The growth of semiconductor nanowires via the vapor-liquid-solid (VLS) mechanism is a complex process governed by surface diffusion dynamics. Central to this mechanism is the role of surface diffusion in transporting adatoms from the vapor phase to the liquid catalyst droplet, where they eventually incorporate into the growing nanowire. The kinetics of this process are influenced by several factors, including adatom mobility, step-edge energetics, and catalyst wetting behavior, all of which collectively determine nanowire morphology and growth rates.

Adatom mobility is a critical factor in VLS growth. Adatoms adsorbed on the substrate or nanowire sidewalls must diffuse toward the liquid catalyst droplet to contribute to axial growth. The diffusion length of these adatoms is determined by the temperature-dependent surface diffusion coefficient, which follows an Arrhenius relationship. Higher temperatures enhance adatom mobility, increasing the likelihood of adatoms reaching the catalyst before desorbing. However, excessive temperatures can also lead to increased desorption rates, reducing the effective adatom supply. Experimental studies on silicon nanowire growth have shown that optimal temperatures balance diffusion and desorption, typically in the range of 400–600°C for gold-catalyzed VLS growth.

Step edges on nanowire sidewalls play a significant role in adatom dynamics. Kinks and ledges act as preferential attachment sites, altering the diffusion pathways of adatoms. The Ehrlich-Schwoebel barrier, an additional energy barrier for adatoms descending step edges, can lead to adatom accumulation on terraces, affecting lateral growth and sidewall roughness. In III-V nanowires, such as GaAs, step-edge energetics influence the transition between axial and radial growth modes. For instance, a high Ehrlich-Schwoebel barrier can suppress lateral growth, promoting straight, vertical nanowires, while a lower barrier may result in tapered morphologies due to enhanced sidewall deposition.

The wetting behavior of the catalyst droplet is another key determinant of VLS growth kinetics. The contact angle between the droplet and the nanowire top facet governs the effective surface area for adatom incorporation. A smaller contact angle increases the liquid-solid interface area, enhancing the collection efficiency of diffusing adatoms. However, it also reduces the droplet curvature, which can affect the supersaturation level in the catalyst. The Gibbs-Thomson effect dictates that smaller droplets exhibit higher solute supersaturation due to their increased curvature, leading to higher nucleation rates at the liquid-solid interface. This interplay between wetting and supersaturation influences nanowire diameter uniformity and growth direction. For example, in silicon nanowires, a contact angle below 90° typically favors vertical growth, while larger angles may lead to kinking or bending.

The combined effects of adatom diffusion, step-edge interactions, and catalyst wetting manifest in the growth rate and morphology of nanowires. Growth rates in VLS systems often follow a linear dependence on precursor flux at low pressures, transitioning to a saturation regime at higher fluxes where surface diffusion becomes rate-limiting. For instance, in germanium nanowire growth, the transition from flux-limited to diffusion-limited regimes occurs at precursor partial pressures around 10^-3 Torr. The morphology of nanowires is further influenced by the relative rates of axial versus radial growth, which are sensitive to the adatom diffusion length and catalyst dynamics. Nanowires with smooth sidewalls typically result from conditions where axial growth dominates, while rough or tapered nanowires indicate significant radial deposition due to limited adatom mobility or low catalyst activity.

In ternary or doped nanowire systems, surface diffusion also affects compositional uniformity. Differences in the diffusivities of constituent atoms can lead to segregation or alloying variations along the nanowire axis. For example, in InGaAs nanowires, indium adatoms exhibit higher surface mobility compared to gallium, leading to inhomogeneous incorporation unless growth conditions are carefully optimized. Similarly, dopant atoms such as silicon or zinc in III-V nanowires may exhibit surface segregation if their diffusion lengths exceed the nanowire diameter, resulting in non-uniform dopant profiles.

The role of surface diffusion in VLS growth extends beyond single nanowires to ensemble behavior in arrays. Inter-nanowire competition for adatoms can lead to growth rate variations, particularly in dense arrays where shadowing and diffusion field overlap occur. Nanowires located at the edge of an array often grow faster than those in the center due to reduced adatom depletion effects. This phenomenon has been quantitatively observed in silicon nanowire arrays, where edge nanowires can exhibit growth rates up to 20% higher than their centrally located counterparts under identical conditions.

Understanding and controlling surface diffusion in VLS growth is essential for tailoring nanowire properties for specific applications. By manipulating growth temperature, precursor flux, and catalyst composition, it is possible to engineer nanowire morphology, crystallinity, and compositional uniformity. Advances in in-situ characterization techniques, such as environmental transmission electron microscopy, have provided direct insights into adatom dynamics during growth, enabling more precise control over VLS kinetics. These insights are critical for the development of nanowire-based devices in electronics, photonics, and energy harvesting, where precise morphological and compositional control is paramount.