Cu atoms begin to migrate into the second layer on Co 25 and compared to Co 50-F the number of Cu atoms that migrates is larger. However no Cu migration to form a third layer is seen. Additionally, we observe a Cu atom detaching from the Cu cluster and migrating towards a Co dopant, while another migrates along the edges of the cluster, but not upwards to form a 3D structure. This indicates good mobility of Cu atoms over the surface. Such Cu atom detachment from the Cu 29 structure was only observed on two other surfaces, the 1 ML Ru passivated surface discussed in references and the W 25 surface. The favourable Cu-substrate interactions, along with the lack of formation of further Cu layers and the Cu atoms that migrate away from Cu 29 suggest that 25% Co-doped TaN is a strong candidate for a combined barrier/liner material that limits 3D island growth.