Preferential heights in the growth of Ag islands on Si(1 1 1)-(7 × 7) surfaces

Growth behavior of thin Ag films on Si substrates at room temperature has been investigated by scanning tunneling microscopy and reflection high energy electron diffraction. In the layer-plus-island growth Ag islands show strongly preferred atomic scale heights and flat top. At low coverage (1 ML), islands containing two atomic layers of Ag are overwhelmingly formed. At higher coverages island height distribution shows strong peaks at relative heights corresponding to an even number (2, 4, 6, …) of Ag atomic layers. Beyond some coverage the height preference vanishes due to the appearance of screw dislocations and spiral growth.     

Edge-dimer row - The reason of three-bilayer steps and islands stability on Si(1 1 1)-7 × 7

A hypothesis of perpendicular dimer row formation along three-bilayer (3 BL) step was suggested. The hypothesis, explains the stability of 3 BL steps on the vicinal Si(1 1 1) surface deflected in direction as well as the limitation of Ge and Si island height by 3 BL at the initial nucleation stages on Si(1 1 1) surface. The detailed examinations of STM images of 3 BL steps were carried out. New peculiarities of atomic structure of 3 BL single step on Si(1 1 1) and 3 BL steps on Si(5 5 7) surfaces were revealed. The results of STM images examination verify the hypothesis of perpendicular dimer row formation along the boundary of the 3 BL step.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

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.     

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.     

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.     

STM and LEED observations of a c(2 × 2) Ge overlayer on Pt(1 0 0)

We have investigated surface structures formed by deposition of 0.2 and 0.5-ML Ge on Pt(1 0 0) by using scanning tunneling microscopy (STM) and low electron energy diffraction (LEED). In addition, their temperature dependence and reactivity to CO have been studied. We observed the formation of disordered domains for Ge adatom coverages below 0.25-ML and complete c(2 × 2) structures at 0.25 to 0.5-ML Ge after annealing at 600-1200 K. Deposition of 0.2-ML Ge on a clean, hexagonally reconstructed (5 × 20)-Pt(1 0 0) substrate at 400 K lifts the reconstruction and ejects excess Pt atoms from the first layer into the adlayer. After annealing this surface to 600 K, the deposited Ge formed Ge adatoms on flat terraces and on round Pt adislands with incomplete c(2 × 2) structures, in addition to the presence of clean (1 × 1)-Pt(1 0 0) domains that were several nanometers across. Some domains of the unreconstructed (5 × 20)-Pt(1 0 0) surface still remained. After the deposition of 0.5-ML Ge and annealing at 600 K, disordered Ge domains disappeared and a c(2 × 2) Ge overlayer was produced all over the surface. Square terraces with square domains of the clean (1 × 1)-Pt(1 0 0) surface extended for nanometers. Annealing this surface to 900 K produced disordered Ge domains, and this was associated with an increase in Ge vacancies. When surfaces with 0.2-ML Ge were heated to 900 or 1200 K, or when a surface with 0.5-ML Ge was heated to 1200 K, larger domains of (5 × 20)-Pt(1 0 0) were formed with the agglomeration of disordered Ge adatoms. Pt clusters were observed in the Ge domains, and we consider these to be composed of those excess Pt atoms formed by lifting the reconstruction of the (5 × 20)-Pt(1 0 0) surface upon Ge agglomeration during cooling. A paper published elsewhere [T. Matsumoto, C. Ho, M. Batzill, B.E. Koel, Physical Review B, submitted for publication.] describes Na⁺-ion scattering spectroscopy (Na⁺-ISS) and X-ray photoelectron diffraction (XPD) experiments that distinguish between Ge present in an overlayer from incorporation into the top Pt layer to form a surface alloy for the surface structures reported here. Furthermore, these investigations revealed that disordered Ge adatoms observed herein might be associated with incomplete c(2 × 2) structures. Therefore, our observations of the formation of complete and incomplete domains of c(2 × 2) Ge adatoms indicate that interactions between Ge adatoms are repulsive at nearest neighbor distances and attractive at second-nearest neighbor distances. Regarding the reactivity of these surfaces, CO does not chemisorb on a Pt(1 0 0) surface with a c(2 × 2)-Ge overlayer and no measurable CO uptake was observed under UHV conditions at 220 K.     

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.     

