Origin of O 1s core-level shifts on oxygen adsorbed Si(111)-(7x7)

Density functional calculations are used to examine the chemical and structural origin of O 1s core-level shifts measured on the initial oxidation stage of Si(111)-(7x7). Our analysis of metastable core-level peaks leads to a conclusive identification of the long-sought metastable oxidation species as a tetrahedral SiO4 unit, formed by two successive O2 adsorptions on a Si adatom. The origin of a higher-binding core-level shoulder is clarified by the presence of a threefold-coordinated subsurface O atom, introduced as a decay product of the metastable SiO4 unit. The present study provides a detailed atomic-scale picture of the initial oxidation process of Si(111)-(7x7).     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

An isotopic labeling study of the diffusion mechanism during oxidation of Si(100) in water vapor by successive oxidation

The diffusion mechanism during the wet oxidation of Si(100) at 1373 K was investigated by successive oxidations finally containing isotopic water. SiO₂ was first thermally grown on Si in non-labeled oxidizing ambient (dry O₂ or H₂O) followed by isotopic water (H₂¹⁸O) to trace ¹⁸O species in SiO₂. The distributions of ¹⁶O and ¹⁸O in the oxide film were analyzed by means of secondary ion mass spectroscopy (SIMS). SIMS depth profiles show that there was a wide overlap of both isotopes (¹⁸O and ¹⁶O) throughout the SiO₂ layer, no matter whether the first oxidation step was carried out in dry O₂ or H₂O, and the concentration gradient of ¹⁸O decreased with increasing oxidation time at the second oxidation step by H₂¹⁸O. The results suggest that the diffusion mechanism in SiO₂ during water vapor oxidation is exchange diffusion; H₂O related oxidizing species diffuse through the network with significant exchange with the pre-existing oxygen in it.     

Na adsorption on the Si111-(7 x 7) surface: from two-dimensional gas to nanocluster array

We have systematically investigated Na adsorption on the Si(111)-(7 x 7) surface at room temperature using scanning tunneling microscopy (STM). Below the critical coverage of 0.08 monolayer, we find intriguing contrast modulation instead of localized Na adsorbates, coupled with streaky noise in the STM images, which is accompanied by monotonic work function drop. Above the critical coverage, Na clusters emerge and form a self-assembled array. Combined with first-principles theoretical simulations, we conclude that the Na atoms on the (7 x 7) surface are, while strongly bound ( approximately 2.2 eV) to the surface, highly mobile in "basins" around the Si rest atoms, forming a two-dimensional gas phase at the initial coverage, and that the cluster at the higher coverage consists of six Na atoms together with three Si adatoms.     

Adsorption of an organic zwitterion on a Si(1 1 1)-7 × 7 surface at room temperature

The couple sulfonato/Si(1 1 1)-7 × 7 leads to remarkable 2D chiral molecular assembly with a stability improved at room temperature. The voltage-dependency of the STM images has been experimentally investigated and the correlation between STM images and PDOS has been studied. The proposed empirical model of the adsorption of molecules on Si(1 1 1)-7 × 7 has been justified by the experimental and theoretical data.     

The Si(001) c(4×4) surface reconstruction: a comprehensive experimental study

We have carried out a comprehensive experimental study of the Si(001) c(4×4) surface reconstruction by scanning tunneling microscopy (STM) (at room temperature and elevated temperatures), Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED). Si(001) samples were kept under ultra-high vacuum (UHV) at around 550°C until the c(4×4) reconstruction appeared. STM contrast of the c(4×4) reconstruction is strongly influenced by electronic effects and changes considerably over a range of bias voltages.

The c(4×4) surface reconstruction is a result of stress which is caused by incorporation of impurities or adsorbates in sub-surface locations. The resulting c(4×4) reconstruction in the top layer is a pure silicon structure. The main structural element is a one-dimer vacancy (1-DV). At this vacancy, second layer Si-atoms rebond and cause the adjacent top Si-dimers to brighten up in the STM image at low bias voltages. At higher bias voltage the contrast is similar to Si-dimers on the (2×1) reconstructed Si(001). Therefore, besides the 1-DV and the two adjacent Si-dimers, another Si-dimer under tensile stress may complete the 4× unit cell. This is a refinement of the missing dimer model.     

STM study of low pressure adsorption of silane on Si(111)7 × 7

We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH₂ species by chemisorption and involves the breaking of Si---Si backbonds of the adatom.     

Correlations between the scanning tunneling microscopy imaged configurations and the electronic structure on stepped Si(111)-( 7x7) surfaces

We have observed the dependence of the scanning tunneling microscopy (STM) imaged atom intensity within the (7x7) unit cell on stepped Si(111) as a function of the tunneling voltage. Pronounced differences from the corresponding atom intensity on the flat surface are observed for the contrast of atoms on the low versus the high side of the step and for the contrast between the faulted versus unfaulted subcells of the (7x7) structure. These differences can be accounted for by changes in the electronic structure within the (7x7) subcells adjacent to the step. Calculations of the local density of states and the STM images using a tight-binding method are in excellent agreement with the experimental results.     

