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

Ion-beam synthesis of beta-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 degrees 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 beta phase was formed on Si(100) substrates for T-a below 930 degrees C, whereas the a phase coexisted with the beta phase for all T-a values even on as-implanted Si(111) substrates. The electrical resistivities of beta-FeSi2 formed on Si(111) substrates with T-a=875 and 915 degrees C were found to be 0.1-0.3 Omega 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 degrees 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 degrees C, only a very broad emission band peaking at 0.8 eV was observed. However, the Si(111) sample with T-a=915 degrees C showed a sharp emission line at 0.837 eV, which was assigned to the intrinsic optical transition of beta-FeSi2.