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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 silicaoverlayer is investigated by X-ray Photoemission Spectroscopy (XPS) and AtomicForce Microscopy (AFM). We show how Ga NDs drill the silica overlayer and makecontact with bulk Si to allow GaAs nanowires (NWs) epitaxial growth by studyingeach of the three steps of the NW growth process sequentially: Ga NDspre-deposition, post-deposition annealing to reach the NW growth temperature,and finally NW growth itself. The pre-deposition temperature allows to controlthe density and morphology of the NDs. A high enough annealing temperatureenhances the reduction of SiOx by Ga oxidation and leads to the formation ofholes which is seen by a new component in XPS spectra, assumed to be theconsequence of the Ga-Si interaction. Finally, these nano-holes act asnucleation 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 withthe SiOx overlayer and Ga arsenide from NWs.

Early stages of GaAs nanowires VLS self-catalyzed growth on silica-terminated silicon substrate: A photoemission study