Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The achievement of sub-Poissonian length distributions (LDs) in VLS-grown III-V nanowire (NW) ensembles, as theoretically predicted by Glas and Dubrovskii, requires synchronized nucleation of all NWs on their substrate. This is especially challenging for self-catalyzed GaAs NWs on a natively-oxidized Si(111) substrate because their nucleation involves a sequence of different physical mechanisms: the formation of Ga droplets at random positions on the substrate, their interaction with SiO_x and the formation of nano-sized holes, and finally the droplet-assisted nucleation of GaAs inside these holes.
Here, we demonstrate that it is possible to achieve highly-synchronized nucleation of MBE-grown GaAs NWs and, thus, very narrow LDs if a simple in situ procedure is employed prior to the growth in order to decouple the formation of SiO_x holes from the subsequent nucleation of NWs. This procedure consists of three steps (substrate annealing – Ga deposition – substrate annealing) and produces SiO_x holes (free of Ga droplets) of controlled size and number density.
Our study compares the LD of GaAs NWs grown on Si substrates with different size or number density of SiO_x holes. The results were fitted with a continuum-growth theoretical model that accounts for nucleation fluctuations, kinetic fluctuations and nucleation antibunching in individual NWs. We have found that the formation of large-enough holes before the initiation of the NW growth can shorten the characteristic nucleation time of the NWs by one order of magnitude and narrow the LD by a factor of 2. The LD was further improved by decreasing the number density of SiO_x holes/GaAs NWs, which is attributed to the suppression of beam-shadowing effects. In the best case, we obtained GaAs NWs that exhibit a remarkably short characteristic nucleation time of 10 ms and a sub-Poissonian LD. All in all, our results (unpublished) not only prove the validity of theoretical considerations about the sub-Poissonian LD for self-catalyzed NWs, but also demonstrate a simple route to low-cost fabrication (without substrate patterning) of GaAs NW-based devices with controllable number density and length uniformity.