Eu- and Yb-induced reconstructions on a vicinal Si(1 0 0) surface

Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated.     

Structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) reconstructions

We have performed the structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) surfaces in the submonolayer regime utilizing low-energy electron diffraction and scanning tunneling microscopy (STM). The almost identical series of one-dimensional chain structures (e.g., 3 × 2/3 × 1, 5 × 1, 7 × 1, 9 × 1, and 2 × 1 phases) are found in order of increasing metal coverage for both adsorbed systems, however, only the Eu/Si system reveals the ‘√3’-like reconstruction before the 2 × 1 endpoint phase. The atomic models of chain structures are proposed and discussed. In particular, our results suggest the odd-order n×1 (n=5,7,9,…) intermediate reconstructions to incorporate the Seiwatz chains and honeycomb chains with the proportion of m:1, where . The statistical analysis of STM images is carried out to examine the correlation of atomic rows on Eu/Si and Yb/Si surfaces. It is found that Eu stabilizes more ordered row configuration compared to Yb, which can be explained in terms of indirect electronic interaction of atomic chains or/and different magnetic properties of adsorbed species.     

Molecular precursor-mediated methanol dissociation on Si(1 1 1)7 × 7: ab initio study

We present an ab initio study of methanol interaction with the Si(1 1 1)7 × 7 surface using a Si(1 1 1)4 × 2 model. The study of the methanol dissociation on Si(1 1 1)4 × 2 shows that pair dissociation on adatom-restatom dangling bonds is largely favoured, in agreement with the experimental observations. The “center” type adatom is slightly more reactive than the “corner” type one, although the difference is weak. Similar behaviour is observed in both adatom types. Our results for a direct CH₃OH dissociation favouring a basic cleavage (adsorption of OH and CH₃ fragments) rather than an acidic one (adsorption of H and OCH₃ fragments), we are finally led to take a kinetic effect into consideration to reconcile theory with experiment. We show that the presence of molecular precursor states is possible. Different orientations with respect to the silicon dangling bonds of these molecular precursors are investigated. However, the corresponding energies are very close and, considering their relative energies, it is finally difficult to discriminate between acidic and basic cleavages.     

Transformation to soluble model for structural phase transition from (4 × 1) to (8 × “2”) of In-adsorbed Si(1 1 1) surface

The Ising model proposed previously for the structural phase transition from (4 × 1) to (8 × “2”) of In-adsorbed Si(1 1 1) surface, Hamiltonian of which is consisting of a two-spin interaction as well as a four-spin interaction is shown to be equivalent in thermodynamic properties to a soluble Ising model with two-spin interactions. Temperature dependence of the long range order and the transition temperature can now be determined from the exact formulae. Comparison between the simulation results and those from the exact formulae is made to see accuracy of the simulation.     

Relaxations in the 1 × 5 reconstruction of Pt(1 1 0)

Pt(1 1 0) is one of the most closely investigated metal surface structures because it displays a variety of “missing-row” reconstructions, which are only marginally stable. The ground state is usually found to have 1 × 2 translational symmetry, but a 1 × 3 form has also been seen. Between 1 × 2 and 1 × 3, a series of disordered structures has been recorded, which shows a slight preference for 1 × 5 periodicity. Under the preparation conditions used in this study, a stable 1 × 5 structure was found for Pt(1 1 0). Investigation by surface X-ray diffraction has led to a complete three-dimensional structure, which closely resembles an alternation of 1 × 2 and 1 × 3 unit cells. Pt(1 1 0) shows an interesting example of two “homometric” structures that are indistinguishable by diffraction, but are distinguishable by virtue of their subsurface relaxation pattern.     

Self-assembly formation of the ordered nanostructure arrays induced by Be interaction with Si(1 1 1) surface

Formation of the beryllium (Be) submonolayers on the Si(1 1 1)7 × 7 surface has been studied using scanning tunneling microscopy. It has been found that Be interaction with Si(1 1 1) at 500-700 °C results in a self-assembly formation of the four various types of the highly-ordered nanostructure arrays. The nanostructure arrays develop on top of the “soft” silicide layer, which period and orientation alter with the nanostructure growth: the shorter the nanostructure period, the larger the rotation angle. The main structural parameters of the silicide layer and nanostructure arrays have been established.     

