Mechanisms of preferential adsorption on the Si(1 1 1)7 × 7 surface

The surface relaxation mechanisms governing the preferential adsorption of metal atoms onto the faulted half-cells of a 7 × 7 reconstructed Si(1 1 1) surface are studied by rate equations and kinetic Monte Carlo simulations. The versatility of these mechanisms to control the formation of quasi-perfect 2D arrays of metal clusters is revealed via the optimization of the deposition/annealing conditions as a function of operating mechanisms, the Si(1 1 1)7 × 7 energy landscape, and the thermal stability of the created clusters. The influence on the formation process of such nanoarrays of the balance between kinetic limitations, which are especially relevant on Si(1 1 1)7 × 7, and thermodynamic tendencies is discussed.     

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.     

Vanadium nanoclusters on Si(1 1 1) 7 × 7 surface studied by scanning tunneling microscopy

The size distribution and shape transition of self-assembled vanadium silicide clusters on Si(1 1 1) 7 × 7 have been investigated by scanning tunneling microscopy. Nanoclusters were formed by submonolayer vanadium deposition at room temperature followed by subsequent annealing (solid phase epitaxy - SPE). At room temperature, initially V-nanoclusters are formed which occupy sites avoiding the corner hole parts of the unit cells in the Si(1 1 1) 7 × 7 surface. Upon annealing, strong metal-silicon reaction occur leading to the formation of vanadium silicide nanoclusters. As a function of temperature, both, flat (2D) and three dimensional (3D) clusters have been obtained. After annealing at temperatures around 900 K many faceted clusters are created, whereas at higher annealing temperature, around 1300 K, predominantly 3D clusters are formed. The size distribution of SPE grown clusters could be well controlled in the range of 3-10 nm. The cluster size depends on the annealing temperature as well as on the initial vanadium coverage. Based on high resolution STM images a structure model for one kind of vanadium disilicide clusters exposing atomically flat surfaces was proposed.     

Interaction of copper with sulfur on the sulfur-terminated Si(1 1 1)-(7 × 7) surface

The adsorption of S₂ on the Si(1 1 1)-(7 × 7) surface and the interaction of copper and sulfur on this sulfur-terminated Si(1 1 1) surface have been studied using synchrotron irradiation photoemission spectroscopy and scanning tunneling microscopy. The adsorption of S₂ at room temperature results in the passivation of silicon dangling bonds of Si(1 1 1)-(7 × 7) surface. Excessive sulfur forms S_n species on the surface. Copper atoms deposited at room temperature directly interact with S-adatoms through the formations of Cu-S bonds. Upon annealing the sample at 300 °C, CuS_x nanocrystals were produced on the sulfur-terminated Si(1 1 1) surface.     

Site selective atomic chlorine adsorption on the Si(1 1 1)7 × 7 surface

The spontaneous dissociation of trichloroethylene molecules on the Si(1 1 1)7 × 7 surface was investigated using STM. Chlorine atoms were identified by using voltage dependent imaging and by observing voltage dependent tip-induced diffusion. At low coverage, we identify one chlorine that dissociates and binds to an adatom, leaving a nearby chlorovinyl group as the other product bound to the surface. Chlorine atoms show strong site selectivity for corner adatoms and some preference for the faulted half of the unit cell. This result differs significantly from previous studies of chlorine on this surface and a site-selective mobile precursor model is used to explain this discrepancy. The observed site-selectivity is consistent with the high electronegativity value for chlorine.     

Core level photoemission and STM characterization of Ta/Si(1 1 1)-7 × 7 interfaces

We present the results of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) of the Ta/Si(1 1 1)-7 × 7 system after deposition of Ta at substrate temperatures from 300 to 1250 K. The coverage of Ta varied from 0.05 up to 2.5 of a monolayer (ML). STM shows that at 300 K and coverage less than 1 ML, a disordered chemisorbed phase is formed. Deposition on a hot surface (above 500 K) produces round 3D clusters randomly distributed on the surface. Cluster height and their diameter are found to change drastically with annealing temperature and the Ta coverage. Analysis of photoemission data of the Si 2p core levels shows that at room temperature and at coverage ?1 ML core level binding energy shifts and intensity variations of Si surface related components are observed, which clearly indicate that the reaction starts already at 300 K. Shifts in the binding energy, changes of the peak shapes and intensity of the Ta 4f doublet at higher temperatures can be explained by the formation of stable silicide on the surface.     

