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

Polycrystalline semiconducting beta-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 degrees C or three-step annealing (3SA) up to 1100 degrees C, are discussed. beta-FeSi2 grown by 3SA has shown a typical direct band-gap energy (E-gdir) of 0.88 eV and a high localized defect density (N-0) of 1.0 x 10(18) cm(-3), the latter being due to crystallographic mismatches or relevant defects at grain boundaries introduced during the transformation process from beta to alpha. On the contrary, beta-FeSi2 grown by 2 SA has shown a lower E-g(dir) of 0.80eV and a smaller N-0 of 1.7 x 10(17) cm(-3), the former arising from a deviation of the stoichiometric composition to the Si-rich side. Broad PL bands near 0.8eV have been observed at 2K from both 2SA and 3SA samples, and we assign these PL bands to optical radiative transitions intrinsic to beta-FeSi2.