Magic clusters on group IV surfaces 1

CHAPTER Chapter 1: 1.1 Introduction Introduction Silicon (Si) and Silicon Carbide (SiC) are studied as the primary substrate materials in this work. As the incumbent raw material in the micro-electronics industry, the study of Si surfaces remains important, especially with the growing interest in the use of mono-dispersed particles in the fabrication of substrate supported nanostructures. Si(111) is the substrate of choice because of the presence of faulted/unfaulted halves of the (7x7) reconstruction which possess available dangling bonds which in turn can function as adsorption sites in promoting the self assembly of ordered nano particles. SiC on the other hand is fast gaining recognition as a substrate material for electronic device applications in areas where common silicon technology finds limitations. Due to its unique properties such as wide band gap, high thermal conductivity and high breakdown field strength, SiC based semiconductors electronic devices are presently being developed for use in high-temperature, high power and high radiation conditions under which conventional semiconductors cannot adequately perform [1-4]. In particular, we focus on the 6H-SiC(0001) substrate which exhibits a wide range of surface reconstructions and is also one of the more commonly used substrates [5-6]. 17 CHAPTER In this work, we focus on the formation of low dimensional structures on these substrates as the electronics industry has always been driven by the pursuit of faster and more efficient devices via reduction in device dimensions. Examples of typical low dimensional structures used in electronic applications include Quantum Dot Light Emission, Single Electron Transistors and Quantum Computing Arrays [7-8]. In particular, the advancement of silicon quantum dot nanostructures in solar photovoltaics is of significant interest due to the urgency in developing clean energy alternatives [9-10]. Amorphous silicon is currently used to fabricate solar cells, but has always been plagued by material degradation as prolonged illumination introduces defects which decrease the solar cell efficiency [11-12]. In this respect, nano-crystalline silicon is much more stable and has a broader absorption range in the visible part of the solar radiation [13-14]. Furthermore the optical properties could be tuned by affecting the band gap via changing the size of the Si nano-crystals [15]. Hence the challenge is to be able to manufacture semiconductor structures with greatly reduced sizes (in the regime of nanometers) on Si and SiC surfaces controllably and reproducibly. As the properties derived from these nano-structures are directly dependent on feature size, shape, distribution as well as interface sharpness, it would therefore be important to be able to re-produce these attributes with a high degree of uniformity. However current top down methodologies such as optical lithography have encountered limitations in feature size reduction due to radiation wavelength [16-17] while bottom up techniques such as quantum dot formation via strain induced MBE thin film growth and SAMs (self-assembled mono-layers) have struggled to generate particles with narrow size 18 CHAPTER distribution [18-21]. The key question therefore is whether we can deliberately prepare and assemble specific sizes of materials such as metals/semiconductors on supporting Si and SiC surfaces. It is well known that small atomic clusters exhibit unique size-dependent physical and chemical properties that differ from those of bulk materials [1] and maybe utilized to form the desired nano-structure architecture. Hence there have been much experimental and theoretical interest on atomic clusters, however most of the work was focused on freestanding clusters [22] which not address the issues in the formation of substrate supported nano-structures. Interestingly the solution may lie in a unique phase of atomic clusters termed as “magic clusters”. Magic clusters have been found to exist in a specific size and structure consisting of a “magic” number of atoms exhibiting closed shell structures leading to enhanced local stability [23]. This in turn leads to a high degree of uniformity in physical, chemical and electronic properties being retained by magic cluster species. Thus, the ability to fabricate substrate supported magic clusters, would address the challenges related to fabrication of atomic structures and presents a progressive platform for device miniaturization. In fact, a considerable number of work [24-29] on magic clusters of various material systems have been reported in a bid to create well ordered mono-dispersed nanostructures for potential applications in micro-electronics, magnetic data storage and atomically precise manufacturing [30-32]. These studies range from homogeneous metallic systems (e.g. Ag/Ag(100) and Pt/Pt(110)[28-29]) to semiconducting systems (e.g. 19 CHAPTER Si/Si(111) and Si/SiC [33-36]) as well as heterogeneous systems comprising of mixed metal/semi-conducting materials (e.g. In/Si(001) and Ga, In, Ag, Mn, Pb, Co-Si/Si(111) [37-41]). However there is still a bereft of studies concerning the origin and formation, atomic structure, adsorption and diffusion characteristics, physical and electronic properties of magic clusters. We therefore contribute to this area by studying the physical and electronic properties of Si and binary magic clusters in terms of their size distribution, chemical information, formation and transport behaviour on Si surfaces such as Si(111) and SiC(0001) substrates as outlined in the following Research Objectives. 