Electrical and optical properties of thin β-FeSi2 films on Al2O3 substrates

Polycrystalline β-FeSi2 thin films of about 600 nm were prepared by simultaneous electron beam evaporation of Si and Fe (1.6 < SiFe < 2.4) onto Al2O3-ceramic and saphire substrates. The films were deposited at 100°C and subsequently annealed between 500 and 950°C. Films crystallized below 870°C predominantly consist of the semiconducting phase β-FeSi2 with a p-type conductivity. At higher temperatures only the metallic monosilicide ϵ-FeSi could be detected probably due to a chemical reaction of Fe, Si and the substrate material. The influence of the crystallization temperature and of deviations from the stoichiometric composition SiFe = 2 on the grain size and some electrical and optical properties are studied. A SiFe ratio between 2.1 and 2.2 is correlated with maximum grain sizes and thermoelectric power values and minimum conductivities. A minimum optical subband absorption below the gap energy is also characteristic for this composition. The Hall voltage between room temperature and 10 K was too low to be resolved. Therefore Hall mobilities are estimated to be rather small (≤ 0.1–0.2 cm2/Vs) and high defect densities and carrier concentrations are concluded. A photoconductive effect could not be demonstrated without any doubt. Due to these results the suitability of such films for photovoltaic applications seems to be questionable up to now.