CHAPTER Chapter 5: Si magic clusters on Si(111)-(7x7) This chapter discusses the data obtained from STM which was used to probe the formation and structure of Si magic clusters on the Si(111)-(7x7) surface in UHV. The first part of this chapter focuses on the formation of Si magic clusters during the phase transformation from “1x1” to (7x7) reconstruction. When the Si(111) surface is quenched rapidly from 1200oC to room temperature, non-DAS cluster-like Si particles possessing uniform shapes and sizes are observed to be spontaneously formed during this process. These magic clusters appear to facilitate the propagation of (7x7) domains, where by excess Si adatoms on the “1x1” phase are removed and form magic clusters before being transported to the step edges. In the second part of this chapter, we will attempt to selectively grow Si magic clusters by depositing Si adatoms from Si solid source evaporator on the Si(111)-(7x7) and annealing the surface structure instead. By carefully controlling the substrate temperature, the structural evolution leading to formation of Si magic clusters is studied. We also probe the formation of various pre-cursor structures and a mechanism is proposed to account for their formation. We will show that the assembly of tetra-clusters leads to the formation of a magic cluster. By analyzing the shape and size of the cluster at different biasing conditions, the structures of both clusters are elucidated. 215 CHAPTER 5.1 Si magic clusters on Si(111): via heat and quench 5.1.1 Global Morphology As discussed in the literature review in Chapter 2, it has been reported that the Si(111)(7x7) reconstruction disorders at about 870oC into the “1x1” phase and reverts back into the (7x7) structure as the surface is cooled below this transition temperature. Localized metastable DAS structures have been observed during this phase transition. However STM observation of this process at such high temperatures is difficult due to a high thermal drift effect which renders inconsistency in scan frame capture. The imaging of the surface is thus best suited to room temperature scanning. However due to the high surface mobility associated with Si adatoms, the adatoms are likely to diffuse very quickly to the step edges which they will attach and integrate into, thus making characterization of surface adatoms difficult. In order to overcome this problem, the surface to be studied would have to be cooled very quickly in order to trap the adatoms on the surface. This trapping process is only possible when the time required to quench to low temperatures is shorter than the time for the Si adatoms to diffuse across the terraces to the steps. We first show the starting surface template in Fig. 5.1a, which is a 1000nmx1000nm large scan STM image of a clean Si(111) surface. This surface was obtained by flashing a Hterminated Si(111) substrate to 1200°C and cooled down to room temperature before STM scanning. The image shows large terraces (~ 300nm to 400nm wide) with step edges running along the direction. Zoom-in images of this surface shows well-ordered (7x7) surface reconstruction scanned under varying tunneling voltages, V=-2.0V (Fig. 5.1b) and V=+2.0V (Fig.5.1c). No surface features such as islands or clusters are observed on the terraces or at the 216 CHAPTER step edges. Hence we use this (7x7) reconstruction as the starting surface template, to generate the “1x1” phase by heating it to 1200oC before quenching rapidly to room temperature at various estimated cooling rates, in order to ascertain the most effective cooling rate as “1x1” transits into (7x7) phase. (a) (b) 3.0nm (c) 3.0nm 200nm Figure 5.1: (a) 1000nmx1000nm STM image of Si(111) flashed to 1200oC and scanned at room temperature and 15nmx15nm zoom-in images of clean Si(111)-(7x7) scanned at (b) -2.0V and (c) +2.0V. The clean Si(111) surface was flashed to high temperatures of 1200oC and subsequently cooled at different rates of ~ 1oC/min, ~ 50oC/min and 100oC/min to room temperature before being scanned using STM, as shown in Fig. 5.2(a), (b) and (c) respectively. 