β-FeSi<sub>2 </sub>has been attracting a great deal of attention because of its compatibility with Si-based light-emitting diodes (LEDs). It has also been reported that Β-FeSi<sub>2</sub> thin film undergoes a direct transition with a band gap of about 0.85 eV.
However, some authors have reported that β-FeSi<sub>2</sub> has an indirect band gap structure on the basis of several first
principles calculations. This difference is thought to be induced by the stress of the β-FeSi<sub>2</sub> interface acting on Si by
lattice mismatch, which affects the band structure of the β-FeSi<sub>2</sub> thin film. To investigate the effect, we evaluated the
optical properties of β-FeSi<sub>2</sub> grown on lattice matched Si (001) substrate 8°off toward the <110> direction. All samples
were formed by depositing Fe on the Si substrate to grow β-FeSi<sub>2</sub> thin film by electron beam deposition. From Raman
measurements, it was observed that the samples prepared on inclined substrate shifted to higher wave numbers. From the
optical absorption measurement, we observed that the band structure was changed by the Si off substrate. Additionally,
the PL intensity at about 0.8 eV for the samples grown on the Si off substrate was increased.