Atomic structure of the Si(103)1 × 1-In surface

To test the model that was originally proposed for the Si(103)1 × 1-Al facets and was later on tested with STM to be correct for the Ge(103)1 × 1-In facets, in the present paper we have studied the Si(103)1 × 1-In surface by means of the QKLEED/CMTA technique. A unit cell of the model consists of an indium atom, which sits in an adatom position and forms three sp²-like bonds with bulk silicon atoms, and a surface silicon atom with a dangling bond. The model has passed the QKLEED/CMTA test and the best parameters of it have been obtained. It has been noticed in the experiment that the clean Si(103) surface has a surprisingly high thermal stability.     

A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

The role of repulsive interactions in molecular bromine adsorption and patterning of Si(100)-2×1

Scanning tunneling microscopy (STM) was used to investigate the role of repulsive interactions in the adsorption and patterning of molecular bromine on the Si(100) surface. At room temperature and low coverage, chemisorption of bromine occurs dissociatively on the same side of adjacent dimers of the same row. Using the STM tip as a probe, we demonstrate the existence of repulsive interactions at adjacent sites on the Si(100)-2×1 surface. These repulsive interactions also contribute to the arrangement of adatoms on the surface. In particular, we report the presence of a stable c(4×2) surface phase that results after exposing the Si(100) surface to bromine under certain conditions. This phase involves adsorption on non-neighboring dimers and is stabilized by repulsive interactions that force bromine adatoms to occupy alternating dimers within rows with an out-of-phase occupancy between adjacent rows.     

Nucleation behaviour during silicon UHV-CVD on Si(111)7×7

We report a UHV-CVD homoepitaxy study on the Si(111)7×7 surface investigated with Scanning Tunnelling Microscopy (STM). We have investigated the two-dimensional island density in the temperature range from 450 to 550°C and silane pressure range from 2 to 8×10⁻⁴ Torr. Contrary to experiments using molecular beam epitaxy, we find that the two-dimensional island density in UHV-CVD cannot be directly described by the standard nucleation theory. We discuss this point and show that the variation of the steady-state hydrogen coverage on the surface during pressure- or temperature-dependent experiments can explain the observed two-dimensional island density behaviour.     

Oxygen adsorption on Pt(110)-(1×2): new high-coverage structures

From an interplay between scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, a comprehensive picture is obtained for oxygen adsorption on the Pt(110)-(1×2) surface, from single isolated oxygen atoms chemisorbed in FCC sites along the platinum ridges to the formation of a new high-coverage oxide-like structure with a local coverage of two oxygen atoms per platinum surface atom. We find that the repulsive O–O interactions for the O/Pt(110) system are compensated by an effective O–O attractive interaction originating from a strong coupling between oxygen adsorption and platinum lattice distortions.     

The adsorption of C6H5Cl on Si(111)7 × 7 studied by STM

The adsorption of chlorobenzene on Si(111)7 × 7 at room temperature was studied by scanning tunneling microscopy (STM). Selective chemisorption was observed at different adatom sites. It was found that the center adatoms were more reactive than the corner adatoms, and the faulted half of the unit cell was more reactive than the unfaulted. The mechanism is discussed in terms of the electronic and atomic structures in Si(111)7 × 7. Both preferences indicate that chlorobenzene was present initially in a mobile precursor state.     

Reconstruction-Determined Diffusion of Ag Adatoms on the Si(111)-(7X7) Surface

Surface diffusion is one of the basic processes determining morphology of a growing film. In the case of metal heteroepitaxy on Si(111)-(7X7) the diffusion is strongly affected by the presence of surface reconstruction, which introduces additional constraints into the motion of deposited atoms. To determine diffusion parameters we used two different approaches: i) interpretation of experimentally observed morphologies by a coarse-grained kinetic Monte Carlo model, ii) direct observation of adatom movement using UHV STM. The attempt frequency and the barrier to hopping of a single Ag atom between half-unit cells of the reconstruction were estimated in both cases. Obtained values are compared and 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.     

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.     

Scanned-probe topological and spectroscopic study of surface states on clean and Si-deposited GaAs (0 0 1)-c(4×4) surfaces

Microscopic topological and spectroscopic properties of MBE-grown GaAs c(4×4) surfaces without and with monolayer Si deposition were investigated by the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Empty state STM images on as-grown surface showed bright and dark cells, and they exhibited strong correlation with the spatial distribution of normal and anomalous conductance gaps of the STS spectra. Bias dependent STM images indicated presence of pinning areas with continuous space and energy distribution of surface gap states. By deposition of monolayer Si, dark areas reduced a great deal and the rate of finding normal STS spectra increased, indicating large reduction of surface states.     

Bi_2Ti_2O_7/Si薄膜的制备及C-V特性研究

采用化学溶液分解法 (CSD)在Si衬底上制备了Bi2 Ti2 O7薄膜 .X射线双晶衍射和原子力显微镜检测表明 ,所制备的薄膜主要为Bi2 Ti2 O7相的多晶材料 .同时还研究了Au Bi2 Ti2 O7 n Si(10 0 )结构的电容 电压 (C V)特性 ,结果表明 ,在Bi2 Ti2 O7薄膜中同时存在固定的与可移动的负电荷 ,可移动的负电荷导致了C V曲线的回线效应     

Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.