Optical, electrical and structural properties of polycrystalline β-FeSi₂ thin films fabricated by electron beam evaporation of ferrosilicon

Ferrosilicon (FeSi₂) grains (99.9%) were evaporated at room temperature onto the (100)-oriented n-type FZ Si substrates using electron beam deposition technique. Optical, electrical and structural properties were systematically investigated as a function of subsequent isochronal (2 hrs) annealing temperature (T_a) in the range of 400 to approximately 950 °C. X-ray diffraction and Raman scattering analysis suggested the formation of polycrystalline β-FeSi₂ above T_a = 500 °C, whereas above T_a = 800 °C, Si agglomeration was observed to form. The electrical resistivity of these samples reached a maximum (0.542 Ω·cm) at T_a = 700 °C, and then it decreased with increasing T_a. Its decrease process was explained by considering the creation of Si vacancies, which could presumably be acting as holes. It is of great interest that in T_a = 600 to approximately 800 °C, the majority carrier converts from n- to p-type. Typical carrier concentrations and mobilities were determined to be μ_n = 39.4 cm²/V·sec, n_e = 6.59×10¹⁷ cm^-3 for n-type β-FeSi₂ with T_a = 600 °C and μ_h = 20.3 cm²/V·sec, n_h = 2.22×10¹⁸ cm^-3 for p-type β-FeSi₂ with T_a = 850 °C. Optical absorption measurements revealed that the nature of the bandgap varies from an indirect to direct one with increasing Ta.