Grains, Growth, and Grooving

We report the in situ investigation of grain growth and grain boundary migration, performed with a variable-temperature scanning tunneling microscope (STM) on a polycrystalline gold film. Atomic step resolution allowed us to identify the individual grains and, thus, also the grain boundaries. Our special, thermal-drift-compensated STM design made it possible to follow the same sample area over large temperature intervals. In this way, we have directly observed grain boundary migration and grain growth. In a first quantitative analysis we correlate the observed, unexpected changes in surface roughness with the evolution of the grain and grain boundary configuration.

Figure 1

STM images (a),(c) immediately after deposition at 293 K and (b),(d) at 748 K. Notice the larger scale in panels (b),(d). We used a differential filter for (a) and (b) in order to obtain a high step contrast, and a special filter for (c) and (d) in order to enhance the contrast on the grain boundaries.

Figure 2

Four images (172 nm) selected from an STM movie illustrate GB migration at 413 K. The time (sec) is indicated in the images. Notice the flatter, more (111) oriented region, which is left behind by the moving GB.

Figure 3

(a) The graph of the rms height variation as a function of the length scale $L$ levels off at $\sim 0.93\mathrm{n}\mathrm{m}$. This value represents the roughness $R$ of the film at this stage. (b) Film roughness $R$ as a function of the average grain size. Each individual point has been determined with the procedure illustrated in (a). Notice that the roughness first decreases, then stays almost constant, and later increases again.

Figure 4

Typical STM height lines of the Au film (a) immediately after deposition, (b) at 668 K, and (c) at 748 K. Each dome is an individual grain. Vertical lines indicate the grain boundary locations. Note the different height scales. In panels (b),(c) individual steps and terraces are visible.

Figure 5

(a) The equilibrium angles at a triple line are determined by the interface free energies. Large values of $f_{\mathrm{G}\mathrm{B}}$ correspond to small angles ${\alpha }_{\mathrm{G}\mathrm{B}}$. The dotted surface contour represents an earlier stage of the grooving. (b) Sketch of the grooving of a polycrystalline surface for a stationary grain size configuration. (c),(d) Film evolution resulting from changes in the GB configuration; the dotted lines represent the initial situation (c) a lower average GB energy results in a smoother surface, while (d) shows an increase in the average grain size results in a rougher surface.