During the thin film growth process using low-energy metal PVD, below room temperature, transition events with high activation barriers do not play a crucial role in the growth process, while transitions with low barriers occur freely. However, there are many cases in which the deterministic evolution processes are led by the acceptance frequency of certain transitions within a metastable energy barrier range.

In their work, the researchers constructed a virtual atomistic transition network and introduced this effective transition energy range – all transition barriers are measured relative to a certain small energy level considered as an "unstable transition level". The transition barriers are either taken from literature values or calculated using a simplified environmentally dependent many-body potential.

Identifying key mechanisms

The figures show the comparison of Ag nanostructures (upper left) produced by oblique PVD on ripple-like pre-patterned glass at room temperature with the group's simulation model (upper right) at 300 K. The arrows indicate the azimuthal deposition angle. The lower snapshots (i–iii) give the Ag cluster growth process simulated by the KMC. In this case, a value for the unstable transition energy border, the boost energy, of 0.20 eV is employed. This accelerates the simulation speed by a factor e0.2/kT ~ 2290.

Transition events with barriers above 0.20 eV, such as the Ag(100), Ag(110)-in channel surface ad-monomer migration, ad-monomer re-evaporation from the substrate and overcoming the Ehrlich-Schwoebel barrier are considered as metastable transition events and dominate the cluster growth process. Transition attempts below the boost energy such as Ag(111) surface ad-monomer migration are accepted freely without any appropriation for the simulation time step. This allows much longer KMC simulations to be performed than in the traditional approach and allows the key mechanism for nanowire formation to be identified, in this case the interplay between the arrival flux rate on the surface and re-evaporation.