There has been an explosion in research on semiconductor nanowires (NWs) in recent years, primarily due to their variety of potential electronic and optoelectronic applications, including photodetectors, electrically-driven lasers, nanoscale transistors, and solar cells. Recent success in the fabrication of axial and radial core/shell heterostructures on NWs, composed of one or more layers with different properties, has enabled greater control of device operation for optoelectronics and solar cells. Since interfaces between different layers in heterostructured NWs strongly influence their properties and in turn device performance, it is especially important to understand carrier dynamics in these quasi-one dimensional (1D) systems.
Here, we use ultrafast optical microscopy (UOM) to directly examine carrier dynamics and diffusion currents in both single silicon (Si) core and Si/SiO2core/shell NWs with high spatial and temporal resolution in a non-contact manner. By measuring the time-resolved photoinduced change in transmission (DT/T) at a specific position and varying the relative pump and probe positions, we can track carrier relaxation and determine the diffusion current along the NW. A striking difference in carrier dynamics and diffusion current maps was observed for Si NWs with or without SiO2, due to trapping in unpassivated surface states. Moreover, our experiments enable us to extract several fundamental parameters in these NWs, including the surface recombination velocity, diffusion coefficients, and diffusion velocities, without the influence of contacts. Finally, we also observed strong acoustic phonon oscillations in both Si and Si/SiO2 NWs for the first time, independent of the separation distance. This research has potential applications in NW-based devices, optoelectronics, and sensitive photodetection by combining measurements at both nanometer distance and femtosecond time scales to reveal the intrinsic properties of these quasi-one-dimensional nanosystems.