temperature of 570 K. The formation of Co-Ge alloy indicates that some Ge atoms have exchanged with Ag atoms. Thus, the depth of Ag atoms is deeper than the one before Co-Ge alloy formation, which suggests a decreasing in Ag Auger intensity.
Fig. 3-1-8 The Auger intensity of Ag and Co in related to the annealing temperature. [236]
As the temperature increases above 650 K, the islands with periodicity transform to those with periodicity. From the previous studies, one believes that Co islands with periodicity are higher than those with periodicity that reduces the area of Co islands [222-225]. Thus, there are more Ag atoms revealed on the topmost layer than at the
3-1 Thermal evolution of Co on the coexisting …
39
annealing temperature of 570 K. It results that the Ag Anger intensity reaches the amount similar to the one pre-annealing.
Ag/Ge(111)- and Ag/Ge(111)- domains were observed by LEED after Co diffusion into bulk at the temperature of 830 K, however, their boundaries located differently comparing to those before Co deposition. Fig. 3-1-9 and Fig. 3-1-10 respectively show the process of Co interaction with Ag/Ge(111)- phase and Ag/Ge(111)- phase. Although the formation of islands commonly occurs in both phases, it involves different underlying processes according to each condition. The interpretation presenting above demonstrates the stages of Co interacting with Ag- or Ag- surface. Below, we briefly encapsulate the proposed scenario, which is specific to the phase of Ag/Ge(111) surface.
(i) Co on Ag/Ge(111)- surface (Fig. 3-1-9): Firstly, the deposited atoms form clusters on the surface (a). Co atoms are prevented from diffusing into bulk by the intermediate Ag layer.
When the temperature increases, Co clusters commence to nucleate (b). Secondly, annealing the surface, Ge atoms locally exchange with Ag atoms to form the alloy with Co atoms. This reaction leads to the creation of CoxGey alloy islands, which is proved by the observed
periodicity on the top and the drop of Ag AES intensity (c). Thirdly, Ag and Ge atoms exchange again so that the CoxGey- islands are transformed into Co- islands.
The new-formed islands are higher, whereas their lateral dimensions are smaller comparing to the counterparts (d). As the islands experience thermal degradation, the Co atoms diffuse into the Ge bulk (e). Further temperature increase leads to the gradual surface deterioration due to the fact that Ag atoms desorbed from the surface. Finally, the whole surface will be converted into the Ge(111)-c surface, from which Co atoms are not detectable (f).
Fig. 3-1-9 The model of 1.0 ML Co coverage on Ag- surface.
The annealing temperature rising from 300 K to 930 K. [236]
Ag- Ge
Ge Ge Ge Ge Ge
CoxGey- Co- Ag atoms
Co atoms
a c d e f
T:300 K T:420 K T:570 K T:650 K T:830 K T:930 K
b
3-1 Thermal evolution of Co on the coexisting …
41
(ii) Co on Ag/Ge(111)- surface (Fig. 3-1-10): Firstly, the deposited Co atoms form clusters on the surface (a), as the case previously discussed. However, in contrast to the example on Ag/Ge(111)- surface, the intermediate Ag- layer does not prevent the deposit from diffusion inward because half of the phase is composed of Ge atoms only. Thus, a certain number of Co atoms dissolve into bulk as the temperature rises.
Meanwhile, Ag atoms were removed from the beneath of Co atoms, which leads to a partial conversion of the phase to the ( phase (b). Secondly, further annealing the surface, Ge atoms exceed the layer of Ag atoms to form CoxGey- . It is to suggest that the Ag atoms were forced to move to other locations at the topmost layer, which could cause the phase transformation from Ag- to Ag- . In other words, at this stage, it is the formation of CoxGey- leads to transformation from Ag- phase to Ag- phase (c). Thirdly, Ag and Ge atoms exchange again so that CoxGey- islands are transformed into Co- islands (d). As the islands encounter thermal degradation, the increasing number of Co atoms diffuses inward through the Ge bulk. Finally, the surface is transformed back to
Ag- phase (e). The Ag atoms desorbed from the surface, forming Ge(111)-c surface again.
Fig. 3-1-10 The model of 1.0 ML Co coverage on Ag- surface.
