Abstract
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.