CHAPTER CHAPTER 4: Role of Si magic clusters in the phase transformation of 6H-SiC(0001): (3x3) →(6x6) clusters→(6x6) rings This chapter discusses the data obtained from STM and XPS which was used to probe the local atomic structure of the 6H-SiC(0001) surface in UHV. The evolution of surface structures is observed with progressive annealing; starting from (3x3) phase at 850oC → Si rich (6x6) clusters at 1000oC → (6x6) rings at 1100oC. This chapter will also include STM analysis of the various surface features such as defects, tetramer formation as well as tetramer agglomeration leading to the formation of the various surface structures. The data is co-related with XPS information in order to propose a mechanism for the observed phase transformation. 147 CHAPTER 4.1 Global morphology of 6H-SiC(0001): STM and XPS The SiC(0001) sample is prepared in–situ and ex-situ as described in Chapter before being annealed for 30min at various temperatures and scanned correspondingly using the STM and XPS. This section focuses on the global morphological evolution as the surface is progressively annealed. Figure 4.1a shows the STM image of the surface after initial deposition of Si at room temperature. The surface is characterized by three-dimensional islands, which appear to be round in shape, and possesses an average island size and height of 50±10.0nm and 20.0±5.0nm respectively. On annealing the surface to 800°C for 30min (Figure 4.1b), the island density decreases while the average island size increases. Flat terraces as shown in Figure 4.1c are beginning to form when the surface was further annealed to 850°C. At this stage, annealing the surface progressive to higher temperatures of 900°C (Figure 4.1d), 950°C (Figure 4.1e), 1000°C (Figure 4.1f) and 1050°C (Figure 4.1g) results in the formation of increasingly larger terraces respectively. STM line profile analysis across adjacent terraces reveals the average step height to be in multiples of 15Å, which is the height of a 6H-SiC structure in the c-direction. This indicates formation of an ordered surface with progressive annealing. 148 CHAPTER (a) (b) (c) (e) (f) 200nm (d) (g) (c)(i) 10nm (f)(i) 10nm (g)(i) 10nm Figure 4.1: A series of 1000nmx1000nm STM images of the global surface morphology after Si was deposited at (a) room temperature and after annealing to temperatures of (b)800°C, (c)850°C, (d)900°C, (e)950°C, (f)1000°C and (g) 1050°C respectively. Zoom in 50nmx50nm images of (c), (f) and (g) shows (c)(i) (3x3) reconstruction, (f)(i) (6x6)clusters and (g)(i) (6x6)-ring-like structures respectively. While the global morphology of the surface annealed from 850°C to 1050°C (Fig. 4.1c-g) shows flat terraces and is not visibly different, the microstructure revealed by high resolution STM images (see Fig. 4.1(c)(i), (f)(i) and (g)(i)) show the surface undergoing significant atomic re-arrangement from the Fig 4.1(c)(i)(3x3) phase → (f)(i) 149 CHAPTER clusters (6x6) → (g)(i) ring-like (6x6) structure. In the following sections we will discuss the (3x3) reconstruction first observed at 850oC followed by the self assembly of (6x6) clusters at 1000oC leading to the eventual formation of the ring-like (6x6) structure at 1050oC. Figure 4.2 shows the XPS trend from the initial deposition of Si up to annealing at 1050°C corresponding to the surface morphology shown in Figure 4.1. Si-C at 103.0eV Intensity(Arbitrary Units) Elemental Si at 99.3eV 1050°C 1000°C 950°C 900°C 850°C Deposit Si As-received 104 102 100 98 96 Binding Energy (eV) Figure 4.2: The evolution of the Si 2p XPS peak (i.e. elemental Si component at 99.3eV and the Si-C component at 103.0eV) after annealing the substrate to different temperatures. 150 CHAPTER The XPS spectrum of the Si 2p peak consists of elemental Si and Si-C signals at 99.3eV and 103.0eV respectively. The peaks were normalized with respect to the Si-C signal from the as-received wafer. It is evident that with progressive annealing to higher temperatures, the Si-C signal increases while the elemental Si signal decreases. The Si-C content is thus greater due to the increasing presence of Si-C bonding at the surface. This trend could be a consequence of excess Si desorbing from the surface and/or a reaction between the deposited Si and bulk C to seed the growth of 6H-SiC(0001) polytype. Both processes would lead to the formation of flat terraces, which have step heights in multiples of 15Å. As it is well known that annealing at elevated temperatures (>1200oC) often induces graphitization, the desorption of Si is expected to eventually dominate. It should be noted in the present study that no graphitization was detected and the surfaces observed are still in the Si rich regime. 