Droplet dance on top of a pillar
24/04/2014, Miroslav Kolíbal
Here, using real time in-situ scanning electron microscopy during germanium nanowire growth we show that the droplet unpinning from the growth interface may also result in a growth of straight nanowires.
In VLS grown nanowires the atoms of the semiconductor material are supplied to the metal droplet either from a gas phase (chemical vapor deposition - CVD) or atomic vapor (physical vapor deposition - PVD). The supersaturation in the alloy droplet increases and after it reaches certain value, the nucleation of the excess material is favorable at the liquid/solid interface. If the growth criteria are fulfilled, upon completion of a monolayer of a semiconductor material the droplet slides up and another monolayer is grown, giving rise to a nanowire with the collector droplet on top. Naturally, in this growth mode the droplet-nanowire interface is crucial for determining the final morphology of the wire. Ideally, the droplet shape would be a spherical cap due to energy minimization. But since the semiconductor surfaces are generally faceted, the nanowire cross-section has usually the shape of a convex polygon instead of a circle. Using in-situ TEM it has been observed that the growth interface is not planar, but truncated, and the small side facets serve as preferential nucleation sites for oscillatory growth. If a new facet is introduced to the system, it rapidly grows and the nanowire kinks in a new direction, because the droplet stays pinned to the triple phase line (TPL, the interface between the three phases) and the former interface facet winds out. However, as more complicated nanowire morphologies are being fabricated, it is speculated that under specific conditions the droplet might repeatedly pin and unpin from the TPL. Here, we demonstrate such droplet behaviour using real-time observations in scanning electron microscope (SEM), where one can easily get a three-dimensional information of the sample morphology by tilting the sample and observing it from different angles.