Tailoring the diameter and density of self-catalyzed GaAs nanowires on silicon

Abstract

Nanowire diameter has a dramatic effect on the absorption cross-section in the optical domain. The maximum absorption is reached for ideal nanowire morphology within a solar cell device. As a consequence, understanding how to tailor the nanowire diameter and density is extremely important for high-efficient nanowire-based solar cells. In this work, we investigate mastering the diameter and density of self-catalyzed GaAs nanowires on Si(111) substrates by growth conditions using the self-assembly of Ga droplets. We introduce a new paradigm of the characteristic nucleation time controlled by group III flux and temperature that determine diameter and length distributions of GaAs nanowires. This insight into the growth mechanism is then used to grow nanowire forests with a completely tailored diameter-density distribution. We also show how the reflectivity of nanowire arrays can be minimized in this way. In general, this work opens new possibilities for the cost-effective and controlled fabrication of the ensembles of self-catalyzed III-V nanowires for different applications, in particular in next-generation photovoltaic devices.S Online supplementary data available from stacks.iop.org/NANO/26/105603/mmedia

Regimes of radial growth for Ga-catalyzed GaAs nanowires

“…The Ga-catalyzed GaAs NWs described in Refs. [10,14,15] were straight from base to top, with the uniform radius either increasing [10,15] or self-equilibrating [14] with time. This suggests a certain mechanism of coreshell growth on the NW sidewalls to be present that adjusts the NW radius to the droplet radius and at the same time maintains the radius uniformity along the NW length, similarly to Ref.…”

“…[12,[17][18][19] and means simply that the arsenic adsorption on the droplet surface is equalized by its desorption and the solid growth. Only in such case, the NW axial growth rate is given by the kinetic of the arsenic growth species, as observed experimentally [4,[10][11][12]15], while the kinetics of the droplet volume depends on the balance between the incoming and outgoing gallium atoms.…”

“…In other words, an almost pure Ga droplet emits a given temperature-dependent diffusion flux onto the NW sidewalls. Due to this feature, the elongation rate of Ga-catalyzed GaAs NWs (and self-catalyzed III-V NWs in general) is determined by the kinetics of arsenic species [4,[10][11][12][13][14], whereas the radial growth at the NW top is dictated by the kinetics of gallium species [10,[13][14][15].…”

“…[12], Glas et al presented a growth model based entirely on the kinetics of the arsenic growth species, combined with the nucleation modeling for two-dimensional GaAs islands emerging from a supersaturated Ga-As droplet. However, the droplet size was considered time-independent, while many experimental works reported that the droplet either swells [4,9,10,14,15] or shrinks [6] during growth. Furthermore, in Ref.…”

Surface optical phonon propagation in defect modulated nanowires

“…5 Recently, Au free self-catalyzed GaAs nanowires were grown using molecular beam epitaxy (MBE) which allows tailoring and controlling the crystal structure and defect density. 6,7 Bulk GaAs crystallizing in cubic zinc blende (ZB) structure is the thermodynamically stable phase with a closed pack ABCABC stacking sequence along the [111] direction. 8 However, when grown as a nanowire, depending on the growth conditions like partial pressure, temperature, and catalyst droplet size, segments of hexagonal wurtzite (WZ) arrangement with the alternate stacking sequence of planes A and B along the [0001] direction can co-exist.…”