Two-dimensional materials for electronic applications
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Introduction The experimental demonstration of the isolation of graphene and a study of its properties in 2004 by Novoselov et al. and Berger et al. triggered tremendous worldwide scientific interest and activity on this material over the past decade.1,2 This prototype two-dimensional (2D) material is made of carbon atoms in a hexagonal structure and displays extraordinary intrinsic properties relevant to microelectronic,3 optoelectronic,4 and nanoelectromechanical systems (NEMS)5 applications (see the MRS Bulletin special issue on graphene, December 2012). These include high carrier mobility,6 high current density,7 broadband optical absorption,8 tensile strength in excess of 1 TPa,9 and high thermal conductivity.10 In addition, graphene is largely compatible with current semiconductor technology, so it can be easily co-integrated with silicon devices.11 Particular promise is seen for integration at the back end of the line, where graphene could provide added functionality to the existing complementary metal oxide semiconductor (CMOS) platform. The most notable disadvantage of graphene is the absence of an electronic bandgap, which limits its applicability as a transistor channel material. Here, other 2D materials may step in, in particular semiconducting transition metal dichalcogenides (TMDs) with the composition MX2 (M = Mo or W; X = S, Se, or Te, Figure 1). Single layers of TMDs are
only three atoms thick and extremely flexible and transparent. Depending on their actual composition, the bandgap of TMDs varies from metallic to those of wide bandgap semiconductors such as n-type MoS2 (bandgap energy Eg ∼ 1.8–2.4 eV).12,13 Phosphorene, another single layer 2D material exfoliated from black phosphorus crystals, has been found to perform well as the channel material in p-type transistors. Preliminary results show that the hole mobility is thickness dependent, with reported values up to 300 ∼ 1000 cm2/Vs.14,15 All 2D materials share issues of high contact resistance and channel mobility reduction due to dielectric interfaces. Fundamental concerns about the former have been somewhat eased by several recent reports of low resistance contacts.16,17 For the latter, another 2D material may come to the rescue, as insulating hexagonal boron nitride (h-BN) dielectrics show weak interaction with graphene and help maintain intrinsic properties.18 This article reviews fabrication methods for graphene and 2D materials, describes challenges in contacting these materials electrically, and discusses the potential for their application as transistors.
Material synthesis and fabrication technology Growth Upon its conclusive experimental discovery, graphene was mechanically exfoliated—or peeled—from graphite crystals.1 While this method still yields the highest graphene quality
Max C. Lemme, University of Siegen, Germany; [email protected] Lain-Jong Li, Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan; [email protected] Tomás Palacios, Department of Electrical Engineering and Computer Science, Massachuse
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