Direct assessment of p?n junctions in single GaN nanowires by Kelvin probe force microscopy

Making use of Kelvin probe force microscopy, in dark and under ultraviolet illumination, we study the characteristics of p?n junctions formed along the axis of self-organized GaN nanowires (NWs). We map the contact potential difference of the single NW p?n junctions to locate the space charge region and directly measure the depletion width and the junction voltage. Simulations indicate a shrinkage of the built-in potential for NWs with small diameter due to surface band bending, in qualitative agreement with the measurements. The photovoltage of the NW/substrate contact is studied by analyzing the response of NW segments with p- and n-type doping under illumination. Our results show that the shifts of the Fermi levels, and not the changes in surface band bending, are the most important effects under above band-gap illumination. The quantitative electrical information obtained here is important for the use of NW p?n junctions as photovoltaic or rectifying devices at the nanoscale, and is especially relevant since the technique does not require the formation of ohmic contacts to the NW junction.