Luminescence from $\beta -{\mathrm{FeSi}}_2$ precipitates in Si. II: Origin and nature of the photoluminescence

In this paper we present photoluminescence, photoreflectance, and absorbance measurements on silicon samples with $\beta -{\mathrm{FeSi}}_2$ precipitates, as structurally characterized in the first part of this paper [M. G. Grimaldi et al., Phys. Rev. B 66, 085319 (2002)]. By comparing the photoluminescence measurements in different experimental conditions and with excitation energy above and below the silicon threshold, by considering the direct gap estimations by photoreflectance and absorption, we argue that the $1.54\hspace{0.5em}\mu \mathrm{m}$ photoluminescence peak in the spectra is produced by an indirect transition in the disc-shaped precipitates. However, the latter ones are predicted to be the most efficient configuration, acting as a trapping well for carriers generated in the silicon matrix, and displaying a high structural quality with no dangling bonds at the $\beta -{\mathrm{FeSi}}_2/\mathrm{Si}$ interface. Our simple model, based on band lineup at the interface, is also able to explain the temperature quenching of the photoluminescence peak.