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Self-catalyzed GaAs nanowire growth was obtained by molecular beam epitaxy on GaAs(001) substrates after predeposition of subnanometer-thick Si layers. Two substrate preparation methods are presented, the first based on the epitaxial growth of Si on GaAs and subsequent exposure to atmosphere, and the second on the direct deposition of Si on epiready GaAs substrates. X-ray photoemission spectroscopy shows that both methods result in a thin Si oxide layer that promotes the growth of GaAs nanowires aligned along the 〈111〉 direction. High densities of nanowires were obtained at substrate temperatures between 620 and 680 °C. Systematic electron microscopy studies indicate that nanowire growth is associated with the formation of Ga nanoparticles on the substrate surface, which act as a catalyst in the vapor-liquid-solid growth mechanism frame. The majority of the nanowires have a pure zinc-blende structure, and their photoluminescence is dominated by a photoluminescence peak 3 to 5 meV in width and centered at 1.516 to 1517 eV.