Themopower, electrical and hall conductivity of undoped and doped iron disilicide single crystals

摘要

The electrical transport properties of beta -FeSi_2 single crystals have been investigated in dependence on the purity of the source material and on doping with 3d transition metals. The transport properties included are electrical conductivity, Hall conductivity and thermopower mainly in the temperature range from 4K to 300K. The single crystals have been prepared by chemical transport reaction in a closed system with iodine as transport agent. In undoped sinle crystals prepared with 5N Fe both electrical conductivity and thermopower depende on the composition within the homogeinity range of beta -FeSi_2 which is explained by different intrinsic defects at the Si-rich and Fe-rich phase boundaries. In both undoped and doped single crystals impurity band conduction is observed at low temperatures but above 100K extrinsic behaviour determined by shallow impurity states. The thermopower shows between 100K and 200K a significant phonon drag contribution which depends on intrinsic efects and additional doping. The hall resistivity is considered mainly with respect to an anomalous contribution found in n-type single crystals and tin films. In addition doped single crystals show at temperatures below about 130K an hysteresis of the Hall voltage. These results make former mobility data uncertain. Comparison will be made between the transport properties of single crystals and polycrstalline material.