Rest Atoms Research Articles

The initial stage of epitaxial growth of Ge on Ge(111) as studied by X-ray diffraction

We present an in situ X-ray scattering study of the evolution of surface structure and morphology during deposition of Ge on Ge(111). The substrate temperature (300 K) was such that the growth proceeds through two-dimensional nucleation. We determined the sites on the c(2 × 8) reconstructed Ge(111) surface to which the first deposited Ge atoms bond. The arrangement of the deposited atoms was inferred from an analysis of the diffracted intensities along the integer-order rods of Bragg scattering perpendicular to the surface plane. We find the first deposited Ge atoms to occupy adsorption sites above the “rest atoms” in the reconstructed unit cell. For higher coverages islands of bilayer height are formed.

Translationally and vibrationally activated reaction of CO2 on Si(111)7×7

The interaction of CO2 molecules with the Si(111)7×7 surface for translational energies between 0.2 and 1.6 eV and varying vibrational energies has been studied with time-resolved electron-energy-loss spectroscopy (TREELS), temperature-programed desorption, and Auger electron spectroscopy. Energy from the normal component of translational motion has been found to strongly increase the dissociation probability of CO2 on the surface. TREELS has been used to tentatively identify the resulting surface complex as O on a Si adatom with CO bonded in a bridging site to a next-layer Si rest atom. This complex decomposes at 400 K to a surface oxide and gas-phase CO. In addition, vibrational excitation has been found to increase the initial sticking coefficient for normal translational energies less than 0.5 eV.

Atomic Arrangement of the Clean Si(110) 5x1 Surface Derived by Low Energy Scattering Spectroscopy

The atomic arrangement of the Si(llO) 5x1 reconstructed surface has been studied by low energy ion scattering. The experimentally measured incident angle scan data were compared with the results of computer simulations for various structural models. Although detailed quantitative interpretation of the full range of ion scattering data is difficult to achieve, there is clear evidence for a major reconstruction of the surface. A reasonable fit for the adatom model between the experimental results and computer simulation suggests that the clean Si(llO) 5x1 surface consists of a mixture of 5x1, 5x4 and 5x5 structures, possibly partially disordered along the [flO] direction. The different adatom and rest atom sites have been determined by fitting the calculation with the experimental data. However, there is no other existing experimental evidence, at this time, to support the adatom or rest atom height.

Ab initio total energy calculations on parallel computers: application to the Takayanagi reconstruction

An overview is presented of the method used to parallelize a set of total energy pseudopotential codes on a 64-node i860 Meiko Computing Surface and a 32-node Intel iPSC/860 Hypercube. These codes have been used to calculate the surface energies and relaxed structures of the 3 × 3, 5 × 5 and 7 × 7 Takayanagi reconstructions of the (111) surface of silicon. It is found that the 7 × 7 reconstruction has the lowest energy and that structural trends across the series of reconstructions can be related to the degree of charge transfer from the adatoms to the rest atoms.

Growth and morphology of Pb on Si(111)

Tunneling microscopy, thermal desorption, Rutherford backscattering, and low-energy electron diffraction are used to study the structures and coverages of the phases of Pb on the Si(111)7 × 7 surface. For room-temperature deposition at low coverage on the 7 × 7 surface, Pb atoms occupy sites above the rest atoms and between the Si adatoms (with a preference for the faulted half of the unit cell). At 0.6 ML, the Pb forms an ordered overlayer based on the 7 × 7 unit cell. The Pb grows epitaxially up to 3 ML at which point Pb crystals start to form. On annealed samples at low coverage, Pb atoms occupy Si(111)7 × 7 adatom sites. At 1 6 ML, a new √3 × √3 mosaic phase consisting of alternating chains of Pb and Si adatoms with a high melting point is produced. At 1 3 ML the standard √3 × √3 phase is observed. Between 1 3 and 1 ML, a 1 × 1 Pb overlayer is found on annealed samples, while above 1 ML, a rotated incommensurate phase is observed. As more Pb is added above 1 ML, the 2D Pb density is increased, and the incommensuration is reduced, until island formation begins. We link our discussion of the atomic structure of the interface to the variations in Schottky barrier heights observed for Pb/Si(111) diodes.

