Optimizing electrical contacts on individual p-i-n GaAs nanowires by site-specific focused ion beam processing

GaAs nanowires with built-in p-n junctions possess unique properties, making them suitable as basic components in a range of device applications, such as solar cells and photodetectors. Establishing reliable nanoscale electrical contacts on individual nanowires is crucial for characterizing and optimizing nanowire-device performances, but it is challenging due to the nanoscale material dimensions. Here, we show the optimization of electrical contacts on individual GaAs nanowires with built-in radial p-i-n junctions using site-specific nanoscale ion beam milling. We quantitatively studied the effect of ion beam treatment on the contact structure and properties using electrical characterization, theoretical modelling and transmission electron microscopy (TEM) measurements. The electrical current rectifying effect at the electrode/nanowire contacts is diminished by the ion beam treatment, as evidenced by both dark and illuminated current-voltage (I-V) characteristics of the individual nanowires. The optimized nanoscale contacts allow the study of the intrinsic photovoltaic performance of the individual GaAs nanowires. Different from the conventional method of reducing rectifying contact by increasing semiconductor doping at the contacts, the contact improvement by ion beam processing is found to be mainly due to a decrease (up to 3 orders of magnitude) in the shunt resistance at the electrode/nanowire Schottky barrier. The combined electrical and structural measurements suggest that the shunt resistance change could be attributed to the modification of the microscopic structure at the contacts: the native oxide on the surface of the nanowires was removed by the ion milling; ion implantation creates a defect-rich surface layer on the nanowires at the contacts. The observations in this study provide new insights into the effect of ion beam processing on nanoscale electrical contacts and may contribute to the fabrication of reliable electrical contacts on not only nanoscale p-n junctions but also semiconducting nanostructures and devices in general.