Near-field control and imaging of free charge carrier variations in GaN nanowires
Samuel Berweger, Paul T. Blanchard, Matthew D. Brubaker, Kevin J. Coakley, Norman A. Sanford, Thomas M. Wallis, Kristine A. Bertness, Pavel Kabos
Despite their uniform crystallinity, the shape and faceting of semiconducting nanowires (NWs) can give rise to variations in structure and associated electronic properties. Here we investigate local variations in electronic structure across individual n-doped GaN NWs integrated into a transistor device using a hybrid scanning probe microscope methodology. We locally gate and control the free charge carrier density with an applied tip bias and monitor its effect on the wire current while we simultaneously perform scanning microwave microscopy to directly image the resulting local charge carrier concentration. In particular we find significant variations in free carriers across NWs, with a reduced carrier density at the wire facets. By increasing the local carrier density we find that the tip injects current into theNWwith strongly localized current with the tip positioned over the wire vertices. Lastly we use photoconductivity measurements to study the influence on wire current transients from the passivation and activation of trap states, with increased illumination intensity significantly improving the rate of current recovery following local gating. These results suggest that the strong variations in electronic properties observed within NWs have significant implications for device design and may lead to new paths to optimization.