Charge transfer and asymmetry on Ge(111)-c(2 x 8) studied by scanning tunneling microscopy.

Scanning-tunneling-microscopy images of annealed Ge(111) show large {ital c}(2{times}8) domains with protrusions that match closely the arrangement of adatom and rest-atom dangling bonds in the simple adatom model. The unit cell, however, appears asymmetric or buckled;'' namely, the two adatoms or the two rest atoms within the unit cell show significant differences in apparent height. Both the adatom and rest-atom dangling bonds can be simultaneously observed with either bias polarity, indicating that both are partially filled. These results will be discussed in relation to the proposed charge transfer between the adatoms and rest atoms.

Surface properties of Si(111)7 × 7 upon NH 3 adsorption and vacuum annealing

The adsorption of NH 3 on a clean Si(111)7 × 7 reconstructed surface and the effect of vacuum annealings of the NH 3-saturated surface have been studied by low energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. The analysis of the surface order, of the nitrogen coverage and of the workfunction, ionization energy and filled surface state band changes upon NH 3 exposure or annealing temperatures gives the following results: the adsorption is dissociative into NH 2 + H and occurs in a two-step process, a fast one with an initial sticking coefficient equal to one up to half saturation, then a much slower one. At saturation, 10 ± 1 NH 3 molecules per 7 × 7 unit cell are adsorbed, the NH 2 radicals occupying the 7 rest atom dangling bonds plus three sites on the average which are attributed to the presence of steps or missing adatoms. Annealings lead to a two-step process, first a hydrogen desorption from 450 °C to 700 °C then a surface nitridation beyond 800 °C.

Adatom-based (2 × 2 n) reconstruction on Ge(111)

Atomic cluster calculations are reported which model adatom generated (2 × 2) and c(2 × 8) reconstruction on the germanium (111) surface, for both the T4 and H3 adsorption sites. The clusters are four atomic layers deep and the positions of atoms in the first three atomic layers are found by minimising the total energy calculated with the MNDO quantum chemical Hamiltonian, according to the prevailing symmetry. Both closed and open shell wavefunctions are considered. We find at least four different electronic states with similar energies but different geometries. The lowest energy is found for the c(2 × 8)H3 configuration with a p-like dangling bond on the rest atom, while for the (2 × 2) structure an s-like dangling bond orbital on the adatom is most favourable.

Chemical titration of clean silicon surfaces with N2O and O2: Atomic nature of ‘‘5×1’’ reconstructed Si(110)

Using spectroscopic differential reflectometry (SDR), Auger electron spectroscopy (AES), and low‐energy electron diffraction (LEED) we have studied the room temperature adsorption behavior of N2O and O2 at the clean low‐Miller index Si surfaces. Decomposition of the nitrous oxide molecule, resulting in Si–O bond formation, only occurs at the top layer Si atoms which have an unsaturated dangling bond. In this way it has been possible to determine the number of broken bonds at these clean reconstructed surfaces using AES. From a comparison of the site‐specific adsorption of N2O with the non‐site‐specific adsorption of O2 we propose a model of the atomic nature of the Si(110) surface showing a prominent 5×1 reconstruction which is in agreement with previously published LEED results and recent scanning tunneling microscopy (STM) and ultraviolet photoemission spectroscopy (UPS) work. This model is based on a reduction of dangling bonds which is due to the formation of adatoms. The model also includes a kind of rest atoms and atoms in nearest‐neighbor top layer chains along the [1̄10] direction (A type).We have found that the ratio adatoms/rest atoms/A atoms per reconstructed unit cell determines the primary superstructure. This ratio also accounts for a more or less ‘‘apparent’’ coexistence of higher order reconstructions very often reported in LEED studies, their presence being in the form of streaks at various fractional order positions along the [1̄10] direction. A real coexistence of various reconstructions with partial order at the Si(110) surface cannot be excluded as evidenced by STM, a result which makes it the most complicated low‐Miller index Si surface to study. Complementary information concerning the energy position of the unoccupied surface states has been obtained from SDR. Our clean Si(110) surface shows partial order along the [1̄10] direction, indicating the coexistence of 5×2 and 5×5 superstructures in addition to the observed prominent 5×1 LEED reconstruction. It has been derived that this surface has empty states at about 0.9 and 1.7 eV above the Fermi level (EF). The occupied states at about 0.8 and 2.4 eV below EF could be identified with rest atoms and A‐type atoms, respectively.

