Quantifying Mn diffusion through transferred versus directly-grown graphene barriers

We quantify the mechanisms for manganese (Mn) diffusion through graphene in Mn/graphene/Ge (001) and Mn/graphene/GaAs (001) heterostructures for samples prepared by graphene layer transfer versus graphene growth directly on the semiconductor substrate. These heterostructures are important for applications in spintronics; however, challenges in synthesizing graphene directly on technologically important substrates such as GaAs necessitate layer transfer and anneal steps, which introduce defects into the graphene. \textit{In-situ} photoemission spectroscopy measurements reveal that Mn diffusion through graphene grown directly on a Ge (001) substrate is 1000 times lower than Mn diffusion into samples without graphene ($D_{gr,direct} \sim 4\times10^{-18}$cm$^2$/s, $D_{no-gr} \sim 5 \times 10^{-15}$ cm$^2$/s at 500$^\circ$C). Transferred graphene on Ge suppresses the Mn in Ge diffusion by a factor of 10 compared to no graphene ($D_{gr,transfer} \sim 4\times10^{-16}cm^2/s$). For both transferred and directly-grown graphene, the low activation energy ($E_a \sim 0.1-0.5$ eV) suggests that Mn diffusion through graphene occurs primarily at graphene defects. This is further confirmed as the diffusivity prefactor, $D_0$, scales with the defect density of the graphene sheet. Similar diffusion barrier performance is found on GaAs substrates; however, it is not currently possible to grow graphene directly on GaAs. Our results highlight the importance of developing graphene growth directly on functional substrates, to avoid the damage induced by layer transfer and annealing.