Effect of residual impurities on transport properties of β-FeSi2 epitaxial films grown by molecular beam epitaxy

Unintentionally doped β-FeSi2 epitaxial films were grown on silicon-on-insulator substrates by molecular beam epitaxy using a high-purity (5N) Fe source to investigate the effect of residual impurities on the transport properties of β-FeSi2. From secondary ion mass spectroscopy analysis, impurities of As, Al, and Mn (∼10¹⁷ cm^-3); P and B (∼10¹⁶ cm^-3); and Cr and Pb (∼10¹⁵ cm^-3) were detected in the epitaxial layer. In Hall measurements at room temperature, the films exhibited n-type conduction with a carrier density of 4–6 × 10¹⁶ cm^-3 and a Hall mobility of 400–440 cm²/Vs. In the temperature (T) dependence of the transport properties, a transition from band conduction to hopping conduction was observed at approximately T = 230 K. At temperatures of 110–150 K, both negative and positive magnetoresistance (MR) were observed depending on the temperature and magnetic field. The MR exhibits mixed conduction of defect band conduction and band conduction in this temperature range.