STM/nc-AFM investigation of (n×6) reconstructed GaAs(0 0 1) surface

We have investigated the n×6 reconstructed GaAs(0 0 1) surface with scanning tunneling and non-contact atomic force microscopy techniques (STM/nc-AFM). For the first time atomically resolved nc-AFM images of that surface are shown. The images confirm the presence of rows of arsenic dimers in the topmost layer as predicted by the current model of n×6 reconstructed surface. However, in contrast to previous reports we found that postulated As dimer sites are not fully occupied. Moreover, the images suggest that ×6 symmetry is present on the surface even in absence of the dimers. We show that due to probing of different surface properties nc-AFM and STM are complementary tools for complex surfaces investigation.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

The mesoscopic and microscopic structural consequences from decomposition and desorption of ultrathin oxide layers on Si(100) studied by scanning tunneling microscopy

The spatially inhomogeneous decomposition and desorption reaction of oxide layers with coverage 1-0.3 monolayers (ML) from a silicon (100) surface has been studied using scanning tunneling microscopy (STM). After desorption, microscopic changes to the (2 × 1) reconstruction produce two variations on the dimer row reconstruction with decreased surface atom density. A (2 × n) vacancy chain reconstruction and a c(4 × 4) incomplete row reconstruction were observed; a structure for the latter is proposed. Both reconstructions are metastable, reforming the (2 × 1) reconstruction upon heating. At greater length scales during desorption from an initial 1.0 ML coverage, the mesoscopic changes to the surface structure include pitting and roughening, with up to a measured 20 fold increase in the edge density as compared to the clean Si(100) surface.

These structural changes suggest a reaction mechanism involving a substantial rearrangement of the substrate silicon. From an initial 1.0 ML oxygen coverage, using measured void size distributions at total desorption levels of 13% and below — before voids have begun to coalesce — the evolution of void sizes during initial desorption can be followed. A mechanism for desorption is proposed in which silicon atoms must diffuse from adjacent clean surface area to the oxide boundary, producing a reactive complex from which SiO is desorbed. Void growth rates derived from two rate limiting cases for this desorption reaction mechanism can be compared to measured results. We show that the measured void area evolution is consistent with a reaction mechanism where the rate limiting step for monolayer desorption is the promotion of a silicon atom in a lattice site to a mobile monomer within the void.     

Rotational epitaxy of a hexagonal layered material on a square substrate: PbI2 on InSb(001)

PbI₂ has been adsorbed on the clean InSb(001)-(4 × 1) reconstructed surface and on the InSb(001)-(1 × 3)-Pb lead covered reconstructed surface. On the clean surface epitaxial growth occurred with the unit mesh of the layered PbI₂ aligning exactly with both the substrate [110] and [1 0] directions. On desorption a reaction occurred between the last layer of PbI₂, and the substrate, forming a series of structures which finished with a well-formed (1 × 3)-Pb structure in which the surface is depleted/enriched in In/Sb compared to the clean (4 × 1). The Pb in this structure is thought to partially replace surface In. Epitaxial adsorption also occurred on the (1 × 3)-Pb surface generating a single, well-formed structure with the hexagonal net of the PbI₂ aligned with just the [1 0] substrate direction. The structures and reactions are discussed and a row matching model is proposed to explain the single epitaxial orientation of PbI₂ on the (1 × 3)-Pb surface.     

Carbidic carbon on Ni(110): an STM study

The interaction of carbon with the Ni(110) surface has been investigated by scanning tunneling microscopy (STM). Dissociation of ethylene at elevated temperatures resulted in the formation of two carbidic carbon structures, (4 × 3) and (4 × 5), both of which are formed by a significant reconstruction of the surface involving a long-range mass transport of 0.5 ML Ni. The density of C in the reconstructed phases was estimated based on Auger electron spectroscopy (AES). Atomic models, which are consistent with all existing experimental observations for carbidic carbon on Ni(110), are proposed for the (4 × 3) and (4 × 5) phases.     

The chemisorption of hydrogen on silicon and silicon carbide (1 0 0) surfaces

The chemisorption of hydrogen onto semiconductor surfaces is examined. The hydrogen bonds to the dangling bond of a surface atom. These dangling bonds also dictate the reconstruction of the crystal surface. The chemisorbed hydrogen therefore modifies the reconstructed surface topology. In this work theoretical calculations of the surface structures of both covalent elemental silicon and polar silicon carbide are presented. The periodic MINDO method is employed to determine the topologies for the 2 × 1 reconstructed (1 0 0) surfaces. These topologies are obtained from the minimisation of the total energy, for silicon and silicon carbide films of 14 layer thickness, with respect to the atomic co-ordinates of the hydrogen adsorbate and the first four layers of the substrate. The results show that the formation of the hydrogen bond to the substrate leads to a general lengthening of the surface dimer bond. In addition, the buckling of the silicon dimer determined for silicon terminated SiC is removed by hydrogen chemisorption.     

