  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.
