Secondary Nucleation and Unstable Roughening in Fe Layers Electrodeposited on Si

Thin iron films are interesting for fundamental studies and have applications in sensors, actuators, magnetic memory, and spintronic devices. Here, the electrodeposition of thin iron films on Si(100) is investigated with microscopy techniques and a modeling approach. Atomic force microscopy images of films with thicknesses from 6 to 180 nm show initial island growth, and after the formation of a continuous film, a decrease in the lateral correlation length is associated with a decrease in the width of surface undulations. In thicknesses above 100 nm, a rapid increase in the roughness suggests the onset of unstable growth. Transmission electron microscopy (TEM) images of 180 nm thick films reveal the secondary nucleation of Fe grains, a feature that was not previously observed in electrodeposited Fe films. These results are interpreted in light of simulations of an electrodeposition model that accounts for the interplay of diffusive cation flux in the electrolyte and surface relaxation of adsorbed Fe atoms. In the thickest simulated films, the unstable growth is controlled by the diffusive flux, and the simulated samples have surface cliffs that resemble those of TEM images. The model suggests that the sharp decrease in the correlation length of the Fe films must be related to some transition in the surface dynamics, consistent with the observed secondary nucleation. These results may help the control of Fe film morphology in future studies.