151 CHAPTER 4.2 6H-SiC(0001)-(3x3) reconstruction STM images of the SiC(0001) surface after annealing at 850°C for 30min are shown in Figures 4.3a and 4.3b below. The images obtained are of the same surface area at albeit at different sample bias of +3.0V and -3.0V for Figure 4.3a and 4.3b respectively. (a) [2110 ] [1210] (i) (i) 9.03Å (ii) 9.07Å Figure 4.3: A 40nmx40nm STM scan of the SiC(0001)-3x3 surface at sample bias of (a) +3.0V and (b) -3.0V. Inset figures show the (3x3) surface at higher magnification (12nmx12nm) in both respective biases. Line profiles illustrate the 3x periodicity of the surface. 152 CHAPTER The (3x3) reconstruction was imaged as bright protrusions in the empty state and as small dark depressions or bright rings in the filled state. The difference in appearance is further highlighted in the respective inset figures. The separation between each adjacent bright protrusion or between neighboring dark depressions measured along the [2110] and [1210] azimuths is ~ 9.0Å. This periodic arrangement is 3x the SiC(0001)(1x1) unit cell (lattice parameter =3.08Å). These observations may be explained using the (3x3) structural model proposed by J. Schardt et al [1]. Figure 4.4b shows the ball and stick models corresponding to the plane view and side view of the surface reconstruction. The (3x3) reconstructed surface is believed to result from arrangement of Si-tetra clusters. These Si tetra-clusters are described as having one Si adatom sitting above a Si trimer structure in the second layer. The Si tetra cluster is positioned above a full Si adlayer which is arranged on top of the Si terminated 6H-SiC(0001) substrate. The (3x3) reconstruction as shown in Figure 4.4a would consist of one tetra-cluster per unit cell. All the Si atoms are four fold coordinated except for the top Si adatom in the tetra-cluster, which has a single dangling bond. The bright protrusions observed in empty state imaging on the (3x3) surface could therefore be attributed to tunneling into the empty states associated with the single dangling bond on each of the top Si adatom. The observation of the rings with depressions in the center under filled state imaging would therefore be attributed to contribution from the back bonding of the 2nd layer Si trimer and third layer Si adlayers. 153 CHAPTER 1.6 1.4 (a) [2110 ] 1.2 Z[Å] (i) 5.0±0.5Å 0.8 0.6 0.4 [1210] 0.2 0 0.5 1.5 2.5 X[nm] (ii) 5.0±0.5Å Z[Å] 1.5 0.5 0 0.5 1.5 2.5 X[nm] 2.4nm 1.4 1.2 (ii) 5.0±0.5Å Z[Å] 0.8 0.6 0.4 (b) 0.2 0 5.0±0.5Å (b) 0.5 1.5 2.5 X[nm] 3.08Å 1.5Å I II IV [2110] [1210] III [1010] [0001] [2110] First layer: Si adatom with one dangling bond Second layer: Si trimers Third layer: Si adlayer in (1x1) position Fourth layer: Si atoms in top layer of bulk Fifth layer: C atoms Figure 4.4: (a) A 8nmx8nm STM image of (3x3) surface structure after annealing at 850°C. Line profiles analysis shows that the (3x3) tetra clusters are about 5.0±0.5Å in size and 1.5 ± 0.5Å in height. (b) A plane and side view of the ball and stick model illustrating the (3x3) surface structure [1]. 154 CHAPTER Figure 4.4a shows a high-resolution 8nmx8nm STM picture of the (3x3) reconstruction. Line profiles (i) and (ii) as indicated in the STM image measures the average full width half maximum size of a protrusion in the (3x3) unit cell to be ~ 5.0 ± 0.5Å while line profile (iii) shows the average height of the protrusions to be ~ 1.5 ± 0.5Å. These average scan sizes were obtained from the line profile measurements of the same features scanned under different biases in different directions. The plane view of the surface as illustrated in Figure 4.4b shows the structure with the Si-adlayer atoms occupying bulk SiC positions and the Si atoms within the tetra-cluster having a Si-Si bond lengths of ~2.3Å [1]. In this configuration, the Si tetra-cluster