Growth map for Ga-assisted growth of GaAs nanowires on Si(111) substrates by molecular beam epitaxy

Abstract

For the Ga-assisted growth of GaAs nanowires on Si(111) substrates by molecular beam epitaxy, growth temperature, As flux, and Ga flux have been systematically varied across the entire window of growth conditions that result in the formation of nanowires. A range of GaAs structures was observed, progressing from pure Ga droplets under negligible As flux through horizontal nanowires, tilted nanowires, vertical nanowires, and nanowires without droplets to crystallites as the As flux was increased. Quantitative analysis of the resulting sample morphology was performed in terms of nanowire number and volume density, number yield and volume yield of vertical nanowires, diameter, length, as well as the number and volume density of parasitic growth. The result is a growth map that comprehensively describes all nanowire and parasitic growth morphologies and hence enables growth of nanowire samples in a predictive manner. Further analysis indicates the combination of global Ga flux and growth temperature determines the total density of all objects, whereas the global As/Ga flux ratio independently determines the resultant sample morphology. Several dependencies observed here imply that all objects present on the substrate surface, i.e. both nanowires and parasitic structures, originate from Ga droplets.

Lithography-free positioned GaAs nanowire growth with focused ion beam implantation of Ga

“…14,21,22 Consequently, the nanowire diameter is found to be widely insensitive to the parameters of the implantation process. Minor deviations in the diameter with respect to nanowires at random positions can mainly be explained by a local variation of the nanowire density and therefore the III/V ratio.…”

“…13 These droplets then served as nucleation centers for the nanowires, while the native oxide of the Si substrates, being stable up to 900 C, reduces parasitic nanowire formation in between. GaAs nanowire growth was performed under Ga-rich growth conditions 10,14 with typical equivalent layer Ga growth rates of 0.1 lm/h. Finally, the samples were cooled to room temperature under residual As 4 flux.…”

“…More nanowires locally lead to a reduction of the III/V ratio and therefore to larger nanowire diameters. 14 …”

“…[10][11][12] Without position control, however, the nucleation process leads to a rather broad diameter distribution, as well as nanowire growth in random spots with local variations in their density. 13,14 Various lithographic processes for the positioning of catalyst metal have been developed. Yet, they all share the fact that they require organic substances as the etch mask, lift-off mask, imprint material, or simply as solvent.…”

Analysis of incubation time preceding the Ga-assisted nucleation and growth of GaAs nanowires on Si(111)

“…Independent of the detailed mathematical description, the change of the incubation time with As flux implies a variation in the Ga droplet size at the point of nucleation, as explained in the following. From our previous study [15] we know that the number density of all objects on the sample does not depend on As flux. In addition, we deduced that all objects form from the initial Ga droplets.…”