The dense α-√3 × √3Pb/Si(1 1 1) phase: A comprehensive STM and SPA-LEED study of ordering, phase transitions and interactions

The T-θ phase diagram for the system Pb/Si(1 1 1) was determined in the coverage range 6/5 ML < θ < 4/3 ML from complementary STM and SPA-LEED experiments. This coverage is within the range where a “Devil’s Staircase” (DS) has been realized. The numerous DS phases answer conflicting information in the Pb/Si(1 1 1) literature and update the previously published phase diagram. The measurements reveal the thermal stability of the different linear DS phases with the transition temperature found to be a function of phase period. Because of additional complexity in the experimental system (i.e. two-dimensionality and 3-fold symmetry) the linear DS phases transform at higher temperature into commensurate phases of 3-fold symmetry HIC (historically named “hexagonal incommensurate phase”). Different types of HIC phases have been discovered differing in the size of the supercell built out of √3 × √3 domains separated by domain walls of the √7 × √3 phase. The detailed structures of these HIC phases (coverage, binding site, twist angle, etc.) have been deduced from the comparison of STM images and diffraction patterns. After heating the system to even higher temperature the HIC phase transforms into the disordered phase. For sufficiently high coverage a SIC (“striped incommensurate phase” which is also built from √3 × √3 domains but meandering √7 × √3 domain walls) is observed which also disorders at high temperatures.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

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.     

In situ Video-STM study of the potential-induced (1 × 1) → “hex” transition on Au(1 0 0) electrode surfaces in Cl containing solution

The potential-induced (1 × 1) → “hex” transition on Au(1 0 0) electrodes in 0.01 M Na₂SO₄ + 1 mM HCl was studied by in situ scanning tunneling microscopy at high time resolution (Video-STM). According to these observations the elementary units of the “hex” surface reconstruction, hexagonally-ordered strings in the Au surface layer, are highly dynamic nanoscale objects. Isolated “hex” strings exhibit dynamic fluctuations in structure and position on the millisecond timescale. These fluctuations exceed the mobility of multistring “hex” domains by several orders of magnitude and can be explained by collective dynamic processes within the strings. Furthermore, the observations reveal a novel 1D mass transport mechanism along the strings, details on the nucleation and growth of “hex” strings and complex string restructuring processes, facilitating “hex” domain ripening.     

Asymmetric adsorption-site of potassium atoms in the (3 × 2)-p2mg structure formed on Cu(0 0 1)

The adsorption of K atoms on Cu(0 0 1) has been studied by low-energy electron diffraction (LEED) at room temperature (RT) and 130 K. At RT, a (3 × 2)-p2mg LEED pattern with single-domain was observed at coverage of 0.33, whereas the orthogonal two-domain was found at 130 K. At 130 K, a c(4 × 2) pattern with orthogonal two-domain was observed at coverage 0.25. Both the (3 × 2)-p2mg and c(4 × 2) structures have been determined by a tensor LEED analysis. It is demonstrated that K atoms are adsorbed on surface fourfold hollow sites in the c(4 × 2), while in the (3 × 2) structure two K atoms in the unit cell are located at an asymmetric site with a glide-reflection-symmetry. The asymmetric site is at near the midpoint between the exact hollow site and bridge-site but slightly close to the hollow site. A rumpling of 0.07 Å in the first Cu layer was confirmed, which might stabilize K atoms at the asymmetric site. Surface structures appearing in a coverage range 0.25-0.33 are discussed in terms of the occupation of the asymmetric site with increase of coverage.     

Atomic structure of the Si(5 5 12)-2 × 1 surface

Based on the results of scanning tunneling microscopy studies of the reconstructed Si(5 5 12)-2 × 1 surface, its atomic structure has been found. It turns out that Si(5 5 12)-2 × 1 consists of four one-dimensional structures: honeycomb (H) chain, π-bonded H′ (π) chain, dimer-adatom (D/A) row, and tetramer (T) row. Its period is composed of three subunits, i.e., (i) (3 3 7) unit with a D/A row [D(3 3 7)], (ii) (3 3 7) unit with a T row [T(3 3 7)], and (iii) (2 2 5) unit with both a D/A and a T row. Two kinds of adjacent subunits, T(3 3 7)/D(3 3 7) and D(3 3 7)/(2 2 5), are divided by H chains with 2× periodicity due to buckling, while one kind of adjacent subunits, T(3 3 7)/(2 2 5), is divided by a π chain with 1× periodicity. Two chain structures, H and π chains, commute with each other depending upon the external stresses perpendicular to the chain, which is the same for two row structures, D/A and T rows. It can be concluded that the wide and planar reconstruction of Si(5 5 12)-2 × 1 is originates from the stress balance among two commutable chains and two commutable rows.     

