Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with Pump–Probe Microscopy

Surface trap density in silicon nanowires (NWs) plays a key role in the performance of many semiconductor NW-based devices. We use pump–probe microscopy to characterize the surface recombination dynamics on a point-by-point basis in 301 silicon NWs grown using the vapor–liquid–solid (VLS) method. The surface recombination velocity (<i>S</i>), a metric of the surface quality that is directly proportional to trap density, is determined by the relationship <i>S</i> = <i>d</i>/4τ from measurements of the recombination lifetime (τ) and NW diameter (<i>d</i>) at distinct spatial locations in individual NWs. We find that <i>S</i> varies by as much as 2 orders of magnitude between NWs grown at the same time but varies only by a factor of 2 or three within an individual NW. Although we find that, as expected, smaller-diameter NWs exhibit shorter τ, we also find that smaller wires exhibit higher values of <i>S</i>; this indicates that τ is shorter both because of the geometrical effect of smaller <i>d</i> and because of a poorer quality surface. These results highlight the need to consider interwire heterogeneity as well as diameter-dependent surface effects when fabricating NW-based devices

Imaging Charge Separation and Carrier Recombination in Nanowire p‑i‑n Junctions Using Ultrafast Microscopy

Silicon nanowires incorporating p-type/n-type (p-n) junctions have been introduced as basic building blocks for future nanoscale electronic components. Controlling charge flow through these doped nanostructures is central to their function, yet our understanding of this process is inferred from measurements that average over entire structures or integrate over long times. Here, we have used femtosecond pump–probe microscopy to directly image the dynamics of photogenerated charge carriers in silicon nanowires encoded with p-n junctions along the growth axis. Initially, motion is dictated by carrier–carrier interactions, resulting in diffusive spreading of the neutral electron–hole cloud. Charge separation occurs at longer times as the carrier distribution reaches the edges of the depletion region, leading to a persistent electron population in the n-type region. Time-resolved visualization of the carrier dynamics yields clear, direct information on fundamental drift, diffusion, and recombination processes in these systems, providing a powerful tool for understanding and improving materials for nanotechnology

Imaging Charge Separation and Carrier Recombination in Nanowire p‑i‑n Junctions Using Ultrafast Microscopy

Silicon nanowires incorporating p-type/n-type (p-n) junctions have been introduced as basic building blocks for future nanoscale electronic components. Controlling charge flow through these doped nanostructures is central to their function, yet our understanding of this process is inferred from measurements that average over entire structures or integrate over long times. Here, we have used femtosecond pump–probe microscopy to directly image the dynamics of photogenerated charge carriers in silicon nanowires encoded with p-n junctions along the growth axis. Initially, motion is dictated by carrier–carrier interactions, resulting in diffusive spreading of the neutral electron–hole cloud. Charge separation occurs at longer times as the carrier distribution reaches the edges of the depletion region, leading to a persistent electron population in the n-type region. Time-resolved visualization of the carrier dynamics yields clear, direct information on fundamental drift, diffusion, and recombination processes in these systems, providing a powerful tool for understanding and improving materials for nanotechnology