KINETIC FACETING IN HOMOEPITAXY OF FE(110) ON FE(110)

The homoepitaxial growth of Fe(110) on atomically smooth Fe(110) surfaces was analyzed using high angular resolution, spot profile low energy electron diffraction (SPA-LEED). Spot profiles were observed near normal incidence for films consisting of up to 10 atomic layers, grown at temperatures between 200 and 500 K. The profiles show a clear separation of a central spike of instrumental width from a background consisting of two broad shoulders, arranged symmetrically with respect to the central spike along the [110BAR] direction. The distance between the shoulders varies linearly with the scattering vector, being zero for in-phase and maximum for out-of-phase conditions. The surface structure therefore consists of a quasi-periodic sequence of up and down staircases along [110BAR], with ridges along [001]. It forms by a facetting process governed by free diffusion on atomic planes in combination with an energy barrier which prevents hopping between the planes (layer restricted diffusion, LRD). The one-dimensional facet structure results from the lowered symmetry of the surface. Apparently, an anisotropic sticking probability of atoms on atomic steps, being high at steps with edges along [110BAR] directions and low at steps with edges along [001], results in nucleation by stripes elongated along [001]. The distance of these stripe nuclei defines the period of the ridge structure which therefore is roughly independent of film thickness. Apparently, the growth virtually consists in a step flow of atoms towards the ridges, with subsequent nucleation of new planes on top of them, and a roughness which increases with the square root of the thickness. The period increases with increasing preparation temperatures. Annealing of the facets at 600 K restores the initial flat surface, thus confirming the kinetic nature of the facetting process.