Low-Contact-Resistance Contacts to Graphene via Metal-Mediated Etching
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Low-Contact-Resistance Contacts to Graphene via Metal-Mediated Etching Wei Sun Leong and John T.L. Thong* Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576
ABSTRACT The theoretically-predicted enhancement of metal-graphene contacts using the “endcontacted” configuration is studied. Graphene edges at the source/drain regions are created via a CMOS process compatible metal-assisted etching technique. The on-resistance of a graphene device with cobalt-etched-graphene contacts shows 6 times improvement compared to pristine graphene device. Apart from that, four-point contacted graphene devices with nickel-etchedgraphene contacts were fabricated and tested under ambient conditions. The proposed graphene devices exhibit contact resistance as low as 14 Ωµm, with an average of 90 Ωµm. Thus, forming metal-etched-graphene contacts is a promising method to obtain low-contact resistance metal contacts to graphene.
INTRODUCTION Arising from its superior electrical properties, graphene has attracted enormous research interest from the electronic device community. One of the critical factors that limits the performance of a graphene field-effect transistor (GFET) is poor metal-graphene contact1. The standard micro-fabrication process to form the metal-graphene contacts typically involves defining source/drain regions using either electron beam lithography (EBL) or photolithography followed by metal deposition. There are several experimental studies2-5 on metal-graphene contacts but the reported values show a large variation in contact resistance. Experimentally measured contact resistances at this critical interface of graphene devices range from hundreds to hundreds of thousands of Ωµm2. Different metals such as Cr/Au, Ti/Au, Pd/Au and Ni have been used to fabricate GFET. Theoretically, it has been shown that metals like titanium, scandium, calcium, cobalt and nickel could form strong chemical bonds with graphene6. Among these 5 transition metals, only titanium, cobalt and nickel are commonly used in the semiconductor industry. However, titanium is often only used as an adhesion layer for noble metals such as gold and a previous experimental study has shown that Ti/Au and Ti/Al induce opposite doping effects on graphene7. Another theoretical study has demonstrated that the end-contacted metal-graphene provides much lower contact resistance compared to that of side-contacted contacts by up to a few orders of magnitude8. However, conventional metallization schemes place the metal electrode on top of the graphene channel resulting in a side-contacted configuration, except for a small amount of edge coverage. The end-contacted configuration is limited by the amount of exposed graphene edges at the source/drain regions of the devices. In 2012, Wang et al. 9 showed that cobalt can perform crystallographic etching on graphite surface. Through annealing, a thin cobalt film on graphite can segregate and ball up due to surface tension and subsequently etch the graphite surface forming m
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