- 學位論文
新穎石墨烯電子元件: 由磊晶成長技術至高速電晶體應用
Graphenetronics: from Growth to High-Speed Transistors
摘要
在2004-2005年左右，"石墨烯"開啟了嶄新的研究大門，這個名詞引起了全球各個研究機構的關注，儼然形成一股研究熱潮，此後，數以千計的的文獻和研究相繼發表在國際知名期刊上。 石墨烯以單原子碳層構成蜂窩狀晶格的二維材料，引發了眾多新穎研究的可能性；尤其它自身的物理特性與特殊的能帶結構，於相對論物理學上探索所有具足輕重的引響，更有機會應用在下一世代的電子產品。然而早在一開始，基於Mermin-Wagner 所提出之理論，得知在熱力學中，室溫下二維晶體結構是極其不穩定地，被認為是不可能單獨存在的事情；但自從A. K. Geim 和K. S. Novoselov 等人於曼徹斯特大學研究團隊中，想出一個在室溫下分離出單元子碳層並轉印在二氧化矽基板上，此方法為:以機械剝離法將高定向熱裂解石墨（HOPG）層層分離，而用的就是大家耳熟能詳的"Scotch tape"，這樣如此簡單的膠帶材料，不但推翻了這根深柢固的想法，也是第一個成功製備石墨烯的實際例子，更為他們贏得了諾貝爾獎此桂冠。也因此，學術界積極開啟了一扇對二維材料研究的大門。 石墨烯具備眾多特性優點，在電學特性上，室溫下的載子電流遷移率達到20,000 cm^2/V.s和長程彈道傳輸；光學特性上，在可見光範圍內的光吸收率只有2.3%，為一個高度透明導電材料；且具有42 Nm^-1的機械特性以及Young's modulus 為 1.0TPa 的斷裂強度，眾合以上特性，石墨烯成為目前為止難得一見的可貴材料。 本論文重點針對以石墨烯為基礎所開發之多種電子元件應用，包含石墨烯基礎成長，石墨烯電晶體、利用互聯技術結合石墨烯和銅導線使其積體化與石墨烯軟性高頻電子元件。本論文分成三個主題，第一個主題包含第一、二章，講解了石墨烯的晶體結構、能帶結構、電學特性和製備等；第二個主題包含第三、四章，將石墨烯以低溫電子迴旋共振化學氣象沉積法直接成長於積體化的銅導線上，在奈密尺度的微縮下，依舊能保有優異的電流程載密度，更重要的是有效降低其操作阻抗，使未來元件朝奈米尺度不斷微縮的應用上，提供更穩定的可靠度；第三個主題包含第五、六、七章，首先介紹微波元件的基礎理論以及重要方程式之推導，接著深入解析石墨烯製備之軟性高頻電子元件製程應用，最重要的是利用本實驗室研發之自我對準設計，使得石墨烯電晶體得以在極高的兆赫頻率下操作，並進一步完成積體化之高頻接收器(包含低雜訊放大器、混波器)與倍頻器，最後進行元件分析。 末節，對整個論文著作進行回顧，在未來我們對石墨烯等二維新穎材料持續寄予厚望，希望有朝一日為下一世代電子產品開創另一個高峰。
並列摘要
The modern era of graphene“gold-rush”started around 2004–2005, when it became possible to fabricate samples with the toddler’s best friend – the Scotch tape. Since then, the publication trends in this area have been nearly exponential- with tens of thousands of publications just in the past few years. Graphene, an isolated mono-atomic carbon layer conformed into two-dimensional honeycomb lattice building blocks, has triggered off numerous novel research possibilities, due to its intriguing physics and as an emerging paradigm for relativistic condensed matter physics as well as showing great promise for its application in next generation electronics. Before A. K. Geim and K. S. Novoselov et al. envisioned a plausible method to isolate a single atomic carbon layer on SiO2; based on the Mermin-Wagner theorem, two-dimensional crystals were predicted as thermodynamically unstable formations and were thought of as non-existing in ambient environment that had so far been known only as an integral part of larger three-dimensional systems. The first successful example of monolayer graphene was achieved by using mechanical cleavage of highly oriented pyrolytic graphite (HOPG) [1] thus making an unprecedented accomplishment in 2-D material science to this day. The door opened by this first isolation of a 2-D crystal has opened countless doors for previously unknown applications. For instance, graphene has a host of characteristics that show great promise for the development of post silicon electronics [1–4], including a large roomtemperature carrier mobility [5] (20,000 cm^2/V.s) and long-range ballistic transport [6]. In addition to its electrical properties, graphene is also an highly transparent material with an absorption of 2.3 % within visible light range [7]. Its thermal conductivity is measured to be 5,000 W mK^-1 for a monolayer graphene at room temperature [8]. The intrinsic mechanical properties of free-standing monolayer graphene have been examined to be a breaking strength of 42 N m^-1 and a Young's modulus of 1.0 TPa, indicating that it is one of the strongest materials ever measured [9]. This thesis focuses on the various electrical applications of graphene-based devices, integrated with graphite/metal bishell interconnects and graphene-FET applied on analogue microwave circuits. Besides, the fundamental physics of graphene and an innovative strategy for high quality CVD-graphene synthesis that are introduced at the beginning not only lead us to a sufficient understanding in material science but also introduce the state-of-the-art of graphenetronics. This thesis content is categorized in to three parts and organized as follows. The first part presents the fundamental physics of graphene, graphene synthesis by chemical vapor deposition (CVD) and electrical calibration, addressing its emerging application in large scale in flexible electronics. Chapter 1 starts with the fundamentals of graphene, including the crystal structures as well as its energy band structure. Subsequently, we present an explanation and simplified mechanism of the Raman