would have a diameter 5.0 ± 0.5Å and a height of 1.5 ± 0.2Å. This size is equivalent to the measured average size of each protrusion occupied by one Si tetra-cluster in the (3x3) unit cell as labeled in Figure 4.4a. 155 CHAPTER 4.3 6H-SiC(0001)-(6x6) clusters 4.3.1 Formation of (6x6) clusters from the (3x3) reconstruction This section discusses STM data of the 6H-SiC(0001) surface evolution as the starting (3x3) surface is progressively annealed from 850ºC to 1050°C. A series of ball and stick diagrams are used to illustrate the formation of the various surface features at the corresponding annealing temperatures. Figure 4.5a(i) shows a 40nmx40nm STM image of the (3x3) surface after annealing to 850ºC for 10 minutes. The surface shows long range ordering of (3x3) reconstruction on the surface with low defect density. At higher resolution (Figure 4.5a(ii)), the STM data shows the appearance of recessed protrusions existing amongst the bright protrusions, previously attributed to the Si tetra-clusters. Line profile measurements of these recessed protrusions reveal an average depth of only ~ 0.6 ± 0.2Å. The recessed protrusion is likely to be caused by a missing Si adatom from the tetracluster, as illustrated in the corresponding ball and stick diagram shown in Figure 4.5a(iii). The remaining underlying Si atoms exposed would therefore contribute to the “shallow” appearance in the STM image. These defects are identified as “shallow” holes. Subsequent annealing of this surface to 900ºC results in the formation of two distinctive defects. Figure 4.5b(i) shows that the surface is characterized by regions of depressions (dark features) and clusters (bright features) coexisting with the (3x3) reconstruction. The bright clusters, which are identified as “A”, are observed to be round 156 CHAPTER Motivated by the above observations, we will attempt to propose a mechanism, taking into consideration the role of Si atoms in the form of M x Si tetra clusters (where M is the number of clusters) as a building block, to describe the structural transformation observed. As we have already shown that 8x Si tetra-clusters are involved in the formation of the (6x6)-cluster phase [6], we will only describe how the formation of the ring structure and the occurrence of type A, B, C, D1 and D2 features develop as we heat the SiC(0001)-(6x6) surface from 1000oC to 1100oC in the next section. We will show that by selectively moving M x units from the structural models at each respective annealing temperature, we will be able to account for the basic morphological features observed in this regime and hence deduce the minimum number of Si atoms required to reproduce the basic structural framework of a ring feature and its (6x6) periodicity on the SiC surface. 199 CHAPTER 4.4.5 Structural model for cluster mediated formation of (6x6) rings In order to form the final (6x6) ring structure, we will need to remove at least a total of M=19 Si tetra clusters (76 Si atoms) per unit cell from the initial (6x6) cluster structure. The process by which this occurs is shown schematically in Figures 4.19, 4.20 and 4.21. The ball and stick models presented are closely based on the STM data in terms of physical dimensions measured and the structural evolution observed as the surface is annealed from 1000°C to 1100°C. When the surface is annealed to 1030°C, disordering of the (6x6) cluster phase takes place while the formation type “A”, “B” and “C” features are observed (Figure 4.14(i)). In order to account for this observation, we remove M=7 tetra-clusters per unit cell in a staggered manner from the original (6x6) cluster arrangement (Figure 4.19(i)). In doing so, we obtain the schematic diagram in Figure 4.19(ii) which depicts a surface morphology consisting of residual type “B” clusters and a majority of type “A” clusters and “C” features which we have identified as the Si adlayer atoms (i.e. 3rd layer Si adlayer of the U. Starke model). In fact, the reduction in area coverage of the type “B” clusters also exposes the underlying type “C” features which is synonymous with the STM observation in Figure 4.14(i). The complete removal of the M=7 tetra clusters from each unit cell would result in a unit cell consisting of M=1 tetra cluster and the Si