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The growth of cobalt silicide nanostructures on clean Si(001) was studied using scanning tunnelling microscopy and transmission electron microscopy. Two types of CoSi2 nanostructures, flat and ridge-type islands, were formed when 0.1 ML Co was deposited onto clean Si(001) between 500 °C and 800 °C. These islands form elongated islands along [110] directions and grow into the Si-substrate within the temperature range. The formation of the two types of islands arises primarily due to the type of CoSi2{111}–Si{111} interface formed between the island and the substrate. Flat islands are bound by CoSi2{111}–Si{111} Type-A interfaces such that CoSi2(001)//Si(001) and CoSi2[001]//Si[001]. Ridge islands, on the other hand, are bound by a “twinned” CoSi2{111}–Si{111} Type-B interface such that CoSi2(221)//Si(001) and CoSi2 [ 1 1 ¯ 0 ] //Si [ 1 1 ¯ 0 ] . This leads to the formation of three less energetically-favourable interfaces: CoSi2 ( 1 ¯ 1 ¯ 1 ¯ ) –Si ( 11 5 ¯ ) , CoSi2 ( 1 ¯ 1 ¯ 2 ¯ ) –Si ( 11 2 ¯ ) , and CoSi2 ( 1 ¯ 1 ¯ 5 ¯ ) –Si ( 11 1 ¯ ) . Analysis of the interfacial energies through dangling bond counting per interfacial area for each interface shows that the formation of the Type-B interface is energetically more favourable compared to the rest of the interfaces. As a result, the island elongates preferentially along the Type B interface leading to the formation of long nanowires with large length–width aspect ratio of 20:1. However, this formation is only achieved at high growth temperatures due to the presence of corner-barriers constraining the growth at low temperatures. Conversely, flat islands are slightly elongated at low growth temperatures with aspect ratio reaching 7:1 at 650 °C. As temperature increases towards 760 °C, they are brought closer to equilibrium and hence become less elongated with aspect ratio reduced to 1.6:1.

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi 2 -like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.

Scanning tunneling microscopy (STM) is used to probe the local atomic structure of the 6H-SiC(0 0 0 1) surface together with X-ray photoelectron spectroscopy (XPS) in UHV. We report STM observation of localized clusters assembled in an ordered (6 × 6) arrangement after annealing the (3 × 3) phase at 1000 °C. These clusters are round in shape and possess a diameter of 14.3 ± 0.5 A and a height of 2.3 ± 0.2 A. Further annealing of the surface beyond 1000 °C caused the clusters to disappear and we observed the formation of a (6 × 6) ring-like structure at 1050 °C. XPS data show that the surface is still Si rich, unlike the (6√3 × 6√3) phase or graphitic honeycomb structures which normally formed above this temperature. Observation of defect and cluster formation, as well as tetramer agglomeration, suggests the rearrangement of the (3 × 3) structure as temperature increases. STM is used to study this mechanism leading to the formation of (6 × 6) clusters. We propose a model utilizing Si tetra-cluster units as building blocks to elucidate the structure of the (6 × 6) clusters and explain the structural transformation observed.

In situ X-ray photoelectron spectroscopy (XPS) reveals that deposited Co atoms at room temperature react with Si at the growth front to form a thin “CoSi 2 -like” layer on both clean and hydrogen passivated Si(0 0 1) and Si(1 1 1) surfaces. Improvement in the silicides crystallinity upon annealing was characterised by an appreciable decrease in the full width at half maximum of the Co 2p 3/2 XPS spectra and a shift in binding energy towards Co in the bulk CoSi 2 crystal structure. Unlike the Si(0 0 1) substrate, the presence of hydrogen on the Si(1 1 1) surface appears to delay the decrease in the peak area ratio of Co 2p 3/2 /Si 2p as annealing temperatures increase. Ex situ surface morphology imaged by atomic force microscopy suggests a reduced adatom mobility on the Co/H-passivated Si surface compared to the Co/clean Si surface, as evidenced by a higher and smaller size CoSi 2 islands density observed on Co deposited/H-terminated Si surfaces after annealing to 700 °C.

We demonstrate the preferential formation and self-assembly of monodisperse Si magic clusters $({X}_{4})$ of size $\ensuremath{\sim}13.5\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{\AA{}}$ on $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface using scanning tunneling microscope. The growth process is observed to occur via a stepwise assembly of planarized Si tetramers $({X}_{1})$ formed from Si adatoms deposited at room temperature, leading to Si tetraclusters $({X}_{2})$ (size $\ensuremath{\sim}4.6\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{\AA{}}$) and culminating in tetracluster dimer $({X}_{3})$ and trimer $({X}_{4})$ formations as the surface is being annealed progressively to $150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The respective cluster species density distribution at each annealing temperature also shows the preferential formation of ${X}_{1}\ensuremath{\rightarrow}{X}_{2}\ensuremath{\rightarrow}{X}_{3}\ensuremath{\rightarrow}{X}_{4}$ at higher temperatures, which we describe using surface reaction schemes; ${X}_{1}\ensuremath{\rightarrow}{X}_{2}$, ${X}_{2}+{X}_{2}\ensuremath{\rightarrow}{X}_{3}$, and ${X}_{2}+{X}_{3}\ensuremath{\rightarrow}{X}_{4}$. We determine the activation and formation energies for respective cluster species and elucidate the formation energetics and dynamics of tetraclusters which function unequivocally as fundamental building blocks in the self-assembly of stable Si magic clusters. Finally, we resolve the structure of the Si magic cluster to comprise three tetraclusters or $n=12$ Si atoms taking into consideration (i) cluster symmetry and alignment, (ii) close packing, and (iii) minimization of dangling bonds.