Optical Investigations Of Semiconducting Fesi2 Using Photoreflectance Spectroscopy

Photoreflectance (PR) spectra obtained from epitaxial films of semiconducting beta-FeSi2 exhibit complex lineshapes resulting from a variety of optical transitions. The main features of the PR spectra can be described by transitions involving the ground and first excited state of the free exciton, in agreement with low temperature absorption measurements. However, additional lower energy (below band gap) resonances and the effects of stress, not observed in single crystal beta-FeSi2, are seen in the PR spectra of thin epitaxial films. The effect of tensile stress, resulting from thermal contraction of the epitaxial film on the Si substrate, is manifest as a substantial redshift of the main spectral feature. The assignment of tensile stress is confirmed by XRD and Raman measurements. In addition to the main PR spectral feature, the appearance of a replica is observed at higher temperatures. The shape of the replica is found to be almost identical to that of the main spectral feature, shifted down in energy by 72 meV, and is attributed to the assistance of an LO phonon.