Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with Pump-Probe Microscopy
Emma E. M. Cating, Christopher W. Pinion, Joseph D. Christesen, Caleb A. Christie, Erik M. Grumstrup, James F. Cahoon, John M. Papanikolas
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 (S), a metric of the surface quality that is directly proportional to trap density, is determined by the relationship S = d/4 tau from measurements of the recombination lifetime (tau) and NW diameter (d) at distinct spatial locations in individual NWs. We find that S 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 tau, we also find that smaller wires exhibit higher values of S; this indicates that tau is shorter both because of the geometrical effect of smaller d 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.
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