A strained film grown on a vicinal substrate: Steps bunch or not to bunch?
When a strained film is grown on a vicinal substrate, the steps advance like a train when the deposited atoms have sufficient mobility to reach the step edges. However, as the steps advance, the strain-induced force monopoles associated with the steps cause the steps to attract to each other (J. Tersoff, PRL 74, 4962, (1995)), resulting in a thermodynamic instability of the steps in the form of step bunching (J. Tersoff, et al., PRL 75, 2730 (1995)).
Recently, it was shown that this bunching instability can be suppressed with proper control of the growth kinetics, taking advantage of the standard step-edge barrier effect (commonly known as the Ehrlich-Schwoebel (ES) barrier effect), and a morphological phase diagram was constructed to obtain smooth films in the regime of persistent step-flow growth (Wei Hong et al., PRL 95, 095501 (2005); see also an earlier post by Wei at http://imechanica.org/node/313).
A more recent twist on this topic was the report by Mina Yoon et al. that, even in the unstable bunching regime with the ES barrier effect already taken into account, the bunching instability won't be triggered unless the system reaches a critical film thickness; therefore, smooth films could still be obtained in an otherwise unstable regime, or effectively in a broader volume/area of the phase space defined by the growth parameters (M. Yoon et al., PRL 99, 055503 (2007)). It was further shown that one could exploit the well-defined scaling properties of the critical film thickness to extract intrinsic energetic parameters for a given system.