Magic clusters on group IV surfaces 2

CHAPTER Chapter 2: Literature Review 2.1 Si Magic Clusters on 6H-SiC(0001) 2.1.1 Silicon Carbide Silicon carbide as a substrate material has unique properties such as high temperature stability, thermal conductivity, fast carrier recombination and large band gap, which has led to the development of SiC based semiconductors electronic devices and circuits. These applications are used in high-temperature, high power and high radiation conditions under which conventional semiconductors cannot adequately perform [1-3] such as the high temperature MOSFET [4] and the blue LED [5]. In particular, the current push for continuing device miniaturization, have led to great interest in nanostructure formation on SiC surfaces. Silicon carbide is the only chemically stable form of silicon and carbon. The basic unit of SiC consists of a covalently bonded tetrahedron of Si (or C) with a C (or Si) at the centre. The bonding of silicon and carbon atoms is 88% covalent and 12% ionic with a distance of 1.89 Å between the Si and C atoms [6]. Each Si(C) atom is surrounded by four C(Si) atoms respectively in strong tetrahedral sp3-bonds as shown in Figure 2.1. The crystalline structure consists of the close-packed stacking of bilayers of Si and C atoms parallel to the surface. One set of atoms (Si or C) is shifted along the main axis of 38 CHAPTER symmetry by a quarter of the distance between the nearest similar layers. The stacking sequence of the double layers follows one of three possible relative positions, arbitrarily labelled A, B and C as in closed packing of hard spheres. One unique aspect of SiC is the stacking sequence of these double layers allows an endless number of different onedimensional orderings (polytypes) without variation in stoichiometry. This is the source of SiC’s large number of crystallographic forms called polytypes. Different crystal forms of the same chemical composition are called polymorphs. Polymorphism commonly refers to a three-dimensional change affected by either a complete alteration of the crystal structure or a slight shift in bond angles. Polytypism is a special type of polymorphism that occurs in certain close-packed structures. Si atom 3.08 Å C-Si C Atom C-Si Figure 2.1: Tetragonal bonding of carbon atom with the four nearest silicon neighbours. The distances between Si atoms and C-Si are about 3.08 Å and 1.89 Å respectively. C-Si 3.08 Å 39 CHAPTER Figure 2.2: Side view of different examples of SiC polytype crystal structure, along the (1120) -direction. (a) Hexagonal bilayer, (b) zincblende structure, 3C-SiC, (c) wurtzite structure, 2H-SiC, (d) 4H-SiC and (e) 6H-SiC. The stacking direction is depicted by the enhanced Si-C bond train parallel to the (1120) -direction (adapted from [7]). SiC exists not as a single crystal type but as a whole family of crystals known as polytypes which are named according to the periodicity of these layers. For example, some of the other more common structures are, 4H and 3C (cubic lattice), as shown in Figure 2.2 [7]. The exact physical, electrical and optical properties of SiC depend on the crystal structure adopted. The stacking sequence of these polytypes can be described by the ‘ABC’ notation, where A, B and C represent the three sites available in separate 40 CHAPTER stacking layers in one sublattice. For example, the cubic form of SiC called beta silicon carbide (β-SiC) or 3C-SiC, has an atom stacking sequence of ABCABCABC. For example, Schardt et al [7] studied the structure of the 3C and 6H polytypes and presented a range of possible stacking sequences, which is shown in Figure 2.3 below. He also investigated the arrangement of atoms on an ideally terminated surface and subsequently named the various site positions at the top, second and third layers T1, T4 and H3 respectively. This is presented in Figure 2.4. Figure 2.3: Cross-sectional view of the linear stacking in the cubic (3C) polytype (β-SiC) and three different staking sequences possible at the surface of the 6H polytype. The different surface terminations are labeled according to the depth of orientation change (S1, CACBABC; S2, BCACBAB and S3, ABCACBA) (adapted from [7]). 