Influence of Silicon on the Nucleation Rate of GaAs Nanowires on Silicon Substrates

Abstract

Despite the unavoidable presence of silicon atoms in the catalyst alloy droplets during the vapor−liquid−solid growth of III−V nanowires on silicon substrates, it remains unknown how the nucleation of nanowires is affected by these foreign atoms. In this work, we present the first attempt to quantify the nanowire nucleation rate versus the silicon concentration in the droplet. We calculate the chemical potential difference per Ga−As pair in the quaternary Au−Ga−As−Si liquid alloy droplet and in solid state, and compare it to the ternary Au−Ga−As droplet without silicon. This allows us to compute the nucleation rates of GaAs nanowires versus the silicon concentration under different conditions. We find that the presence of silicon in the droplet decreases the nucleation probability of GaAs nanowires for gallium-rich droplets (with the gallium contents c GA greater than 0.6) and increases it for goldrich droplets (c GA < 0.6). The model is used to explain our experimental data for hydride vapor phase epitaxy of gold-catalyzed GaAs nanowires, which easily nucleate on Si(111) covered with different SiO 2 layers but do not grow on the bare Si(111). In the latter case, more silicon is etched from the substrate and enters the gallium-rich droplets, which suppresses the nanowire nucleation. We discuss other relevant data, including the known difficulties in obtaining self-assisted GaAs NWs on silicon by chemical epitaxy techniques. These results may be useful for the fine-tuning of III−V nanowire properties and integrating them with silicon electronics.

Thermodynamics of the Vapor–Liquid–Solid Growth of Ternary III–V Nanowires in the Presence of Silicon

“…The calculated values are always positive, showing that the presence of Si in In-rich catalyst droplets is beneficial for nucleation of NWs regardless of their composition x, including almost pure GaAs solid (x 0). This is different from the results reported in Reference [ 29 ], where pure GaAs NWs were grown from pure Ga droplets. The effect is due to the presence of In in the catalyst droplet which, even at small x, may have a great impact on the growth thermodynamics due to the interactions with the co-existing species in liquid.…”

“…After adding Si atoms to the droplet, the initial atomic fraction of element q must be reduced by the factor ( ), that is, [ 29 , 30 ]. In the present study, we do not consider any possible influence of Si dopant on the chemical potential of solid NW.…”

“…Due to the exponential dependence of the nucleation probability for each NW monolayer formation on , a small fluctuation of the liquid chemical potentials results in an extremely steep change of . This change is given by [ 29 ] where is the critical nucleus size of classical nucleation theory and hence must be much larger than unity. The probability determines the nucleation rate of NWs emerging from catalyst droplet and the axial growth rate of developed NWs.…”

“…It is well-known that chemical potential driving the liquid–solid transition strongly depends on the liquid composition [ 27 , 28 ] and hence on the Si concentration in the droplet. We have already reported how the presence of Si atoms influences the nucleation of some binary III–V and III–N NWs [ 29 , 30 ], but this has not been done for ternaries so far.…”

Does desorption affect the length distributions of nanowires?

“…Although the MBE growth conditions for the considered samples are similar, they were obtained in different growth runs and on different substrates. According to [30], the chemical potential of arsenic atoms in the droplet is strongly affected by the presence of silicon coming from the substrate. The effect is so strong that GaAs NWs cannot grow at all when the silicon concentration is too high.…”