Investigation of hydrogen interaction with the Si(1 1 1)-7 × 7 surface by STM with Bi/W tips

The STM technique with a special Bi/W tip was used to study the interaction of hydrogen atoms with the Si(1 1 1)-7 × 7 surface. The reactivity of different room temperature (RT) adsorption sites, such as adatoms (A), rest atoms (R), and corner holes (CH) was investigated. The reactivity of CH sites was found to be ∼2 times less than that of R and A sites. At temperatures higher than RT, hydrogen atoms rearrange among A, R, and CH sites, with increased occupation of R sites (T <  300 °C). Further temperature increase leads to hydrogen desorption, where its surface diffusion plays an active role. We discuss one of the possible desorption mechanisms, with the corner holes surrounded by a high potential barrier. Hydrogen atoms have a higher probability to overcome the desorption barrier rather than diffuse either into or out of the corner hole. The desorption temperature of hydrogen from CH, R, and A sites is about the same, equal to ∼500 °C. Also it is shown that hydrogen adsorption on the CH site causes slight electric charge redistribution over neighbouring adatoms, namely, increases the occupation of electronic states on A sites in the unfaulted halves of the Si(1 1 1)-7 × 7 unit cell. Based on these findings, the indirect method of investigation with conventional W tips was suggested for adsorbate interaction with CH sites.     

Early stages of vanadium deposition on Si(1 1 1)-7 × 7

We investigate the low-coverage regime of vanadium deposition on the Si(1 1 1)-7 × 7 surface using a combination of scanning tunnelling microscopy (STM) and density-functional theory (DFT) adsorption energy calculations. We theoretically identify the most stable structures in this system: (i) substitutional vanadium atoms at silicon adatom positions; (ii) interstitial vanadium atoms between silicon adatoms and rest atoms; and (iii) interstitial vanadium - silicon adatom vacancy complexes. STM images reveal two simple vanadium-related features near the Si adatom positions: bright spots at both polarities (BB) and dark spots for empty and bright spots for filled states (DB). We relate the BB spots to the interstitial structures and the DB spots to substitutional structures.     

Si nucleation on Si(1 1 1)-7 × 7: From cluster pairs to 2D islands

Nucleation of 2D islands in Si/Si(1 1 1)-7 × 7 molecular beam epitaxy is studied using scanning tunneling microscopy (STM). A detailed analysis of the population of small amorphous clusters coexisting on the surface with epitaxial 2D islands has been performed. It is shown that small clusters tend to form pairs. The pairs serve as precursors for 2D islands as confirmed by direct STM observations of the smallest 2D islands covering two adjacent half-unit cells of the 7 × 7 reconstruction. It is proved with scaling arguments that the critical nucleus for 2D island formation consists not only of the pair itself, but also includes additional adatoms not belonging to the stable clusters.     

K-induced surface structural change of Si(1 1 1)-7 × 7 probed by second-harmonic generation

The growth of thin K films on Si(1 1 1)-7 × 7 has been investigated by selecting the input and output polarizations of second-harmonic generation (SHG) at room temperature (RT) and at an elevated temperature of 350 °C. The SH intensity at 350 °C showed a monotonic increase with K coverages up to a saturated level, where low energy electron diffraction (LEED) showed a 3 × 1 reconstructed structure. The additional deposition onto the K-saturated surface at 350 °C showed only a marginal change in the SH intensity. These variations are different from the multi-component variations up to 1 ML and orders of magnitude increase due to excitation of plasmons in the multilayers at RT. The variations of SHG during desorption of K at 350 °C showed a two-step decay with a marked shoulder which most likely corresponds to the saturation K coverage of the Si(1 1 1)-3 × 1-K surface. The dominant tensor elements contributing to SHG are also identified for each surface.     

Desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips

We investigated desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips. The hole-induced desorption of chlorine atoms had a threshold bias voltage corresponding to the energy position of the S3 surface band originated in Si backbonds. The chlorine atom desorption rate was almost proportional to the square of the tunneling current. We have discussed possible mechanisms that two holes injected into Si surface states get localized at the backbonds of chlorinated Si adatoms, which induces the rupture of Cl-Si bonds to result in chlorine atom desorption.     

Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C2H5OH

Metal atom on the Si(1 1 1)-7 × 7 surface undergoes migration by hopping among Si-adatom and Si-rest atom. If the hopping migration is prohibited, how change the deposited metals? In this paper, we studied the deposition of metals on the Si(1 1 1)-7 × 7 surface saturated with C₂H₅OH, on which the whole Si-rest atoms are changed to Si-H so that the hoping migration of metals will be prohibited. We found the growth of ca. 5 nm of crystalline dots by the deposition of Sn, Zn and Ag. Interestingly, Ag dots undergo layer-by-layer growth so that the surface is covered with 5 nm size dots with uniform height. When the hopping migration is prohibited, growth of dots is controlled by the kinetics of precursor state atoms instead of the lattice energy relating to lattice matching or strain.     

Fabrication of uniform Au silicide islands on the Si(1 1 1)-(7 × 7) substrate

The Au silicide islands have been fabricated by additional deposition of Au on the prepared surface at 270 °C where the Si islands of magic sizes were formed on the Si(1 1 1)-(7 × 7) dimer-adatom-stacking fault substrate. The surface structure on the Au silicide islands shows the Au/Si(1 1 1)-√3 × √3 reconstructed structure although the substrate remains 7 × 7 DAS structure. The size of the Au silicide islands depends on the size distribution of the preformed Si islands, because the initial size and shape of the Si islands play important roles in the formation of the Au silicide island. We have achieved the fabrication of the Au silicide islands of about the same size (∼5 nm) and the same shape by controlling the initial Si growth and the additional Au growth conditions.     

Structural study of Si(1 1 1)-6 × 1-Ag surface using surface X-ray diffraction

The surface structure of Si(1 1 1)-6 × 1-Ag was investigated using surface X-ray diffraction techniques. By analyzing the CTR scattering intensities along 00 rod, the positions of the Ag and reconstructed Si atoms perpendicular to the surface were determined. The results agreed well with the HCC model proposed for a 3 × 1 structure induced by alkali-metals on a Si(1 1 1) substrate. The heights of the surface Ag and Si atoms did not move when the surface structure changed from Si(1 1 1)-√3 × √3-Ag to Si(1 1 1)-6 × 1-Ag by the desorption of the Ag atoms. From the GIXD measurement, the in-plane arrangement of the surface Ag atoms was determined. The results indicate that the Ag atoms move large distances at the phase transition between the 6 × 1 and 3 × 1 structures.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

Discrimination of individual atoms on Ge/Si(1 1 1)-(7 × 7) intermixed surface

We have discriminated individual Ge atoms from the intermixed Ge/Si(1 1 1)-(7 × 7) surface using a non-contact atomic force microscope at a room temperature environment. In fact, Si-Ge (IV-IV) binary system is considered as one of the most difficult systems for atomic discrimination among atoms in the IV group because of the similarities in the electronic and chemical properties. However, in this study, we found one of the most attractive tools to discriminate a specific atom from the others even in the difficult Si-Ge system. Ge atoms are shown as dim spots in comparison to Si atoms with bright spots on the intermixed surface by a weak chemical bonding energy and/or a relaxation effect despite large atomic radius and high spatial position in both variable frequency shift and topographic images. The discrimination of individual atoms with respect to the chemical interaction variation will further provide a chance to manipulate different atomic species and assemble various nanostructures in near future.     

