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