Initial growth morphology in a heteroepitaxial system at low temperature: Fe on Ag(100)

The growth of ultrathin iron films on Ag(100) at 135 K has been studied by means of thermal-energy helium diffraction. Film deposition has been monitored by measuring the oscillations of the specular beam intensity during the growth. Information on the island morphology of the deposited film has been obtained by exploiting the dependence of the scattered intensity on the momentum transfer. The set of data indicates that there are definite differences between the growth mode of the first and subsequent layers. The surface of the first monolayer exhibits a two-dimensional morphology, while the estimate of the interface width w [J. W. Evans, Phys. Rev. B 43, 3897 (1991)] from a kinematic analysis of data provides values of w(2) ranging from 0.28 at 2 ML to 0.37 at 5 ML. The obtained values are consistent with the so-called one-hop transient mobility model with adsorption at the fourfold hollow site and show that from the second layer on the morphology gradually deviates towards island growth. The analysis of rocking curves also provides the step height (h') of the growing interface. At 2, 3, 4, and 5 ML, h' assumes values that are slightly larger than the value of bulk bcc iron h(Fe) = 1.43 W. This finding is assigned to the occurrence of intermixing through simple arguments based on the conservation of the atomic volumes. The Ag population at the surface of the film is found to decrease with film thickness consistently with recent ion-scattering measurements.