Growth of ultrathin iron silicide films: Observation of the γ-FeSi2phase by electron spectroscopies

In this paper, we report on the iron silicide formation when annealing 3–7-Å-thick Fe films deposited on Si(111) between 250 and 550 °C. The growing films are characterized by Auger-electron spectroscopy, electron-energy-loss spectroscopy, low-energy electron diffraction, and surface-extended energy-loss fine structure in order to investigate the film composition, crystallography, and local atomic environment. We show that, upon annealing at 500 °C, ${\mathrm{FeSi}}_2$ in the ${\mathrm{CaF}}_2$ phase is formed when the initial Fe coverage is less than 7 Å whereas β-${\mathrm{FeSi}}_2$ is formed when the Fe coverage lies above 10 Å.