The annealing temperature is rising from 300 K to 930 K. [236]
Ge Ge Ge Ge Ge Ge
a b c d e f
T:300 K T:420 K T:570 K T:830 K T:930 K
Ag-
CoxGey- Co-
Ag atoms Co atoms Ag-4
T:650 K
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
43
3-2 Structure of Co- nanoislands on Ag/Ge(111)- surface [238]
Fig. 3-2-1 (a) nm2 STM image showing the coexistence of - and - islands grown on Ag/Ge(111)- surface. (b) nm2 STM image of hexagonal-shaped Co(11-20)- island of 0.4 nm height. (c) nm2 STM image of stripe-like Co(11-20)- island of 3.7 nm height. In (b) and (c), the angles between the atomic rows are given. [238]
Observing the image in Fig. 3-2-1 (a), one notices the desirable reconstructions showing at the top of most Co islands grown on Ag/Ge(111)- surface. Although the unwanted reconstruction is also perceived, the amount of which
a b
c
CoxGey- Co-
is insignificant. According to the previous observation, the structure of the Ge(111) surface significantly influenced the structural properties of the Co islands grown on Ag/Ge(111)- surface [222]. It is the strong coupling between the Co atoms and the Ge(111) surface that contribute to the phenomenon. Thus, it is reasonable to neglect the influence of the Ag layer in the following consideration. Based on the condition, we propose a model for the Co- phase growth on Ge(111) surface.
For Fig. 3-2-1 (a), the Co- islands adopt either hexagonal or stripe-like shapes. Fig. 3-2-1 (b) and (c) demonstrate an insight into the inner structure of the individual island. Roughly inspecting Fig. 3-2-1 (b), the hexagonal island seems to contain unit cell similar to that of Ge(111)- surface in parallelogram shape.
The phenomenon indicates that the substrate surface strongly influences the structure of the growing island. However, studying the image carefully, one notices that the angle of the intersection between the island-rows running in the directions shown in Fig.
3-2-1 (b) is appeared to be larger than the typical of Ge(111)- surface (i.e., 70° vs. 60°). The example represented in Fig. 3-2-1 (c) even demonstrates the intersection angle at 77°.
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
45
Further analyzing a number of images, we are confirmed that the above-mentioned discrepancy is distinct for the stripe-like islands, which are generally higher than their hexagonal counterparts. These observations have important implications on modeling the island growth because we can then speculate the island growth that involves a distortion of the island rows through referring the substrate surface rows. In addition, the degree of the distortion enhance with the increasing island height. Consequently, as the shape of the island unit cells changes noticeably (e.g.
transforming from parallelogram to rectangular), island itself is undergoing a process of inner structure alterations. In other words, as their height grown, the islands are undergoing an evolution, transforming the form hexagonal-shape into stripe-shape. Based on the ideas mentioned above, we focus on the stripe-shaped islands as more evolved phase in an attempt to construct the model demonstrating the growth of the Co- islands.
To ascribe a specific crystallographic structure to the Co- islands, it is important to compare Co- islands with typical bulk Co, which is in hcp structure. From the previous researches, one knows that Co layers are also in hcp form when which is grown on metallic surface [190-191]. However, it was demonstrated that
the Co layer can grow epitaxially on the Au(111) surface in the fcc structure when it is prepared under special conditions [192-194].
Moreover, Tonner et al. have shown that it is the different thickness of the deposited films influencing the established structure of Co layers [195]. From the observations and the previous studies, it is to believe that both fcc and hcp are possible structure of Co layer.
Although the initial Co layer growing on clean Cu(111) surface proceeds in fcc phase, it will transform from the original structure to hcp structure as the film thickness increases beyond two layers [195].
Table 3-2-1 demonstrates the lattice parameters and the value of interlayer spacing from low-index planes with Co in bcc, fcc, and hcp structure respectively, as well as those from Ge(111)- surface. Comparing the lattice parameters of the substrate and the various Co surface phases, we identify the Co-hcp(11-20) and the Co-bcc(111) planes by noticing their smallest lattice mismatch. To recognize the exact phase, it is a must to take account of the unit cell shapes displayed in Table 3-2-1 with those shown in the STM images (see Fig. 3-2-1). Realizing the fact that the rectangular-shaped unit cells of hcp(11-20) plane and the typical shape of the evolved Co- islands are similar, we conclude that
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
47
the islands are oriented by hcp(11-20) plane rather than bcc(111) plane.
Fig. 3-2-2 Schematic diagram of hcp structure. (a) Numbered balls represent atoms which belong to successive atomic layers, perpendicular to [11-20] crystallographic direction. The open circles refer to the layers with odd numbers, while the filled circles represent those with even numbers. (b) The top view of hcp(11-20) structure, the stacking sequence is ABABAB…. [238]
[0 0 0 1]
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
49
Fig 3-2-2 (a) illustrates a schematic diagram for the hcp structure. The lattice parameters of the Co(11-20)- unit are measured 407 pm along [0001] direction, and 434 pm along [1-100]
direction. Observing the fact that the value for the interlayer spacing (125 pm) is by far lower than the lattice parameters (407 pm and 434 pm), it is to suggest that there is a bigger chance that the islands form bilayered structure than single-layered construction.
