ABSTRACT
The understanding of transport properties in graphene has become a topic of great interest not only because of the underlying fascinating physics, but also because of the ¦ourishing graphene-based technologies in •elds such as ¦exible displays, high-frequency devices, composite materials, or photovoltaic applications. Graphene shows an exceptional ability to convey charge carriers (electrons and holes) and displays some of the most exotic quantum transport features of modern condensed matter physics (Geim and Novoselov 2007). The origin of these properties roots in the linear band dispersion of low-energy electrons (developed around two independent K and K′ valleys in the Brillouin zone), and in the presence of a pseudospin degree of freedom; both properties are shared with metallic carbon nanotubes, their onedimensional counterparts (Charlier, Blase, and Roche 2007, Castro Neto et al. 2009). Low-energy electronic excitations behave as massless Dirac fermions, which yield
3.1 Introduction ....................................................................................................65 3.2 Pseudospin Effects and Localization in Disordered Graphene ......................66 3.3 Graphene-Based Field Effect Transistors: The Clean Case............................ 71
3.3.1 Graphene Nanoribbon Electrostatics .................................................. 72 3.3.2 Graphene Nanoribbon Transport Model ............................................. 74 3.3.3 Model Assessment .............................................................................. 76
3.4 Improving Device Performances: Mobility Gap Engineering ....................... 79 3.5 Conclusion ...................................................................................................... 81 Acknowledgments .................................................................................................... 82 References ................................................................................................................ 82
unique quantum phenomena such as Klein tunneling (Katsnelson, Novoselov, and Geim 2006) and weak antilocalization (McCann et al. 2006). These fascinating properties of clean graphene-based materials can be further tuned and diversi•ed in unprecedented ways by chemical modi•cations of the underlying π-conjugated network (Loh et al. 2010). The reported charge carrier mobilities in graphene layers can reach about 200.000 cm2 V−1s−1 at room temperature, which is several orders of magnitude larger than those of silicon. However, an undoped single-layer graphene acts as a zero-gap semiconductor, which is unsuitable for achieving competitive electrostatic gating ef•ciency and further developing all carbon-based nanoelectronics. Experimental measurements reported ratios between the current in the ON state and the current in the OFF state not higher than one order of magnitude. One possibility to increase the (zero) gap of two-dimensional graphene single layers is to reduce the lateral dimensions of the device, fabricating graphene nanoribbons with widths down to a few tens of nanometers, using state-of-the-art e-beam lithographic techniques and oxygen plasma etching, or ion-beam lithography. Graphene ribbons can also be engineering by chemically unzipping carbon nanotubes (Shimizu et al. 2011). These approaches allow some more or less ef•cient con•nement gap engineering despite an interfering contribution of disorder effects (Cresti et al. 2008, Roche 2011). Besides, theoretical simulations suggest that the best accessible energy band gaps remain too small or very unstable in regards to edge reconstruction and defects (Dubois et al. 2010), to envision outperforming ultimate complementary metal-oxide semiconductor •eld-effect transistors (CMOS-FETs) (Lee 2010).
