A scanning tunneling microscopy study of water on Si(111)-(7 × 7): adsorption and oxide desorption

Scanning tunneling microscopy and temperature-programmed desorption have been used to investigate the chemistry of water on Si(111)-(7 × 7) substrates which were misoriented 2° toward the [ 10] direction. Upon room temperature exposure to water, the adatoms of the (7 × 7) unit cell are still evident even after high exposures, implying that major modifications of the substrate do not occur. At high coverages, the distribution of reacted adatoms shifts from one controlled by dissociative adsorption across the adatom-rest atom pair to a statistical distribution based on the availability of dangling bonds. Desorption of the oxide layer which remains after water adsorption and the desorption of hydrogen have also been characterized. The oxide desorption occurs along well-defined wavefronts which originate at step edges and advance in directions consistent with the underlying substrate symmetry, primarily the [ 2] direction (i.e. the wave vector points in the [ 2] direction). In regions of the surface where the oxide has desorbed, the (7 × 7) unit cell can be seen clearly. Vacancies resulting from the loss of surface silicon atoms (via the etching) coalesce into islands in the clean regions of the terraces, but unlike desorption of oxide layers from Si(100), the desorption does not occur from the boundaries of these vacancy islands.     

Bonding configurations and collective patterns of Ge atoms adsorbed on Si(111)-(7 x 7)

We report scanning tunneling microscopy observations of Ge deposited on the Si(111)-(7 x 7) surface for a sequence of submonolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, until small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. Thus, the observed structures are indeed more thermodynamically stable.     

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.     

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

Growth mechanism of the Pd(100)-p(2×2)-p4g-Al surface alloy

We have studied the growth mechanism of a Pd(100)-p(2×2)-p4g-Al surface alloy by scanning tunneling microscopy (STM). The surface alloy has a bilayer structure and is formed by annealing at 450–700 K (depending on the initial aluminum coverage) after the deposition of aluminum on Pd(100) at room temperature. The ratio of the surface-alloy coverage to the initial aluminum coverage is found to be constant (0.44) irrespective of the initial aluminum coverage from 0.5 monolayers (ML) up to 2 ML. The growth mechanism of the surface alloy is proposed on the basis of the STM measurements at various annealing temperatures. Upon annealing at 450 K, some of the surface aluminum atoms migrate into the bulk and, instead, palladium atoms come out to the surface. These palladium atoms react with aluminum atoms remaining on the surface to form a surface alloy. When the initial aluminum coverage is less than 1 ML, bilayer-high islands of the surface alloy with an average area of 100 nm² are formed at 450–500 K, which diffuse on the terrace at 500–700 K and coalesce to form larger islands. A possible role of the percolation transition of aluminum islands in the formation of the surface alloy is discussed.     

Random distribution of islands explains the oscillatory intensity in He/Si(111)-(7 × 7)

It is shown that the oscillatory specular beam intensity observed in He/Si(111)-(7 × 7) scattering is consistent with a random distribution of islands of height ~3.05 Å. Each of these islands may be reconstructed, totally or partially, in a (7 × 7) structure according to the more favourable models proposed by LEED. This could be a possibility to explain, with the same (7 × 7) unit cell, He scattering and LEED experiments.     

Effect of oxygen on the stability of Ag islands on Si(111)-7 × 7

We have used scanning tunneling microscopy to probe the effect of oxygen exposure on an ensemble of Ag islands separated by a Ag wetting layer on Si(111)-7 × 7. Starting from a distribution dominated by islands that are 1 layer high (measured with respect to the wetting layer), coarsening in ultrahigh vacuum at room temperature leads to growth of 2-layer islands at the expense of 1-layer islands, which is expected. If the sample is exposed to oxygen, 3-layer islands are favored, which is unexpected. There is no evidence for oxygen adsorption on top of Ag islands, but there is clear evidence for adsorption in the wetting layer. Several possible explanations are considered.     

Peculiarities of the initial stage of growth of niobium-based nanostructures on a Si(111)-7 × 7 surface

The initial stage of growth of nanoislands prepared by thermal deposition of niobium on the reconstructed surface of Si(111)-7 × 7 in ultrahigh vacuum is experimentally investigated. The morphological and electrophysical properties of niobium-based nanostructures are studied by means of low-temperature scanning tunneling microscopy and spectroscopy. It is found that upon the deposition of niobium on a substrate at room temperature, clusters and nanoislands are formed on the silicon surface, having a characteristic lateral size of 10 nm with the metallic type of tunneling conductivity at low temperatures. Upon the deposition of niobium on a heated substrate, quasi-one-dimensional (1D) and quasi-two-dimensional (2D) structures with typical lateral dimensions of up to 200 nm and three-dimensional pyramidal islands with semiconducting type of tunneling conductivity at low temperatures are formed.     

Chemisorption and ordering of naphthalene and azulene on Pt(s)[7(111) × (100)]: The effect of periodic defects on long range order

Naphthalene and azulene are molecularly adsorbed on the stepped platinum surface Pt(s)[7(111)×(100)] at room temperature. The (111) terraces of this surface are wide enough to accommodate the unit cell of ordered naphthalene on Pt(111). The periodic defects of this surface disrupt the long range ordering seen for both naphthalene and azulene on Pt(111). There appears to be no correlation between ordered islands on neighboring terraces. In the case of naphthalene, a LEED intensity anomaly allows us to place restrictions on the relative orientations of neighboring naphthalenes and the orientation of the naphthalene islands on each terrace. The naphthalene molecules adjacent to the steps have the long axis of the molecule close to perpendicular to the step.     

