Carrier transport in semiconductors is of both fundamental and technological significance, as it not only reflects fundamental aspects such as electron-phonon interactions, but also controls electronic and opto-electronic device characteristics. Minority carrier transport is particularly important, as it determines the performance of p-n junction based devices. A fundamental understanding of carrier transport properties, especially those of minority carriers, provides a critical basis for material engineering and device design efforts.

In advancing semiconductor nanowire-based device technologies, a quantitative knowledge of carrier transport parameters, such as the carrier diffusion length, is required for a rational design of devices with controlled performance. From a fundamental perspective, in semiconductor nanowires, the one-dimensional confinement of carriers and phonons, together with the high surface-to-volume ratio, can render carrier transport characteristics significantly different from those in the bulk. Here, using the near-field scanning photocurrent microscopy technique, we have directly measured the minority carrier diffusion length in single ZnO nanowires. In particular, a near-field scanning optical microscope was used to locally generate minority carriers in single nanowire Schottky diodes; the spatial variations of the resulting photocurrent images near the Schottky contact were used to obtain the minority carrier diffusion length, L_D. The diameter dependence of L_D suggests a diameter-dependent surface electronic structure, particularly an increase in the density of mid-bandgap surface states with the decreasing diameter. This diameter dependence of the surface electronic structure might be a universal phenomenon in wurtzite-type nanostructures, and is critical in interpreting and understanding the effects of surfaces on various material properties.