Temperature-dependent top-layer composition of ultrathin Pd films on Cu(100)

In-Situ Observation of Stress in Cu/Pd Multilayers

Stress evolution was studied during growth of Cu/Pd multilayers using an in-situ, laserbased wafer curvature technique which allowed measurement of stress changes associated with sub-monolayer thickness increases. The apparent stress in the Cu layers changes during growth, from compressive, for thickness less than about 0.5 nm, to tensile for thicknesses above this. The stress behavior in the Pd layers depends on the thickness of the underlying Cu layer. Pd deposited on thick (2 nm) Cu layers is under an apparent compressive stress, while Pd deposited on thin (< 1 nm) Cu layers is initially under a tensile stress but changes to a compressive stress at about 0.5 nm. The overall compressive stress maxima observed in multilayers at a bilayer period of 2 nm is explained by this in-situ behavior. The stress behavior in this system is consistent with either island growth of Cu, or thickness dependent alloying behavior, or both.

Palladium diffusion into bulk copper via the (100) surface

Using low-energy electron microscopy, we measure the diffusion of Pd into bulk Cu at the Cu(100) surface. Interdiffusion is tracked by measuring the dissolution of the Cu(100)-c(2 × 2)-Pd surface alloy during annealing (T>240 °C). The activation barrier for Pd diffusion from the surface alloy into the bulk is determined to be (1.8 ± 0.6) eV. During annealing, we observe the growth of a new layer of Cu near step edges. Under this new Cu layer, dilute Pd remaining near the surface develops a layered structure similar to the Cu(3)Pd L 1(2) bulk alloy phase.

Diffusion kinetics in the Pd/Cu(001) surface alloy

We use atom-tracking scanning tunneling microscopy to study the diffusion of Pd in the Pd/Cu(001) surface alloy as a function of temperature. By following the motion of individual Pd atoms incorporated in the surface, we show that Pd diffuses by a vacancy-exchange mechanism. We measure an activation energy for the diffusion of incorporated Pd atoms of 0.88 eV, which is in good agreement with our ab initio calculated energy of 0.94 eV.