Excitonic transitions in β-FeSi2 epitaxial films and single crystals

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of beta-FeSi2 at low temperatures (Tless than or equal to180 K). A model based on the results of low-temperature absorption [M. Rebien , Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12-180 K, we extract a free exciton binding energy of (0.009+/-0.002) eV and a direct energy gap at T=0 K of (0.934+/-0.002) eV. (C) 2004 American Institute of Physics.