Chemical reaction induced carrier localization in nanometer-thin Al/Ru, Al/Co, and Al/Mo superlattices

It is well-known that the electrical conductivity of a metallic film reduces dramatically when the film becomes very thin. This effect is mainly attributed to surface scattering of the conducting carriers. In a multilayer structure, interface scattering also reduces the conductance, but chemical reactions at the interfaces can have equal or bigger effects. The extent of chemically induced carrier localization at the metallic interfaces has not been explored or reported. We have grown superlattices consisting of nm-thin, alternating Al and transition-metal layers (Al/Ru, Al/Co and Al/Mo) by magnetron sputtering, and measured the electrical conductance of the superlattices in-situ during the growth. We observed a sharp conductance drop at the start of each transition metal layer and a pause in conduction increase at the start of each Al layer, neither of which is predicted by the surface scattering model. We show that these abnormal conductance changes can be explained by localization of Al free carriers at the interfaces to facilitate the formation of intermetallic bonds. The magnitude of the measured conductance drops suggests that one monolayer of compound is formed at each interface at room temperature. Annealing the superlattices to 300 °C caused a modest decrease in conductance, attributed to further chemical reactions. In contrast, a superlattice involving two fully miscible transition metals, Ru and Co, exhibited no carrier localization, resulting in a conductance more than three times that of superlattices containing Al layers.