Effect of contacts, graphene type and underlying substrate on the transport properties of graphene.

摘要

Graphene is a two dimensional system, comprising of a monolayer of carbon atoms arranged in a hexagonal lattice structure. Ultrathin graphite has been studied for several decades. In 2004, single layer graphene was isolated and its electronic properties have been extensively studied since then. The primary focus of this work was related to the effect of contacts, channel dimensions, and graphene type (exfoliated vs. deposited) on the measured electrical characteristics of graphene. A process flow was developed to enable electron beam based field effect- transistor (FET) fabrication on exfoliated graphene and photolithographic based FET fabrication on chemical vapor deposited (CVD) graphene. The metal-graphene contact resistance (Rc) was observed to dominate the total measured resistance at high vertical fields. The extracted Rc was determined to be independent of metal type and contact dimensions, in contrast to a typical metal-semiconductor system. Reduction of contamination and residue at the interface between the metal and graphene was observed to reduce the Rc. The lowest value of contact resistance obtained for exfoliated and CVD graphene was about 200 Ω. Effective mobility of CVD graphene was studied as a function of domain size (6 &mgr;m - 20 &mgr;m). For room temperature measurements in air, the extracted mobility was observed to be weakly dependent of the domain size. The extracted effective mobility was additionally observed to be dependent on channel dimension and underlying oxide thickness. This anomalous dependence is shown to be the partial cause of observed high mobility values in graphene devices. The dependence is primarily due to the graphene device behaving as a strip capacitor. Overall, the results of the research presented here provide important insights necessary to develop numerous types of graphene-based devices.