With a band gap value of 1.7 eV, Al 0.2 Ga 0.8 As is one of the ideal III-V alloys for the development of nanowire-based Tandem Solar Cells on silicon. Nevertheless, growing self-catalysed AlGaAs nanowires on silicon by solid-source molecular beam epitaxy is a very difficult task due to the oxidation of Al adatoms by the SiO 2 layer present on the surface. Here we propose a nanowire structure including a p.i.n radial junction inside an Al 0.2 Ga 0.8 As shell grown on a p-GaAs core. The crystalline structure of such self-catalysed nanowires grown on an epi-ready Si(111) substrate (with a thin native SiO 2 layer) was investigated by transmission electronic microscopy and photoluminescence. I(V ) measurements performed on single nanowires have shown a diode-like behaviour corresponding to the radial p.i.n junction inside the Al 0.2 Ga 0.8 As shell. Moreover, a current generation under the electron beam was evidenced over the entire radial junction along the nanowires by means of electron beam induced current (EBIC) microscopy. The same structure was reproduced on patterned substrates with a SiO 2 mask, producing an ordered hexagonal array. High and uniform yields from 83% to 87% of vertical nanowires were obtained on 0.9×0.9 cm 2 patterned areas. EBIC mapping performed on these nanowires confirmed the good electrical properties of the radial junction within the nanowires.

It is well known that the crystalline structure of the III-V nanowires (NWs) is mainly controlled by the wetting contact angle of the catalyst droplet which can be tuned by the III and V flux. In this work we present a method to control the wurtzite (WZ) or zinc-blende (ZB) structure in self-catalyzed GaAs NWs grown by molecular beam epitaxy, using in situ reflection high energy electron diffraction (RHEED) diagram analysis. Since the diffraction patterns of the ZB and WZ structures differ according to the azimuth [11 ̅ 0], it is possible to follow the evolution of the intensity of specific ZB and WZ diffraction spots during the NW growth as a function of the growth parameters such as the Ga flux. By analyzing the evolution of the WZ and ZB spot intensities during some NW growths with specific changes of Ga flux, it is then possible to control the crystal structure of the NWs. ZB GaAs NWs with a controlled WZ segment have thus been realized. Using a semi-empirical model for the NW growth and our in situ RHEED measurements, the critical wetting angle of the catalyst droplet for the structural transition is deduced.