Mechanism of GeH4 dissociation on Si(1 1 1)-(7 × 7)

Results of an STM study of dissociative GeH₄ adsorption on Si(1 1 1)-(7 × 7) at 300 K show that GeH₄ adsorbs under scission of two Ge-H bonds according to GeH₄(g) + 4db → GeH₂(ad) + 2H(ad). GeH₂ binds to two adatom dangling bonds in a bridged configuration, while the two released hydrogen atoms saturate two additional dangling bonds. The GeH₄ sticking coefficient under these conditions is 1.2 × 10⁻⁶, one order of magnitude smaller than for SiH₄.     

A scanning tunneling microscopy study of PH3 adsorption on Si(1 1 1)-7 × 7 surfaces, P-segregation and thermal desorption

PH₃ adsorption on Si(1 1 1)-7 × 7 was studied after various exposures between 0.3 and 60 L at room temperature by means of scanning tunneling microscopy (STM). PH₃-, PH₂-, H-reacted, and unreacted adatoms can be identified by analyzing empty-state STM images at different sample biases. PH_x-reacted rest-atoms can be observed in empty-state STM images if neighboring adatoms are hydrogen terminated. Most of the PH₃ adsorbs dissociatively on the surface, generating H- and PH₂-adsorbed rest-atom and adatom sites. Dangling-bonds at rest-atom sites are more reactive than adatom sites and the faulted half of the 7 × 7 unit cell is more reactive than the unfaulted half. Center adatoms are overwhelmingly preferred over corner adatoms for PH₂ adsorption. The saturation P coverage is ∼0.18 ML. Annealing of PH₃-reacted 7 × 7 surfaces at 900 K generates disordered, partially P-covered surfaces, but dosing PH₃ at 900 K forms P/Si(1 1 1)- surfaces. Si deposition at 510 K leaves disordered clusters on the surface, which cannot be reordered by annealing up to 800 K. However, annealing above 900 K recreates P/Si(1 1 1)- surfaces. Surface morphologies formed by sequential rapid thermal annealing are also presented.     

Germanium growth on Br-terminated Si(1 0 0)

The consequences of Ge deposition on Br-terminated Si(1 0 0) were studied with scanning tunneling microscopy at ambient temperature after annealing at 650 K. One monolayer of Br was sufficient to prevent the formation of Ge huts beyond the critical thickness of 3 ML. This is possible because Br acts as a surfactant whose presence lowered the diffusivity of Ge adatoms. Hindered mobility was manifest at low coverage through the formation of short Ge chains. Further deposition resulted in the extension and connection of the Ge chains and gave rise to the buildup of incomplete layers. The deposition of 7 ML of Ge resulted in a rough surface characterized by irregularly shaped clusters. A short 800 K anneal desorbed the Br and allowed Ge atoms to reorganize into the more energetically favorable “hut” structures produced by conventional Ge overlayer growth on Si(1 0 0).     

Spectroscopic interpretation for Na-induced phase transitions on Na/Si(1 1 1)3 × 1

Na adsorption at room temperature causes the Na/Si(1 1 1)3 × 1 surface with Na coverage of 1/3 monolayer (ML) to transit into the Na/Si(1 1 1)6 × 1 surface at 1/2 ML and sequentially into the Na/Si(1 1 1)3 × 1 surface at 2/3 ML. The phase transition was studied by Si 2p core-level photoemission spectroscopy. The detailed line shape analysis of the Si 2p core-level spectrum of the Na/Si(1 1 1)3 × 1 surface (2/3 ML) is presented and compared to the Na/Si(1 1 1)3 × 1 surface (1/3 ML) which is composed of Si honeycomb chain-channel structures. This suggests that as additional Na atoms form atomic chains resulting in the Na/Si(1 1 1)3 × 1 surface (2/3 ML), the inner atoms of the Si honeycomb chain-channel structure is buckled due to the additional Na atoms.