  In this paper the early stages of the self-catalyzed Vapor-Liquid-Solid (VLS)
growth of GaAs nanowires on epi-ready Si substrates by Molecular Beam Epitaxy
(MBE) are studied. The interaction of Ga nano-droplets (NDs) with the silica
overlayer is investigated by X-ray Photoemission Spectroscopy (XPS) and Atomic
Force Microscopy (AFM). We show how Ga NDs drill the silica overlayer and make
contact with bulk Si to allow GaAs nanowires (NWs) epitaxial growth by studying
each of the three steps of the NW growth process sequentially: Ga NDs
pre-deposition, post-deposition annealing to reach the NW growth temperature,
and finally NW growth itself. The pre-deposition temperature allows to control
the density and morphology of the NDs. A high enough annealing temperature
enhances the reduction of SiOx by Ga oxidation and leads to the formation of
holes which is seen by a new component in XPS spectra, assumed to be the
consequence of the Ga-Si interaction. Finally, these nano-holes act as
nucleation sites for the epitaxial GaAs VLS growth. During the GaAs growth,
three different chemical environments of Ga are identified in Ga2p core level:
metallic Ga due to the droplets at the top of the NWs, Ga oxide in contact with
the SiOx overlayer and Ga arsenide from NWs.
