Local photocurrent mapping as a probe of contact effects and charge carrier transport in semiconductor nanowire devices
Schematic of procedures A (a) and B (b) for fabricating Schottky and Ohmic contacts, respectively, on CdS nanowires. Contact regions are defined on resist-coated CdS nanowires by e-beam lithography (i). In (a), CdS nanowires are etched by HF before Ti metal evaporation (ii) followed by lift-off (iii). For procedure B (b), CdS nanowires are subject to bombardment (ib) followed by steps (ii) and (iii). The typical diameter of CdS nanowires is and the electrode thickness is around with a width of . For clarity, the schematics shown here are not to scale.
Typical current-voltage characteristics of (a) Schottky-barrier devices (device A), (b) Ohmic devices (device B), and (c) Schottky diodes (device C) under weak room light (dashed lines) and fiber lamp illumination (solid lines). and represent the contact resistance and the nanowire resistance, respectively. In the inset to (c), the curve of the Schottky diode in (c) is plotted on a semilogarithmic scale (open circles) and fit (solid lines) to thermionic-diffusion theory.
(a) Schematic of SPCM setup. Photocurrent images of (b) a Schottky-barrier device (device A), (c) an Ohmic device (device B), and (d) a Schottky diode device (device C) under various biases. The channel lengths are 0.8, 5.8, and , respectively. The normalized photocurrent profile perpendicular to the nanowire channel is plotted in (e) for the measurements using a near-field scanning microscope [open circles, from (b)] and a scanning confocal microscope [solid circles, from (c)] and is fit by Gaussian functions (solid lines).
Semilogarithmic plot of the photocurrent profiles of (a) the Schottky-barrier device (device A) under biases of (open circles), (open squares), and (open triangles); (b) the Ohmic device (device B) under biases of (open circles), (open squares), and (open triangles); and (c) the Schottky diode (device C) under forward biases of (open circles) and (open squares), and under reversed biases of (solid circles) and (solid squares). The profiles are offset along the axis for clarity.
Fractional photocurrent peak position as a function of (a) and (b) , where is the device channel length, for device B1 (solid circles, ) and device B2 (solid squares, ).
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