Nanoscale Conductivity Measurements of Biased Silicon Nanowires with Infrared Near-Field Optical Microscopy

Vapor-liquid-solid (VLS) growth of Si nanowires (SiNWs) allows for precise control of dopant density and type. These encoded dopant superlattices have enabled microscopic diodes for a wide range of optoelectronic applications. However, performance of these diodes depends critically on the behavior of carriers at junctions, which is influenced by dopant activation, geometry, and defect states. Furthermore, junctions in SiNWs are abrupt (<100 nm), making it difficult to apply traditional characterization methods. Here we use mid-infrared scattering-scanning near-field optical microscopy (s-SNOM) to image the carrier distribution in axial p-i-n junction SiNWs in operando. The high spatial resolution (< 20 nm) allows us to directly measure the free-carrier concentration arising from not only the native doping, but also from band bending. Combined with finite element modeling, we can semi-quantitatively determine the local carrier concentration and mobility, and examine the effect of bias on junction lengths and space-charge regions. These measurements will demonstrate that s-SNOM can be used to study the I-V characteristics of SiNWs with unprecedented spatial resolution.