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.     

Formation of ordered arrays of Ag nanowires and nanodots on Si(5 5 7) surface

The ordered arrays of Ag nanowires and nanodots have been grown in ultra-high vacuum on the Si(5 5 7) surface containing regular steps of three bilayer height. Formation of Ag nanostructures have been studied by scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy at room temperature. It was shown that a sample exposure in the vacuum before Ag growth affects the shape of the forming Ag islands. This effect is caused by oxygen adsorption on the silicon surface from the residual atmosphere in the vacuum chamber. When Ag is deposited on the clean silicon surface the islands, overlapping several (1 1 1) neighboring terraces, form. The arrays of silver nanowires elongated along steps and silver nanodots, arranged in lines parallel to the steps, can be formed on the Si(5 5 7) surface depending on the amount of adsorbed oxygen.     

Au island growth on a Si(1 1 1) vicinal surface

Au island nucleation and growth on a Si(1 1 1) 7 × 7 vicinal surface was studied by means of scanning tunneling microscopy. The surface was prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255-430 °C substrate temperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au-Si reconstructed islands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density, size and position on substrate temperature and on Au coverage is investigated. At 340 °C and above, hemispherical 3D islands nucleate systematically on the step edges.     

Comparison of the Si(111) (7 × 7) and (1×1)surfaces

High-resolution electron energy-loss spectroscopy and monochromatic low-energy electron diffraction have been applied to the study of the Si(111)(7 × 7) surface and the thermally-quenched Si(111) (1 × 1) surface. For the (1 × 1) surface, the inelastic continuum, observed for the (7 × 7) surface, due to the Drude absorption of electrons in the dangling-bond surface states is not existent, which indicates that the surface-state band associated with the dangling-bonds of the (1 × 1) surface is insulating. The observed electronic transitions indicate that the (7 × 7) and (1 × 1) surfaces have similar local band geometries and that they differ only in long-range order. The (1 × 1) surface is considered to have a disordered structure. The defect model is favored for the (7 × 7) structure.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.     

Initial stages of Ge epitaxy on Si(111) under quasi-equilibrium growth conditions

The results of the structural and morphological studies of Ge growth on a Si(111) surface at the initial stages of epitaxy by means of scanning tunneling microscopy and high-resolution transmission electron microscopy are presented. Epitaxy of Ge has been performed in the temperature range of 300 to 550°C under the quasi-equilibrium growth conditions and low deposition rates of 0.001–0.01 bilayers per minute. The stages of the formation and decay of the nanoclusters as a result of the redistribution of the Ge atoms into two-dimensional pseudomorphic Ge islands before the formation of the continuous wetting layer have been experimentally detected. The positions of the preferable nucleation of three-dimensional Ge islands on the wetting layer formed after the coalescence of the two-dimensional islands have been analyzed. The c2 × 8 → 7 × 7 → c2 × 8 phase transitions due to the lateral growth of the islands and the plastic relaxation of the misfit strains occur on the surface of the three-dimensional Ge islands when their strain state changes. The misfit dislocations gather at the interface and two types of steps lower than one bilayer are formed on the surface of the three-dimensional islands during the relaxation process.     

Link between adatom resonances and the Cu(111) Shockley surface state

Low-temperature scanning tunneling microscopy and spectroscopy at 7 K was used to assemble and characterize native adatom islands of successive size on the Cu(111) surface. Starting from the single adatom we observe the formation of a series of quantum states which merge into the well known two-dimensional Shockley surface state in the limit of large islands. Our experiments reveal a natural physical link between this fundamental surface property and the sp(z) hybrid resonance associated with the single Cu/Cu(111) adatom.     

Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.     

Structural analogy between GeSi(111)-5 × 5 and Si(111)-7 × 7 surfaces

Low energy electron diffraction (LEED) patterns for the GeSi(111)-5 × 5 surface are reported and compared to those for the Si(111)-7 × 7 surface. Parallels between the observed LEED patterns are explained by a structural analogy between GeSi(111)-5 × 5 and Si(111)-7 × 7 surfaces. Both the (5 × 5) and (7 × 7) patterns are shown to be consistent with structural models of the triangle-dimer type previously proposed for Si(111)-7 × 7 surface.     

Ge(111) 7 × 7 surface structure induced by Sn

When tin was deposited by 0.3–0.8 monolayer thick on Ge(111) clean surface at room temperature and annealed at 345°C, 7 × 7 superlattice structure appeared on the surface. Features of the diffraction pattern of the 7 × 7 structure are strikingly similar to those of the diffraction pattern of Si(111) 7 × 7 surface structure, which suggests that atomic arrangements in the two 7 × 7 structures resemble each other very well.     

Growth of Ge islands on prepatterned Si (0 0 1) substrates

We report on the growth and properties of Ge islands grown on (0 0 1) Si substrates with lithographically defined two-dimensionally periodic pits. After thermal desorption and a subsequent Si buffer layer growth these pits have an inverted truncated pyramid shape. We observe that on such prepatterned substrates lens-like Ge-rich islands grow at the pit bottoms with less Ge deposition than necessary for island formation on flat substrates. This is attributed to the aggregation of Ge at the bottom of the pits, due to Ge migration from the pit sidewalls. At the later stages of growth, dome-like islands with dominant {1,1,3} or {15,3,23}, or other high-index facets [i.e. {15,3,20} facets] are formed on the patterned substrates as shown by surface orientation maps using atomic force microscopy. Furthermore, larger coherent islands can be grown on patterned substrates as compared to Ge deposition on flat ones.