GaAs nanowires grown by Ga-assisted chemical beam epitaxy: Substrate preparation and growth kinetics

Growth kinetics of GaAs nanowires (NWs) on Si(111) substrates by Ga-assisted chemical beam epitaxy is studied as a function of growth conditions such as substrate temperature (Ts), V/III flux ratio and catalyst dimension. The preparation method for Si(111) substrates is optimized in order to obtain a thin surface oxide with a thickness around 0.5 nm, allowing both the decomposition of metalorganic precursors and GaAs nucleation at oxide pinholes. The use of thinner oxides enables the growth of a GaAs layer whereas the utilization of thicker oxides could even inhibit GaAs nucleation. The successful self-formation of Ga droplets over this slightly oxidized Si surface has been observed by scanning electron microscopy (SEM), whose initial size is demonstrated to affect both the NW growth rate and the resultant NW aspect ratio. The formation of these droplets is crucial to enable the catalytic growth of NWs whose morphology is thoroughly analyzed by SEM, showing a self-organized array of vertically aligned match shaped GaAs NWs with a hexagonal footprint. In addition, the crystalline structure of NWs is monitored in-situ by reflection high energy diffraction, showing pure zincblende phase along the whole NW stem. In terms of better NW aspect ratio, higher crystalline quality and faster growth rates, the best NW growth conditions are found at Ts=580 °C, using an effective flux ratio V/III≈0.8. Moreover, NW growth kinetics is demonstrated to be improved when using a pre-deposited Ga coverage of 7.5 monolayers, stabilized for 90 s prior to the NW growth