Structure and energetics of Ge(111) c(2 × 8) reconstruction

We present self-consistent MNDO calculations of atomic clusters modelling the structure of adatom generated 2 × 2, √3 × √3 and 2 × 8 reconstructions of the Ge(111) surface. Comparisons are made of the T4 and H3 adsorption sites and of closed shell singlet and open shell (spin triplet) RHF wavefunctions. We find that √3 × √3 prefers the T4 site energetically, but 2 × 2 prefers H3. Subsurface relaxation up to three layers deep plays a crucial role in the energetics and we also find significant charge transfers between rest atoms and adatoms as well as between layers. A consistent interpretation can be given of the similarities and differences between the configurations in terms of bond rehybridization and polarization effects as well as the different surface topologies.

Atomic nature of ‘5 × 1’ reconstructed Si(110) as revealed by chemical titration with N 2O and O 2: AES and LEED results

By comparing the site-specific adsorption of N2O with the non-site-specific adsorption of O2 at the clean Si(110) surface which shows a prominent 5 × 1 superstructure (both at 300 K), employing Auger electron spectroscopy (AES) and low energy electron diffraction (LEED), a model of the atomic nature of this surface has been derived which is in agreement with previous LEED and recent scanning tunneling microscopy (STM) work. The description of the real ‘5 × 1’ reconstructed Si(110) surface is based on a reduction of dangling bonds which is due to the formation of adatoms. The model also includes a kind of rest atoms, and atoms in nearest neighbour top layer chains along [110] (A). It is the ratio adatoms/rest-atoms/A-atoms which determines the primary superstructure. The spatial variation of this ratio at the same Si(110) surface is examined, which cannot be excluded as evidenced by STM, and is found to itself account for the more or less visible streaks (or even spots) very often reported in LEED studies.

Cluster calculations of the Si(111) 7 x 7 dimer-adatom-stacking-fault structure.

We have studied the dimer\char21{}adatom\char21{}stacking-fault (DAS) 7\ifmmode\times\else\texttimes\fi{}7 reconstruction on Si(111) using quantum-chemical calculations of large triangular clusters representing the two halves of the surface unit cell (SUC). We calculate the DAS structure to be 0.17 eV per 1\ifmmode\times\else\texttimes\fi{}1 SUC more favorable than the 2\ifmmode\times\else\texttimes\fi{}1 chain reconstruction. It is found that the electronic structure is quite sensitive to vertical displacements of the surface atoms, even though the surface energy remains relatively undisturbed. We associate the phenomenon with the pairing of the rest atoms and the central adatoms which resembles the Jahn-Teller effect. We then show that the effect is important for a realistic model of the DAS reconstruction energetics, which explains the occurrence of 7\ifmmode\times\else\texttimes\fi{}7 reconstruction rather than other odd-by-odd surface patterns. Regarding the bonding configuration, we find (somewhat delocalized) bonds between the adatom and first-layer atoms, and also between these and the second-layer atom directly below the adatom. However, we find there is no bond between the adatom and the second-layer atom. Instead, there is an s-like dangling-bond orbital on the adatom. We expect the adatom backbonds to be weakened single bonds.

Surface structure and long-range order of the Ge(111)-c(2 x 8) reconstruction.

