In situ IR and RHEED study of carbon monoxide surfactant assisted growth of fcc-Fe on Cu(1 0 0)

Infrared reflection absorption spectroscopy (IRRAS) and reflection high-energy electron diffraction (RHEED) have been applied to the in situ characterization of fcc-Fe epitaxial growth on Cu(1 0 0) substrates under very low carbon monoxide (CO) pressures. When CO is not admitted into the deposition chamber, the layer-by-layer epitaxial growth up to 14 monolayer (ML) thickness takes place on the substrate at room temperature. The layer-by-layer growth extends up to 18 ML at 2 × 10^-10 Torr of CO, for which no IR absorption features due to CO adsorption on the growing film surface can be observed. In contrast, layer-by-layer growth becomes unstable for deposition under 1 × 10^-8 Torr CO. In this case, an IR band due to CO adsorption appears at 1950 cm^-1 only in the early stages of deposition. For deposition at 90 K, on the other hand, no layer-by-layer growth occurs on clean Cu(1 0 0). However, pre-adsorption of CO on Cu(1 0 0) followed by Fe deposition without CO admission into the chamber brings about the layer-by-layer growth up to 8 ML. The Fe deposition leads to an intensity reduction of the band due to adsorbed CO on the substrate with the emergence of the band ascribable to CO on the deposited Fe, indicating that the pre-adsorbed CO molecules on Cu(1 0 0) migrated in part to the deposited Fe surface. Proceeding Fe deposition results in an intensity reduction of the band due to CO on the Fe. In contrast, the layer-by-layer growth hardly takes place when 2 × 10^-10 Torr CO was admitted in the growth atmosphere. These observations indicate that the molecular adsorption of surfactant CO at the growing film surface as well as the dissociation of the adsorbed CO is a key process to promote the layer-by-layer growth of fcc-Fe.