Nov 1, 2011
Growth of nanowire structures on pre-patterned surfaces
The application of functional nanostructures is of high interest especially for transparent conductive electrodes for advanced solar cells. For industrial applications their effective and stable reproducibility is important. Recently, self-assembled Ag nanostructures were successfully produced by oblique physical vapour deposition (PVD) utilizing electron beam evaporation on ripple-like pre-patterned glass substrates. For fast investigation of the effect of various experimental parameters on the cluster growth patterns, scientists have developed an extended three-dimensional lattice kinetic Monte Carlo (KMC) simulation method. The correlation between the experimental and theoretical results as well as the essential idea of the simulation technique is presented in J. Phys.: Condens. Matter 23 222203.
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.
About the author
Satoshi Numazawa is a theoretical researcher working with Karl-Heinz Heinig at the Ion-Beam Physics and Materials Research Institute in the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The experimental side of this work is one of the topics in the thesis of Mukesh Ranjan, a former co-worker of Stefan Facsko's group at the same institute. Ranjan is now a postdoctoral researcher of FCIPT, Institute for Plasma Research, Gandhinagar, India. Roger Smith is in the Department of Mathematical Sciences, Loughborough University, UK and spent six months at the HZDR in 2010 thanks to a Global Research award from the Royal Academy of Engineering.