CRYSTALLIZATION OF COEVAPORATED BETA-FESI2 THIN-FILMS

Iron disilicide thin films (300-1100 nm thick) were prepared by simult aneous electron beam evaporation of Fe and Si onto sapphire and Cornin g 7059 glass substrates. The influence of the deposition temperature o n the crystallization of the films was investigated in the range of 50 to 900-degrees-C. At low deposition temperatures (< 250-degrees-C) Fe /Si films are amorphous. After deposition amorphous layers were anneal ed at 400 to 700-degrees-C. The crystallization starts with the nuclea tion of small semiconducting beta-FeSi2 grains. The succeeding crystal growth is accompanied by a significant decrease of the electrical con ductivity as a consequence of the phase transition. A low annealing te mperature of 400-degrees-C is sufficient to crystallize the layers. Fr om optical transmittance a direct bandgap of 0.9 eV as well as a high absorption coefficient in the order of 10(5) cm-1 near the absorption edge were evaluated. Due to these properties beta-FeSi2 is an interest ing material for photovoltaic applications. Such beta-FeSi2 films show a considerable subband absorption probably due to crystal imperfectio ns giving rise to defect states in the bandgap. With increasing anneal ing temperature the subband absorption was reduced significantly. Inve stigation by TEM showed a lamellar defect structure and spherical Si-r ich particles which are supposed to consist of amorphous Si. Most of t he large-angle grain boundaries are oriented perpendicularly to the su bstrate. No dislocations could be found.