217 CHAPTER (a) (b) (c) Step edges Step edges Trails (i) Step edges 200nm Disordered “1x1” (ii) (7x7) 20nm Disordered Cluster (iii) (9x9) Cluster 6nm (7x7) (7x7) (5x5) (7x7) (2x2) (7x7) o Figure 5.2 shows STM images of Si(111) after flashing to 1200 C and cooled at (a) ~ 1oC/min, (b) ~ 50oC/min and (c) ~ 100oC/min in (i) 1000nmx1000nm (ii)100nmx100nm and (iii) 30nmx30nm scans. The 1000nmx1000nm scans are shown in Fig. 5.2a-c(i). Fig. 5.2a(i) and Fig. 5.2b(i) share similar global morphology which is dominated by flat terraces which are about ~300400nm wide, with step edges running in the direction. In contrast, Fig. 5.2c(i) shows large 218 CHAPTER triangular domains with trails leading from the domain apexes to the step edges on the terrace surface. Zoom-in 100nmx100nm observations of these respective surfaces are shown in Fig. 5.2a(ii), Fig. 5.2b(ii) and Fig. 5.2c(ii). No significant difference is seen between the two surface morphologies in Fig. 5.2a(ii) and Fig. 5.2b(ii), which show large terraces and step edges. However Fig. 5.2c(ii) shows large triangular (7x7) domains co-existing with bright regions of disordered “1x1” phase on wide terraces. A closer examination of the surface reveals well resolved cluster-like particles which appear bright and round in shape residing in the “1x1” regions while the (7x7) domains are clearly characterized by large and well ordered triangular domains showing well defined (7x7) unit cells. Zoom-in 30nmx30nm images are shown in Fig. 5.2a(iii), Fig. 5.2b(iii) and Fig. 5.2c(iii). Well ordered (7x7) reconstruction are observed to dominate the terrace surfaces with no other features present as shown in Fig. 5.2a(iii). This could be attributed to the slow cooling rate of ~ 1oC/min, which have allowed sufficient time for Si adatom diffusion. While Fig. 5.2b(iii) also shows long range (7x7) ordering, Si cluster-like particles are now observed on the surface, albeit accumulated at the step edges away from the terraces. This shows that while the cooling rate of ~ 50oC/min allows for the observation of these features, it is still sufficient for Si diffusion to the step edges resulting in the particles decorating the steps. Fig. 5.2c(iii) shows a considerable number of these same cluster-like particles existing on the disordered “1x1” phase, indicating that the cooling rate of ~ 100oC/min is quick enough to capture sufficient particle populations on the terrace surface. Further inspection of the surface shows initial ordering of Si adatoms among the disordered “1x1” phase and the presence of metastable DAS phases such as (5x5), (7x7) and (9x9) as well as non-DAS phases such as (2x2). It is interesting to note from the STM data, that 219 CHAPTER in addition to the meta-stable structures and single Si adatoms, the feature with the largest occurrence existing on top of the “1x1” phase appears to be cluster-like particles. We obtain the dimensions of each cluster-like particle by taking the average of STM line profile measurements to represent the estimated size and height of each particle. An example of the cluster size measurement is shown in Fig. 5.3(a). The separation across the area occupied by the bright maxima of the cluster is measured by line profile in directions along the direction. The average of these readings is the estimated diameter of each cluster. Similarly, the height difference measured from the peak of each cluster maxima to the neighboring (7x7) Si adatom peak is used to obtain the average height of each cluster, as shown in Fig. 5.3(b). The average cluster sizes are counted as shown in Fig. 5.3(c) and tabulated into a histogram showing the cluster size distribution for each scan (about ~ 100±5 clusters per scan). The statistical data collected from scans shows a narrow cluster size distribution with the largest occurrence of the estimated average cluster size to be 14.0±0.5Å. This information coupled with the STM observation of the same clusters consistently possessing a uniformly round shape suggests that each of these particles are magic clusters. This identification of the Si magic clusters is significant, as these clusters are consistently present during the surface evolution during (7x7) reconstruction domain growth from “1x1”. As the size of features observed under STM are sensitive to changes in the tunneling bias used during scanning, we will examine the shape and size of the Si magic cluster under varying tunneling biases in the following section. 