Thickness dependence of the initial oxidation behaviors of Gd films grown on Si by laser molecular beam epitaxy

Gd thin films with different thickness (about 10 nm and 0.5 nm) were deposited on Si(100) by laser molecular beam epitaxy (LMBE). Thickness dependence of the initial oxidation behaviors of Gd films was studied based on the in situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) analysis under ultra-high vacuum (UHV) condition. When the thin film is around 10 nm, the XPS results show that Gd is extremely reactive with oxygen forming Gd oxides and the oxides of Gd are easily hygroscopic. The UPS results show that the Gd 4f has a double-peak structure and the double-peak structure of Gd 4f evolves into a single-peak feature after exposing to air. When the thickness of the Gd film decreases to about 0.5 nm, the reactivity of Gd film with oxygen is decreased by the diffusion of Si component into Gd layers based on the XPS and UPS results. It is suggested that the silicon atoms segregate at the grain boundaries of Gd film to form a barrier, which block the further diffusion of oxygen and water vapor into the Gd layers.