Low reactivity of molecular oxygen with Si(1 1 1)-c(2 × 8)

The adsorption of molecular oxygen on the c(2 × 8) reconstruction of quenched Si(1 1 1) surfaces has been studied at the atomic scale using scanning tunneling microscopy (STM) at room temperature (RT). It has been found that clean well reconstructed c(2 × 8) adatoms do not react with O₂ molecules but that a limited oxidation can start where adatom sites arranged in reconstructed structures are present. Comparison between O₂ adsorption on Si(1 1 1)-c(2 × 8) and Si(1 1 1)-7 × 7 reconstructions coexisting on the same quenched silicon surface has been carried out in detail. For each atomic site present on the surface the variation of reacted sites with exposure has been measured. For low O₂ exposures, bright and dark oxygen induced sites appear on the Si(1 1 1)-7 × 7, while Si(1 1 1)-c(2 × 8) does not oxidized at all. At high O₂ exposures, large oxidation areas have spread on the 7 × 7 reconstruction, preferentially on the faulted halves of the unit cell, and smaller oxidation areas induced by topological defects have grown all around clean un-reacted c(2 × 8) regions.     

Anisotropic kinetics on growing Ge(0 0 1) surfaces

Reflection high-energy electron diffraction (RHEED), reflectance difference spectroscopy (RDS), and scanning tunneling microscopy (STM) have been used to study the anisotropic kinetics on the growing Ge(0 0 1) surface. While switching of dimer direction in alternate (2 × 1)/(1 × 2) layers causes the bilayer-period oscillations in RD response, RHEED oscillations are governed by variations in surface step densities. We show that the RHEED oscillations are strongly affected by the growth front morphology: when the growth front becomes distributed over several layers, the transition from bilayer- to monolayer-period occurs in RHEED oscillations.     

Hopping motion of chlorine atoms on Si(1 0 0)-(2 × 1) surfaces induced by carrier injection from scanning tunneling microscope tips

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of V_S∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si-Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si-Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms.     

Dynamics of copper atoms on Si(1 1 1)-7 × 7 surfaces

This study investigated the dynamics of copper atoms adsorbed on Si(1 1 1)-7 × 7 surfaces between 300 K and 623 K using a variable-temperature scanning tunneling microscope (STM). The diffusion behavior of copper clusters containing up to ∼6 atoms into a particular half unit cell of the 7 × 7 reconstructed Si(1 1 1) surface was considered. The movements and the formation of copper clusters were tracked in detail. The activation energies and pre-exponential factors for various diffusion paths were estimated. Finally, the Cu-etching-Si process and the quasi-5 × 5 incommensurated phase of Cu/Si islands were discussed.     

Phase discrimination of iron-silicides on Si(0 0 1) surfaces by three-dimensional reciprocal-lattice mapping

Various iron-silicides are grown on clean Si(0 0 1) surfaces by solid phase epitaxy, a process which involves the deposition of iron and subsequent annealing [6]. Among them, we studied the structure of three-dimensional (3D) elongated islands, which are the major silicide type produced at lower Fe coverage (∼1 monolayer) and ?500-600 °C annealing. We applied a newly developed method of azimuth-scan reflection high-energy electron diffraction (RHEED) to obtain 3D reciprocal-lattice mapping. We succeeded in discriminating an α-FeSi₂ phase from controversial bulk phases of the islands, and we were also able to determine the orientation relation as and , where the lattice mismatches are −1% in direction and +34% in direction. The attenuation of the incident electron beam along the length direction of the islands leads to extremely weak spots in the RHEED pattern. We emphasize that such an analysis of the reciprocal-lattice mapping is also useful in studying other 3D island structures. Using scanning tunneling microscopy, we showed that the island’s elongated directions are perpendicular to the dimer rows of the substrate located under the islands. The islands are located near the S_B step edges. The elongation lengths of the islands are almost the same as the widths of the Si substrate terraces. We discussed the formation mechanism of the 3D-elongated islands. From an atomic image of the facet and edge of a 3D-elongated island, we proposed an atomic-structure model of the island facet and edge: a Si adatom on the hollow site of four Si atoms of an unit, with ordering in the direction of the elongation, forming an facet locally.     

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.     

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.     

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.     

STM study of Si(1 1 3) 3 × 2-Ga surface

The Ga-adsorbed structure on Si(1 1 3) surface at low coverage has been studied by scanning tunneling microscopy (STM). The bright protrusion corresponding to the position of the dimer without the interstitial Si atom of the clean surface disappeared in the filled-state STM image after Ga adsorption, although the protrusion due to the Si adatom still remained. On the basis of the adatom-dimer-interstitial (ADI) model, this result indicates that the Ga atom is adsorbed interstitially at the center of another pentamer that does not have the interstitial Si atom. An ab initio calculation was performed and STM images were simulated.     

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.     

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.