adlayer (type C feature) as shown in either Figure 4.19(iii) or Figure 4.19(iv). We note that this structure has not been observed experimentally. Instead, the STM data obtained at 1050°C shows formation of a shallow ring structure with a hole in the centre coexisting 200 CHAPTER with type A clusters and type C feature. The size of the hole is ~ 15.5±0.5Å and depth is ~ 5.0±0.5Å (Figures 4.15 and 4.16). In order to account for this ring size, we will need to remove from Figure 4.19(iii) or Figure 4.19(iv) another 20 Si atoms or M=5 tetra-cluters. This shallow ring structure can be generated by removing the Si atoms from either two hexagonal regions between the type “A” clusters, as shown in Figures 4.19(iii) and (iv) respectively. Hence, we will introduce two possible routes by which loss of Si atoms from the surface occurs and also show that the two pathways will lead to the formation of an identical (6x6) ring structure involving the same number of tetra-clusters. 201 CHAPTER (i) Type “B” cluster [2110] (ii) Type “C” cluster [1120] (6x6) unit cell Type “B” cluster Type “A” cluster 10Å (iii) (iv) Figure 4.19: A plane view of the ball and stick model illustrating the surface structural evolution as a function of temperature. Figure 4.19(i) shows the (6x6) cluster phase consisting of type “B” clusters at 1000°C. Figure 4.19(ii) illustrates the staggered removal of M=7 tetra clusters per unit cell from the surface in Figure 4.19(i), leading to the formation of a surface consisting of type “A”, “B” and “C” features, corresponding to STM images at 1030°C. Figure 4.19(iii) and (iv) show the surface after the complete removal of M=7 clusters. Two regions from which 20 Si atoms are removed to generate the shallow ring structure as highlighted respectively in Figure 4.19(iii) and (iv). 202 CHAPTER 4.4.5.1 Loss of Si via pathway-1 The loss of Si atoms via the first route would require removing 20 Si atoms which are highlighted as shown in Figure 4.19(iii). This would generate a shallow ring like structure with a diameter of ~ 15.5Å and depth of ~5.3Å. The removal of these 20 Si atoms will also expose the underlying Si atoms (i.e. 4th layer Si atoms of the U Starke model) and they will also be bordered by the remaining Si atoms of the original 3rd layer. In doing so, preferential sitting of the type “A” clusters at the corner sites of each ring structure will also be observed. This ring structure with a shallow depression is indicated by the dotted hexagon as shown in Figure 4.20(i). The resulting structure is consistent with the surface topography as imaged by the STM shown previously in Figure 4.15(i). Proceeding further, we will need to account for the prevalent existence of type “C” Si atoms which form the D1 and D2 triangular features. These features are in turn arranged into ordered networks which generate the resultant (6x6) ring structure as observed at 1100oC. In order to describe these structural characteristics, we will need to remove another 28 Si atoms (M=7) from the unit cell. The following Si atoms are removed; (1) Si atoms (M=1) i.e. type “A” cluster which existed at the corner sites as shown in Figure 4.20(i) is removed. This would give rise to the surface structure shown in Figure 4.20(ii), which is consistent with the absence of the type “A” clusters in the STM data at this stage; (2) In order to account for the observation of D1 and D2 features, we selectively remove Si atoms per unit cell from Si atoms from the 3rd and 4th layer (M=1) as indicated in Figure 4.20(ii). In doing so, the remaining 3rd layer Si adatoms 203 CHAPTER would form the boundary of the ring structure shown in Figure 4.20(iii). By following registry of the SiC (1x1) structure, the arrangement of remaining Si adlayer adatoms would form equilateral triangular structures of sizes with side lengths measuring ~ 3.08Å and ~ 9.3Å respectively as outlined by dotted triangles in Figure 4.20(iii). These dimensions match closely with the measured lengths of the D1 (size~3.1±0.5Å) and D2 (size~9.0±0.5Å) triangular features in the STM scans. By arranging these two features in an alternating hexagonal structure, would result in a hexagonal ring structure with an average uniform diameter of ~ 15.5Å that follows the (6x6) periodicity; (3) Finally, to account for the vertical profile analysis earlier in Figure 4.16, we will need to remove another 20 Si atoms (M=5) from the exposed 4th layer of Si atoms as indicated in Figure 4.20(iii). This would give a ring structure as shown in Figure 4.20(iv) with a depth of ~ 3.5±0.5Å measured against the peak of D1 and D2 features. A total of 19 tetra-clusters is thus involved. 