We have performed an x-ray diffraction study of the Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) surface. From the analysis of 43 in-plane, symmetry-inequivalent structure factors, a simple adatom model with subsurface relaxation is deduced. The relaxations around the adatoms are similar to those observed in the adatom complexes on Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) and Sn/Ge(111)-(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ), -(5\ifmmode\times\else\texttimes\fi{}5), and -(7\ifmmode\times\else\texttimes\fi{}7), suggesting that the adatoms are located in threefold sites above the second-layer atoms (${T}_{4}$ sites). Significant distortions around the rest atoms are seen due to rehybridization of the dangling bonds. The diffraction peaks from the eighth-order reflections are displaced away from their nominal positions. This is explained by a model where the long-range order of the c(2\ifmmode\times\else\texttimes\fi{}8) structure is disturbed by faults due to rows of 2\ifmmode\times\else\texttimes\fi{}2 unit cells.

Atom-resolved surface chemistry using scanning tunneling microscopy.

We use scanning tunneling microscopy to investigate the reactivity of the dangling-bond states of Si(111)-(7\ifmmode\times\else\texttimes\fi{}7). The reaction with ${\mathrm{NH}}_{3}$ is used as a prototype. We find that Si rest atoms are more reactive than Si adatoms and that center adatoms are more reactive than corner adatoms. Using atom-resolved electronic spectra, we probe the dangling-bond states on both clean and ${\mathrm{NH}}_{3}$-exposed surfaces. We find significant interactions and charge transfer between sites which strongly influence surface reactivity.

ChemInform Abstract: THE CRYSTAL AND MOLECULAR STRUCTURE OF SALICYLALDEHYDE‐4‐MORPHOLINOTHIOSEMICARBAZONE

Crystals of salicylaldehyde-4-morpholinothiosemicarbazone, , are orthorhombic with space group Pna21. Unit-cell dimensions are a = 11.85(5), b = 15.45(5) c = 7.18(3) with z = 4. Three-dimensional intensity data were collected from the multiple-film equi-inclination Weissenberg photographs taken with radiation. The intensities were estimated visually. The structure was solved by Patterson and Fourier methods and refined by the block-diagonal least-squares methods until the final R value becomes 0.11 for the 1064 observed independent reflections. The morpholine ring has a chair form. The rest atoms of salicylaldehyde-4-morpholinothiosemicarbazone molecule excluding morpholine ring and sulfur atom approximately lie on a plane. The hydroxyl group of the salicylaldehyde and the nitrogen atom of the thiosemicarbazone form an intramolecular hydrogen bond, , of 2.67. The short intermolecular distances all appear to be normal van der Waals contacts.

935. Electron-impact-induced effects in a nitrogen ad-layer on tungsten, investigated by means of FEM: W Ermrich,Nuovo Cimento Suppl, 5 (2), 1967, 582–585

Temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM) were used to probe the atomic arrangement on clean Ge(011)-c(8×10), as well as the desorption kinetics and pathways from hydrogen-adsorbed surfaces. For the TPD measurements, the samples were heated at 2°C s−1 after adsorbing atomic deuterium at 100°C to coverages θD ranging up to saturation. Low-energy electron diffraction (LEED) and STM show that saturation deuterium coverage results in a (1×1) structure with the surface composed of randomly distributed adatom islands. TPD spectra exhibit three second-order peaks corresponding to D2 desorption from multideuterides, adatom monodeuterides and rest-atom monodeuterides. Desorption from the multideuteride phase (with an activation energy Ea of 1.61 eV) begins at 200°C and, by 270°C, only the D–adatom and D–rest-atom monodeuteride phases remain. D2 begins to desorb from adatom sites (Ea=1.76 eV) above 230°C and from rest-atom sites (Ea=1.83 eV) above 240°C. From quantitative analyses of the TPD spectra, the adatom density on the clean surface is ≥0.47. This high adatom density — similar to that of Si(111)-(7×7), Si(011)-(16×2) and Ge(111)-c(2×8), all of which contain adatoms and rest atoms as primary building blocks — appears to rule out previously proposed models for the Ge(011)-c(8×10) surface structure, for which the adatom density is 0.064.