Three-dimensional carrier profiling of individual Si nanowires by scanning spreading resistance microscopy

Individual silicon nanowires (Si NWs) grown by molecular beam epitaxy and in-situ doped with boron were investigated by scanning spreading resistance microscopy (SSRM), a technique based on the conductive atomic force microscopy. The carrier profiles of the NWs were derived from the measured spreading resistance values and calibrated with the known carrier concentration of the underlying epi-layer. The three-dimensional SSRM profile of a NW was obtained by measuring the NW cross sections at different depths along the radial direction. Scanning the same NW with a controlled force on the SSRM tip can abrade material from the cross-sectional surface and the tip moves deeper into the volume of the NW after each image scan. Repeated stripping of the material from the NW results in a "thinning" of the remaining NW segment and a corresponding increase of its resistance which can be addressed by an appropriate data correction. The achieved three-dimensional carrier profile reveals a multi-shell structure of the carrier distribution across the NW diameter which consists of a lower doped core region, a higher doped shell region and a carrier depleted sub-surface region.