AbstractWe report a detailed investigation of the structural and chemical characteristics of thin evaporated Al₂O₃ tunnel barriers of variable thickness grown onto single-layer graphene sheets. Advanced electron microscopy and spectrum-imaging techniques were used to investigate the Co/Al₂O₃/graphene/SiO₂ interfaces. Direct observation of pinhole contacts was achieved using FIB cross-sectional lamellas. Spatially resolved EDX spectrum profiles confirmed the presence of direct point contacts between the Co layer and the graphene. The high surface diffusion properties of graphene led to cluster-like Al₂O₃ film growth, limiting the minimal possible thickness for complete barrier coverage onto graphene surfaces using standard Al evaporation methods. The results indicate a minimum thickness of nominally 3 nm Al₂O₃, resulting in a 0.6 nm rms rough film with a maximum thickness reaching 5 nm.