204 CHAPTER (i) (ii) 15.5Å (iii) 4.4.7.2 (iv) Loss of Si via pathway-2 Figure 4.20(i)-(iv) describe the progressive removal of M=19 clusters per unit cell to obtain the final (6x6) ring structure through pathway-1. Figure 4.20(i) shows the surface after removal of M=7 clusters. The 20 atoms highlighted per unit cell are removed from the 3rd and 4th layer of the U. Starke structure (M=5) to generate the resultant shallow ring structure with ring size~15.5Å at 1050°C as indicated in Figure 4.20(ii). Figure 4.20(ii) also shows the highlighted type “A” clusters which are removed (M=1) to account for their absence in the STM images. Figure 4.20(iii) shows the 3rd layer atoms forming the resulting boundary of the ring structure. atoms highlighted within this layer are removed to account for the measured triangular D1 ~3.1 Å and D2 ~ 9.3 Å sizes (M=1) shown in Figure 4.23(iv). Figure 4.20(iv) shows the removal of 20 Si atoms from the 4th layer to account for the deep holes observed (M=5). This results in the final ring structure which involves M=19 cluster in total. 205 CHAPTER 4.4.5.2 Loss of Si via pathway-2 In the second route, we selectively remove 20 Si atoms (M=5) from the 3rd layer which are highlighted in the unit cell as shown in Figure 4.21(i) (these atoms are equivalent to those highlighted in Figure 4.19(iv)). By releasing these atoms highlighted in Figure 4.21(i), from each unit cell, we will again obtain a shallow ring structure of size ~15.5Å and depth ~ 5.3Å while exposing the 4th layer Si atoms. The resulting ring structure shown in Figure 4.21(ii) will consist of the remaining Si adlayer atoms from the original 3rd layer as well as type “A” clusters which sits at the corner of the ring corresponds closely to the STM image of the surface observed at 1050oC. Similar to earlier discussion, we will need to account for the absence of type A clusters, the prevalent existence of type “C” Si atoms which forms D1 and D2 triangular features and the resultant (6x6) ring structure which is observed at 1100oC. To describe these features, we again need to remove another 28 Si atoms (M=7) from this unit cell. The absence of type “A” clusters at 1100oC, means we need to remove one type “A” cluster (M=1) from each unit cell as shown in Figure 4.21(ii), to generate the resultant surface shown in Figure 4.21(iii). However, the presence of the D1 (size~3.1±0.5Å) and D2 (size~9.0±0.5Å) features observed by STM further dictates the removal of Si atoms (M=1) from the 3rd and 4th layer from each unit cell as highlighted in Fig. 4.17(iii). Consequently, the surface structure shown in Figure 4.21(iv) will now consist of smaller triangular features (size~3.08Å) and larger triangular features (size~9.3Å) consisting of co-planarly bonded Si atoms, which correspond to the observed D1 and D2 features respectively. As the STM line profile analysis have demonstrated a ring depth of ~ 206 CHAPTER 3.5±0.5Å stemming from the loss of Si from the 4th layer, hence we will need to remove the remaining 20 atoms or M=5 tetra-clusters from the exposed 4th layer Si atoms as indicated in Figure 4.21(iv). This would generate a ring structure of respective depth ~ 3.8 Å and size ~ 15.5 Å with triangular D1 and D2 features (Figure 4.21(v)) which accounts for the STM image analysis. The final ring structure obtained here is the same as the one obtained through pathway-1 and again involves a total of 19 tetra-clusters. (i) (ii) 15.5Å (iii) (iv) 207 CHAPTER (v) Figure 4.21(i)-(v) describe the progressive removal of M=19 clusters per unit cell to obtain the final (6x6) ring structure through pathway- 2. Figure 4.21(i) shows the alternative region, as indicated by dotted hexagon, from which 20 Si atoms are removed (M=5). Figure 4.21(ii) also shows the highlighted type “A” clusters which are removed (M=1) to account for their absence in the STM images. Figure 4.21(iii) shows