41 CHAPTER Figure 2.4: Adatom (or molecular) site models on a SiC(0001) surface: T1 represents the atop site; T4 represents the 3-fold site with an additional neighbor in the low-half top bilayer; H3 represents the 3-fold site without additional coordination (adapted from [7]). While there are numerous polytypes (~250 different polytypes) that dictate different stacking sequences of the Si-C bilayer which results in varied and complex surface atomic arrangement as well as chemical identity, one of the more commonly used polytypes is the hexagonal 6H-SiC crystallographic bulk structure, which consists of stacking of Si-C bi-layers with the ABCACB sequence along the C-axis direction of the hexagonal structure and where each A, B or C corresponds to a SiC bilayer, as shown in Figure 2.5. In an ideally bulk truncated configuration, the (0001) surface is terminated by Si atoms whereas the (0001) surface is terminated by C atoms. 42 CHAPTER A B C A C 15Ǻ B A Si atom C atom Figure 2.5: Side view of 6H stacking sequence of 6H-SiC(0001) As the surface structure and composition are strongly dependent on surface preparation conditions, the methodology of surface preparation is essential in determining the cleanliness, morphology and chemical make-up of the surface. Hence the next section would be devoted towards reviewing the different approaches towards achieving clean and well ordered 6H-SiC(0001) surfaces in UHV. Depending on annealing temperature, the 6H-SiC(0001) polytype exhibits a rich series of surface reconstructions ranging from Si-rich (3x3), (√3x√3)R30°, (5x5), (9x9), (2√3x2√13) to Si deficient (6√3x6√3) R30°. 43 CHAPTER These surface structures have been previously studied by a variety of techniques such as Low Energy Electron Diffraction (LEED), Reflection High Energy Electron Diffraction (RHEED), Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS) and STM [8-16]. We would discuss the various literature in this area of work focusing on the significant surface structures which in turn would form the basis for our study of Si magic clusters on 6H-SiC(0001) substrate surfaces. 44 CHAPTER 2.1.2 SiC surface cleaning and preparation SiC surfaces are strongly dependent on the methods in which the sample is prepared. Conventional annealing in UHV yields a surface, which is considerably carbonized or graphitised. As early as 1975, Van Bommel et al [17] showed that atomically clean ordered surfaces of 6H-SiC could be obtained by annealing in UHV at 800OC, but surface precipitation of graphite was observed even at this relatively low temperature, presumably associated with Si loss. Annealing at higher temperatures required to volatilise surface oxides caused structural changes, which could not be reversed by additional annealing, since they are caused by the depletion of surface Si. Subsequent work has confirmed that, while annealing in UHV can yield oxide-free surfaces, this preparation method suffers from three distinct disadvantages [18]. Firstly, the results obtained are dependent on the type of sample. Second, the surface phase transformations can proceed only in the direction of successively Si-poorer compositions. Third, the most Si-poor phase was found to be accompanied by graphite precipitation and considerable disorder [18]. Since annealing in vacuum causes irreversible phase changes and can conceal low temperature phases, an alternative method of surface preparation based on the chemical reduction of surface oxide at modest temperatures was applied. Kaplan et al [19] reported that virtually all oxide from 6H samples disappeared after several minutes exposure at 850OC to a Si flux. With this method, Kaplan et al was able to obtain clean surfaces without heating above 1000OC. The arriving Si atoms convert surface oxide to SiO, 45 CHAPTER which is volatile in this temperature range, thus removing the oxide without otherwise disturbing the surface. Contaminant C is similarly converted to SiC, thereby becoming part of the crystal structure. By means of further Si exposure or annealing, the whole range of SiC surface structures from Si-terminated to C-terminated phases could be obtained and studied. This method, although successful in obtaining clean SiC surfaces, does result in a high defect density. We will consequently show that by adopting this method of surface cleaning we are able to prepare well ordered 6H-SiC(0001)-(3x3) reconstruction. 