20 CHAPTER 1.2 Research Objectives (I) Si Magic Clusters on 6H-SiC(0001) One of the primary issues is to be able to prepare well ordered substrate surfaces on which we can grow magic clusters. Hence we first study SiC surfaces in order to be able to produce high quality surface templates repeatedly. 6H-SiC(0001) exhibits several surface reconstructions ranging from (3x3), (√3x√3), (5x5), (2√3x2√13), (6x6) and (6√3x6√3)R30° as shown by various techniques [5-6]. In this work, we report new STM and XPS data that revealed the formation of an ordered (6x6) arrangement of Si clusters (diameter~14.3±0.5Å) [42]. These clusters possessed a uniform shape and size, which were attributed to Si magic clusters due to the Si-rich nature of the surface. This was surprising as the occurrence of magic clusters on SiC surfaces have not been reported before. We showed that these clusters were derived from the ejection of the Si-tetra cluster unit of the initial (3x3) reconstruction when the surface is heated to elevated temperatures. The result of this study is interesting, as it implies that the (6x6) structure was likely to be due to a self assembly of Si magic clusters rather than a reconstruction. The formation and ordering of the (6x6) structure also appears to be mediated by clusters rather than by adatoms. Despite the myriad of surface reconstructions observed on the SiC surface, there has been very little coherent work discussing the evolution of these structures and especially involving the occurrence of Si magic clusters. In particular, the role of silicon adatoms or magic clusters in terms of their atomic re-arrangement, bond breaking and 21 CHAPTER formation within the top few layers beginning with the SiC(0001)-(3x3) phase have been neglected. Therefore objective (I) of this PhD program focuses on aspects of magic cluster phenomenon on the SiC(0001) surface; 1) The first aim is to probe the origin and formation of Si magic clusters on the 6HSiC(0001) substrate surface. We intend to proceed with this work by annealing the SiC(0001) surface progressively beyond 800°C when the (3x3) surface is formed, to study the (3x3) phase transition to Si-rich (6x6) clusters with in-situ STM and XPS. While the STM will provide real space evidence of the micro-structural regime, the XPS will be able to provide information on binding energy shifts as well as changes in peak areas as the surface is annealed. This will allow us to address key issues in the formation of these clusters, such as whether this process occurs via the nucleation of Si adatoms or re-arrangement of the (3x3) surface or the ejection and agglomeration of whole tetraclusters. 2) The observation of these Si magic clusters would suggest a preference for Si atoms to exist as clusters with a specific magic number of atoms with similar atomic structure which promotes stability on the SiC(0001) surface. The second intention is to utilize insitu STM, with variation in tunneling biasing, to obtain real space information and consequently elucidate the structure of these magic clusters. By analyzing physical cluster dimensions such as size and geometry derived as a function of tunneling voltage, we intend to estimate the “magic” number of atoms in each cluster through a statistical study of cluster size distribution. This will lead to the elucidation of the magic cluster structure using high resolution images and we will eventually propose a model of the 22 CHAPTER cluster through the co-relation of STM data with the (3x3) tetra-cluster model as reference. 3) The third aim is to investigate the role these clusters play in the facilitating of phase transformations as the 6H-SiC(0001) surface is annealed. By co-relating STM data on structural evolution with corresponding XPS spectra, we intend to trace the Si and C signal trends as the surface is progressively annealed from 800°C to 1200°C. By doing so we would be able to probe the chemical information of different surface structures at various temperatures and study the transition of a Si-rich surface consisting of predominantly Si-Si bonding to a C-rich one dominated by Si-C bonding. Together with high resolution STM, we will also be able to analyze the different structures arising from the surface evolution as well as the self assembly process of Si magic clusters on 6HSiC(0001) surface, by analyzing the final adsorption sites as well as direction and separation of ordering. In particular, there has been much controversy surrounding the high temperature surface phases of SiC(0001) when it is heated beyond 1000°C. The existence of a (6√3x6√3)R30° surface structure upon heating to ~1200°C [43-44] have been reported by several diffraction studies, which also attributed the structure to an incommensurate graphite overlayer [45-46]. However, recent XPS studies suggested that the (6√3x6√3) R30° reconstruction could be attributed to carbon in a Si deficient environment and did not yet amount to surface graphitization, which only appeared after heating beyond 1200°C [47]. Despite exhaustive studies on this structure, real space STM evidence of the (6√3x6√3) R30° reconstruction is still not obvious [48]. Hence, in an effort to resolve the surface structures observed at higher temperatures, we intend to 23 CHAPTER investigate the surface structural evolution further by heating the 6H-SiC(0001) surface beyond 1000oC. We will study again with STM, how the clusters participate in surface re-ordering and evolution leading towards the formation of high temperature surface structures. In order to explain the structural transformation observed with the occurrence of magic clusters, we will first analyze the geometry and atomic structure of the surface at various temperatures using high resolution STM and propose a model to describe the observations. We will use J. Schardt et al’s (3x3) atomic model as a basis for discussion with our STM observations [49]. Specifically, we will use the tetra-clusters as basic-units to propose structural models, to account for the observation of the clusters and to map the microstructure evolution arising from the (3x3) phase to the self organized (6x6) cluster arrangement and subsequent high temperature phases. We will also determine if each structural phase observed can be accounted for by moving or removing M x Si atoms (where M=number of clusters). This will complete objective (I). 24 CHAPTER (II) Si Magic Clusters on Si(111) It is interesting to note that the size and shape of the Si magic clusters on 6H- SiC(0001) is similar to that of Si clusters observed on Si(111) surface [42]. While the existence of Si magic clusters on the Si(111)-(7x7) surface have already been reported [33-35], there has been no work addressing the origin and formation of these clusters. Although the literature suggest a preference for Si atoms to exist as clusters with a magic number of atoms with similar atomic structures on both surfaces, there have been intriguing lack of evidence of magic cluster ordering on Si(111) unlike SiC(0001) and how clusters influence surface structural changes. Even fewer reports carry experimental studies addressing the fine structure and magic atom number of these substrate supported clusters on Si(111). Therefore objective (II) of this PhD program will address key aspects of Si magic cluster work on the Si(111) surface; 1) While the occurrence of Si magic clusters on Si(111) have been recently reported [3335], it is still unknown how these clusters are formed and the conditions of these clusters’ origin. However we observe cluster-like particles of a uniform size occurring during the cooling transition from the high temperature “1x1” disordered phase to the more stable (7x7) reconstruction. This observation is important as it suggests that these particles could be Si magic clusters which mediate structural transformation and function as vehicles for mass transport across surfaces. Hence the role of Si magic clusters has important implications on the nucleation and growth processes of epitaxial Si surfaces. 25 CHAPTER Therefore, our first aim in objective (II) is to use in-situ STM to determine if the particles are magic clusters and address the formation and origin of these clusters on Si(111) as well as the role it plays in facilitating the ordering process of (7x7) from “1x1”. However as the clusters are metastable and diffuse very quickly, we intend to create clusters by fast quenching the Si(111) surface from high temperatures of > 800°C, through means of kinetically limiting the diffusion of clusters. Having formed domains of ordered (7x7) and “1x1”, we would be able to probe the gradual formation of ordered (7x7) from “1x1” using high resolution STM in real time. This may be achieved via slow annealing at slightly elevated temperatures of ~ 400°C in order to slow down the kinetic processes. In this way, we will able to trace the initial stages of Si adatoms undergoing self re-arrangement into magic clusters as a function of annealing time. This would form the basis of our study of substrate induced formation of magic clusters during the phase transformation. 2) Since Si magic clusters appear to form spontaneously from surface Si adatoms, in the second part of this work, we will try to grow Si magic clusters from Si adatoms deposited on Si(111). Unlike previous work, where typical cluster-based evaporating sources are used [22, 50-51], we avoid depositing clusters of different sizes by using a Si solid source evaporator instead. As the growth of these clusters appear to arise naturally as a tendency of atoms on the substrate surfaces to self-assemble under specific preparation conditions [27, 26], we will use STM to study the formation of Si magic clusters from adatoms, as the surface is being annealed. We will also probe if spatial ordering of Si magic clusters can be achieved on the Si(111) template using the available dangling bonds on the 26 CHAPTER faulted/unfaulted halves of the (7x7) reconstruction. Similarly we intend to investigate the mechanism and energetics leading to the formation of magic clusters, and probe if the Si4 tetra-cluster observed on 6H-SiC(0001) is also involved as a building block in cluster formation. This allow us to address the issues of control over cluster fabrication much needed in cluster applications previously mentioned. 