A structural parallel between Ge- and Si-induced 4 × 4 and 3 × 3 reconstructions on SiC(0 0 0 1) drawn from comparative RHEED oscillations

The initial Ge growth stages on a (√3 × √3)R30°-reconstructed SiC(0 0 0 1) surface (√3) have been studied using a complete set of surface techniques such as reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), atomic force microscopy (AFM) and photoemission and compared with similar Si surface enrichments in place of Ge. The investigations essentially focus on the wetting growth-regimes that are favoured by the use of the √3 surface as a starting substrate, this surface being the closest to a smooth and ideally truncated Si-terminated face of hexagonal SiC(0 0 0 1). Depending on temperature and Ge or Si coverages, varying surface organizations are obtained. They range from unorganized layer by layer growths to relaxed Ge(1 1 1) or Si(1 1 1) island growths, through intermediate attempts of coherent and strained Ge or Si surface layers, characterized by 4 × 4 and 3 × 3 surface reconstructions, respectively. RHEED intensity oscillation recordings, as a function of Ge or Si deposited amounts, have been particularly helpful to pinpoint the limited (by the high lattice mismatch) existence domains of these interesting coherent phases, either in terms of formation temperature or surface coverages. Prominently comparable data for these two Ge- and Si-related reconstructions allow us to propose an atomic model for the still unexplained Ge-4 × 4 one. It is based on a same local organization in trimer and ad-atom units for the Ge excess as admitted for the Si-excess of the 3 × 3 surface, the higher strain nevertheless favouring arrangements, for the Ge-units, in 4 × 4 arrays instead of 3 × 3 Si ones. Admitting such models, 1.25 and 1.44 monolayers of Ge and Si, should, respectively, be able to lie coherently on SiC, with respective lattice mismatches near 30% and 25%. The experimental RHEED-oscillations values are compatible with such theoretical ones. Moreover, these RHEED coverage determinations (for layer completion, for instance) inform us in turn about the initial Si richness of the starting √3 reconstruction and help us to discriminate between earlier contradictory atomic models proposed in the literature.     

Theoretical STM images of alkaline-earth metal adsorbed Si(1 1 1)3 × 2 surfaces

We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

Growth of Si nanostructures on Ag(0 0 1)

The first stages of the growth of silicon on Ag(0 0 1) at moderate temperatures start by the formation of a p(3 × 3) superstructure, which continuously evolves with increasing coverage toward a more complex superstructure. In this paper, the atomic arrangement of the p(3 × 3) and of the “complex” superstructure has been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 × 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(0 0 1) surface layer. For higher coverages, i.e., when the “complex” superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure.     

First-principles study of indium on silicon (1 0 0)—the structure, defects and interdiffusion

The atomic structures of indium (In) on silicon (Si) (1 0 0)-(2 × 1) surface are investigated by the local density approximation using first-principles pseudopotentials. Total energy optimizations show that the energetically favored structure is the parallel ad-dimer model. The adsorption energy of In on ideal Si(1 0 0)-(1 × 1) surface is significantly higher than that on reconstructed Si(1 0 0)-(2 × 1) surface, suggesting that In adsorption does not break the Si-Si dimer bond of the substrate. When Si surface contains single dimer vacancy defects, In chain will be interrupted, leading to disconnected In nanowires. Displacive adsorption of In on Si(1 0 0) is also considered, and the calculation suggests that interdiffusion of In into Si substrate will not be favorable under equilibrium conditions.     

Atomic structure of Cs layer grown on Si(0 0 1)(2 × 1) surface at room temperature

The atomic structure of Cs atoms adsorbed on the Si(0 0 1)(2 × 1) surface has been investigated by coaxial impact collision ion scattering spectroscopy. When 0.5 ML of Cs atoms are adsorbed on Si(0 0 1) at room temperature, it is found that Cs atoms occupy a single absorption site on T3 with a height of 3.18 ± 0.05 Å from the second layer of Si(0 0 1)(2 × 1) surface, and the bond length between Cs and the nearest Si atoms is 3.71 ± 0.05 Å.     

Effects of electronic confinement and substrate on the low-temperature growth of Pb islands on Si(1 0 0)-2 × 1 surfaces

The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate.