Self-catalyzed growth of GaAs and GaP nanowires by molecular beam epitaxy is often performed on processed SiOx/Si(111) substrates with regular arrays of lithographically defined holes. Ga droplets form in the holes during Ga pre-deposition step in the absence of As supply. It was considered evident that the Ga diffusion flux in such a process is directed from the oxide surface into the holes. Here, we show that it is not always true and that the diffusion flux can change its direction depending

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... on the growth conditions. Our model can be useful for modeling the incubation and growth times of group III droplets within the holes and explains long nucleation delays of the droplets.