STABILITY AND TOPOLOGICAL TRANSFORMATIONS OF LIQUID DROPLETS ON VAPOR-LIQUID-SOLID NANOWIRES

 

Edwin J. Schwalbach, Stephen H. Davis, Peter W. Voorhees, James A. Warren, Daniel Wheeler

 

The Vapor-Liquid-Solid (VLS) method is one of the most popular techniques for growing semiconducting nanowires, and the stability of the liquid droplet is an important factor controlling wire morphology and, ultimately, functionality. Earlier theoretical work on axisymmetric systems indicates that the lowest-energy liquid configuration varies with surface energies, wire radius, and fluid volume. We test these predictions with a fully dynamic phase-field model that incorporates viscous fluid flow. Under conditions predicted by earlier theoretical work, we observe the pinning of the liquid to the top face of a nanowire, a condition necessary for wire growth. To study the stability of the droplet, we apply perturbations to the liquid shape and find that the system can transition to a metastable configuration, a local minimum in the energy landscape. Furthermore, the transition pathway to this local minimum depends on the magnitude of the perturbations. Under conditions that favor a liquid on the sidewalls of the wire, we observe a spontaneous transition of the liquid from a droplet to an annular configuration through an intermediate state that is not predicted by theory. The time scales and contact-line speeds for these transitions are determined through simulation and are consistent with approximations based on simple dimensional analysis.