OPTICAL-PROPERTIES AND PHASE-TRANSFORMATIONS IN ALPHA-IRON AND BETA-IRON DISILICIDE LAYERS

Ion beam synthesis (IBS) has been used to fabricate semiconducting beta-FeSi2 and metallic alpha-Fe0.82Si2. For all of the doses studied a photoluminescence (PL) signal is observed at 1.54 mu m. This signal is first seen after annealing at 800 degrees C and increases in intensity with a commensurate decrease in full width half maximum (FWHM) as the anneal temperature is increased up to 920 degrees C. Likewise the intensity increases and FWHM decreases as the anneal time at 920 degrees C is increased up to 18 h. Optical absorption measurements reveal a linear relationship between the square of the absorption coefficient and the incident photon energy, indicating a direct allowed transition from a semiconductor (beta-FeSi2) with a band gap of about 0.87 eV. After annealing at 1000 degrees C no PL or absorption is observed in this spectral region; this is because a thicker, conducting layer of alpha-Fe0.82Si2, containing similar to 18% Fe vacancies has then been formed. If an alpha-Fe0.82Si2 layer is subsequently annealed below the phase transition temperature(similar to 950 degrees C) then the PL signal reappears as the layer is largely reconverted back to the beta-phase.