Speaker
Description
Low- and medium-energy ion beam irradiation of surfaces induces a variety of nanoscale morphologies, depending on the irradiation conditions [1]. Under specific conditions, hexagonally ordered dot or pit arrays, checkerboard patterns, and periodic ripple structures oriented either perpendicular or parallel to the ion beam direction can form spontaneously during continuous surface erosion by ion sputtering.
On amorphous surfaces, pattern formation is primarily governed by the interplay between roughening mechanisms, e.g., curvature-dependent sputtering, ballistic mass redistribution, and composition changes in multicomponent materials—and smoothing mechanisms, including surface diffusion and viscous flow.
An additional surface instability emerges above the recrystallization temperature, where ion-induced bulk defects are dynamically annealed and amorphization is suppressed. In this regime, the diffusion of ion-induced vacancies and adatoms on crystalline surfaces is influenced by the Ehrlich–Schwoebel (ES) barrier, i.e., an additional energy barrier for interlayer mass transport across terrace steps. As a result, vacancies and adatoms become trapped on terraces and can nucleate into extended pits or islands, respectively [2].
Patterns formed in this so-called “reverse epitaxy” regime exhibit well-defined crystalline facets, and their symmetry reflects the underlying crystal structure. Nevertheless, ballistic effects can still contribute significantly to morphology evolution on crystalline surfaces. This has been demonstrated for high-temperature irradiation at oblique incidence angles [3], as well as for normal incidence in the intermediate regime between checkerboard and isotropic patterns [4].
The fundamental understanding of surface pattern formation under these non-equilibrium conditions is already well advanced. Atomistic simulations, such as molecular dynamics (MD) and kinetic Monte Carlo (kMC), as well as continuum modeling approaches, successfully reproduce most experimental observations. In recent years, continuum models have been further refined, achieving increasing predictive capability for both amorphous and crystalline surfaces.
References
[1] Cuerno, R. and Kim, J.-S., J Appl Phys 128, (2020) 180902.
[2] X. Ou, K.-H. Heinig, R. Hübner, J. Grenzer, X. Wang, M. Helm, J. Fassbender, and S. Facsko, Nanoscale 7, 18928 (2015).
[3] D. Erb, R. de Schultz, A. Ilinov, K. Nordlund, R. M. Bradley, and S. Facsko, Phys Rev B 102, 165422 (2020).
[4] D. J. Erb, D. A. Pearson, T. Škereň, M. Engler, R. M. Bradley, and S. Facsko, Phys. Rev. B 109, 045439 (2024).