The Ge(111) surface, prepared under the conditions described in the previous section, shows the tendency to display the c(2 × 8) domains of different orientations in coexistence with small domains of local 2 × 2 and c(4 × 8) symmetry. After deposition of 0.1 ML Ni onto the surface (Figure ), we can observe the formation of brightly imaged clusters. The clusters accumulate predominantly at the boundaries between either the different domains which exist on the surface or the different c(2 × 8) orientations (see inset in Figure ). The abundance of the clusters is also seen at the edge separating the terraces, implying that the RT mobility of Ni is not negligible. The inner parts of domains are covered by clusters at a smaller degree. A closer inspection reveals that most clusters are surrounded by dark holes in the substrate which indicates that even at RT, metallic adsorbate reacts with Ge. The formation of Ni-induced structural defects in semiconductor surfaces has been widely reported in the literature of the subject, e.g., [20
Figure 1 Empty-state STM image showing the formation of clusters after Ni deposition onto Ge(111)-c(2 × 8) surface at RT. The initial Ni coverage is approximately 0.1 ML. The image size and bias voltage are 80 × 80 nm2 and 1.5 V, respectively. (more ...)
Figure shows the Ag/Ge(111)-√3 × √3 surface with 0.1 ML Ni deposited at RT. Here, clusters seem to be randomly distributed without concentrating at the terrace edges, which indicates that the surface diffusion of the species at RT is suppressed. In the area between the clusters, a defect-free √3 × √3 structure is clearly resolved (see inset in Figure ) which suggests that there is no chemical reaction between the deposit and the surface. Therefore, we argue that the clusters are composed of pure Ni atoms rather than Ni-Ge compounds.
Figure 2 Filled-state STM image taken after deposition of 0.1 ML Ni onto Ag/Ge(111)-√3 × √3 surface at RT. The image size is 80 × 80 nm2, and the bias voltage is -1.6 V. Inset: small-scale (24 × 22 nm2) image showing that (more ...)
Annealing the surfaces with deposited materials within the range from 470 to 770 K results in the appearance of a variety of objects. While most of them appear only on either Ni/Ge(111)-c(2 × 8) surface (Figure ) or Ni/Ag/Ge(111)-√3 × √3 surface (Figure ), some structures commonly form on both of them (Figure ).
Figure 3 STM images showing Ni-induced structures on Ge(111)-c(2 × 8) surface. (a) Ring-like defects in single and trimer configurations. Inset: 7 × 7 nm2 filled-state image taken at a sample bias of -0.6 V, showing ring-like defects. (b) 2√7 (more ...)
Figure 4 Empty-state STM images showing Ni-containing structures on Ag/Ge (111)-√3 × √3 surface. (a) Triple-hole defects which appear after annealing between 470 and 570 K. (b) Long islands (enclosed by circles) which appear after annealing (more ...)
Figure 5 Empty-state STM images showing Ni-containing structures. (a) Hexagonal island on Ge(111)-c(2 × 8) surface. (b) Hexagonal island on Ag/Ge(111)-√3 × √3 surfaces. (c) 7 × 7 island on Ge(111)-c(2 × 8) surface. (more ...)
First, we focus on the structures typical of the Ni/Ge(111)-c(2 × 8) surface. They are presented in Figure along with proposed schematics of the structural models. The models are drawn on a background of the Ge(111)-c(2 × 8) lattice. Figure a is a small-scale empty-state STM image showing ring-like defects. By analyzing a number of images, we have found that the structures emerge in single, dimer, or trimer configuration. In an attempt to explain the origin of the structures, we shall recall that ring-like clusters frequently develop after annealing the Si(111) surfaces containing trace amounts of Ni [1
], Co [3
], and Fe [6
]. Depending on the adsorption system, the authors ascribed the rings to precursors to either metal-induced reconstruction of the substrate surface or metal-containing islands which grow on the substrate surface. The ring-like defects, however, were not reported on the Co/Ge(111)-c(2 × 8) surface [10
By referring the STM image to the structural model of the Ge(111)-c(2 × 8) (Figure a), we notice that the rings are likely to represent missing Ge adatoms. In filled-state images, however, the rings are brighter in contrast to the substrate. This effect is particularly distinct for the sample bias -0.6 V at which no local density of states exists for the Ge(111)-c(2 × 8) surface (see inset in Figure a). This observation leads us to conclude that the ring-like defects are more likely to belong to Ni atoms sitting at Ge atom positions rather than represent missing adatoms.
Besides the ring-like defects, annealing the Ni/Ge(111)-c(2 × 8) surface produces flat-topped islands with atomically resolved corrugations, forming a 2√7 × 2√7 pattern (islands enclosed with solid circles in Figure b) and a 3 × 3 pattern (in Figure b, the island enclosed with a dotted circle). The islands typically have a height within the range from 0.15 to 0.2 nm and adopt approximately triangular, hexagonal, and trapezoidal shapes. However, a few islands are observed with irregular shapes. The islands with the 3 × 3 are observed at higher densities as compared to their counterparts. The distances between the islands and ring-like objects as well as their location on the surface are random. More detailed features of the different islands are shown in the insets in Figure b as well as in Figure c. We shall notice that both islands have empty-state images markedly different from the filled-state ones. This indicates that the islands have semiconducting properties rather than metallic. The unit cell of the 3 × 3 structure is aligned parallel to the c(2 × 8) unit cell (right part of Figure c) while that of the 2√7 × 2√7 structure is rotated by an angle of 19.1° to the c(2 × 8) unit cell, as illustrated in Figure b.
Figure shows structures which grow on the annealed Ni/Ag/Ge(111)-√3 × √3 surface, but do not appear on the Ni/Ge(111)-c(2 × 8) surface. After annealing the surface above 470 K, numerous dark holes appear in the surface (Figure a). Interestingly, some of them are housing rather unusual objects: triangular islands which contain triangular-shaped protrusions in each apex. We refer to them as triple-holes and speculate that they contain Ni. After annealing the surface above 670 K, large islands with elongated shapes (hereafter long islands) develop in coexistence with the triple-holes. Some long islands are enclosed by circles in the large-scale image in Figure b, and an example island is zoomed in the left part of Figure c. It is seen that the edges of the long islands are aligned in three different directions, i.e., [-101], [1–10], and [01–1], indicated in the schematic diagram of the approved structural model of the Ag/Ge(111)-√3 × √3 surface (Figure c, lower right part).
Figure shows structures which are commonly observed on the Ge(111)-c(2 × 8) and Ag/Ge(111)-√3 × √3 surfaces. One group includes three-dimensional hexagonal-shaped islands with no distinct pattern at their tops (Figure a,b). The other group contains islands with a 7 × 7 pattern (hereafter 7 × 7 islands) and somewhat triangular shape (Figure c,d).
Figure summarizes STM images of the Ni/Ge(111)-c(2 × 8) (top of Figure ) and Ag/Ge(111)-√3 × √3 surfaces annealed within the range from 470 to 770 K (bottom of Figure ). The hexagonal-shaped islands and those with the 7 × 7 reconstruction are common, but the others are typical of individual surfaces: ring-like structures, the 2√7 × 2√7 islands, the 3 × 3 on the Ni/Ge(111)-c(2 × 8) vs. triple-holes and long islands on the Ag/Ge(111)-√3 × √3. A brief description of the individual structures is presented above. The notations for the structural phases are indicated in Figures ,,. Below, we encapsulate our observations in terms of the thermal evolution of the surfaces:
Phase diagram for Ni/Ge(111)-c(2 × 8) and Ni/Ag/Ge(111)-√3 × √3 along with corresponding STM images. The notations for the structural phases are indicated in Figures ,,.
1. Ni/Ge(111)-c(2 × 8) surface. Even at RT, deposited Ni atoms react with the substrate forming Ni-containing clusters. When the temperature reaches 470 K, the reaction proceeds to create Ni-containing islands with the 2√7 × 2√7 and 3 × 3 reconstructions as well as the ring-like defects. At 670 K, in addition to the latter structures, the hexagonal and 7 × 7 islands appear here and there within the c(2 × 8) matrix. An increase in temperature causes the hexagonal islands to grow in size at the expense of all other types of islands. Finally, at 770 K, only the hexagonal islands remain on the surface. In the inter-island area, the ring-like features are clearly resolved.
2. Ni/Ag/Ge(111)-√3 × √3 surface. At RT, Ni nucleation is determined by the formation of clusters. At around 540 K, the triple-holes and the 7 × 7 islands commence forming. The latter are also observed to form on the Ni/Ge(111)-c(2 × 8) surface but at a higher temperature. After annealing at 670 K, the hexagonal and the long islands form in coexistence with all above-mentioned structures. It is likely that the clusters which were initially trapped in the triple-holes develop into regular islands upon annealing. The islands grow in size with the increase in temperature at the cost of 7 × 7 islands. Finally, at 770 K, the hexagonal and long islands coexist with the triple-holes.
The formation of defects, differing in appearance (i.e., the ring-like defects on the Ge(111)-c(2 × 8) surface vs. the triple-hole defects on the Ag/Ge(111)-√3 × √3 surface), indicates that the mixing between Ni and Ge proceeds on both surfaces through different mechanisms. Generally, however, the presence of 1 ML Ag on the Ge(111) surface retards the inter-diffusion between Ni adatoms and Ge substrates, at least at temperatures below 670 K. This is why the formation of the Ni-containing 2√7 × 2√7 and the 3 × 3 islands is prevented on the Ag/Ge(111)-√3 × √3 surface.
By analyzing a number of images taken after annealing at the final temperature, we have found that the total volume of islands is several times greater than the volume which should be expected from the amount of deposited Ni. This means that Ni reacts with Ge atoms to form Ni-containing islands, perhaps the long islands and/or the hexagonal islands.
The formation of the long islands indicates that the Ag/Ge (111)-√3 × √3 surfaces provide Ni, Ge, and NixGey clusters with a lower surface diffusion energy. As a result, the formation of the long islands takes place only on the Ge(111) surface with an Ag buffer layer.