46 CHAPTER 2.1.3 6H-SiC(0001)-(3x3) The 6H-SiC(0001) (3x3) reconstruction is a much-studied surface, with no less than atomic structural models being proposed to describe it. In this section, these different models will be reviewed and later compared to the STM results obtained on (3x3) reconstruction. By using a Si flux to anneal Si-terminated 6H-SiC(0001) samples at 1120K, Kaplan et al first observed the formation of a SiC(111)-(3x3) LEED pattern [19]. Kaplan identified the (3x3) phase with an adsorbed Si bilayer on the Si-terminated surface. From LEED and EELS results, Kaplan et al proposed a (3x3) structural model based on the DAS (Dimer-Adatom-Stacking Fault) model of the Si(111)-(7x7) [20]. This unit cell contains two adatoms per surface unit cell, six “rest” atoms in a first layer and eight atoms in a second layer on top of a silicon-terminated surface, as shown in Figure 2.6. The existence of the Si bilayer from the proposed EELS studies indicated a Si rich nature of SiC(111)-(3x3) surface, and therefore the similarities between the Si(111)-(7x7) and the SiC(111)-(3x3) were assumed. From previous inspection of the Si(111)-(7x7) model, it was also shown that the same type of reconstructions can develop with a (2n+1)x(2n+1) periodicity (n=1,2,3,4….) [21]. Periodicities (5x5), (9x9), (11x11) and (13x13) have been reported on Si surfaces [22-23]. Therefore, a similar DAS reconstruction seemed a good candidate to account for the SiC(111)-(3x3). By modifying the stacking structure in the substrate, the DAS model also seemed to offer a good description for 4H- and 6H-SiC(0001)-(3x3) surface. 47 CHAPTER 2.2.6 Summary The observation of Si magic clusters on Si(111) surface has generated much interest in resolving the structure of these clusters and their role in growth and ordering of Si surfaces. Both 6H-SiC(0001) and Si(111) surfaces possess rich surface reconstructions which have been well reported in the literature. However there has been a lack of understanding as to how clusters actually function in the transition of surface structures. The observation of Si magic clusters on Si(111) have been previously reported by Tsong et al [46] as well as on the 6H-SiC(0001) surface by Ong et al [70]. While Tsong et al studied the diffusion characteristics of Si magic clusters in terms of the cluster hopping mechanism and electro-migration effects [51-56]. They did not however discuss the origination nor the role of these magic clusters in the surface micro-structural evolution of Si(111) at high temperatures. It is also interesting to note that the magic clusters observed on the SiC(0001) surface are also similar in shape and size as the Si magic clusters observed on the Si(111) surface [70]. Although these results 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). 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Silicon forms stable metallic and semiconducting compounds with metals known as metal silicides, which show low resistivity and high thermal stability, thus making them suitable for ULSI application. Applications of metal silicides for polysilicon silicide (polycide) gate structure and self-aligned silicide (salicide) [1-2] are well known, while WSi2 [3-4], TaSi2 [5] and MoSi2 [6] are commonly used as polycides due to their compatibility with processing chemicals. TiSi2 and CoSi2 (~ 14 µΩ.cm) are commonly employed in salicide applications because of their low resistivity [7, 8]. In addition to ULSI applications, 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 [9-12]. Their unique electronic and magnetic properties at reduced size may be useful for information storage applications [13]. Hence we are interested in correlating the unique properties associated with these clusters in terms of their size distribution, atomic structure, formation and transport behavior. 92 CHAPTER 2.3.1 Structural properties of Co and silicide phases Cobalt is a transition metal with a hexagonal close-packed (HCP) crystal structure at room temperature. The lattice parameters are a = 2.50 Å and c = 4.06 Å. However, Co will undergo polymorphic transformation above 450°C into a FCC crystal structure of lattice parameter 3.54 Å [14]. The composition of bulk Si and Co is described by the phase diagram as shown given in Figure 2.14 [15]. The main phases of Si and Co are Co-rich silicide (Co2Si), the monosilicide (CoSi) and the disilicide (CoSi2), depending on the relative atomic percentage of Si and Co. Of the Co silicide phases, Co2Si is the only phase that belongs to the orthorhombic crystal system, while CoSi and CoSi2 both belong to the cubic system. In particular, CoSi2 has a lattice parameter of 5.37 Å [16] and a CaF2 – type crystal structure, which is shown in Figure 2.13. The Co atoms take up the position of “Ca”, while Si atoms occupy the “F” position in the unit cell. The CoSi2(001) plane will be terminated alternately by Co and Si (assume bulk-termination). Figure 2.13: Crystal structure of CoSi2. 