Thickness-related instability of Cu thin films on Ag(1Â 0Â 0)

Copper and silver have a sufficiently large mismatch of lattice parameters that the growth of Cu on Ag(1 0 0) substrates stabilizes the metastable body-centered cubic (BCC) form of Cu. This paper reports that layer-by-layer growth of the BCC film occurs up to a critical thickness of 10 ML, beyond which a structural instability arises in the thin film. This leads to a uniaxial laterally modulated phase giving rise to satellite peaks in the diffraction from both film and substrate. The characteristic dependence of satellite spacing versus film thickness is interpreted in terms of four possible variants of the modulated film structure. Only one of these variants is consistent with the observed growth behavior.