the resulting ring structure due to the boundary formed by 3rd layer atoms. atoms highlighted within this layer are removed to account for the measured triangular D1 ~3.1 Å and D2 ~ 9.3 Å sizes (M=1) shown in Figure 4.21(iv). Figure 4.21(iv) shows the removal of 20 Si atoms from the 4th layer to account for the deep holes observed (M=5). This results in the final ring structure in Figure 4.21(v) which involves M=19 cluster in total. Given the present experimental results, we are not able to resolve the dominating pathway. Nevertheless, it is clear that irrespective of the route in which Si may be loss, the same number of Si atoms equivalent to M = 19 tetra-clusters were removed from the initial (6x6) cluster phase to obtain the final ring structure shown in Figure 4.20(iv) or Figure 4.21(v). In addition, the resulting ring structure which is formed also involves the removal of Si atoms from the original 4th layer Si atoms of the U Starke’s model. Hence underlying top layer of C atoms of the SiC bulk would also be exposed for the first time 208 CHAPTER at this stage. This would result in greater Si-C bonding as detected by XPS, which indicates a more intense Si-C signal as compared to the Si-Si at this temperature. In principle, formation of C-C bonds within the hole may result since it will reduce the number of carbon dangling bonds. We were not able to image these carbon atoms within the holes. However, it is interestingly to note at this stage, the C 1s peak is still dominated by the Si-C chemical environment. The shift in binding energy to 285eV only occurs later at 1200oC where a graphitic surface has been reported [10-11] to dominate the SiC surface. This would imply further loss of Si atoms from the 4th layer leading to the eventual formation of a carbon rich surface. We have shown that by selectively moving M x units from the structural models at each respective annealing temperature, we are able to account for the basic morphological features observed and also deduce the minimum number of Si atoms required to reproduce the basic structural framework of a ring feature and its (6x6) periodicity on the SiC surface. By defining a unit volume as a (6x6) unit cell area consisting of the top layers where Si-Si bonding dominates, the (3x3) structure would consist of 88 Si atoms per unit volume. It is clear from the structural evolution illustrated by two pathways in Figure 4.20 and Figure 4.21, a removal of at least M = 15 clusters or 60 atoms per unit volume from the initial (3x3) reconstruction is therefore necessary to obtain a (6x6) ring structure which describe the STM data. It will be interesting to compare this value (i.e. loss of 60 Si atoms) with that deduced by comparing the XPS intensity (I(3x3)) obtained for elemental Si at 850oC (where ordered 3x3 reconstruction is observed) and that (I(6x6) rings) at 1100oC (where ordered (6x6) ring structure is observed). 209 CHAPTER The peak area intensity for the elemental Si 2p XPS peak is related to the atomic concentration of Si atoms per unit volume (NSi) by the following expression ISi = NSi×ASF×TF where ISi is the peak area of Si 2p for Si-Si, ASF=atomic sensitivity factor and TF=transmission function). Since ASF and TF are constants for the same element, we have; I(3×3) I(6×6)rings = N (3×3) N (6×6)rings Experimentally I(3x3) was determined to have a value of 726 counts. As previously defined, the (3x3) structure would consist of 88 Si atoms per unit volume (N(3x3)= 88). Since the value of the XPS peak area intensity for the (6x6) ring structure i.e. I(6x6) rings, is equal to 236 counts, we can therefore estimate the value of N(6x6) rings to be 28. The value is coincidentally equivalent to the number of Si atoms found within the model describing the (6x6) ring structure. Consequently, the difference in the number concentration of Si per unit volume between the two phases is 60. Quantitative analysis of the XPS data not only shows a clear loss of Si with annealing resulting in increased exposure of the SiC bulk, but also agrees with the proposed model of Si loss via selective removal of M tetraclusters. 