體和背面金屬接觸的介面插入重摻雜的p型石墨烯。p型石墨烯不僅可以產生背面電場和加速電洞傳輸,有助於收集的光生電洞,也可誘發p型石墨烯和半導體之間的導電帶能階差異作為光生電子反射層。我們利用此一背面電場概念增強奈米脊狀氧化鋅/矽光探測器響應率和單晶矽太陽能電池功率轉換效率。 Graphene, an atomically thin film composed of a single layer of carbon, has attracted significant attention due to its exceptionally electrical and optical properties. In contrast to indium tin oxide (ITO), graphene possesses high electron and hole mobility, high transparency and high robustness. Therefore, graphene has been considered as a promising conducting material, offering low cost and high transparency in comparison with other frequently used conducting materials, such as Ni/Au and ITO. Based on the novel properties of graphene, in this thesis, we develop the potential applications of graphene on optoelectronic devices. The highlight of our scientific achievement is briefly described as follows. 1. An advanced economical alternative for electrically tunable light emitters: graphene/SiO2/p-GaN diodes A newly designed metal-insulator-semiconductor (MIS) light-emitting diode (LED) has tunable emission spectra when used under forward and reverse biases. The high performance of our MIS-LED is made possible by taking advantage of the excellent transparency and conductivity of graphene. The observed electroluminescence spectra under both forward and reverse biases can be interpreted by the tunneling of electrons and holes through the insulating layer, respectively. This approach may aid in the development of economical alternatives to current tunable LEDs and represents a novel application of graphene in MIS-based optoelectronic devices. In addition, the use of graphene as a versatile electrode could enable the integration of transistors and memory devices into the presented MIS structures and open up a new area of research. 2. Bistable light emitting diodes: smart memory devices with electric and optical detections In traditional memory devices, the reading process is only detected electrically in serial sequence. In this work, we made the attempt of a light-emitting memory (LEM) which enables data communication via optical detection and provides both parallel reading process and high data bandwidth simultaneously. The LEM was achieved by developing the metal-insulator-semiconductor (MIS) light-emitting diode (LED) in tandem with a transparent SiO2 based memory cell. We fabricated MIS-LED using graphene layer as a metal contact on p-GaN substrate with an insulating SiO2 interlayer. When the device is biased above a certain voltage, it switches from a high resistance state (HRS) to a low resistance state (LRS). The switching mechanism is due to the formation of metal filaments. Moreover, the luminescence intensity can be simply controlled by adjusting the resistance state. Tunnel injections of holes from graphene through thin SiO2 barrier into p-GaN consistently explain the origin of luminescence spectra. Based on the integration of light emitters and memories, our work shown here could open up a new route for the optical communication, digital memories and recordable display panels. We stress here that the newly developed LEM device possesses a simple structure with easy fabrication process, which integrates several advantages of the composed materials, and should be very useful for practical applications. 3. Enhanced photocurrent in silicon photovoltaics using chemically doped p-type graphene By taking the advantage of doping modulation and high transparency of graphene simultaneously, the power-conversion efficiency of crystalline-silicon (c-Si) solar cells can be significantly improved by depositing p-type graphene on the front surface. Here, CVD graphene was doped as a function of time with constant AuCl3 concentration. The underlying mechanism can be interpreted quite well in terms of the fact that the p-type graphene can help the collection of photogenerated holes and enhance the short-circuit current (Jsc). 4. Enhanced performance of photodetector and photovoltaic based on carrier reflector and back surface field generated by doped graphene The major recombination loss arises from the back-surface recombination created by the poor interface quality between active layer and metal contact. With the insertion of a heavily doped p-type graphene between semiconductor and back side metal contact, we anticipate that the p-type graphene not only can generate a back surface field (BSF) and accelerate the transport of holes, but it also can induce a conduction band barrier due to the band alignment between p-type graphene and semiconductor to serve as a blocking layer for photogenerated electrons. We report the influence of carrier reflector and back surface field generated by doped graphene on n-ZnO nanoridges/p-silicon photodetectors and silicon solar cells. It is found that the p-type graphene not only acts as an electron blocking layer, but also helps the collection of photogenerated holes. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/61664 |
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顯示於系所單位: | 電子工程學研究所 |
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