GaAs-based nanowires (NWs) can be grown without extrinsic catalyst using the Ga-assisted vapor-liquid-solid method in an epitaxy reactor, on Si(111) substrates covered with native oxide. Despite its wide use, the conventional method fails to provide a good control over uniformity, reproducibility, and yield of vertical NWs. The nucleation of GaAs NWs is very sensitive to the properties of the native oxide such as chemical composition, roughness and porosity. Consequently, samples grown under the same conditions on Si(111) substrates from different manufacturing batches often produce dramatically different growth results. In order to remove the dependence on wafer batch, a controlled chemical oxidation process is developed to replace the native oxide on Si(111) substrate with a reproducible chemical oxide. A high yield (exceeding 90%) of vertical GaAs NWs is achieved with excellent uniformity on chemical oxide-covered substrate. As an added advantage, the crystalline quality is significantly improved over that of GaAs NWs grown on native oxide-covered substrate, and pure zinc blende crystal structure can be achieved with minimal defects. In addition, the chemical oxide can be used as a template for producing different combinations of NW densities and sizes in parallel on the same wafer using the same growth conditions.