210 CHAPTER The progressive loss of Si in terms of M x Si atom units will also be consistent with other key experimental observations of tetra clusters during the phase transformation of the 6H-SiC(0001)-(3x3) surface to (6x6) ring structure; (1) The observation of type “A” clusters being released from the (3x3) reconstruction. This suggest a natural tendency for Si adatoms to come together to form a Si4 cluster; (2) The agglomeration Si4 clusters to form the new (6x6) cluster phase [7], the formation of same sized type “A” clusters after the disappearance of type “B” clusters due to heating and the eventual formation of the ring structure also suggest that this unique cluster is an important vehicle for Si mass transport in facilitating the surface structural transformation. These experimental observations point towards the existence of type “A” cluster as a unique stable structure for Si atoms on the SiC(0001) surface. This is surprising given that the Si atoms within the cluster are likely to possess non-ideal tetrahedral bonding unlike that of Si atoms in the bulk. Recently, Grass et al [16] reported the existence of Si magic clusters consisting of Si atoms on HOPG. The Si4 clusters however were deposited from a cluster source, and together with theoretical calculations showed that these clusters are stable against coalescence at room temperature. It will thus be of an interest to study these magic clusters from a theoretical approach and hence derive a minimum energy structure to rationalize their existence on the SiC(0001) surface. 211 CHAPTER 4.5 Summary In this chapter, we have found that the phase transition of (3x3) → (6x6) clusters → (6x6) rings with progressive annealing is facilitated by the formation of Si magic clusters. This phase transformation occurs with Si first popping out from the (3x3) surface as tetra-clusters, driven by the breaking of highly strained co-planar Si-Si bonds. The agglomeration of these clusters forms larger cluster species of uniform shape and size, which self assemble to form (6x6) cluster structures at higher temperatures. We show that this process is motivated by the minimization of dangling bonds and a consequent lowering of surface energy. We also propose a hexagonal cluster structure model consisting of sub-units of tetra-clusters to account for the observed cluster shape, size and (6x6) periodicity. The formation of (6x6) rings from the (6x6) clusters at higher temperatures is shown and the absence of the (6√3x6√3) R30º reconstruction is established from the line profile and auto co-relation measurements. XPS shows loss of Si during annealing, however the ring surface is still Si-rich and not graphitized. STM reveals that smaller type “A” clusters (tetra-clusters) form from type “B” and leads to the formation of (6x6) ring structure consisting of type “C” Si adatoms when the surface is heated beyond 1000 °C. This shows that type “A” clusters participate in the phase transformations by mediating the surface structural transformation. 212 CHAPTER We propose a model to account for this surface evolution through the selective removal of M x Si atoms (Where M=number of clusters) at various annealing temperatures. By removing M=19 per unit cell, we obtain a structural model of the ring structure consisting of locally organized type “C” Si adatoms arranged into hexagonal D1 and D2 formations, which accounts for the experimental observations. It should be noted that the occurrence of similarly sized and shaped Si magic clusters was also observed on Si(111). However the formation and behavior of these clusters on Si(111) are not resolved, hence we would address these issues as well as compare both spieces of cluster on the different surface templates in the following chapter. 213 CHAPTER 4.6 References [1] J. Schardt, J. Bernhardt, U. Starke and K. Heinz, Phys Rev B 62 (2000) 10335 [2] I. S. 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Lett. 81 (2002) 3810 214 [...]... the A2, A4 and A6 tetra -clusters 165 CHAPTER 4 will need to occupy positions directly above atoms labeled as 3, 5 and 9 respectively, as shown in Figure 4. 7b 2 37 26 25 1 10 A1 24 11 15 13 A2 23 12 31 A3 8 21 20 30 29 16 7 36 22 A6 38 14 27 32 28 33 17 A5 4 A4 34 18 19 35 6 Measured cluster diameter ~ 14. 3±0.5Å (b) Figure 4. 7(b): Movement of tetra -clusters A2, A4 and A6 with corresponding bond breaking... shown in Figure 4. 