GaAs NW typically adopt Cubic zince blende (ZB) structure or hexagonal wurzite (WZ) structure. \cite{Glas_2007} Each crystal structure is responsible for different properties of the NW and each crystal structure is composed by staking bilayers.\cite{Zhou_2014} The ZB has a ABCABC stacking sequence, and WZ has a ABABAB stacking sequence and forms a hexagonal prism.\cite{Morral_2011} When GaAs NW have diameters below 40 nm the WZ structure is adopted, and for diameters greater than 40 nm the ZB structure tends to be adopted.\cite{Shtrikman_2009} Two of the most common defects that are observed are “twinning" (two or more intergrown crystals are formed in a symmetrical fashion), and stacking fault (SF) (incorrect stacking of crystal planes). \cite{Andr__2014}
Axial and radial growth are the two directions of growth taking place during NW growth for MOCVD and VLS. Radial growth occurs when species are absorbed on the sidewalls of the NW. It is possible for these species to be further absorbed towards the Au catalyst causing them to be incorporated into the axial growth. “Tapering" of NW occur because the base of NW are exposed to precursors for longer times, and so grow for longer times. But also, adatoms can collect on the substrate which is where the base of the NW is located and so adatoms are likely to become absorbed onto the NW here. \cite{Pankoke_2012}