220 CHAPTER (A) (a) 1.8 C 1.6 1.2 1.4 Z[Å] 0.8 0.6 1.5 0.8 0.6 0.4 0.5 0.4 0.2 0.2 0 [110] 1.6nm 1.2 Z[Å] A 1.4 Z[Å] B 0.5 1.5 2.5 0 0.5 X[nm] [112] 1.5 2.5 X[nm] A) 14.0Å 0.5 1.5 2.5 X[nm] C) 14.2Å B) 14.0Å (B) (b) 1.6 1.4 1.2 Z[Å] D 0.8 0.6 0.4 0.2 [110] 1.6nm 0.5 1.5 2.5 X[nm] [112] D) 2.1Å (C) (c) Number of Clusters 70 58 60 50 40 30 19 20 10 11 [110] 20nm 13 13.5 14 14.5 15 15.5 Cluster Size (A) [112] Fig. 5.3(a)-(b) is a 8nmx8nm scan showing (a) the measurement of the separation between corresponding bright maxima to obtain the average diameter of each cluster to be ~ 14.0±0.5Å (b) the average height of cluster obtained from the peak of the maxima to the trough of the neighboring “1x1” phase to be 2.1±0.1Å . Fig. 5.3(c) shows the tabulation of all measured average cluster diameters from a 100nmx100nm STM scan to show the cluster size distribution where the average size of 14.0±0.5Å has the highest occurrence. 221 CHAPTER 5.1.2 Dual Biasing STM Analysis: Si magic cluster from heat and quench (b) (a) Using the STM, we scan the Si(111) surface at room temperature which has been quenched from 1200oC , where the morphology is observed to comprise of ordered (7x7) and dis-ordered “1x1” domains. We focus the STM scan frame on the same area which shows a domain boundary with both (7x7) and “1x1” structures (Fig. 5.4). The scan frame is determined to be of the same area by the identification of consistent features such as the kink in the (7x7) domain boundary as indicated by the arrow in each scan frame. We vary the tunneling voltage of the scan from V=-1.8V to +1.8V, in steps of 0.2V, to capture a series of 30nmx30nm STM images of this area as shown in Fig. 5.4. The presence of the (7x7) reconstruction serves as a reference feature (See Chapter – Appendix 1), as the physical appearance of the unit cells change with respect to different tunneling voltages. The STM scans from V=-1.8V to -1.0V show the (7x7) reconstruction appearing as unit cells consisting of triangles with contrasting brightness. The “1x1” domain is clearly resolved to show a region of disordered small round dots with the same brightness as the (7x7) structures. The high resolution scans also pick up the other features residing in the ‘1x1” domain such as metastable DAS (9x9) structures and Si magic clusters which appear brighter than the background surface and (7x7) surface. It is interesting to note at this point that the Si magic clusters appear as bright blobs which are round in shape and found to be sitting on top of the “1x1” surface. 222 CHAPTER At voltage (V) = -0.8V, the “1x1” region appear to be slightly darker than the (7x7) domain, while no changes to the intensity of the other features were noted. From V=-0.6V onwards, the “1x1” region becomes gradually darker until it can no longer be resolved at V=0.05V and appears as dark patches on the STM scan. However the Si adatoms of the metastable DAS structures can still be observed albeit now at the same intensity as the (7x7) domains, which suggests that the DAS structures could be sitting on top of the “1x1” phase. It is also at this stage where the top layer Si adatoms of the (7x7) reconstruction begin to appear brighter which eventually leads to the appearence of (7x7) unit cells as hexagonal rather than triangular arrangements of Si adatoms. However the contrast difference between the Faulted and Unfaulted Halves of the unit cell is still discernable albeit less obvious than previously. It is observed that the Si magic clusters still remain as the brightest features in the scan frame and still retain the same round appearance as previously seen in higher negative voltage scans. When the tunneling voltage is switched over to low positive values of V=+0.05V, the surface appearance is observed to be similar to that when scanned at V=-0.05V. However, as the positive tunneling voltage is being increased from V=+0.2V to +1.0V, the previously dark patches of disordered “1x1” areas are being gradually resolved and begin to appear as faint small dots amidst the (7x7) and DAS structures which remain at the same brightness and appearance. The Si magic clusters not change in appearance and are still observed to be brighter than the other features, even though the brightness of the clusters is now reduced. 