Comparison of the Co growth behavior on the Ag/Si(111)-√ × √

Before the Co is evaporated on the surfaces, the Ag-√3 × √3 surface is prepared on the Si(111)-7×7 and the Ge(111)-c(2×8) surfaces. Whatever the Ag-√3 × √3 structure on the Si(111)-7×7 or the Ge(111)-c(2×8) surface, Co atoms disperse on the Ag-√3 × √3 surface at RT as shown in Figure 4-20(a) and (f). According to the serial images, it is apparent that the Ag/Si(111)-√3 × √3 surface shows the complex morphology due to the silver replacement effect compared to the Ag/Ge(111)-√3 ×

√3 surface. Even upon annealing up to 200^oC, there is no obvious nucleation reaction of Co clusters on the Ag/Ge(111) surface. However, Co clusters on the Ag/Si(111) surface tend to diffuse to the boundaries of the upper and the lower Ag-√3 × √3 layers. The ratio of Co clusters on the boundaries and the terraces of the Ag-√3 × √3 surface (i.e. AreaCo on the boundaries/AreaCo on the terraces) is about 2.2 at RT, and about 2.7 at 100^oC. They are almost equal. However, the ratio obviously increases up to about 6.2 after annealing to 200^oC. The sharp variation shows that Co clusters are provided with enough energy to overcome the diffusion barriers on the Ag/Si(111)-√3 × √3 surface

-500 0 500

during the 200^oC annealing process. When annealing temperature is higher than 300^oC, Co clusters diffuse and nucleate to form the 2D islands on the Ag/Ge(111) surface. For the Co/Ag/Ge(111) surface, the STM images are shown in Figure 4-20(h) and (g). As annealing temperature is elevated, Co islands begin to integrate other islands into the larger islands. While annealing temperature reaches 500^oC, Co 2D islands become higher and larger on the Ag/Ge(111) surface. For the Co on the Ag/Si(111) surface as shown in Figure 4-20(c) to (e), the sizes of Co clusters also becomes bigger as annealing temperature is increased. The average height of the islands increases from a 0.06  0.01 nm in Figure 4-20(c) to a 0.31  0.14 nm in Figure 4-20(e). The small height 0.06 nm is realized from two factors. One is low Co coverage (0.02 ML). The other is that Co atoms are inserted into the centers of the Ag or Ge trimers resulting in the lower height (0.06 nm) than one Co layer (0.2 nm).

Further, increasing Co coverage to a 0.3ML on the Ag/Si(111)-√3 × √3 surface still leads to the cluster formation after annealing to 500^oC as shown in the inset image of Figure 4-20(e); the average height of the clusters rises to about 0.8 nm. Therefore, the stable Co shapes are clusters (3D islands) and 2D islands on the Ag/Si(111)-√3 × √3 and the Ag/Ge(111)-√3 × √3 surfaces, respectively. We choose the Co with low coverage absorbed on the Ag/Si(111)-√3 × √3 surface because it is easy to analyze the growth behavior of Co clusters such as the favorite diffusion positions. The topmost layers of the 2D islands of the Co on the Ag/Ge(111)-√3 × √3 surface are constructed with periodic structures of the √13 × √13R14° and 2×2 reconstructions of the 2D island surfaces with different heights discussing in section 4.2.¹⁰⁰ In additon, the structure of the Ag buffer layer is also important. If the Ag on the Ge(111) surface dose not form a stable √3 × √3 reconstruction, the intermixing of the Co and the Ge can occur even at low annealing temperatures.

Figure 4-19. Two serial

What mechanism results in different growth behavior of the Co on the Ag/Si(111)-√3 × √3 and the Ag/Ge(111)-√3 × √3 surfaces? Submonolayer Ag on the Si(111) and the Ge(111) surfaces are studied to answer this question.

Figure 4-21. (a) A submonolayer Ag on the Si(111)-7×7 surface constructs a local

√3 × √3 layer (Vs = -1.7 V, 14.1×12.3 nm²). The height profile is taken along the L-L’ line. The height is 0.08  0.01 nm between the original Si adatoms level and the higher or the lower Ag-√3 × √3 layers. (b) the Ag/Si(111)-√3 × √3 surface (1.0 ML Ag, Ubias = -1.0 V, 100.0×100.0 nm²). (c) A submonolayer Ag on the Ge(111)-c(2×8) surface construct a local √3 × √3 layer (Ubias = +2.0 V, 14.1×12.3 nm²). There are the 4×4 and the 3×1 reconstructions of the Ag on the Ge(111)-c(2×8) surface as indicated by the dimmer triangle regions.⁸¹

Figure 4-21(a) shows that a submonolayer Ag on the Si(111) surface constructs a fractional √3 × √3 layers after annealing to 500^oC. From the height profile along the L-L’, the height is 0.08  0.01 nm between the original Si adatoms layer and the higher Ag-√3 × √3 layer or the lower Ag-√3 × √3 layer. This phenomenon reflects that Ag atoms replace the original Si adatoms to from the upper and the lower √3 × √3 layers mechanism of which is described in detail in section 4.1.⁷⁶ Here, we find that the replacement process occurs in the local small regions individually. This is why the boundaries of the Ag-√3 × √3 surface on the Si(111) surface are so irregular. Figure 4-21(b) shows a large scale image of the Ag/Si(111)-√3 × √3 surface. The topography of the Ag/Si(111)-√3 × √3 surface is very lumpy. On the contrast, a

Ag-√3 × √3 surface is directly formed on the Ge(111) surface after annealing to 300^oC as shown in Figure 4-21(c). We cannot find different heights of the Ag-√3 ×

