Synthesis of β-FeSi₂ for optical applications by Fe triple-energy ion implantation into Si(100) and Si(111) substrates

Ion-beam synthesis of β-FeSi₂ layers was performed by Fe triple-energy ion implantation into Si(100) and Si(111) substrates and subsequent two-step annealing. By keeping the first-step annealing temperature constant at 600 °C, the physical properties were characterized as a function of the second-step annealing temperature (T_a). X-ray diffraction measurements revealed that only the β phase was formed on Si(100) substrates for T_a below 930 °C, whereas the a phase coexisted with the β phase for all T_a values even on as-implanted Si(111) substrates. The electrical resistivities of β-FeSi₂ formed on Si(111) substrates with T_a = 875 and 915 °C were found to be 0.1-0.3 Ω cm. It was shown by optical absorption measurements that the band gap decreased from 0.83 to 0.78 eV with increasing T_a. Samples with both Si(100) and Si(111) substrates with T_a = 875 °C exhibited a defect-related optical absorption band at around 0.7 eV. In the 2 K photoluminescence spectra of these two samples with T_a = 875 °C, only a very broad emission band peaking at 0.8 eV was observed. However, the Si(111) sample with T_a = 915 °C showed a sharp emission line at 0.837 eV, which was assigned to the intrinsic optical transition of β-FeSi₂.