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

Thermal conductance of metal contacts on transferred graphene (trG) could be significantly reduced from the intrinsic value of similar contacts on as-grown graphene (grG), 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 to understand heat transfer across metal contacts on transferred graphene. Our samples are polycrystalline graphene grown on Cu foils by chemical vapor deposition (CVD) and CVD-grown graphene transferred to evaporated Cu thin films. 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 carefully rule out the contributions of residues, native oxides and interfaces roughness, and attribute the difference in the thermal conductance of as-grown and as-transferred CVD graphene 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 successfully improve the conformity of the as-transferred graphene to the substrates by annealing the samples at 300{\deg}C, and thus enhance the thermal conductance of the transferred graphene to the intrinsic value. From the temperature dependence measurements of G of Al/trG/Cu and Al/grG/Cu interfaces, we also confirm that phonons are the dominant heat carries across the metal/graphene/metal interfaces despite a substantial carrier concentration of 3x10^{12} cm^{-2} induced in the graphene.