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(a)-(c) SEM pictures with a tilt angle of 30° of as grown NWs for different Si flux: (a) undoped, (b) low and (c) high doping. (d) Cross-section SEM picture of as grown NWs with medium doping level. The scale bars are 500 nm. (e) Scheme of the growth sequence. (f) Average NW diameter and length as a function of Si flux, extracted from measurements over 10 NWs using SEM pictures similar to (a)-(c). The diameter is measured about 1 μm above the surface, and the length measured from the surface to the top of the gold particle. Errors bars are the standard deviations. The solid line is a linear fit of the diameter vs. Si flux.
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(a) Conductivity σ (filled dots) and NW diameter D (open triangles), and (b) electron density n (filled dots) and FET mobility (open triangles) extracted from characteristics, as a function of the distance to the gold seed d for a set of NWs grown at low doping. The solid line in (a) is a linear fit of σ vs. d. The error bars result from the resolution of the pictures used for extracting the diameter. (c) SEM picture of a typical NW contacted with five contacts, with the distance d to the gold seed represented. The scale bar is 500 nm. (d) Typical ISD vs. VBG characteristics at a fixed taken for increasing and decreasing gate voltage for a NW grown at low doping. (e) n vs. d for a set of undoped NWs. (f) n (filled dots) and D (open triangles) vs. d for a set of NWs grown at high doping. In (b), (e), and (f) the solid line is a fit of n vs. d, and the dashed line represents the average electron density in undoped NWs (see panel (d)). (g) Slope of n vs. d (extracted from linear fit) as a function of Si flux. Error bars are the 95% confidence bounds of the linear fit.
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(a) Scheme of the growth sequence for the NWs of types I and II: type I with bottom grown undoped and the top grown with Si flux, and type II with bottom grown with Si flux and top undoped. (b)-(c) Scanning electron microscope pictures with a tilt angle of 30° of as grown nanowires of (b) type I and (c) type II for high doping. The scale bars are 500 nm long. (d) Nanowire diameter as a function of position compared to the tip for two typical NWs of type I (filled diamonds) and type II (open triangles). (e) Electron density n extracted from the threshold voltage as a function of the distance d to the gold seed for NWs of type I (filled diamonds) and type II (open triangles). The solid lines are linear fits of parts of each data set, and the dashed line represents the average electron density in undoped NWs (see Fig. 2(d) ).
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