In a matter of seconds, fully-formed metal islands have been observed exploding out of a silicon surface wetted with lead atoms. This island formation vastly differs from the classical model of nucleation, where islands form from the gradual aggregation of randomly moving surface atoms and can grow slowly over hours. At temperatures well below freezing, lead atoms are systematically deposited onto a thick piece of silicon wafer, forming an amorphous layer known as a wetting layer. The presence of silicon increases the lead density and allows more atoms to squeeze together and interact than if lead were deposited on lead. When the wetting layer reaches critical density, growth happens suddenly, and multi-height, crystalline, fully-formed islands mushroom up in a matter of seconds. The speed and uniformity of the growth involves millions of atoms moving collectively across the surface, a process that clearly deviates from the usual random motion of particles. These observations shed light on a new type of fundamental nucleation mechanism, leading to better control of high-quality thin film growth.
Non-Classical ‘Explosive’ Nucleation in Pb/Si(111) at Low Temperatures