223 CHAPTER When the tunneling voltage is increased from V = +1.0V to +1.8V, the previously faint “1x1” regions now gradually become brighter and eventually regain the same intensity as it was first observed at V=-1.8V. In fact, the Si adatoms in the disordered “1x1” areas can now be resolved and appear to possess the same brightness as the (7x7) domains and DAS structures. While the DAS structures remain the same in appearance, the Si adatoms in the (7x7) reconstruction which are still seen to be arranged in the hexagonal configuration, now appear uniform in intensity without distinction between adatoms existing in either Faulted or Unfaulted unit cell halves. The Si magic clusters are observed to remain similar in shape and size, although the cluster intensity is lower than when scanned at V=-1.8V. In spite of the change in tunneling voltages, the Si magic clusters remain as the brightest features, while the other structures have been observed at similar intensities/brightness at some stage of scanning. This suggests that the clusters are likely to be the highest structures existing amongst the (7x7) and “1x1” domains. In order to study the clusters more closely, we use STM to zoom in onto one specific Si magic cluster which is circled in Fig. 5.4 and capture high resolution images as shown in Fig. 5.5 (cluster is identified by the arrow). In the series of images in Fig. 5.5, we scan the same Si magic cluster over the voltage range of V=-1.8V to +1.8V in steps of 0.2V. From voltage scans of V=-1.8V to -0.8V, the magic cluster is observed to be a bright feature possessing a round shape. When the tunneling voltages enter the V=-0.6V to +0.05V range, the Si magic cluster retain its shape but appear to be less bright compared to the previous voltage scans (V=-1.8V to -0.8V). When the STM continues the 224 CHAPTER A B 1.2 C 0.9 0.8 1.2 0.7 0.8 0.6 Z[Å] Z[Å] 0.8 0.6 Z[Å] (A) 0.6 0.4 0.4 0.5 0.4 0.3 0.2 0.2 0.2 0.1 [110] 2.0nm [112] 0.5 1.5 2.5 3.5 0.5 1.5 2.5 0.5 1.5 2.5 X[nm] X[nm] X[nm] A)13.5±0.5Å B) 13.5±0.5Å C) 13.5±0.5Å (B) 0.35 0.3 0.8 0.8 0.25 0.2 0.15 0.6 Z[Å] F Z[Å] Z[Å] D E 0.6 0.4 0.4 0.2 0.2 0.1 0.05 0 [110] 2.0nm 0.5 1.5 0.5 X[nm] 1.5 X[nm] D) 7.5±0.5Å [112] 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 X[nm] F) 5.7±0.5Å E) 5.7±0.5Å (C) G G HI 0.35 1.2 0.3 0.8 0.25 0.8 0.15 0 [112] 0.4 0.2 0.2 0.05 2.0nm 0.6 0.4 0.1 [110] 0.6 Z[Å] 0.2 Z[Å] Z[Å] IH 0.2 0.4 0.6 0.8 1.2 1.4 1.6 X[nm] G) 4.6±0.5Å 0 0.5 1.5 X[nm] H) 4.7±0.5Å 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 X[nm] I) 4.5±0.5Å Figure 5.18 shows line profile measurements of (a) average size of X4 to be ~ 13.5±0.5Å, (b) average separation of sub-unit clusters of X4 to be ~ 5.7±0.5Å and (c) average size of each subunit cluster to be ~ 4.6±0.5Å. 268 CHAPTER We now proceed to resolve the structure of the magic cluster, by considering (i) the symmetry and alignment cluster with respect to (7x7) template, (ii) close-packing and (iii) minimization of dangling bond exhibited by X2, X3, and X4 species. In Fig. 5.19, we summarize the measured average dimensions of the tetra-cluster features as indicated by the red dotted circles. Fig. 5.19a shows the size of X2 to be ~4.6±0.5Å. Fig. 5.19b shows the size of each individual cluster within X3 to be ~4.6±0.5Å. The X3 tetra-cluster pair is also shown to be aligned in the direction along the (7x7) unit cell boundaries with a separation of ~5.7±0.5Å. Fig. 5.19c shows the size of X4 scanned in negative tunneling bias to be ~13.5±0.5Å. Fig. 5.19d shows the size of each of the clusters to be ~4.6±0.5Å when scanned in positive tunneling bias. The tetra-clusters resolved within the X4 species in Fig. 5.19d is also observed to be arranged in an isosceles configuration with separations of 5.7±0.5Å, 5.7±0.5Å and 7.5±0.5Å. In order to rationalize the size of an X2 tetra-cluster, we co-relate the size occupied by a single tetra-cluster with a similar spatial area on the (7x7) DAS structure. When we scan the clean (7x7) surface under different biases ranging from -0.2V ≤ V ≤ +2.0V, the average size of the electron cloud associated with the Si adatom of the (7x7) reconstruction is shown to be ~4.6±0.5Å. This is evident from the line profiles taken from the 10nmx10nm STM scans of the (7x7) surface scanned under tunneling bias V=+2.0V and V=-2.0V as shown in Fig. 5.20a. As this electron cloud comprises of electronic state contributions from the Si atoms bonded directly below in the rest atom layer, hence the spatial area occupied by a Si adatom can be attributed to a Si atom sitting on top of atoms in a tetrahedral configuration. When we analyze 269 CHAPTER the (7x7) unit cell Dimer-Adatom-Structure (DAS) structure as shown in Fig. 5.20b(i) plane and b(ii) side view, we find that the observed area occupied by a X2 tetra-cluster (size~4.6±0.5Å) when super-imposed on the (7x7) unit cell as a dark