Dimer elongation on the monohydride-covered Ge/Si(100) surface

Using transmission ion channeling, we have made the first measurement of the Ge dimer geometry for the monohydride-covered Ge/Si(100)-2×1 surface. Comparison of calculated angular scans with experimental angular scans near the 100 and 110 directions has resulted in a measured Ge dimer bond length of 2.8 Å, which is 8% longer than the corresponding dimer bond length reported for Ge on Si(100) in the absence of H. This elongation is similar to that reported for Si dimers on the Si(100) surface. Also, relative to the (100) surface plane, the dimers change from tilted without H to untilted with H.     

Asymmetric versus symmetric dimerization on the Si(001) and As/Si(001)2 × 1 reconstructed surfaces as observed by grazing incidence X-ray diffraction

The atomic structures of the clean Si(001) and As/Si(001)2 × 1, 1 × 2 reconstructed surfaces prepared in situ have been obtained by grazing incidence X-ray diffraction under ultra high vacuum. In the former case an asymmetric dimer inclined by 7.4° on the surface plane with a slightly contracted dimer bond length (−1.5% compared to the bulk value) has been indentified as the structural basis, whereas a symmetric dimer is found after adsorption of one monolayer of As at 350 ° C. The induced displacements in the first subsurface silicon layer have been derived in both cases. These results are compatible with the models proposed on the basis of spectroscopic and direct imaging methods.     

The surface structure of the metallic sodium tungsten bronze Na0.667WO3(001)

Scanning tunnelling microscopy has been used to identify a number of surface reconstructions on the (001) surface of the cubic metallic sodium tungsten bronze, Na_0.667WO₃. Which is dominant has been found to depend critically on sample preparation. As well as a reconstruction that bears a striking similarity to that of the parent material, tungsten trioxide, regions of (2×1) periodicity are observed that can only be explained in terms of an Na_yO surface layer. In the current work, we relate the effect of sample preparation on the surface electronic structure of Na_0.667WO₃(001) with that on the atomic structure by comparing photoemission spectra with STM images. Particular interest is focused on band gap defect states in photoemission spectra which, in contrast to similar states in spectra from WO₃, do not appear to correlate with the appearance of localised defects or highly reduced terraces in STM images. The existence of peroxide-like oxygen dimers at the (2×2) reconstructed surface, on the other hand, is characterised by the appearance of identifiable states in the valence band spectrum.     

Diffusion of Pb adatom and ad-dimer on Si(1 0 0) from ab initio studies

The diffusion pathways of Pb adatoms and ad-dimers on Si(1 0 0) are investigated by first-principles calculations. Pb adatoms are found to diffuse on top of the Si(1 0 0) dimer row with an energy barrier of 0.31 eV. However, Pb dimers are energetically more stable. Pb dimers on top of the dimer row have a high energy barrier (0.95 eV) to rotate from the lowest energy configuration to the orientation parallel to the underlying Si(1 0 0) dimer row. Once the ad-dimer is oriented parallel to Si(1 0 0) dimer row, they can diffuse along the dimer row with an energy barrier of only 0.32 eV.     

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.     

A theoretical study of structural and electronic properties of a missing dimer defect on Si- and C-terminated SiC(0 0 1)

We present a theoretical study of the geometrical and electronic properties of the C- and Si-terminated -SiC(0 0 1) surfaces in the vicinity of the missing dimer defect. The experimental results suggest that the atomic structures of these two surfaces may be considerably modified by external stress. In our present study we have considered the possible influence of this factor on the surface geometry of both systems. We have shown that the structural differences between the C- and Si-terminated surfaces lead to their different behaviour in the presence of a missing dimer and applied stress. In the case of the C-terminated c(2×2) surface, the missing dimer defect causes the buckling of the adjacent carbon dimers lying in the line of the defect (dimer atoms adjacent to the defect have vertical positions lower by 0.18 Å). This effect becomes more pronounced in the presence of compressive stress — the stress of 8% leads to the buckling of these two dimers of around 0.5 Å. The vertical positions of silicon atoms located directly below the defect were increased by 0.2 Å. We have also found that the missing dimer influences the structure of the carbon dimers on the neighbouring lines of dimers. Contrary to the C-terminated surface, the missing dimer defect on the Si-terminated SiC(0 0 1)-p(2×1) surface remains neutral for silicon dimers located in the line of defect, i.e. the dimers do not change their geometrical properties in unstrained structure nor in the presence of a tensile stress. On the other hand, this defect modifies considerably the geometry of the dimers from the two neighbouring lines of dimers by reducing their bond lengths and vertical positions. Changes in the geometrical properties of the second neighbour dimers (with respect to the defect) in these two lines are also noticeable. Moreover, we have found that the presence of a missing dimer modifies significantly the positions of the adjacent subsurface carbon atoms.     