3) Having studied cluster formation, it is also important to understand the structure of these magic clusters. In the structural elucidation of Si magic clusters, several proposed structures include the Si10 trigonal prism, the Si11 pentagonal antiprism and the Si12 hexacapped antiprism, where a Sin cluster contains n number of Si atoms. These theoretical calculations have showed that magic clusters with different numbers of Si atoms display different bonding structures and hence not share similar electronic or even physical properties [52-55]. However it should be noted that these atomic models could only provide information of freestanding clusters in idealized minimum energy structures, and hence not accurately represent these magic clusters physically when they exist on reconstructed surfaces. It is therefore not surprising that the bonding characteristics of these proposed Si clusters would be altered with the introduction of an underlying substrate surface potential. We contribute to this work by using the approach outlined in objective (I) to elucidate the structure of Si magic clusters. This will allow us to investigate the critical number of atoms within the clusters and how these atoms are arranged to give rise to the enhanced stability instead of existing as adatoms. By using a comparison in physical features as a 27 CHAPTER function of tunnelling voltage, we intend to use the STM to probe the size and geometry of the clusters. This will allow us to estimate the “magic” number of atoms in each cluster through a statistical analysis of cluster size distribution, leading to the elucidation of the final structure using high resolution images. Using this information, we will propose a model of the cluster through the co-relation of STM data with the DAS structure of the Si(111)-7x7 reconstruction as reference. This understanding of the formation and structure of the magic clusters and its contribution to surface ordering and mass transport on Si(111) surface will hence complete research objective (II). 28 CHAPTER (III) Co-Si Magic Clusters on Si(111) The implication of realizing objectives (I) and (II) is a selective tendency for Si adatoms to cluster in specific sizes. The next issue is whether we can selectively prepare and assemble not only specific sizes of Si clusters on Si(111) but also binary clusters such as Co/Si on similar Si substrates. The purpose of this section of the work is thus aimed at achieving an atomic level understanding of the electronic and physical properties of these naturally occurring inorganic/metallic magic clusters formed on Si(111) surfaces. The motivation to study the formation of metallic Co clusters also stem from the fact that metal clusters are expected to have different electronic/magnetic properties from the bulk [2, and 39]. Their unique electronic and magnetic properties at reduced size may be useful for information storage applications [1]. Again, our interest is in correlating the unique properties associated with these clusters to its physical properties in terms of their size distribution, atomic structure, formation and transport behaviour. As discussed earlier, Si magic clusters appear to play an important role in relation to mass transport and surface ordering on Si(111) surfaces. Hence our aim is to study how the clusters contribute toward material transport, self assembly characteristics and growth on the surface as a function of various kinetic parameters such as annealing temperature, time and coverage. We intend to first form Co-Si magic clusters by depositing Co by the use of a solid source onto an ordered Si(111)-(7x7) surface at room temperature and annealing to ~ 400°C [56]. Consequently, we will identify the regime 29 CHAPTER under which self ordering of magic clusters take place using STM, as the surface is annealed. By observing this self assembly process with fast speed STM during real time scanning, we would also be able to probe the number of atoms/clusters which may exist as critical nuclei in promoting surface growth of nanostructure and at the same time obtain statistical evidence of this observation. In the second part of this section, we will adopt the approach described in (II) to elucidate the structure of the clusters through statistical study of cluster size distribution as well as analysis of high resolution STM images of the clusters with respect to various tunneling potentials. This understanding of the dynamic behavior as well as structure of the clusters will allow us to formulate some order of control over the formation of these structures in completing objective (III). 30 CHAPTER 1.3 Outline of thesis The organization of this thesis is detailed as follows; Chapter consists of a review of existing literature related to work done on Si magic clusters on SiC(0001), Si magic clusters on Si(111) and Co-Si magic clusters on Si(111). This study will identify issues that will be addressed via experimental work in Chapters 4, and 6. Chapter describes the operating principles and procedures for the main analytical tools used in this work, such as STM and XPS as well as other experimental preparation which include in-situ and ex-situ sample preparation, STM tip etching, calibration of substrate temperature and calibration of the electron beam evaporator flux. The first experimental results are shown and discussed in Chapter 4, which uses the STM and XPS to investigate the evolution of surface structures with progressive annealing; starting from (3x3) phase at 850oC → Si rich (6x6) clusters at 1000oC → (6x6) rings at 1100oC. Detailed analysis of surface features such as defects, tetramers and tetramer agglomeration during this surface evolution is co-related with chemical information to provide an atomic model which can account for the structural transformation by moving/removing M x Si atoms (where M = number of tetra-clusters clusters). 