The correlation of residual contamination with void formation at the contact/SiO2 interface for single GaN nanowire (NW) devices was investigated. The morphology of the metal/SiO2 interface was studied by removing annealed Ni/Au films from the SiO2 surface with carbon tape, mounting the films for scanning electron microscopy and imaging. Formation of voids at the metal/SiO2 interface, which can negatively affect device performance, was shown to occur during annealing of Ni/Au contacts to NWs on SiO2. It was discovered that residual contamination, both from the NW solution and from photolithographic processes on the NW-SiO2 surface prior to metal deposition, can cause significant increases in the number of voids observed at the metal/SiO2 interface. Various predeposition cleaning methods were investigated in order to minimize the amount of void formation associated with residual contamination. The degree of void formation at the metal/SiO2 interface is used to evaluate the effectiveness of each cleaning method. It was determined that the most effective cleaning method for removing residual contamination from the NW-SiO2 surface was an ultraviolet ozone treatment followed by a dilute HCl bath, immediately preceding contact deposition.