√3 layers which indicate the replace mechanism occurring. In addition, the Ag/Ge(111) surface has the 4×4 and 3×1 reconstructions simultaneously according to the phase diagram of low Ag coverage as indicated in the dimmer triangle regions of Figure 4-21(c).¹⁰¹

Figure 4-22(a) shows the detailed boundary structure of the Ag/Si(111)-√3 × √3 surface. Two important features are noted. One is that the boundaries along the boundaries of the Ag-√3 × √3 islands are brighter than other Ag-√3 × √3 regions.⁷⁷ This character can be also found in Figures 2-8(b) and (c), especially the boundaries with the greater local curvature. The other is that the structures of the neighboring domains A and B as shown in Figure 4-22(a) cannot become identical by translating their unit cells, but by the half unit cell according to the white line. It means that the boundaries can be classified as antiphase boundaries (APBs).^79,102 When the bias is positive, the boundaries appear bright. While the bias is negative, the boundaries appear dark as shown in Figure 4-20(b) and (c).⁷⁸ This voltage dependence is dueto the unsaturated states from the silicon atoms.

Figure 4-22. STM images of the Ag/Si(111)-√3 × √3 surface. (a) with the two Ag-√3 × √3 domains A and B (Ubias = +2.0 V, 9.0×9.0 nm²).⁸¹ (b) and (c) show the boundaries appearing bright or dark at +1.0 V and -1.0 V, respectively. The image sizes of (b) and (c) are 30.0×30.0 nm².

The APBs on the Ag/Si(111)-√3 × √3 surface with the unsaturated states prefer to trap Co atoms. From section 4.1, it knows that the absorbed cobalt is great influenced

Domain B

Domain A

by the APBs rather than the necks after the higher annealing temperatures. In general, the atom diffusion barriers at near steps may be higher than at the terraces.¹⁰³ Therefore, the binding force is stronger on the boundaries than on other regions.

Especially for the boundaries with the greater local curvature, there is a stronger trapping ability for Co clusters. After annealing to 500^oC, the ratio of Co clusters occupying the APBs is about 14 times than the terraces on the Ag-√3 × √3 surface.

The favorite growth behavior is to form the 3D islands and the favorite growth positions are on the APBs of the Ag/Si(111)-√3 × √3 surface for Co clusters.

Figure 4-23(a) shows a special structure for the Co/Ge(111) surface after annealing to 400^oC. It is very similar to the Co/Ag/Ge(111)- √13 × √13R14° reconstruction as shown in Figure 4-23(b). Their unit cells have the same √13 × √13 periods and the side lengths are equal to 1.44 nm as indicated by the black tetragon in Figure 4-23.

Further, the four corners of their unit cells also appear brighter. Therefore, we believe the special structure is the same as the Co/Ag/Ge(111)- √13 × √13R14°

reconstruction. However, the present probability of the Co/Ge(111)-√13 × √13R14°

reconstruction to be observed is very lower, and hence, the LEED pattern does not show the reconstruction spots. Therefore, the Co-Ge alloy is almost formed on the Ge(111) surface, but for Co/Ag/Ge(111) surface all Co atoms easily construct the

√13 × √13R14° reconstruction just after the 300^oC annealing without the Co-Ge alloy according to the dark defect-liked character. Therefore, the Ag-√3 × √3 layer indeed retards the formation of the Co-Ge alloy efficiently. In addition, the Ge(111) substrate also plays an import role to influence the growth behavior of Co atoms. The Co/Ge(111)-√13 × √13R14° reconstruction is an evidence.

Figure 4-23. (a) and (b) show the unit cell of the Co-√13 × √13R14°reconstruction as indicated by the black tetragon for the Co/Ge(111) (0.6 ML Co, U_bias = +1.0 V) and the Co/Ag/Ge(111) (0.35 ML Co, U_bias = +2.0 V) surfaces, respectively. The image sizes are 15.0×15.0 nm².⁸¹

Co clusters start to diffuse on the Ag/Si(111)-√3 × √3 surface after annealing to 200^oC. However, the obvious diffusion motion of Co clusters on the Ag/Ge(111)-√3 × √3 surface is found only after annealing to 300^oC. Therefore, the diffusion barrier of the Co is greater on the Ag/Ge(111)-√3 × √3 surface than on the Ag/Si(111)-√3 × √3 surface. The reason is that there is a stronger coupling of Co atoms with the Ge(111) surface than with the Si(111) surface. From Figure 4-23(a), the Co-√13 × √13R14° reconstruction clearly shows that periodic surface structure of Co islands is influenced by the Ge(111) surface. As annealing temperature becomes above 300^oC to provide enough energy to enable an ascending motion, Co islands grow up higher to decrease the surfaces free energy. The surface free energy of the Ag (1.3 J/m²) is only half that of the Co (2.71 J/m²).^80,82 Therefore, the increasing areas of the Ag-√3 × √3 surface can decrease the surfaces free energy of the whole surface.

For the Ag/Si(111)-√3 × √3 surface, the surface has many APBs with the extra electron states which attract Co atoms to form the clusters. The clusters grow up after annealing above 300^oC. This behavior also results in the surface free energy going down because of the enlarging areas of the Ag surfaces. Therefore, the rich boundaries of the Ag-√3 × √3 surface and the unsaturated states of the APBs result in different growth behaviors of the Co on the Ag/Si(111)-√3 × √3 surface versus the Ag/Ge(111) -√3 × √3 surface.