We report on the structural characterization of GaAs nanowires integrated on Si(001) by template-assisted selective epitaxy. The nanowires were grown in lateral SiO2 templates along  with varying V/III ratios and temperatures using metal-organic chemical vapor deposition. The nanowires have been categorized depending on the growth facets which typically consisted of (110) and (111)B planes. Nanowires exhibiting a (111)B growth facet were found to have high density planar defects for all growth conditions investigated. However, GaAs nanowires with a single (110) growth facet were grown without the formation of planar stacking faults, resulting in a pure zinc blende crystal.