Towards Quantitative Conductivity Measurements of Doped Silicon Nanowires with Infrared Near-Field Optical Microscopy

Performance in semiconducting structures, such as silicon nanowires (SiNWs), depends on nanometer-scale electronic and structural properties, including surface roughness, mobility, or dopant density. We report the use of mid-infrared scattering-type scanning near-field optical microscopy (s-SNOM) as a non-destructive optical method to extract nanoscale conductivity maps in semiconductors. Combining atomic force microscope (AFM) characterization with mid-infrared spectroscopic analysis, we can extract quantitative local dielectric properties. Using this technique, we can detect local changes in the electrically-active doping concentration from the free-carrier absorption in both n-type and p-type doped SiNWs. The high spatial resolution (< 20 nm) allows us to directly measure free-carrier concentrations arising from B and P dopants in single and multi-junction SiNWs. Combined with finite element analysis, we can analyze the local carrier concentration and mobility, and correlate nanoscale variations in doping, strain, crystallinity and structure with growth conditions. This capability will enable the use of s-SNOM as an advanced platform for exploratory research and practical characterization of semiconducting structures.