scattering, which is an important tool to examine the quality of graphene. We introduce the basic knowledge of Raman scattering and the phonon dispersion relation of graphene. The transport properties such as electric field-effect, minimum conductivity and scattering mechanism within graphene will also be explained briefly. Chapter 2 presents graphene growth mechanism and electrical transport analysis in graphene-based FETs. In brief, we show a new facile growth process to improve graphene quality by using CVD technology. In order to examine isolated graphene, the transfer technique and FET fabrication process are represented in following sections. To extract field effect mobility, Drude model is employed to describe the electrical behavior of graphene devices. Starting from the second part, we show a novel interconnect technique: metal/graphite conformal bi-shell booster via plasma-assisted technology to synthesize well-controlled graphene sheets. Chapter 3 starts with a introduction of electron cyclotron resonance (ECR) chemical vapor deposition (CVD) of graphene and Chapter 4 presents the fabrication and characterizations of novel electrical interconnect test lines made of Cu/Graphite bi-shell composite with the graphite cap layer grown by ECR-CVD. The graphite layer can boost the composite structure current-carrying capacity to 10^8 A/cm^2, more than an order of magnitude higher than that of bare metal lines, further reducing resistivity of fine test lines by 20 %. Raman measurements reveal that physical breakdown occurs at 680 – 720 ◦C. Modeling the current vs: voltage curves up to breakdown shows that the maximum current density of the composites is limited by self-heating of the graphite, suggesting the strong roles of phonon scattering at high fields and highlighting the significance of metal counterpart for enhanced thermal dissipation. The third, and final, part shows that state-of-the-art graphene-based microwave transistors can be implemented on diverse substrates, including both flexible PET and rigid AlN substrates,thus further assuring the feasibility of advanced applications in high-speed analogue circuits. These results indicate that self-aligned graphene FETs can provide remarkably improved deviceperformance and stability for a range of applications in flexible electronics. Chapter 5 starts with fundamental concepts in microwave transistors, including the evolutionary history of microwave transistors and a well-understood two-port networks representation. In Chapter 6, the purpose shows the novel fabrication process for high-performance CVD graphene FETs with self-aligned drain/source contacts have been presented and implemented on flexible PET substrates. It is very promising to apply this new strategy onto flexible high-speed electronics, especially for new generation wireless communication systems. In our process, an Al gate was directly defined on graphene by e-beam lithography, followed by pure O2 exposure, forming a native oxide layer around the Al wire. We also characterize other device properties, such as charge neutrally point shifting, current saturation, RF properties, device performance in diverse bending states, and further applications in microwave integrated circuits such as low noise amplifier, frequency mixer and doubler. As a closure, Chapter 7, shows state-of-the-art of graphenetronics built on rigid substrates. In this work, we propose a novel idea that uses the CVD growth method without pre-deposited metal catalysts can directly synthesize graphene on insulator substrates, which develops into a one-step approach not only allowing to bypass the wet transfer but also to obtain the electronicgrade and large-scale graphene films on which graphenetronics are developed. On the other hand, following well-defined manufacturing process mentioned before, high-speed graphenetronics have achieved recorded unity current gain cutoff frequency of 43 GHz realized with transferred graphene films on AlN substrate. To data, this is still a superlative extrinsic cutoff frequency on graphene-based electronics. In the end, a short conclusion and prospect are represented that graphene has been already showing the remarkable potential to be a one of channel materials in the next generation.