Origin and perspectives of the 1.54 mu m luminescence from ion-beam-synthesized beta-FeSi2 precipitates in Si

The structural and optical properties of beta-FeSi2 precipitates in Si have been analyzed. Float zone Si samples were implanted at 250 degrees C with 350 keV Fe ions to fluences in the range 1-5 x 10(15)/cm(2) and annealed in vacuum at 800 degrees C for times up to 24 h. Detailed morphological analyses of these samples, using transmission electron microscopy, reveal the presence of (i) a band of small (with a diameter < 30 nm) highly strained beta-FeSi2 precipitates centered at a depth of similar to 150 nm, (ii) a band of large (> 100 nm diameter) fully relaxed beta-FeSi2 precipitates centered at a depth of similar to 320 nm, and (iii) residual extended defects. A sharp photoluminescence peak at 1.54 mu m is measured at 17 K. This peak remains unchanged when the region containing the small precipitates is removed, using Ar sputtering. On the other hand, it is also fully suppressed when the large precipitates region is removed and a high concentration of extended defects remains in the samples. This allowed us to identify the large unstrained precipitates as the main sources of the 1.54 mu m luminescence. Finally, we found that carrier recombination within iron precipitates is characterized by a weak oscillator strength resulting in a long decay lifetime of the 1.54 mu m signal at 17 K (similar to 60 mu s) and in a strong temperature quenching of the luminescence yield.