Si/Fe flux ratio influence on growth and physical properties of polycrystalline β-FeSi₂ thin films on Si(100) surface

This work investigates the Si/Fe flux ratio (2 and 0.34) influence on the growth of β-FeSi₂ polycrystalline thin films on Si(100) substrate at 630 °C. Lattice deformations for the films obtained are confirmed by X-ray diffraction analysis (XRD). The volume unit cell deviation from that of β-FeSi₂ single crystal are 1.99% and 1.1% for Si/Fe =2 and Si/Fe =0.34, respectively. Absorption measurements show that the indirect transition (~ 0.704 eV) of the Si/Fe =0.34 sample changes to the direct transition with a bandgap value of ~0.816 eV for the sample prepared at Si/Fe =2. The absorption spectrum of the Si/Fe =0.34 sample exhibits an additional peak located below the bandgap energy value with the absorption maximum of ~0.36 eV. Surface magneto-optic Kerr effect (SMOKE) measurements detect the ferromagnetic behavior of the β-FeSi₂ polycrystalline films grown at Si/Fe =0.34 at T=10 K, but no ferromagnetism was observed in the samples grown at Si/Fe =2. Theoretical calculations refute that the cell deformation can cause the emergence of magnetization and argue that the origin of the ferromagnetism, as well as the lower absorption peak, is β-FeSi₂ stoichiometry deviations. Raman spectroscopy measurements evidence that the film obtained at Si/Fe flux ratio equal to 0.34 has the better crystallinity than the Si/Fe =2 sample.