93 CHAPTER Figure 2.14: Phase diagram of Co – Si system. From the phase diagram, these phases usually form at very high temperatures (>1000°C). However, for thin film/substrate interface reaction, where surface energy term becomes dominant (i.e. no longer bulk-like), the phases can actually form at much lower temperatures. Various epitaxial growth techniques ranging from molecular beam epitaxy (MBE), reactive deposition epitaxy (RDE) and solid-phase epitaxy (SPE) have been used to grow these Co2Si, CoSi and CoSi2 thin films [17]. In MBE, Co metal and silicon are co-evaporated onto a Si substrate held at an elevated temperature, usually between 400oC and 600oC. In RDE, Co is directly deposited on a heated silicon substrate. Unlike MBE or RDE, SPE is a two step process. It involves the initial deposition of a thick Co film (few tens of nanometers) on Si substrate at room temperature followed by thermal processing to induce reaction between Co and Si to form CoSi2. 94 CHAPTER The formation of a particular Co-Si phase is temperature dependent and it is believed to occur through the following stages. o o o ~250 C ~350 C ~650 C Co → Co2 Si → CoSi → CoSi2 A Co-rich phase (Co2Si) is first formed at ~ 250°C, which is unstable at higher temperatures and will transform into a highly resistive monosilicide phase (CoSi) upon annealing to ~ 350°C. By annealing to even higher temperature of more than 650°C, a high-crystallinity and low-resistivity disilicide phase (CoSi2) forms [18]. Many techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and ultraviolet photoelectron spectroscopy (UPS), have been used to characterize the behavior of these Co layers on Si substrates, in particular, Co deposition and annealing on clean Si surfaces [19 – 22]. Most of the work focused on growth related issues pertaining to MBE or SPE-fabricated CoSi2 films on Si surfaces as defects such as grain misorientation, layer non-uniformity and pinhole formation frequently occur [23 – 27]. These defects are undesirable as it would inadvertly lead to degradation of device performance [28]. 95 CHAPTER 2.3.2 Co/Si(111) As most of the attention centered on thin film Co silicide growth on Si(111) as previously mentioned, majority of experimental and theoretical work in this area was focused on issues pertaining to silicide nucleation, formation and surface structure. During initial periods of this area of work in the 1980’s, the earlier experimental works relied heavily on LEED, Angle Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) and XPS core level photoemission spectroscopy, AES, TEM and Rutherford backscattering to probe the structure and nucleation of epitaxially grown CoSi2 on Si(111) [29-32]. It was only from the onset of the early 1990’s, where the development of the STM as a powerful imaging tool capable of atomic spatial resolution allowed for more in depth study of the surface structure of CoSi2 on Si(111). The first STM and RHEED study, by Stalder et al [33], of epitaxial CoSi2 on Si(111) yielded the observation of a (2x1) reconstruction. This led to other similar work which exploited the capabilities of the STM to study the initial reaction on Si(111) when Co was deposited and annealed to 150oC. Preferential silicide nucleation from the faulted halves of the (7x7) unit cells were observed [34], which preceded the formation of flat topped mono-layer silicide islands [35]. These islands were triangular in shape with vertices along the directions and grew in sizes with edge lengths that were integer multiples of the (7x7) unit cells when the surface was annealed. While STM later revealed the surface structure of these islands to be a (2x2) Si adatom configuration [36], global morphological studies showed that the islands developed into large multi-layer 96 CHAPTER CoSi2 islands co-existing with a √7 sub-monolayer phase [34 and 37]. As most of the work thus far had been directed at characterizing silicide islands, it was no surprise when the focus was gradually shifted to the atomistic processes that governed the growth of these islands, such as the type of species facilitating mass transport and surface diffusion mechanisms. As the √7 sub-monolayer phase was suggested to be a precursor phase to CoSi2 island formation, most of the STM studies eventually concentrated on this surface structure. It was first identified by Bennett and his co-workers, who observed the formation of a Si(111)-(√7x√7) reconstructed Co surface when 0.1ML of Co deposited on Si(111) was annealed at 670oC [38-39]. LEED showed √7 spots which was co-related to STM images showing small domains consisting of well ordered bright maximas with (√7x√7) unit cell. Using medium energy ion scattering (MEIS), they determined the observed atomic geometry of the maxima to consist of a Co atom surrounded by Si adatoms. This model was further elaborated upon in theoretical calculation [40], which used local density approximation, and described the structure as a ring cluster containing a single Co atom covered by bridging and capping Si adatoms as shown in Figure 2.15 below. 