Scanning tunneling microscopy study on initial stage of atomic hydrogen adsorption on the Si(1 1 1) 7 × 7 surface

Initial hydrogen adsorption on the Si(1 1 1) 7 × 7 surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Room temperature adsorbed hydrogen on the adatom in the 7 × 7 reconstruction led to depression of adatoms in the STM images. The hydrogen uptake curve at the adatom site as a function of hydrogen exposure time was well represented by Langmuir adsorption. No preferential adsorption was seen among four inequivalent adatoms in the 7 × 7 reconstruction. Adsorption of the adjacent center and corner adatoms respectively showed ∼10% higher adsorption. Even though the number of reacted adatoms in the half unit of the 7 × 7 reconstruction was statistically random, the number of reacted adatoms in the nearest neighbor half unit was enhanced as the number of reacted sites increased in the half unit.     

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.     

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.     

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.     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.     

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.     

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.     

Electron-induced attachment of chlorinated benzenes to Si(1 0 0)2 × 1

Thermal (300 K) and electron-induced reactions of benzene (Bz), chlorobenzene (ClPh), 1,2-dichlorobenzene (1,2-diClPh) and 1,4-dichlorobenzene (1,4-diClPh) with Si(1 0 0)2 × 1 have been examined by scanning tunneling microscopy (STM). Thermal reactions of Bz yielded predominantly the quadruply-σ-bound tight bridge, TB, configuration on top of the Si dimer-rows, For ClPh and 1,2-diClPh, which resembled one another, thermal reaction led with 45-50% yield to the doubly-σ-bound butterfly, BF, configuration, also on top of the dimer-row, and with 20% yield to a novel ‘displaced’, D, configuration to one side of a dimer-row. The adsorbate 1,4-diClPh was alone in favouring a configuration in which neighbouring dimer-rows were ‘linked’ (L) by a bright-feature centrally located between the dimer-rows. By ab initio calculation, we interpret D as due to the rupture of one C-Cl bond per adsorbate molecule, and L to the rupture of two C-Cl’s. The breaking of this weak bond is followed in the former case by attachment of the aromatic ring to one dimer-row, and in the latter to attachment to two adjacent dimer-rows. Application of a −5 V voltage pulse to the STM tip substantially increased the percentage of row-linking structures, L, for 1,4-diClPh, but neither −5 V nor +4-6 V volt pulses resulted in L-type binding of Bz. The postulated L product of 1,4-diClPh, with an aromatic ring linking the two inner Si atoms of adjacent dimer-rows and the two Cl’s on the outer Si atoms of the dimer-rows, is shown to be in accord with ab initio simulation of the observed STM image.     

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.     

Oxygen induced surface structure of Mo(1 1 0) studied by scanning tunneling microscopy

The oxygen induced surface structures formed on Mo(1 1 0) by oxygen exposure at 1300 K in UHV has been studied by scanning tunneling microscopy (STM). Two kinds of oxygen-adsorbed surface structures are observed. One consists of one-dimensional rows running along or directions at substrate molybdenum lattices, and another shows more complex structure including discrete arrangement of large protrusions and zig-zag alignments of small protrusions. This complex structure is probably a further oxygen-adsorbed structure than the well-known p(2 × 2) structure of 0.3 ML coverage. On the basis of STM image, an atomic model is proposed, where adsorbed oxygen atoms occupy both long-bridge and the quasi-threefold sites of molybdenum lattice (0.4 ML coverage). This structure is presumed to be a transient state during site-conversion with increase of oxygen exposure.     

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.     

An STM study of the Si(0 0 1) (2 × 7)-Gd, Dy surface

Both Gd and Dy induce two different reconstructions of the Si(0 0 1) surface with 2 × 4 and 2 × 7 unit cells. Detailed examination by scanning tunneling microscopy shows that the structure of both phases is essentially the same for both metals. Furthermore, the 2 × 7 unit cell contains structural subunits that are the same as the 2 × 4 structure. The similarities and differences between the two superstructures induced by the two metals are discussed.     

