Vapor Phase Synthesis and Characterization of ε- FeSi Nanowires

Over the past decade, one-dimensional inorganic nanostructures have emerged as promising materials for fundamental studies and possible technological applications. These structures exhibit physical and chemical properties distinct from their bulk counterparts as a result of radial confinement, while they retain the advantages of wire-like connectivity. In particular, silicon 5] and silicide nanowires have received considerable attention due to their potential ease of integration into conventional siliconbased electronics. ε-FeSi is a narrow-gap semiconductor with a cubic structure (space group P213) that has been classified as a hybridization-gap semiconductor or Kondo insulator. 16] It has attracted interest for over half a century, mainly because of its unusual magnetic behavior. Moreover, a recent study has identified doped iron monosilicides as potential alternatives to (GaMn)As and (GaMn)N in spintronics applications. Attempts to make FeSi thin films and nanorods 27] embedded in silicon substrates have been reported in the past few years. To our knowledge, however, free-standing FeSi nanostructures have never been prepared, nor have magnetic or transport measurements been performed on embedded rod or thin-film samples. Here we report the synthesis of single-crystalline FeSi nanowires and the characterization of their magnetic and electrical properties. The synthesis was performed using a chemical vapor deposition (CVD) method: silicon substrates were cleaned with 1% buffered HF and placed in a horizontal tube furnace, between the center and the downstream end of the alumina tube. Anhydrous FeCl3 powder (Aldrich, 99.99%) was placed in an alumina boat upstream of the substrates. An inert atmosphere was maintained with a flow rate of 100 sccm N2, and the temperature at the center of the furnace was set to 1100 °C. When the center of the furnace reached the set point, the iron source was hot enough (180 – 250 °C) to produce vapor-phase FeCl3 or Fe2Cl6 without thermal decomposition. The precursor vapors were carried by the N2 flow to the center of the furnace where they reacted with silicon from the substrates. The reaction was held under these conditions for one to two hours, and then the furnace was allowed to cool down to room temperature. When the silicon substrates were taken out for examination, the substrates were covered by a ‘fluffy’ black powder composed of aggregates of FeSi nanowires.