dotted line coincides with the spatial area occupied by the Si adatom and the underlying Si rest-atoms. Hence we illustrate the structure of a tetra-cluster by a yellow ball sitting on top of blue ones occupying a spatial area of ~ 4.6 ±0.5Å in Fig. 5.20b(ii). In order to rationalize the spatial alignment and dangling bond minimization of the X2, X3, and X4 cluster species on the (7x7) surface, we also illustrate in Fig. 5.20b(iii), a surface template comprising of only rest-atom sites from a (7x7) Faulted Half unit cell. We identify the two positions which a tetra-cluster may occupy on this template, as; (i) Top Site (TS), where the tetra-cluster sits directly above a rest-atom or (ii) Hollow Sites (HS), where the tetra-cluster occupies the position between rest-atom sites as shown in Chapter Apendix-4. We study the different possible orientations of TS tetra-clusters as well as HS tetra-clusters and we expect tetra-clusters to prefer to occupy TS positions, as TS tetra-clusters tend to lead to a lower dangling bond density on the (7x7) surface structure (see Chapter Appendix-4). In the case of the X3 species, after considering the alignment and separation (~5.7±0.5Å) of paired tetra-cluster as well as dangling bond minimization, we determine that the configuration of adjacent TS clusters separated by ~ 5.7Å aligned along has the lowest dangling bond density of per half unit cell, after ruling out all other configurations which either have a higher dangling bond density or not fulfill the dimension considerations (shown in Chapter Apendix-4). Similarly, we find that the structure consisting of TS tetra-clusters 270 CHAPTER arranged in a closed packed isosceles formation with separations of 5.7Å, 5.7Å and 7.5Å is the best fit in describing the X4 species, as the other possible structures not meet the observed alignment and dimensions for the X4 species as shown in Chapter Appendix-4. In summary, the resolved structures of X2, X3 and X4 species consisting of tetra-clusters occupying top site positions are thus shown schematically as a ball and stick models in Fig. 5.21(a), Fig. 5.21(b) and Fig. 5.21(c) respectively. In particular, the Si magic cluster structure with a size of ~ 13.5±0.5Å comprising of TS tetra-clusters arranged in a closed packed isosceles formation with separations of 5.7Å, 5.7Å and 7.5Å is therefore determined to consist of n = 12 Si atoms. The ball and stick model proposed represents an attempt to elucidate the structure from consideration of dangling bonds and comparison of sizes of clusters observed from STM measurements. However the experimental observations are confined within the constraints of the STM measurements, as the existence of artifacts due to electronic effects could affect the accuracy of our size measurement. Nevertheless, we took caution during our scans by applying a slow scan speed through a large bias range to eliminate drift and electronic effects in ensuring topographical accuracy of our images. A first principle calculation of the magic cluster structure on a Si(111)-(7x7) surface template will clearly be useful in further confirming the cluster structure. 271 CHAPTER STM image Size(Å) Schematic (a) X2 4.6 0.5 4.6Å 2nm (b) X3 5.7Å 5.7 0.5 (c) X4 13.5 0.5 (d) X4 5.7Å 13.5 0.5 13.5 0.5 Figure 5.19 shows the measured dimensions of tetra-cluster features indicated by dotted circles in (a) X2 (size~4.6±0.5Å, height~0.6±0.2Å) (b) X3 (size~4.6±0.5Å, height~0.6±0.2Å), (c) X4 scanned in negative tunneling bias (V=-2.0V)(size~13.5±0.5Å, height~0.6±0.2Å) and (d) X4 scanned in positive tunneling bias (V=+2.0V)(size~4.6±0.5Å, height~0.6±0.2Å). These features are represented as dotted circles and superimposed onto (7x7) unit cell structures adjacent to the STM images. The proposed schematic models to describe the X2, X3 and X4 species occupying top site positions on the (7x7) unit cell are shown in the third column. 272 CHAPTER V = +2.0V (a) V = -2.0V Size Size 1.6nm Size ~ 4.6 0.5Å Size ~ 4.6 0.5Å (b) (i)Plane View Unfaulted Half Faulted Half - [110] [111] - 4.6Å [112] Dimer layer Rest-atom layer Adatom layer (ii)Side View Dangling bond (iii) Inclined view of rest-atom sites on Unfaulted Half Unit Cell (i) (i) 3.8Ǻ (ii) 5.7Ǻ (iii) (iii) 7.6Ǻ (ii) - - [110] [112] Figure 5.20(a) shows the 10nmx10nm STM image of a (7x7) surface scanned at V = +2.0V and V = -2.0V. Average size and height of Si adatom is measured to be ~4.6±0.5Å and ~1.2±0.2Å (from the dimer layer) for both scans. Figure 5.20(b) shows schematic diagram of the (7x7) unit cell in (i) plane and (ii) side views and (iii) inclined views indicating rest-atom sites. 