31 CHAPTER Chapter focuses on STM which is used to probe the formation and structure of Si magic clusters on the Si(111)-(7x7) surface. In particular, we will investigate the micro-structural change which occurs during the Si(111)-“1x1”→ (7x7) phase transformation at high temperatures. We determine if Si magic clusters exist and probe the origin of these Si particles and how they influence the phase transformation in terms of micro-structure and macro morphology. We will also attempt to grow the same Si particles by depositing Si adatoms from Si solid source evaporator on the Si(111)-(7x7) and annealing the surface structure. Using STM, we will trace the structural evolution leading to formation of Si magic clusters and propose a mechanism to account for this observation. In both approaches, we will use STM dual biasing and line profile to elucidate the structure of the clusters. In Chapter 6, we will use STM and XPS to probe the evolution of Co deposited on clean Si(111)-(7x7) and progressively annealed to higher temperatures. We will characterize the various surface structures such as 3D islands or clusters that could occur at different annealing temperatures as a function of Co flux and therefore determine if a critical Co coverage exists where clusters can form exclusively and propagate (√7x√7) surface structures. We will also use real time STM scanning to investigate the self assembly phenomena of clusters to resolve if the (√7x√7) structure is an intrinsic surface reconstruction or due to self assembly of clusters. 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Phys. 32, R71 (1999). 37 [...]... co-relation of STM data with the DAS structure of the Si (11 1)-7x7 reconstruction as reference This understanding of the formation and structure of the magic clusters and its contribution to surface ordering and mass transport on Si (11 1) surface will hence complete research objective (II) 28 CHAPTER 1 (III) Co-Si Magic Clusters on Si (11 1) The implication of realizing objectives (I) and (II) is a selective... of the clusters will allow us to formulate some order of control over the formation of these structures in completing objective (III) 30 CHAPTER 1 1.3 Outline of thesis The organization of this thesis is detailed as follows; Chapter 2 consists of a review of existing literature related to work done on Si magic clusters on SiC(00 01) , Si magic clusters on Si (11 1) and Co-Si magic clusters on Si (11 1) This... can selectively prepare and assemble not only specific sizes of Si clusters on Si (11 1) but also binary clusters such as Co/Si on similar Si substrates The purpose of this section of the work is thus aimed at achieving an atomic level understanding of the electronic and physical properties of these naturally occurring inorganic/metallic magic clusters formed on Si (11 1) surfaces The motivation to study... Lett. 91, 2 610 4 -1 (2003) [ 41] S.C Li, J.F Jia, R.F Dou and Q.K Xue, I.G Batyrev and S.B Zhang, Phys Rev Lett 93, 11 610 3 -1 (2004) [42] W.J Ong, E.S Tok, H Xu and A.T.S Wee, Appl Phys Lett 80, 3406 (2002) [43] J.E Northrup and J Neugebauer, Phys Rev B 52 (19 95) R170 01 [44] L Simon, J.L Bischoff and L Kubler, Phys Rev B 53 (19 96) 13 793 [45] M.H Tsai, C.S Chang, J.D Dow and I.S.T Tsong, Phys Rev B 45 (19 92) 13 27... mentioned 3) Having studied cluster formation, it is also important to understand the structure of these magic clusters In the structural elucidation of Si magic clusters, several proposed structures include the Si10 trigonal prism, the Si 11 pentagonal antiprism and the Si12 hexacapped antiprism, where a Sin cluster contains n number of Si atoms These theoretical calculations have showed that magic clusters. .. (19 92) 13 27 [46] L Li, I.S.T Tsong, Surf Sci 3 51 (19 96) 14 1 [47] L.I Jonhansson, F Owman, P Martesson, Phys Rev B 53 (19 96) 13 793 [48] F Owman and P Martensson, Surf Sci 330 (19 95) L639 [49] J Schardt, J Bernhardt, U Starke and K Heinz, Phys Rev B 62 (2000) 10 335 36 CHAPTER 1 [50] M Grass, D Fischer, M Mathes, G Gantefor and P Nielaba, Appl Phy Lett 81, 3 810 , (2002) [ 51] B Klipp, M Grass, J Muller,... is used to probe the formation and structure of Si magic clusters on the Si (11 1)-(7x7) surface In particular, we will investigate the micro-structural change which occurs during the Si (11 1)-“1x1”→ (7x7) phase transformation at high temperatures We determine if Si magic clusters exist and probe the origin of these Si particles and how they influence the phase transformation in terms of micro-structure... 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This study. and shape of the Si magic clusters on 6H- SiC(00 01) is similar to that of Si clusters observed on Si (11 1) surface [42]. While the existence of Si magic clusters on the Si (11 1)-(7x7) surface have. the Si (11 1)-7x7 reconstruction as reference. This understanding of the formation and structure of the magic clusters and its contribution to surface ordering and mass transport on Si (11 1) surface