Twin Boundaries Can Be Moved by Step Edges During Film Growth

We track individual twin boundaries in Ag films on Ru(0001) using low-energy electron microscopy. The twin boundaries, which separate film regions whose close-packed planes are stacked differently, move readily during film growth but relatively little during annealing. The growth-driven motion of twin boundaries occurs as film steps advance across the surface—as a new atomic Ag layer reaches an fcc twin boundary, the advancing step edge carries along the boundary. This coupling of the microstructural defect (twin boundary) and the surface step during growth can produce film regions over $10\mu \mathrm{m}$ wide that are twin free.

Figure 1

Scanning tunneling micrograph of 2 ML Ag on Ru(0001) showing a closed-packed hexagonal Ag layer. Each bright spot is a Ag atom. The triangular corrugations result from a dislocation network confined to the 1st Ag layer. The dislocations accommodate the large size misfit between the Ag and Ru atoms.

Figure 2

(a) Schematic illustration of a twin boundary in a Ag(111) film in Ru(0001). A stacking fault in the 3rd Ag layer causes the two film regions next to the boundary to have different stacking sequences. Partial dislocations (blue $T$ symbols) occur at the twin boundary on each film layer where the stacking changes across the boundary. (b) Illustration of growth-induced twin-boundary motion. The twin boundary moves in lock step with the step edge of the advancing 5th Ag layer; the stacking of the 3rd and 4th layers switches, and the stacking fault shrinks.

Figure 3

Twin boundaries being eliminated as Ag steps advance during film growth. (a)–(d) Dark-field LEEM images of (01) diffraction beam showing changes in stacking sequence of 3 ML Ag films during growth at 180 °C. The 3rd Ag layer nucleates randomly in two possible stacking sequences (the black and white domains labeled I and II, respectively) on the 2 ML film (gray) in image (a). By 4.7 ML, image (d), the type II stacking domain has been eliminated. The imaged region contains no substrate steps. (e)–(f) Film-thickness contrast in bright-field images of growing Ag film shown in images (c)–(d). The advancing step edge of the 4th layer (scalloped lines near center marked by white arrows) in image (e) coincides with the twin boundary of image (c). 5 ML-thick regions are colored red. (g)–(h) Schematic cross section of how an advancing film layer causes twin boundaries to move and change the stacking of underlying film layers. Shown are the stacking sequences of the film layers that occur at the location of the dashed horizontal lines shown in images (c)–(f). The dislocations (marked with blue $T$ symbols) that form the twin boundary move in concert with the 4th Ag layer advancing from the left and right. A domain of type II stacking is eliminated as opposite twinning dislocations annihilate.

Figure 4

Possible atomic arrangements that could result from a Ag layer overgrowing a stacking fault. The circles represent three layers of Ag atoms color coded by their stacking arrangement. The horizontal lines show the Ag layers in cross section, also color coded and labeled by their stacking type. In (a) a step advances from the left towards a twin boundary that bounds a stacking fault. The Shockley partial dislocation (the upright and inverted $T$ symbols) at the twin boundary changes the stacking of the 2nd Ag layer from $B$ to $C$. In (b), the step of the 3rd layer has stopped at the twin boundary. In (c)–(e), the step has advanced further, but with three different scenarios of the Ag-layer stacking. In (c) and (d), the original twin boundary does not move. In (c) fcc stacking is maintained on either side of the boundary but the top Ag layer contains atoms in unfavorable twofold bridge sites (vertical arrow) at the twin boundary. (The twin boundary thickens by one layer and the stacking fault in the 2nd layer does not shrink.) In (d) all atoms are in energetically favorable threefold hollow sites but unfavorable hcp ($ACA$) stacking occurs. Scenario (e) matches experiment—the twin boundary moves in concert with the advancing film step and the stacking fault in the 2nd film layer shrinks.