273 CHAPTER (a) Unfaulted Half Hollow site Top site (b) TS 5.7Å Hollow site TS Top site (c) 5.7Å TS TS 5.7Å TS 7.6Å Figure 5.21 shows proposed schematic models of (a) X2, (b) X3 and (c) X4. 274 CHAPTER 5.3 Comparison between Si magic clusters on Si(111) and Si magic clusters on 6HSiC(0001) In this section, we will first compare the Si magic clusters generated via the different methods of heat and quench as well as growing from adatom deposition. We will be analyzing the formation process, dynamic behavior as well as cluster structure. Consequently, we will be applying this comparison with Si magic clusters observed on 6H-SiC(0001) as shown in Chapter 4. 275 CHAPTER 5.3.1 Si magic clusters generated via heat/quench vs via Si adatom deposition Si magic clusters are formed when Si(111)-(7x7) transits to “1x1” phase during annealing at high temperatures (>900oC). The origin of these clusters is attributed to excess atoms which are expelled from the “1x1” surface, and consequently form Si magic clusters. This is due to the difference of ~ 45 Si atoms between the “1x1” and (7x7) unit cell structures when the phase change occurs. In the second approach, Si magic clusters are grown from Si adatom deposition on Si(111)-(7x7) with progressive annealing to 150oC. STM data shows that pre-cursor tetracluster structures such as X2 and X3 cluster species combine to eventually form Si magic clusters. Further annealing of grown Si magic clusters leads to the nucleation of well ordered (7x7) islands We compare the characteristics of clusters formed from the methods as follows; • For cluster formed from heating/quenching there is no evidence of surface ordering. In contrast, clusters grown on Si(111)-(7x7), are observed to exhibit localized ordering on Faulted Half Unit Cell sites, after heating at 150oC. • In both cases, the Si magic clusters are essential features which facilitate surface phase formation such as “1x1” to (7x7) phase transformation and nucleation of (7x7) islands respectively. • The clusters formed from both methods are similar in shape and size, and hence share the same structure as shown by dual biasing STM data. 276 CHAPTER 5.3.2 Si magic clusters on Si(111) vs Si magic clusters on 6H-SiC(0001) As shown in Chapter 4, heating of 6H-SiC(0001)-(3x3) at 900oC leads to ejection of type “A” clusters (tetra-clusters) from the surface, which combine to form larger type “B” clusters. These larger clusters in turn self organize into (6x6) arrays at 1000oC. As these clusters are stable (retain shape and size) at higher temperatures, hence they are described as Si magic clusters. We compare Si clusters on both Si(111) and 6H-SiC(0001) in the following points; • Formation of both types of clusters is similar, as they are formed from excess Si adatoms expelled onto the surface when the substrate is heated to high temperatures. In the case of Si(111), it is due to surface transition from a higher atomic density structure to a lower one coupled with transition from ideal tetrahedral Si-Si bonding to DAS bonding characteristics (i.e. “1x1” to (7x7)). For SiC, it is due to agglomeration of tetra-clusters which are ejected from (3x3) due to lattice mis-match and non-ideal bonding conditions. • There are also similarities in that both Si magic clusters grown on Si(111) from Si deposition as well as Si magic clusters on SiC, nucleate via the assembly of Si tetraclusters. Consequently each Si tetra-cluster structure functions as an essential building block unit in the assembly of a Si magic cluster albeit on different surface templates. This is interesting as Si atoms prefer to exist as tetra-clusters and not as individual adatoms when participating in localized surface re-structuring. However these tetra-clusters are not considered “magic clusters” as they appear to be meta-stable and eventually form larger and more stable Si magic clusters. 