Contact resistance at planar metal contacts on bilayer graphene and effects of molecular insertion layers

TL;DR

This study investigates the origins of contact resistance in bilayer graphene with metal contacts, highlighting the roles of charge transfer, doping, and molecular layers, and proposing strategies for resistance reduction based on contact doping effects.

Contribution

It reveals that contact doping increases DOS and reduces resistance, but molecular insertion layers can increase tunneling resistance, and inter-band hopping dominates when carrier types differ.

Findings

Contact doping enhances DOS and decreases contact resistance.

Molecular dopant layers can increase tunneling resistance.

Inter-band hopping via in-gap states is a major resistance contributor.

Abstract

The possible origins of metal-bilayer graphene (BLG) contact resistance are investigated by taking into consideration the bandgap formed by interfacial charge transfer at the metal contacts. Our results show that a charge injection barrier (Schottky barrier) does not contribute to the contact resistance because the BLG under the contacts is always degenerately doped. We also showed that the contact-doping-induced increase in the density of states (DOS) of BLG under the metal contacts decreases the contact resistance owing to enhanced charge carrier tunnelling at the contacts. The contact doping can be enhanced by inserting molecular dopant layers into the metal contacts. However, carrier tunnelling through the insertion layer increases the contact resistance, and thus, alternative device structures should be employed. Finally, we showed that the inter-band transport by variable range hopping via in-gap states is the largest contributor to contact resistance when the carrier type of the gated channel is opposite to the contact doping carrier type. This indicates that the strategy of contact resistance reduction by the contact-doping-induced increase in the DOS is effective only for a single channel transport branch (n- or p-type) depending on the contact doping carrier type.