  In this work we show that the incidence angle of group-III elements fluxes
plays a significant role on the diffusion-controlled growth of III-V nanowires
(NWs) by molecular beam epitaxy (MBE). We present a thorough experimental study
on the self-assisted growth of GaAs NWs by using a MBE reactor equipped with
two Ga cells located at different incidence angles with respect to the surface
normal of the substrate, so as to ascertain the impact of such a parameter on
the NW growth kinetics. The as-obtained results show a dramatic influence of
the Ga flux incidence angle on the NW length and diameter, as well as on the
shape and size of the Ga droplets acting as catalysts. In order to interpret
the results we developed a semi-empirical analytic model inspired by those
already developed for MBE-grown Au-catalyzed GaAs NWs. Numerical simulations
performed with the model allow to reproduce thoroughly the experimental results
(in terms of NW length and diameter and of droplet size and wetting angle),
putting in evidence that under formally the same experimental conditions the
incidence angle of the Ga flux is a key parameter which can drastically affect
the growth kinetics of the NWs grown by MBE.
