Enhanced electrical properties of nominally undoped Si/SiGe heterostructure nanowires grown by molecular beam epitaxy

Electrical properties of epitaxial single-crystalline Si/SiGe axial heterostructure nanowires (NWs) on Si〈1 1 1〉 substrate were measured by contacting individual NWs with a micro-manipulator inside an scanning electron microscope. The NWs were grown by incorporating compositionally graded Si_1−xGe_x segments of a few nm thicknesses in the Si NWs by molecular beam epitaxy. The I-V characteristics of the Si/SiGe heterostructure NWs showed Ohmic behavior. However, the resistivity of a typical heterostructure NW was found to be significantly low for the carrier concentration extracted from the simulated band diagram. Similarly grown pure Si and Ge NWs showed the same behavior as well, although the I-V curve of a typical Si NW was rectifying in nature instead of Ohmic. It was argued that this enhanced electrical conductivities of the NWs come from the current conduction through their surface states and the Ge or Si/SiGe NWs are more strongly influenced by the surface than the Si ones.