On the structure of Si(113)

The structure of the (113) face of Si has received a large amount of interest recently because of its high stability, despite having a high index. There remains some controversy about the surface reconstruction, various groups having reported observations of both 3 × 1 and 3 × 2 structures. We have obtained atomically resolved STM images of this surface showing areas of 3 × 1 reconstructed surface, the structure of which matches a modified version of a previously proposed model. In addition we have also obtained CITS images showing the complex underlying reconstruction and spectroscopy measurements which are comparable with a recent ARUPS study of the same surface.     

Atomic structures of GaAs(100)-(2 × 4) reconstructions

Atomic structures of the As-rich GaAs(100)-(2 × 4) reconstructions based on converged first-principles total-energy calculations are reported. All geometries are characterized by similar structural elements such as As dimers with a length of about 2.5 Å, dimer vacancies, and a nearly planar configuration of the three-fold coordinated second-layer Ga atoms leading to a steepening of the dimer block. For an As coverage of θ = 3/4 we find the two-dimer β2 phase to be energetically preferred over the three-dimer β phase. A structure with partial replacement of As by Ga in the uppermost layer corresponding to an As coverage of is found to be slightly less favourable than the phase of GaAs(100). Geometry parameters are given for all structures and compared with the available experimental results.     

Hydrogen-induced surface metallization of beta-SiC(100)-(3x2) revisited by density functional theory calculations

Recent experiments on the silicon terminated (3 x 2)-SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of density-functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: in addition to saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, i.e., stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surface.     

Diffraction from reconstructed surfaces with incommensurate domain walls

The kinematic approximation method has shown that the peak intensity and the full width at half maximum (FWHM) of stepped surfaces exhibit an oscillatory behavior for changing incident energy. This paper generalizes the kinematic approximation to an (N × 1) reconstructed surface with a distribution of various types of lateral displacements at a step. A particular solution of this model we call the fixed point solution, yields a clear intuitive understanding of these oscillations as well as an exact solution for the step density of any surface. The specific examples of (5 × 1) and (2 × 1) reconstruction are examined to show the striking differences between the reconstructed surface diffraction patterns. These differences make an examination of the half-maximum (HM) intensity position a powerful tool to determine the surface structure for any incommensurate stepped surface.     

A photoemission study of 4H---SiC(0001)

A photoemission study using synchrotron radiation of the (0001) surface of 4H-SiC is reported. The investigations were concentrated on the (√3 × √3)-R30° and (6√3 × 6√3)-R30° reconstructed surfaces, prepared by resistive heating at a temperature of about 1000°C and 1250°C, respectively. Results from surfaces heated at intermediate temperatures, exhibiting a mixture of these reconstructions, and after heating at a higher temperature, when graphitisation is clearly observed, are also presented. The √3 and 6√3 reconstructed surfaces exhibit characteristic core level and valence band spectra. High resolution core level spectra show unambiguously the presence of surface shifted components in both the Si 2p and C 1s core levels. For the √3 reconstruction, two surface shifted components are observed both in the Si 2p and C 1s level. For the 6√3 reconstruction, the surface region is found to contain a considerably larger amount of carbon. This carbon is found not to be graphitic since surface C 1s components with binding energies different from a graphitic C 1s peak are observed. Graphitisation, as revealed by the appearance of a graphitic C 1s peak, is observed only after heating to a higher temperature than that required for obtaining a well developed 6√3 diffraction pattern.     

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.     

Reversible local-modification of surface structure on clean Ge(0 0 1) by scanning tunneling microscopy below 80 K

The structure of the clean Ge(0 0 1) surface is locally and reversibly changed between c(4×2) and p(2×2) by controlling the bias voltage of a scanning tunneling microscopy (STM) below 80 K. It shows hysteresis for the direction of the sample bias voltage change. The c(4×2) structure is observed with the sample bias voltage V_b?−0.7 V. This structure is maintained at V_b?0.7 V with increasing the bias voltage from −0.7 V. When V_b is higher than 0.8 V, the structure changes to p(2×2). This structure is then maintained at V_b?−0.6 V with decreasing the bias voltage from +0.8 V. The area of the structure change can be confined in the single dimer row just under the STM tip using a bias voltage pulse. In particular, the minimum transformed length is four dimers along the dimer row in the transformation from p(2×2) to c(4×2). The observed local change of the reconstruction with hysteresis is attributed to the energy transfer process from the tunneling electron to the Ge lattice in the local electric field due to the STM bias voltage. A phenomenological model is proposed for the structure changes. It is based on a cascade inversion of the dimer buckling orientation along the dimer row.