Vapor-liquid-solid nucleation of GaAs on Si(111): Growth evolution from traces to nanowires

GaAs nanowires were grown on Si(111) substrates by molecular-beam epitaxy employing Au droplets for the vapor-liquid-solid mechanism. The nucleation happens in three stages, which coincide with the abundance of one of three different GaAs manifestations: first Au-induced lateral traces, then three-dimensional islands, and finally Au-induced vertical nanowires. During deposition of the first 7 ML of GaAs no nanowires grew. By reflection high-energy electron diffraction and transmission electron microscopy, the crystal structure of the GaAs manifestations was examined. Traces and islands adopted the zinc-blende crystal structure and include twins, while nanowires grew in the hexagonal wurtzite structure. The delay in nanowire formation was observed to increase for higher growth temperatures. The gradual covering of the Si by GaAs was comparable to the case of Au-free growth and appears to be linked with the evolution from trace to nanowire growth. During trace formation, Au droplets kept in contact with the Si substrate and were pushed sideways by precipitating GaAs. The nucleation stages could thus be explained by considering that any Au droplet created a trace while in contact with the Si surface and a nanowire after having been pushed onto GaAs. This mechanism of trace growth was explained by the lower interface energy of liquid Au droplets on Si when compared with that on GaAs, as supported by numerical estimates.