Improved topological conformity enhances heat conduction across metal contacts on transferred graphene

Abstract

Thermal conductance of metal contacts on transferred graphene (trG) could be significantly reduced from the intrinsic value due to additional resistance by increased roughness, residues, oxides and voids. In this paper, we compare the thermal conductance (G) of Al/trG/Cu interfaces with that of Al/grG/Cu interfaces. We find that for the Al/grG/Cu interfaces of as-grown CVD graphene, G = 31 MW m−2 K−1 at room temperature, two orders of magnitude lower than that of Al/Cu interfaces. For most as-transferred graphene on Cu films, G ≈ 20 MW m−2 K−1, ≈35 % lower than that of as-grown CVD graphene. We rule out the contributions of residues, native oxides and interfaces roughness, and attribute the difference to different degrees of conformity of graphene to the Cu substrates. We find that a contact area of ≈50 % only reduces the thermal conductance by ≈35 %, suggesting that a small amount of heat transfer occurs across “voids” at graphene interfaces. We improve the conformity of the as-transferred graphene by annealing the samples at 300 °C, and thus enhancing the thermal conductance to the intrinsic value. From the temperature dependence measurements, we also confirm that phonons are the dominant heat carries across the metal/graphene/metal interfaces despite a substantial carrier concentration of ≈3 × 1012 cm−2 induced in the graphene.