Low temperature InSb(0 0 1) surface structure studied by scanning tunneling microscopy

InSb(0 0 1) surface prepared by ion sputtering and thermal annealing has been studied in the temperature range from 77 K up to 300 K using scanning tunneling microscopy (STM). At 300 K the surface is c(8 × 2) reconstructed as indicated by low energy electron diffraction and STM images, and its structure appears to be consistent with the “ζ-model” recently proposed for this surface. Upon lowering of the temperature below 180 K a new phase appears on the surface. This phase is characterized by the surface structure period doubling along [1 1 0], lowering the surface symmetry from c2mm to p2, and appearance of structural domains. Possible origins of the new phase are discussed.     

Atomic structure of Bi-dimer row selectively adsorbed on Si(5 5 12)-2 × 1 surface

In order to understand the atomic structure of nanostructures self-assembled on the template with one-dimensional symmetry, Bi/Si(5 5 12) system has been chosen and Bi-adsorption steps have been studied by STM. With Bi adsorption, the clean Si(5 5 12) is transformed to (3 3 7) terraces with disordered boundary due to mismatched periodicities between (3 3 7) and (5 5 12), and Bi-dimer rows are formed inside the (3 3 7) unit as follows: Initially, when Bi atoms are deposited at the adsorption temperature of about 450 °C, they selectively replace Si-dimers and Si-adatoms and form adsorbed Bi-dimers and Bi-adatoms, respectively. If additional Bi is supplied, the Bi-dimers adsorb on the Bi-dimers and Bi-adatoms in the first layer. These adsorbed dimers in the second layer are facing each other to form a Bi-dimer pair with relatively stable p³bonding. Finally, a single Bi-dimer adsorbs above the Bi-dimer pair in the second layer, at which point the Bi layer thickness saturates. It has been concluded that the Bi-dimer pair with stable p³ bonding is the composing element in the second layer and such site-selective adsorption is possible due to the substrate-strain relaxation through inserting Bi-buffer layer limited to specific sites of the substrate.     

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.     

Growth of Cr on Ir(1 1 1) studied by scanning tunneling microscopy

We have studied the room-temperature growth of Cr on Ir(1 1 1) by scanning tunneling microscopy. Even in the low-coverage regime, up to a total coverage of 2 monolayers (ML), Cr does not grow in the layer-by-layer mode. Instead, we observe islands with local coverages Θ between 1 ML and 5 ML. While the 1st layer growth is pseudomorphic, sporadic defect lines are observed in the 2nd layer. For Θ ? 3 ML periodic one-dimensional dislocation lines appear indicating the onset of strain relief. Scanning tunneling spectroscopy reveals that islands with Θ = 1 ML exist in two modifications. Though their tunneling spectra are qualitatively rather similar, direct comparison shows that the main peak is shifted by about 15 mV, resulting in peak positions of −0.255 V and −0.270 V. We interpret these two modifications as regular fcc Cr and Cr which exhibits a faulted hcp stacking on Ir(1 1 1), respectively. The assignment of fcc to areas directly attached to substrate steps together with the evolution of the ratio of the different ML-areas with coverage leads to the conclusion that hcp is the more favorable stacking.     

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.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

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.     

Study of Si(0 0 1)4 × 2-Ga structure by scanning tunneling microscopy and ab initio calculation

We have studied Si(0 0 1)-Ga surface structures formed at Ga coverages of slightly above 0.50 monolayer (ML) at 250 °C by scanning tunneling microscopy (STM). 4 × 2-, 5 × 2-, and 6 × 2-Ga structures were observed in a local area on the surface. The 4 × 2-Ga structure consists of three protrusions, as observed in filled- and empty-state STM images. The characters of these structures are clearly different from those of other Si(0 0 1)-Ga structures. We also performed an ab initio calculation of the energetics for several possible models for the 4 × 2-Ga structure, and clarified that the three-orthogonal-Ga-dimer model is the most stable. Also, the results of comparing the simulated STM images and observation images at various bias voltages indicate that this structural model is the most favorable.