Upward mass transport and alloying during the growth of Co on Cu(111)

Co growth on Cu(111) was investigated at several temperatures between 120 K and 300 K by variable-temperature fast-scanning tunneling microscopy at submonolayer coverage. Islands nucleate heterogeneously at step edges and homogeneously on terraces. The height and area distribution difference between these two types of differently nucleated islands is attributed to a step edge alloy. Furthermore, the transformation from one-monolayer high islands to two-monolayer high islands is followed in time-lapsed sequences between 145 and 165 K. A surprising low-energy barrier for upward mass transport of $E_{\mathrm{upward}}\approx (0.15\pm 0.04)$ eV is determined for islands on terraces. At 120 and 150 K, the terrace islands are pure Cu; in contrast, at room temperature, terrace islands larger than $\approx 120$ ${\mathrm{nm}}^2$ alloy at their border.

Figure 1

Cu(111) surface before and after Co deposition. (a),(b) STM images of pristine surface before deposition: (a) large-scale image, (b) atomic resolution. (c)–(h) 0.04 ML Co on terraces and at step edges deposited at (c),(d) 120 K. Lower inset: step edge at higher coverage to make nucleation at lower step edge visible; at (e),(f) 150 K and (g),(h) room temperature (RT). Insets: magnification by a factor of 2. Nucleation at lower and at upper step edge is marked. Tunneling parameters: (a) $V_t=-805$ mV, $I_t=0.5$ nA, RT; (b) $-175$ mV, 1.4 nA, RT; (c),(d) $-860$ mV, 0.5 nA, 120 K; Inset: $-884$ mV, 0.53 nA; (e),(f) 1160 mV, 0.07 nA, 150 K; (g),(h) $-860$ mV, 0.33 nA, RT.

Figure 2

Growth of Co islands at 120 K $(-805$ mV, 0.6 nA, 0.03 ML, 120 K): (a) region of surface with small terrace widths, $\langle d\rangle =(25\pm 2.7)$ nm; (b) region with one large terrace of $(121\pm 0.5)$ nm; and (c) relation between island density $\rho$ and terrace width $W$.

Figure 3

Origin of an additional energy barrier at step edges, adapted from Ref. [23] (with permission of author), illustrating a (a) sketch of mobile Co atoms reaching the step edge based on Monte Carlo calculations [23]. The darker orange balls are Cu atoms on the upper terrace, while the lighter orange balls are on the lower terrace. The gray balls are Co atoms. The solid and dashed arrows indicate the trajectory of the mobile Co atoms through the exchange process (E) and diffusion process (D), respectively. (b) Energy profile from static relaxations [23] for the diffusion of a Co atom after the incorporation of other Co atoms at step edges.

Figure 4

Co island growth at step edges at 120 K: (a) STM image of 0.03 ML $(-805$ mV, 0.4 nA, 120 K); (b) Line profile along the line across the step edge, as shown in (a). The dashed line marks the measured height of the Co island at the upper edge of the step. (c) Height distribution. (d) Area distribution. $A_M$: median value. $\langle A\rangle$: mean value.

Figure 5

Growth of Co islands on terraces at 120 K: (a) STM image $(V_t=-800$ mV, $I_t=0.5$ nA, 0.03 ML, 120 K); (b) Line profiles along the lines shown in (a). The dashed line indicates the measured height of the Co island. (c) Height distribution. (d) Area distribution. $\langle A\rangle$: mean value. $A_M$: median.

Figure 6

Transport of atoms from the first to the second layer of an island on the terrace: (a) Three-dimensional (3D) STM image on false-color scale of an island; (b) 3D STM image of the same island after 2 min $(U_t=-860$ mV, $I_t=0.6$ nA, 0.04 ML, 165 K); (c) Arrhenius plot: the red line is an apparent linear fit yielding $E_A\approx (0.19\pm 0.04)$ eV and a prefactor $\nu \approx (1.8\pm 0.5)\times {10}^{11}$/s. Green curved lines show the confident band of the fitting process.

Figure 7

Growth of Co islands at $\approx 150$ K: (a) region of surface with small terraces, $\langle d\rangle =(20.1\pm 7.2)$ nm (860 mV, 0.5 nA, 146 K, 0.03 ML); (b) region of surface with a wide terrace, $d=(244\pm 2)$ nm $(-800$ mV, 0.42 nA, 154 K, 0.04 ML); and (c) dependence of island density $\rho$ on terrace width $W$.

Figure 8

Co island growth at step edges at $\approx 150$ K: (a) STM image with a mean terrace width of $(20.1\pm 7.2)$ nm (860 mV, 0.5 nA, 146 K, 0.03 ML); (b) Line profile across step edges along the lines as shown in (a). Dashed lines mark measured heights of the Co islands. (c) Height distribution. (d) Area distribution of all islands in gray. Bilayer high island distribution in blue. $A_M$ is median value. $\langle A\rangle$ is mean value.

Figure 9

Growth of Co islands on terraces at $\approx 150$ K: (a) STM image of the Co islands $(-1740$ mV, 0.1 nA, 150 K, 0.04 ML); (b) Line profiles along the lines as shown in (a). Dashed lines mark the measured height of Co islands. (c) Height distribution. (d) Area distribution of all islands in gray; only bilayer high island in blue. $A_M$ is the median value. $\langle A\rangle$ is the mean value.

Figure 10

Growth of Co island at room temperature: (a) region of surface with small terraces with $\langle d\rangle =(14.8\pm 4.9)$ nm (860 mV, 0.5 nA, RT); (b) region of surface with a wide terrace of $W=(65\pm 1)$ nm (860 mV, 0.6 nA, 0.04 ML, RT); and (c) dependence of island density $\rho$ on terrace width $W$.

Figure 11

Co island growth on step edges at room temperature: (a) STM image (860 mV, 0.6 nA, 0.04 ML, RT); (b) Line profile across the step edge along the line as shown in (a). Dashed lines mark the measured height of Co islands. (c) Height distribution. (d) Area distribution of all islands in gray. Bilayer islands in blue. $A_M$ is the median value. $\langle A\rangle$ is the mean value.

Figure 12

Growth of Co islands on terraces at room temperature: (a) STM image of islands with compact hexagonal shape (860 mV, 0.5 nA, 0.04 ML, RT); (b) STM image of island with irregular triangular shape $(-860$ mV, 0.6 nA, 0.03 ML, RT); (c),(d) Line profiles along the lines shown in (a) and (b). The dashed lines are measured heights of Co islands. (e) Height distribution for both type of islands: compact hexagonal islands in red, irregular triangular islands in blue. (f) Area distribution. $A_M$ is median value. $\langle A\rangle$ is mean value.

Figure 13

Growth of Co vacancy islands at step edges at room temperature: (a) STM image of region with small terraces with a vacancy island next to a step edge $(-860$ mV, 0.8 nA, 0.03 ML, RT); inset: region of surface with two hexagonal vacancy islands on a terrace (856 mV, 0.6 nA, 0.03 ML, RT). (b) Line profiles along the lines as shown in (a). The dashed lines indicate measured heights of an atomic layer of Cu(111). (c) Vacancy island depth distribution. (d) Vacancy island area distribution.

Figure 14

Comparison of island density and area for three growth temperatures: (a) Dependence of Co islands density $\rho$ on the terrace width $W$. (b) Normalized area histograms at step edges.