Graphene devices with bottom-up contacts by area-selective atomic layer deposition

Graphene field-effect transistor devices were fabricated using a bottom-up and resist-free method, avoiding common compatibility issues such as contamination by resist residues. Large-area CVD graphene sheets were structured into device channels by patterning with a focused ion beam. Platinum contacts were then deposited by direct-write atomic layer deposition (ALD), which is a combination between electron beam induced deposition (EBID) and bottom-up area-selective ALD. This is a unique approach that enables nucleation of Pt ALD on graphene, and therefore these devices are the first reported graphene devices with contacts deposited by ALD. Electrical characterization of the devices confirms ambipolar transistor behaviour with typical field-effect mobilities in the range of 1000-1800 cm2 V-1 s-1. We observe clear signs of strong Pt-graphene coupling and contact induced hole doping, implying good contact properties in contrast to the conventionally weak bonding between Pt and graphene. We attribute these observations to the reduced amount of resist residue under the contacts, the improved wettability of the Pt due to the use of ALD, and the formation of a graphitic interlayer that bonds the Pt more strongly to the graphene. We conclude that direct-write ALD is a very suitable technique for metallization of graphene devices and to study the intrinsic properties of metal-graphene contacts in more detail. In addition, it offers unique opportunities to control the metal-graphene coupling strength