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Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three-parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.

In this letter we present photoluminescence results on β-FeSi2/Si using excitation energies above and below the silicon band gap. These results show that the luminescence emission observed at 1.5 μm can be firmly attributed to band edge related emission from the β-FeSi2. This result confirms the potential of β-FeSi2 as a strong contender for a silicon compatible optoelectronics technology that matches the conventional optical fiber transmission wavelength at 1.5 μm.

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