COEVAPORATED THIN-FILMS OF SEMICONDUCTING BETA-FESI2

Iron disilicide thin films (500-600 nm thick) were prepared by simultaneous electron beam evaporation of Fe and Si onto sapphire and Coming 7059 glass substrates. After deposition the amorphous layers were annealed at 400-700-degrees-C. The crystallization starts with the nucleation of small beta-FeSi2 grains. The succeeding crystal growth is accompanied by a steep increase of the sheet resistance due to the transition from metallic to semiconducting properties. A low annealing temperature of 400-degrees-C is sufficient to crystallize the layers. The optical transmittance, electrical conductivity, thermo-electric power, and grain size are influenced by the deposition temperature during evaporation, deviations from stoichiometry, and the annealing temperature and duration. From optical transmittance a bandgap between 0.85 and 0.95 eV as well as a high absorption coefficient in the order of 10(5) cm-1 near the absorption edge are calculated. Due to these properties beta-FeSi2 is an interesting material for photovoltaic applications.