Abstract
Morphological instability of a planar surface ([111], [011], or [001]) of an ultra-thin metal film is studied in a parameter space formed by three major effects (the quantum size effect, the surface energy anisotropy and the surface stress) that influence a film dewetting. The analysis is based on the extended Mullins equation, where the effects are cast as functions of the film thickness. The formulation of the quantum size effect (Zhang et al., Phys Rev Lett 80:5381, 1998) includes the oscillation of the surface energy with thickness caused by electron confinement. By systematically comparing the effects, their contributions into the overall stability (or instability) is highlighted.
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Notes
Herring–Mullins theory has been used to study the instabilities and morphological evolution of a comparatively thick films (>100 nm). However, the theory does not have a limitation on a thickness per se. The base assumption in the theory is that there is a mobile adsorbate on the crystal surface, i.e., the top layer of the crystal is “fluid”, and the geometrical quantities such as the curvature can be defined for the surface. The top layer is usually considered to be one to three atoms deep. Thus, the theory is applicable to thinner films. It must be noted here that the dewetting of the ultra-thin metallic films is apparently driven by surface diffusion, as the films develop faceted pinholes that extend from the surface down to the substrate, and the pinholes are surrounded at their surface perimeter by the characteristic high rims formed out of the film material that diffused from the pinhole bottom [5].
It was argued recently [20] that in addition to this conventional surface stress of a macroscopic thin film, the ultra-thin metal films also are influenced by the quantum oscillations in the surface stress that are phase shifted with respect to the oscillations of the surface energy (the latter oscillations are termed the QSE oscillations in this paper). The authors of Ref. [20] propose that the coupling of the two oscillations may be responsible for the extension of the surface energy oscillations to thick films (>30 ML).
In Section 5 of his paper, Hirayama [3] cites the experiments where the shift of the critical thickness is arguably achieved through enhancement of charge spillage at the interface after the layer of Al is inserted between Ag film and Si substrate.
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Pierre-Louis O, private communication
Acknowledgments
The author thanks Olivier Pierre-Louis (University of Lyon) for making available his unpublished recent derivation of the electronic contribution to the surface energy. The anonymous Referee is acknowledged for the expert and very thorough multi-stage review of the paper’s manuscript.
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Khenner, M. Interplay of quantum size effect, anisotropy and surface stress shapes the instability of thin metal films. J Eng Math 104, 77–92 (2017). https://doi.org/10.1007/s10665-016-9874-6
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DOI: https://doi.org/10.1007/s10665-016-9874-6