Considering the (11-20) surface, one notices from the schematic diagram that the stacking sequence is ABABAB…: one is represented in the first, the third, and the fifth layer as viewed from the top (layer A), whereas the other is demonstrated in the second, the fourth, and the sixth layer (layer B) (see Fig. 3-2-2 (b)).
To verify the assumption, it is a must to study the line profiles.
Firstly, we measure which along the Co- islands randomly chosen from a number of large-scaled STM images that were acquired under similar tunneling conditions. Secondly, based on their height, we number the islands in increasing order, starting the lowest. In case which two or more islands are measured with the same height, we ascribe them according to their successive numbers. From the obtained data, one notices that the majority of
islands have the equal height of either 320 or 535 pm (see Fig.
3-2-3). To proof the bilayered structure of Co- islands, we propose the strategy applying the discrepancy between the measured and the theoretical height. When the imaging objects are different in conducting properties, inconsistency occurs between the measured height (i.e., the discrepancy of 320 and 535 pm, 215 pm) and the theoretical height of the bilayered Co-(11-20) (i.e., 250 pm) due to the tip-caused effects. Moreover, one finds the above-mentioned issue even more critical in case of a metal/semiconductor system.
0 100 200 300 400
0 500 1000 1500 2000
Height (pm)
Island number
Fig. 3-2-3 Distribution of height within a population of 500 Co- islands. [238]
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
51
Fig. 3-2-4 Schematic diagrams of Co-(11-20) planes. The squares represent the positions of Co atoms in bilayers. The open circle corresponds to electron clouds of groups of four neighboring atoms. The pink filled circles refer to a group of six atoms in lower bilayer, and the yellow filled circles are the group in upper bilayer.
(a) Array of parallelogram-shaped Co- unit cells. Direction of row shift is marked with arrows. (b) Array of rectangular Co- unit cells. (c) Array of parallelogram Co- unit cells against a background of Co- lattice. Two different directions for the island growth are marked with numbered arrows. [238]
60∘
As proposed above, Ge(111)- surface is acting as a template for Co island. Fig. 3-2-4 (a) represents the imagined configuration of the surface, which is an array of parallelogram-shaped unit cells. The unit cells of Co island adopt the shape of the substrate surface in their early growing stage. Yet, referring to the substrate surface rows, the growing island rows will undergo a distortion as they evolved (see Fig. 3-2-4 (a), the direction of the distortion is shown by arrows). Eventually, the unit cells of Co island will adapt to the rectangular shape. Fig. 3-2-4 (b) shows the schematic structure of the resultant Co- . The proposed model (i.e., Co- ) shows no correspondence in the periodicities of Co island (i.e., Co- ). However, referring to the resent findings, it is to believe that under metal/semiconductor epitaxial system, the protrusions appearing on the STM image corresponds to the sites where the electron clouds of atoms group concentrated, rather than representing the individual atoms [239].
To identify the sites, we consider two alternative positions. The yellow filled circles represent the sites at which the electron clouds of six neighboring atoms group are likely to accumulate, whereas the open circles refers to the alternative site at which the electron clouds of four neighboring atoms group accumulated (see Fig. 3-2-4
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
53
(c)). From the image, one observe that the open circles have a lower probability to be detected by the tip, as compared to the yellow filled circles. Therefore, in case when the open circles do not show on the STM image that the protrusions form a distinct pattern, it is because the represented sites are enclosed by the other position, which is indicated by the yellow filled circles.
The arrangements of atoms are the same in the first bilayer (AB) and in the second bilayer (AB) because the stacking sequence of hcp(11-20) surface is ABABAB…. One may suppose that, in STM image, the protrusions in the first bilayer overlap the one in the second bilayer. However, we shall emphasize on the fact which the protrusions in the first bilayer do not overlap the one in the second bilayer as seen in Fig. 3-2-5. Namely, the atomic positions of the sites that the electron clouds are accumulated are not identical in the successive bilayers (11-20).
Observing Fig. 3-2-4 (c), the pink filled circles represent the accumulated sites for the electron clouds of six neighboring Co atoms group positioned in the lower bilayer. Due to charge repulsion, the positions of the electron clouds of similar atoms group shift from the topmost bilayer (represented as yellow filled circles) to the sites, enclosing by the pink filled circles. In addition,
one notice that there are two different directions for the island growth (see Fig. 3-2-4 (c), denoted as 1 and 2). Growing along direction 1, the rows in the topmost bilayer are consistent with respect to those in the underlying bilayer. For direction 2, however, the topmost and the underlying bilayers do shift with respect to each other.