7d 168 CHAPTER 4 2 37 26 25 1 10 A1 24 11 15 36 A2 30 22 23 12 13 A3 16 21 7 20 31 29 A6 38 14 27 4 32 28 33 17 A4 A5 34 18 35 19 6 (c) Figure 4. 7(c): The (6x6) cluster base structure incorporating a centre tetra-cluster 169 CHAPTER 4 2 26 25 1 27 A1 24 10 37 36 A2 22 30 11 15 A3 A6 38 14 31 16 12 7 20 32 28 21 13 4 33 A5 A4 34 18 19 35 6 (d) Figure 4. 7(d): The formation of the (6x6)... atoms through sp2–like co-planar bonds, leading to a twisted bonding configuration The type-“I” and “II” bonding configurations are similar and they differ only in orientation The type “III” atoms are also bonded directly down to the Si atom in the bulk position of the substrate While one bond is linked to the tetra-cluster structure, the remaining two bonds are bonded co-planarly to neighboring Si... (a) formation of (3x3) reconstruction consisting of Si tetra -clusters arranged on top of a full Si adlayer above Si-terminated SiC bulk at 850°C (b) ejection of tetra -clusters onto surface due to breaking of highly strained co-planar Si-Si bonds at 900°C, resulting in shallow defects (c) agglomeration of 8 tetra -clusters to form larger type “B” clusters at 950°C in order to reduce dangling bond density... driven by the formation of a structure with a lower strain energy and minimum number of unsatisfied dangling bonds In the following section, the same approach in analysis will be applied to study the formation of (6x6) ring structures from the (6x6) cluster structure 1 74 CHAPTER 4 4 .4 6H-SiC(0001)-(6x6) rings 4. 4.1 Formation of (6x6) rings from (6x6) clusters: global morphology In the previous section,... through desorption of Si when the substrate is progressively heated to 1100oC from 850oC Apart from STM data, evidence for this loss of Si material can also be inferred from the XPS measurements (Fig 4. 10 and 4. 11) corresponding to the surfaces shown in Figure 4. 9, which is discussed in the following section 178 CHAPTER 4 4 .4. 2 Formation of (6x6) rings from (6x6) clusters: XPS Fig 4. 10 shows the best... the [1210] and [2110] azimuths, we selectively move three clusters A2, A4 and A6 (or A1, A3 and A5) tetra -clusters from the (3x3) reconstruction as shown Figure 4. 7a However, this process alone cannot account for the observation of larger type “B” cluster In order to describe a cluster with a diameter of 14. 3Å would require the agglomeration of tetraclusters on the surface Preserving the integrity of... efficiently In this section, we investigate with STM, the SiC(0001)-(3x3) phase transition to Si-rich (6x6) reconstruction as a function of temperature We were able to observe the formation of (6x6) clusters from the (3x3) surface, after annealing at 1000°C High temperature annealing causes tetra -clusters to “pop-out” of the (3x3) surface reconstruction Agglomeration of these tetra -clusters on the surface occurs... self-organization We have combined J Schardt et al’s (3x3) atomic 173 CHAPTER 4 model as a basis for discussion with STM evidence to demonstrate the phase transition mechanism We have proposed a hexagonal cluster structure model consisting of 8 subunits of tetra -clusters to explain the cluster size and the 6x-periodicity The process involving the ejection of silicon tetra -clusters, agglomeration of the tetra -clusters. .. corresponding ball and stick diagram shown in Figure 4. 5b(iii) Further annealing to 950ºC (Figure 4. 5c(i)) shows that there are now more depressions and clusters on the surface Closer analysis of these defects (Figure 4. 5c(ii)) shows that a second species of clusters, termed as “B”, are now observed along with the “A” type clusters The size of the depressions is now also larger than before These new “B” clusters . co-planar bonds, leading to a twisted bonding configuration. The type-“I” and “II” bonding configurations are similar and they differ only in orientation. The type “III” atoms are also bonded directly. type clusters do not increase in size beyond the dimension of 14. 3±0.5Å. This size and shape are in fact similar to the Si magic clusters reported by Tsong et al [2-3]. The magic Si clusters. (3x3) (ii) (i) (iii) (iv) (c) [ ] 1021 [ ] 2110 (i) 18.6Å (ii) 18 .4 (iii) 14. 3Å (iv) 2.3Å C C H H A A P P T T E E R R 4 4 160 Figure 4. 6: Shows the STM images (40 nmx40nm) of the surface