Effect of multiple-step annealing on the formation of semiconducting β-FeSi₂ and metallic α-Fe₂Si₅ on Si (100) by ion beam synthesis

Polycrystalline semiconducting β-FeSi₂ layers on Si (100) have been formed by ion beam synthesis. Results from two different annealing processes, either two-step (2SA) annealing up to 900°C or three-step annealing (3SA) up to 1100°C, are discussed. β-FeSi₂ grown by 3SA has shown a typical direct band-gap energy (E^dir _g) of 0.88 eV and a high localized defect density (N₀) of 1.0 × 10¹⁸ cm^-3, the latter being due to crystallographic mismatches or relevant defects at grain boundaries introduced during the transformation process from β to α. On the contrary, β-FeSi₂ grown by 2 SA has shown a lower E^dir _g of 0.80 eV and a smaller N₀ of 1.7 × 10¹⁷ cm^-3, the former arising from a deviation of the stoichiometric composition to the Si-rich side. Broad PL bands near 0.8 eV have been observed at 2 K from both 2SA and 3SA samples, and we assign these PL bands to optical radiative transitions intrinsic to β-FeSi₂.