Fig. 3-2-5 contains illustrations of the resolved STM images of the Co(11-20)- islands grown along direction 1 (see Fig. 3-2-5 (a)) and direction 2 (see Fig. 3-2-5 (b)), as well as their corresponding schematic diagrams (Fig. 3-2-5 (c) and (d)). For Fig.
3-2-5 (a) and (b), the rows in the topmost bilayers are indicated by black lines, whereas those in the underlying bilayers are suggested by white lines. Regarding Fig. 3-2-5(c) and (d), filled and open circles represent the protrusions in the topmost and underlying bilayers respectively. Observing the images, one notice a lateral shift between the atomic rows in the topmost and the underlying bilayers as the islands grow along direction 2, whereas no shift is perceived as the islands grow along direction 1.
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
55
Fig. 3-2-5 (a) nm2 STM images showing Co-(11-20)- islands growing along the pseudo-[1-100] direction, and (b) the pseudo-[1-100] direction. The black lines represent the rows in the topmost bilayers while the white lines refer to the rows in the underlying layers. Corresponding schematic diagrams are shown in (c) and (d), respectively. The filled circles refer to the protrusions in the topmost bilayer, whereas the open circles represent the atomic protrusions in the underlying bilayer. [238]
From statistical analysis, it is to understand that 90% of the Co(11-20)- islands grow along direction 2. The phenomenon indicates that there is least lattice inconsistency between the newly grown island and its substrate growing along direction 2. Based on
a b
c d
our observation, the amount of lattice misfit between the unit cell of the rectangular-shaped Co(11-20)- and the Ge(111)-1 is 2% when the islands grow along the direction [0001], and 9% when which grow along the direction [1-100]. From the obtained results, we assign direction 2 as [0001] direction, whereas direction 1 as [1-100] direction. In addition, since ideal rectangular shape is not observed in the unit cells of the growing island, we ascribe the two directions to pseudo-[0001] direction and pseudo-[1-100]
direction.
3-2 Structure of Co-2×2 nanoislands on Ag/Ge(111)-√3×√3 surface
57
Fig 3-2-6 (a) STM image showing the Co-(11-20)- island which is composed of three bilayers. (b) The differentiated image of (a). Lateral shifts between the successive bilayers indicate that the island grew along the pseudo-[0001] direction. [238]
1st bilayer [0001]
2nd bilayer
3rd bilayer a
b
So far, we have focused on the Co- islands composed of two bilayers. For further understanding, one must also consider those Co- islands constituting multi-bilayered systems observed in the STM images. Fig. 3-2-6 shows the island composed of three bilayers. To obtain more detailed information about the islands growth, we study their structure appearing at the boundary between the successive bilayers with the focus on the grown direction of each bilayer. One observe a lateral shift at the boundary between the first and the second bilayers, which indicates that the bilayers have grown at the pseudo-[0001] direction. Moreover, the similar lateral shift has been found also at the boundary between the second and the third bilayers. Based on the information mentioned above, it is to believe that there exists only one preferential direction for a particular island to grow. Having analyzed a number of similar multi-bilayered islands, we conclude that it is correct for Co(11-20)- islands that are grown on Ge(111) surface.
3-3 Strain-relaxation in the structure of Co-2×2 islands on …
59
3-3 Strain–relaxation in the structure of Co- islands on Ag/Ge(111)- surface [240]
Fig. 3-3-1 (a) An empty-state STM image (Vs=0.5 V, size=
nm2). Areas enclosed with square boxes are shown in details in (b) and (c). (b) A typical image of Co- island. (c) STM image of the - surface shows the honeycomb structure. In (b) and (c), the marks indicate the crystallographic directions along which line profiles were measured. [240]
a
c b
Fig. 3-3-1 (a) is a typical empty-state STM image obtained after annealing Ag/Ge(111)- surface with Co of submonolayer coverage at 670 K. From the image, one observes the islands coexisting with those in reconstruction. In the inter-island area, a defect-free surface structure is fully conserved, which indicates that there is no chemical interaction occurs between the deposits and the surface.
As suggested in our earlier paper, one must consider the influence from the substrate surface exhibited on the structure of the resultant islands, even though it is known that there is no chemical reaction between Co atoms and the substrate surface at the annealing temperature ranging from 320 K to 730 K [222-225].
One knows the fact that CoxGey- is formed of Co-Ge alloy and it is lower than its counterparts, Co- in height. Based on the idea, CoxGey- islands are expected to adapt the atomic compactness of the substrate
By analyzing a number of the CoxGey- islands regardless of size, we realize a lattice constant that is fixed to the amounts of 1.44 nm, the value similar to which of the substrate surface. However, for the Co- islands, which are higher than CoxGey- islands, we expect less influence from the