Origins of Nanoscale Heterogeneity in Ultrathin Films

A key challenge in thin-film growth is controlling structure and composition at the atomic scale. We have used spatially resolved electron scattering to measure how the three-dimensional composition profile of an alloy film evolves with time at the nanometer length scale. We show that heterogeneity during the growth of Pd on Cu(001) arises naturally from a generic step-overgrowth mechanism relevant in many growth systems.

Figure 1

(a) 13.1 eV LEEM image recorded after the deposition of $\sim 0.6\mathrm{ML}$ Pd on Cu(001) at $200°\mathrm{C}$. Scale bar, $1\mu \mathrm{m}$. (b) 3D map of the Pd concentration near a surface step. The color images (circled in white) show the concentrations in the first three surface layers after deposition of 0.45 ML of Pd at $200°\mathrm{C}$. The spatial resolution is 8.5 nm. The colored maps are superimposed on the corresponding 13.1 eV gray-scale LEEM image. The upper (U) and lower (L) terraces are marked. Scale bar, 500 nm.

Figure 2

A schematic of step-flow overgrowth. Side views of the Cu surface are shown at 3 times ($t_1<t_2<t_3$) during Pd deposition. The Pd composition in the second (third) layer is shown in green (blue). Step-flow overgrowth converts mobile Pd in the second layer into fixed Pd in the third layer.

Figure 3

13.5 eV LEEM image recorded after the deposition of 0.45 ML of Pd at $200°\mathrm{C}$. Distances along the line scan are given in nanometers. At the start of Pd deposition, the step was located at $x=0$. Measured (computed) $IV$ curves at $A$, $B$, $C$, and $D$ are shown in black (red).

Figure 4

Pd concentration along the line indicated in Fig. 3. (a) Layer-resolved Pd concentrations measured for a total Pd coverage of 0.45 ML. (b) Third-layer Pd concentration measured at three different Pd coverages (i.e., deposition times) during deposition at $200°\mathrm{C}$. During growth, the step advances to the right.