Abstract
We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation.
Highlights
Substrate surface energy [2]–[4]
Since the atomic processes that convert a flat film into 3D islands are poorly known, predicting the time needed for film de-wetting is not possible
We find that atomic steps, both on the film’s surface and at the film/substrate interface, allow a kinetic pathway that avoids the nucleation of holes in film layers to expose the wetting layer or the nucleation of new film layers to thicken the film
Summary
Most experiments were performed in an ultrahigh-vacuum-based (UHV) low-energy electron microscope (LEEM). The Cr deposition rate was measured before and after each de-wetting experiment by observing the time needed to deposit the first complete Cr layer on W(110) at 455 ◦C, a temperature sufficiently high to give perfect step-flow film growth [33, 34]. The local uphill and downhill directions along the substrate staircase were determined by monitoring with LEEM the directions that Cr film steps advanced during deposition under conditions of step-flow growth. For films thinner than about six layers, the local film thickness was determined in two ways—either directly during step-flow growth by tracking film step edges, or by measuring how the electron reflectivity varied as a function of electron energy (i.e., the quantum-size effect [35]). The crystallographic directions of this crystal were determined using STM to image the directions of the atomic rows in the (2 × 1) structure of oxygen adsorbed on W(110) [36]
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