Electrical properties of graphene-metal contacts

Gianluca Fiori,Max C Lemme,Teresa Cusati,Amit Gahoi,Giuseppe Iannaccone,Vikram Passi,Alessandro Fortunelli

doi:10.1038/s41598-017-05069-7

Abstract

The performance of devices and systems based on two-dimensional material systems depends critically on the quality of the contacts between 2D material and metal. A low contact resistance is an imperative requirement to consider graphene as a candidate material for electronic and optoelectronic devices. Unfortunately, measurements of contact resistance in the literature do not provide a consistent picture, due to limitations of current graphene technology, and to incomplete understanding of influencing factors. Here we show that the contact resistance is intrinsically dependent on graphene sheet resistance and on the chemistry of the graphene-metal interface. We present a physical model of the contacts based on ab-initio simulations and extensive experiments carried out on a large variety of samples with different graphene-metal contacts. Our model explains the spread in experimental results as due to uncontrolled graphene doping and suggests ways to engineer contact resistance. We also predict an achievable contact resistance of 30 Ω·μm for nickel electrodes, extremely promising for applications.

Highlights

Low and reproducible metal-graphene contact resistance RC is an imperative requirement for the industrial adoption of graphene in electronics[1,2,3] and for the adoption of other two-dimensional materials, which often rely on the use of graphene-metal interfaces[4]
Liu et al.[5] further analysed the impact of molecular orbitals involved in the contact on transmission, finding that the conductance of the metal-graphene-metal junction is affected by the interfacial binding, and by which molecular orbitals are involved and their symmetry, and that contact resistance decreases with the increase of the contact area at low bias voltage
The contact resistance clearly depends on the Dirac point energy in graphene, and on the charge density in the graphene layer, which is often neither reported nor controlled in the experimental literature

Summary

Introduction

Low and reproducible metal-graphene contact resistance RC (i.e., smaller than 100 Ω × μm) is an imperative requirement for the industrial adoption of graphene in electronics[1,2,3] and for the adoption of other two-dimensional materials, which often rely on the use of graphene-metal interfaces[4]. Ji et al.[18] used this concept in a systematic study of the contact resistance, including both single-sided and double-sided contacts for different graphene-metal systems. They compute graphene-metal tunnelling with the Wentzel–Kramers–Brillouin approximation, that is inadequate for high transmission, and their study is limited to a single geometry. Ma et al.[24] carried out a systematic first-principles study on contact resistance between several metals and graphene, observing the dependence of contact resistance on edge termination, contact area and point defects on the contact region. Metals weakly interacting with graphene (i.e. Au and Ag) are very sensitive to the atomistic configuration at the contact region: edges without chemical terminations, small contact length and point defects result in decreased contact resistance. Matsuda et al.[16] have shown that the different interaction strength can affect the contact geometry and give rise to “edge” configurations

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Conclusion