277 CHAPTER • The occurrence of Si magic clusters on both Si(111) and SiC also coincide with phase transformations of Si(111) and 6H-SiC(0001). This shows that Si magic clusters play a significant role in mediating phase transformations on different Si surfaces, such as; (1) Si magic clusters facilitate the surface phase transformation of (3x3) to (6x6) structures on 6H-SiC(0001); (2) Si magic clusters assist in the transition of “1x1” to (7x7); (3) Si magic clusters diffuse on Si(111) to nucleate the epitaxial growth of (7x7) islands. • Si magic clusters are observed to exist with (6x6) periodicity in localized domains on SiC surface and not appear mobile. When the surface is heated to progressively higher temperatures, there is little evidence to suggest cluster diffusion while XPS data in fact shows loss of Si from the SiC surface resulting in the formation of a Si deficient ring-like surface structure. Si magic clusters grown on Si(111)-(7x7) also exhibit localized ordering on Faulted Half Unit Cell sites, after heating at 150oC. However as annealing of this surface to higher temperatures leads to (7x7) island nucleation, the clusters on Si(111) are therefore more mobile in contrast to the clusters on SiC substrate. The observation of mobile Si magic clusters on Si(111)-(7x7) from heating/quenching are also seen from evidence of cluster trails leading from the apexes of the triangular “1x1” domains to the step edges. However the observations of Si loss on SiC as opposed to cluster diffusion were not unexpected, as the annealing temperatures of SiC were much higher than Si(111) and also closer to temperatures associated with Si bond breaking and adatom loss from the surface. 278 CHAPTER • While clusters on both surfaces are round in shape, however the clusters on SiC have slightly larger average sizes as shown in Table 5.2. In fact the varying of tunneling bias for clusters on S(111) shows single bright particles (negative bias) resolved into smaller features (positive bias), similar scanning of the clusters in SiC does not reveal any change in the physical appearance. While these clusters also comprise of pre-dominantly Si adatoms there is insufficient evidence to indicate that the same similarities exist in terms of structure akin to the Si magic clusters on Si(111)-(7x7). Cluster Type Cluster size (-ve bias) Cluster shape (-ve bias) Tetra-cluster size (+ve bias) Tetra-cluster separation (+ve bias) Clusters from Clusters from heating/quenching deposition 14.0±0.5Å 13.5±0.5Å Si Clusters on 6HSiC(0001) 14.3±0.5Å round round round 4.5 ±0.5Å 4.8±0.5Å - 5.7 ±0.5Å 5.7 ±0.5Å - Table 5.2 shows the average dimensions of respective Si magic clusters taken from line profile measurements. 279 CHAPTER 5.4 Summary In this chapter, we have studied the formation and structure of Si magic clusters on Si(111)-(7x7) in UHV. We first form Si magic clusters from heating and quenching of Si(111). These clusters possess a uniform round shape and an average size of ~ 14.0 ± 0.5Å, and are formed from excess Si adatoms expelled from the surface during“1x1”→ (7x7) phase transformation at high temperatures. These Si magic clusters are also mobile and participate in the mass transport of Si thereby facilitating the removal of excess Si from the “1x1” regions and consequently allowing the propagation of the (7x7) domains. Hence the formation of Si magic clusters is critical to the “1x1” to (7x7) transition as they function not only as features accommodating the excess atoms during structural change but also as vehicles in facilitating mass transport during (7x7) domain nucleation. However this method of generating magic clusters is not ideal as it is difficult to establish control over spatial ordering of these clusters because of the surface disordering during phase transformation. Hence in the second part of this chapter, we address how to selectively fabricate stable monodisperse magic clusters from Si adatoms deposited on (7x7)-Si(111) by; (i) Demonstrating the self assembly of Si magic cluster via the use of an atom source instead of a cluster source. In doing so, we avoid growth issues related to inconsistent cluster size and shape distribution typically attributed to the use of cluster source techniques. 280 CHAPTER (ii) Demonstrating the stability and self assembly phenomena of Si magic clusters to form spatially well–ordered cluster arrays at low temperatures ([...]... 0 .50 X[nm] D 0 .50 0.0 Z[Å] 0 .50 Z[Å] 0.0 Z[Å] 0 .50 C 0.0 B 1.0 A 1.0 1 .5 (A) 1.0 CHAPTER 5 1.0 1 .5 2.0 2 .5 0.0 X[nm] A) 4 .5 0 .5 0 .50 1.0 1 .5 2.0 X[nm] B) 4 .5 0 .5 C) 4 .5 0 .5 E 0.0 0 .50 Z[Å] 0 .50 0.0 Z[Å] 1.0 1.0 1 .5 1.6nm 0.0 0 .50 1.0 1 .5 2.0 0.0 0 .50 X[nm] D) 4 .5 0 .5 (B) F 1.0 1 .5 2.0 X[nm] E) 4 .5 0 .5 G 1.4 1 0.8 H 1.2 0.8 1 0.4 0.6 Z[Å] Z[Å] Z[Å] 0.6 0.4 0.8 0.6 0.4 0.2 0.2 0 0 1.6nm 0.2 0 0 .5. .. 