97 CHAPTER (A) Plane View (B) Side View Figure 2.15: (A) Plane view of model of Si(111)-(√7x√7). Unit cell is outlined and contains a single Co atom (filled circle) and a ring of Si adatoms (large open circles) on top of a (1x1) top Si substrate layer (small open circles) (B) Side view of a unit cell. Dotted circles represent projected positions of atoms not in the plane. Horizontal line represents nominal Si(111) plane (top of substrate layer). Co atom (filled circle) is sixfold co-ordinated and is bonded to bridging atoms (open solid circles) with bond length d and to atoms in the second layer of the bi-layer substrate (nearest dotted circles). Bottom layer consists of H atoms (adapted from [40]). Using low energy electron microscopy (LEEM), some groups were later able to probe the structural evolution of the Co-Si(111) surface as a function of annealing temperature. They found that the well ordered (√7x√7) surface gave way to form a disordered “1x1” cluster phase with lower Co concentration at higher temperatures. It was also observed that the dis-ordered “1x1” phase tends to occur surrounding CoSi2 islands. Further annealing at higher temperatures resulted in the growth of silicide islands with the depletion of the “1x1” phase and eventual formation of Si(111)-(7x7) [41-44], which led to the conclusion that silicide island growth was facilitated by the drawing of metallic Co from the initial (√7x√7) phase to form the “1x1” cluster phase before eventually leaving behind only silicide islands sitting on uncovered Si(111)-(7x7) surface. 98 CHAPTER 2.3.3 Summary While the work thus far had shed some light on low coverage silicide island growth on Si(111), the initial concerns over the surface atomistic processes and dynamics were still yet to be adequately addressed. In fact the reported works so far in general have assumed that the ordering of clusters occurs spontaneously to form the (√7x√7) surface structure and have not addressed whether a stable critical nuclei exists to promote the self assembly process, analogous to that in thin film growth [45-48]. 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(√3x√3)R30O reconstruction pattern by annealing the 6H-SiC sample at 1 120 K in vacuum By using an analogy with adsorption of group III or IV atoms on Si(111) [27 -28 ], he proposed that the (√3x√3) pattern might be due to 1/3 of a monolayer of Si adatoms each bonding to 3 Si atoms of the SiC surface [19] By annealing at 1170K, Owman and Mårtensson [29 ] observed a hexagonal array of protrusions visible with... of the surface Brillouin zone instead of a parabolic relation [43-44] and the electron transport is governed by Dirac’s equation rather than Schrodinger’s equation [ 42, 45-46] In addition, graphene was also shown to have unconventional twodimensional electron gas properties leading to quantum confinement [ 42, 45, 47], which has led to prospective graphene based nano-electronics Graphene had always... and bonds to nearest neighbors in 1st layer ~2. 383 Å, bonds to nearest neighbors in 3rd layer ~2. 15 Å; dimer bond length normal to surface~ 2. 45 Å Energy minimization calculations based on semi-empirical tight binding approach also showed that energy of (7x7) DAS was lower by 0.403eV per (1x1) unit cell than ideally terminated (1x1) [13-14] 71 CHAPTER 2 2 .2. 3 Meta-stable Si(111) reconstructions (2x1)... site with one angling bond each per unit cell • 6 restatoms with one dangling bond each per unit cell • bottom atom at the corner ring has one dangling bond The side and plane view of this model generated from an ideally terminated Si(111) are illustrated in the schematic as shown in Figure 2. 11 and 2. 12 respectively 2. 35 2. 035 (A) Side view of ideally terminated Si(111) surface 2. 15A 2. 349A 1st... 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