Ultrafast Carrier Dynamics in Individual Silicon Nanowires: Characterization of Diameter-Dependent Carrier Lifetime and Surface Recombination with Pump-Probe Microscopy

Ultrafast charge carrier dynamics in silicon nanowires (NWs) grown by a vapor-liquid-solid mechanism were interrogated with optical pump-probe microscopy. The high time and spatial resolutions achieved by the experiments provide insight into the charge carrier dynamics of single nanostructures. Individual NWs were excited by a femtosecond pump pulse focused to a diffraction-limited spot, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Photoexcited carriers undergo both electron-hole recombination and diffusional migration away from the excitation spot on similar time scales. The evolution of the carrier population is monitored by a delayed probe pulse that is also focused to a diffraction-limited spot. When the pump and probe are spatially overlapped, the transient signal reflects both recombination and carrier migration. Diffusional motion is directly observed by spatially separating the pump and probe beams, enabling carriers to be generated in one location and detected in another. Quantitative analysis of the signals yields a statistical distribution of carrier lifetimes from a large number of individual NWs. On average, the lifetime was found to be linearly proportional to the diameter, consistent with a surface-mediated recombination mechanism. These results highlight the capability of pump-probe microscopy to quantitatively evaluate key recombination characteristics in semiconductor nanostructures, which are important for their implementation in nanotechnologies.