Main steps of device fabrication.

Epitaxial electrical contact to graphene

Surface electrical contacts to two-dimensional materials suffer from the poor coupling between the 2D surface and the 3D metal. The situation is further degraded by contamination in the lithographic processing and/or layer transfer. The best innovation is the recent realization of one-dimensional edge-contacts to graphene possibly combined with large doping. However, the improvement of contact resistance is made at the expense of technological simplicity since the contact fabrication necessitates several steps.

TEM image of edge contacts.Edge bonding and large electron transfer are known to occur during the growth of graphene on SiC and could be exploited for electrical contacts if SiC was replaced by a similar yet conducting material. Conducting carbides appear as good candidates since they have similar chemical properties and since they could allow new functionalities owing to additional material properties such as magnetism or superconductivity.

The growth of graphene on carbides other than SiC was first demonstrated by Foster, Long and Strumpf. In 1958, they showed that “aluminum carbide dissociates in the vicinity of 2200-2500 ◦ C, at atmospheric pressures, to aluminum vapor and pure single crystals of graphite” establishing that other carbides could potentially be used for graphene technology. Nevertheless, this subject has remained unexplored owing to the lack of commercial substrates. In this work, we have demonstrated that few graphene layer can be grown on a metallic carbide by thermal annealing of a carbide forming metal film (niobium or tantalum) on SiC in high vacuum circumventing the problem of metallic carbide substrate availability. Based on this discovery we have described a resist-free and scalable method to fabricate few graphene layers (FGL) with electrical contacts in a single growth step. The combined effect of edge-contact and partially-covalent surface epitaxy between graphene and the metallic carbide allowed us to fabricate devices in which low contact-resistance and Josephson effect were observed.

More info in T. Le Quang et al to appear in Carbon 121 48 (2017).