Al deposition on Fe: Formation of an iron aluminide surface alloy

Abstract

An investigation has been made of the formation of a surface iron-aluminum alloy through aluminum adsorption (and subsequent reaction) on Fe(100) single-crystal and polycrystalline Fe surfaces. On the Fe(100) surface, Auger electron spectroscopy and low-energy ion scattering spectroscopy (LEISS) studies indicate that at low Al coverages (<1 ML), and a very low Al deposition rate (<0.03 ML/min), a surface alloy grows uniformly at 25 °C to yield an aluminide with an approximate average stoichiometry of FeAl3. At higher Al exposures the surface becomes more aluminum rich. This Al enrichment is due to a kinetic limitation in the formation of the surface alloy. Heating the surface (300 °C during or after Al deposition) partially overcomes this kinetic limitation, and the topmost surface layer changes to a stable stoichiometry with some reduction in the relative aluminum concentration. Low-energy electron diffraction observations made during the Al deposition (or subsequent heating experiments) gave no indication of long-range order, suggesting the formation of a disordered surface alloy. LEISS experiments show that at 25 °C, the final stoichiometric configuration of the surface is highly dependent on the average arrival rate of Al at the surface. A higher deposition rate ultimately leads to a more aluminum-rich surface than for an equivalent dose at a lower rate. This may be rationalized by noting that at higher deposition rates the Al atoms are more likely to encounter other Al atoms at the surface and may proceed to form two-dimensional microclusters of Al instead of reacting to form a surface or subsurface aluminide alloy. The thermodynamic implications of these observations in the context of the formation of intermetallic alloy phases seen here will be discussed.