0 0 .5 1 1 .5 2 0 0 X[nm] F) 5. 7±0 .5 0 .5 1 1 .5 X[nm] G) 5. 7±0 .5 2 0 0 .5 1 1 .5 2 X[nm] H) 7 .5 0 .5 (C) 4 .5 (101) (110) 14.0Å Figure 5. 7(a) shows line profile analysis of the smaller sub-units resolved within the Si magic cluster (A), (B) and (C) to have an average size of ~4 .5 ±0 .5 each, and the average size of the neighboring Si adatoms of the “1x1” region to be also ~4 .5 ±0 .5 Figure 5. 7(b) shows... function of annealing temperature 244 CHAPTER 5 5.2 Selective growth of Si magic clusters on (7x7)-Si(111) 5. 2.1 Formation of Si Magic Clusters From the previous section, we demonstrated that we are able to grow Si magic clusters spontaneously from Si adatoms popping up from Si(111) surface via heating and quenching treatment Hence in this section, we will now explore if we can selectively grow Si magic. .. we cannot conclude whether Si pops up as single adatoms and form tetra -clusters /magic clusters or pop up as Si tetra -clusters and form magic clusters Therefore we will address these questions regarding the formation mechanism by studying the progressive formation of Si magic clusters from deposited Si adatoms on Si(111)-(7x7) instead of heating/quenching the Si(111)-(7x7) surface in Section 5. 2 The observed... analysis of the average separation between each smaller sub-unit resolved within the Si magic cluster to be ~5. 7 ±0 .5 , 5. 7 ±0 .5 and 7 .5 ±0 .5 Figure 5. 7(c) shows a schematic of the Si magic cluster 231 CHAPTER 5 5.1.3 Real Time STM: Formation of Si magic clusters In order to probe the surface structural changes as “1x1” phase transforms to (7x7) phase, fast scanning STM (5 sec/ frame) was used to image... sub-unit clusters arranged in an isosceles triangle with an estimated separations of ~5. 7 ±0 .5 , ~5. 7±0 .5 and ~7 .5 ±0 .5 as illustrated in Fig 5. 7(c) We also establish that the size of the Si magic cluster is estimated to be ~14.0 ±0 .5 and exits as a single round entity when imaged in negative biases We will use this information to elucidate the structure of the Si magic cluster sitting on a (7x7)... each Si magic cluster consists of 3 smaller clusters of size ~4 .5 , hence Si adatoms could pop up from the “1x1” surface as Si tetra -clusters and consequently agglomerate into the larger Si magic cluster species (size~14 .5 ) comprising of several Si tetra -clusters in a formation mechanism similar to the nucleation of Si clusters on SiC [5- 6] However as we are unable to observe this formation mechanism... Therefore these observations suggest that Si magic clusters play an important role in facilitating the “1x1” to (7x7) phase transformation Since excess Si adatoms can pop up onto the surface and form Si magic clusters, hence it would be interesting to see if we can selectively grow magic clusters from Si adatoms deposited on (7x7) surface, in the following section Chapter 5. 2.1 242 CHAPTER 5 (a) S te i) 1 min... excess Si adatoms pops onto the “1x1” surface as “1x1” → (7x7) and forms Si magic clusters on the surface 4) Si magic clusters diffuse along trails towards step edges as (7x7) domains propagate 5) Magic clusters dissociate and incorporate into step edges thereby perpetuating step edge growth 6) Hence formation and diffusion of Si magic cluster is a critical step in the nucleation and growth of (7x7)... the 100nmx100nm image (Fig 5. 10(c)(vi)), trails comprising of Si magic clusters running across well ordered (7x7) reconstruction and leading towards the step edges From the STM observation, these cluster trails are only discernable when the “1x1” bright areas are sufficiently small enough Once again, Si magic clusters are not found on the (7x7) terraces but exist only along the trails observed By estimating . 4 .5 0 .5 C) 4 .5 0 .5 D) 4 .5 0 .5 E) 4 .5 0 .5 H 0 0 .5 1 1 .5 2 0 0.2 0.4 0.6 0.8 1 X[nm] Z[Å] 0 0 .5 1 1 .5 2 0 0.2 0.4 0.6 0.8 X[nm] Z[Å] F) 5. 7±0 .5 G) 5. 7±0 .5 H) 7 .5 0 .5 (A) (B) 0 0 .5 1 1 .5. 1 .5 X[nm] Z[Å] A) 4 .5 0 .5 0.0 0 .50 1.0 1 .5 2.0 2 .5 0.0 0 .50 1.0 X[nm] Z[Å] 0.0 0 .50 1.0 1 .5 2.0 0.0 0 .50 1.0 X[nm] Z[Å] 0.0 0 .50 1.0 1 .5 2.0 0.0 0 .50 1.0 1 .5 X[nm] Z[Å] 0.0 0 .50 1.0 1 .5 2.0 0.0 0 .50 1.0 X[nm] Z[Å] B). magic cluster to be ~5. 7 ±0 .5 , 5. 7 ±0 .5 and 7 .5 ±0 .5 . Figure 5. 7(c) shows a schematic of the Si magic cluster. 1.6nm F 1.6nm A CB D E G 0.0 1.0 2.0 3.0 0.0 0 .50 1.0 1 .5 X[nm] Z[Å] A) 4 .5 0 .5 0.0