Contact potential induced carrier localization in nanometer-thin Cu/Ru, Cu/Co, and Cu/Mo superlattices

Abstract

In a metallic multilayer structure, interfacial scattering of carriers reduces the electrical conductance, but carrier localization at the interfaces can have equally significant effects. The authors have grown superlattices consisting of nanometer-thin, alternating Cu and transition metal layers (Cu/Ru, Cu/Co, and Cu/Mo) by magnetron sputtering and measured the electrical conductance of the superlattices in situ during the growth. They observed a sharp conductance drop at the start of each transition metal layer and a lower-than-expected conductance with the growth of each Cu layer, neither of which can be explained by carrier scattering mechanisms. They show that this abnormal conductance behavior can be attributed to the movement of Cu free carriers across the interfaces, creating a local dipole potential to equalize the Fermi levels of the Cu and transition metal layers. The measured conductance drop and deficit suggest that on average ∼1 carrier per Cu atom is localized at the Cu/Ru and Cu/Co interfaces, while only ∼0.4 carrier per Cu atom is localized at the Cu/Mo interface.