2, Georgia Institute of Technology, Atlanta, Georgia, United States
Spectroscopy methods working in the mid-infrared wavelength range allow for addressing several kinds of fundamental material properties as molecular vibrations and the excitation of phonons and free charge carriers. However, the downscaling of electronic and photonic devices raises the need for characterization techniques with nanoscale resolution. Conventional spectroscopy methods lack this high resolution as they are limited by the diffraction limit to about half the wavelength. In scattering-type scanning near-field optical microscopy (s-SNOM), the high resolution of atomic force microscopy (AFM) is combined with the capability of infrared spectroscopy methods to address fundamental material excitations. A metallized AFM tip is illuminated by laser light. Near-fields are excited at the tip apex leading to a confinement of the light to a region in the order of the tip radius (~25 nm), which is the only limitation for the lateral resolution. Parallel to the acquisition of the samples topography, local changes in the dielectric response of the sample are mapped. In the mid-infrared wavelength range this response is connected with the carrier properties of doped semiconductors as described by the Drude model. Due to this, s-SNOM has proven great potential for the quantification of the amount of activated free carriers in doped semiconductor nanowires. 
By performing sequential spectroscopy in the range of the plasma edge we present on the ability to determine the charge carrier concentration profile along silicon nanowires with phosphorous-doped segments in between intrinsic silicon that have been grown with the vapor-liquid-solid (VLS) technique. We show that the combination of the shape of line profiles along the nanowire axis with the spectroscopic evaluation at one point gives us precise information about the level of free charge carriers in the doped segment as well as the sharpness of the boundaries between intrinsic and doped regions. We show that this determination is even possible if the plasmon resonance of the free carriers is overlaid by a phonon resonance of the native SiO2-layer covering the nanowire, by using a multilayer model  for the tip-sample interaction.
 J. Stiegler et al., “Nanoscale free-carrier profiling of individual semiconductor nanowires by infrared near-field microscopy”, Nano Letters 10, 1387 (2010).
 B. Hauer et al., “Quasi-analytical model for scattering infrared near-field microscopy on layered systems”, Optics Express 20, 13173 (2012).