ELECTRICAL AND OPTICAL-PROPERTIES OF THIN BETA-FESI2 FILMS ON AL2O3 SUBSTRATES

Polycrystalline beta-FeSi2 thin films of about 600 nm were prepared by simultaneous electron beam; evaporation of Si and Fe (1.6 < Si/Fe < 2.4) onto Al2O3-ceramic and sapphire substrates. The films were deposited at 100 degrees C and subsequently annealed between 500 and 950 degrees C, Films crystallized below 870 degrees C predominantly consist of the semiconducting phase beta-FeSi2 with a p-type conductivity. At higher temperatures only the metallic monosilicide epsilon-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 Si/Fe = 2 on the grain size and some electrical and optical properties are studied. A Si/Fe 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 (less than or equal to 0.1-0.2 cm(2)/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.