Arsenic Pathways in Self-Catalyzed Growth of GaAs Nanowires

Self-catalyzed growth of GaAs nanowires by molecular beam epitaxy on (111)Si substrates is investigated by introducing AlxGa1-xAs time markers. The nanowire elongation rate is found to be radius-independent, constant at substrate temperatures below 650 degrees C and linearly increasing with the incoming arsenic flux. The basic question of which pathways are followed by the arsenic species contributing to nanowire growth is clarified. The flow rate of As atoms directly impinging on the Ga catalyst drop is significantly smaller than the As consumption by nanowire growth. Thus, supplementary As atoms are necessary to explain the actual elongation rate. We show that surface diffusion of adsorbed As-x species toward the catalyst cannot account for the missing atoms. On the other hand, the reevaporation of As-x species from the substrate and from nanowire sidewall surfaces can act as an efficient secondary arsenic source. We argue that a sufficient amount of these species can be intercepted by the Ga drop and add up with the direct As impingement to explain the actual elongation rate.