Temperature dependent low energy electron microscopy study of Ge growth on Si(1 1 3)

We investigated the initial Ge nucleation and Ge island growth on a Si(1 1 3) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 °C and 590 °C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

Phase discrimination of iron-silicides on Si(0 0 1) surfaces by three-dimensional reciprocal-lattice mapping

Various iron-silicides are grown on clean Si(0 0 1) surfaces by solid phase epitaxy, a process which involves the deposition of iron and subsequent annealing [6]. Among them, we studied the structure of three-dimensional (3D) elongated islands, which are the major silicide type produced at lower Fe coverage (∼1 monolayer) and ?500-600 °C annealing. We applied a newly developed method of azimuth-scan reflection high-energy electron diffraction (RHEED) to obtain 3D reciprocal-lattice mapping. We succeeded in discriminating an α-FeSi₂ phase from controversial bulk phases of the islands, and we were also able to determine the orientation relation as and , where the lattice mismatches are −1% in direction and +34% in direction. The attenuation of the incident electron beam along the length direction of the islands leads to extremely weak spots in the RHEED pattern. We emphasize that such an analysis of the reciprocal-lattice mapping is also useful in studying other 3D island structures. Using scanning tunneling microscopy, we showed that the island’s elongated directions are perpendicular to the dimer rows of the substrate located under the islands. The islands are located near the S_B step edges. The elongation lengths of the islands are almost the same as the widths of the Si substrate terraces. We discussed the formation mechanism of the 3D-elongated islands. From an atomic image of the facet and edge of a 3D-elongated island, we proposed an atomic-structure model of the island facet and edge: a Si adatom on the hollow site of four Si atoms of an unit, with ordering in the direction of the elongation, forming an facet locally.     

Mixed layer formation of copper overlayers on Ni(1 1 0) surface

Copper overlayer formation on the Ni(1 1 0) surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Atom-resolved STM images showed that initially deposited Cu is replaced with surface Ni atoms forming atom-size depressions on the Ni(1 1 0) terraces and a Ni-rich quasi-one-dimensional island along the direction. Further Cu deposition yields a mosaic structure on the islands, indicating Cu/Ni mixed layer formation. From the quantitative measurement of the Cu/Ni ratio on the substrate and the islands, impinging Cu will be replaced with surface Ni whereas expelled Ni and directly impinging Cu to the island form the mixed island. The number of Cu atoms in the islands, however, more than the directly impinging Cu, indicate significant Cu/Ni replacement at the periphery of the island.     

Epitaxial growth of ultra-thin NiO films on Cu(1 1 1)

The very first stages of the growth of NiO on Cu(1 1 1) is examined on a microscopic scale. The paper focuses on the morphological and structural characterization of nanostructures formed in the 0-1 Å thickness range. Ultra-thin NiO films, obtained through evaporation of a Ni rod under an oxygen atmosphere were grown at 550 K. In the early stages of the growth the oxide film morphology shows 10-30 nm large, monolayer high, islands with a partial incorporation of metallic Ni in the first Cu(1 1 1) surface plane. The first layer is formed by an epitaxial atomic layer exhibiting a STM contrast similar to the one observed on adsorbed oxygen on Cu(1 1 0). A NiO cluster nucleation and coalescence mechanism is proposed in order to explain the formation of the second NiO layer. A α-Ni₂O₃ hexagonal phase, or a structural distortion of the NiO(1 1 1)()R30° structure could both explain the complex LEED patterns.     

Dynamics of NF3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF₃ on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF₃ adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF₃ is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F⁺ central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF₃ (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F⁺, N⁺, NF⁺, and ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the and ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF₃ molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF₃ crystal. In this process, some NF₃ molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.     

Morphology of Ni ultrathin films on Mo(1 1 0) and W(1 0 0) studied by LEED and STM

The morphology of ultrathin Ni films on Mo(1 1 0) and W(1 0 0) has been studied by low-energy electron diffraction and scanning tunneling microscopy. Ni films grow pseudomorphically on Mo(1 1 0) at 300 K for a coverage of 0.15 ML. A (8 × 1) structure is found at 0.4 ML, which develops into a (7 × 1) structure by 0.8 ML. The film undergoes a structural change to fcc Ni(1 1 1) at 6 ML. The growth mode switches from layer-by-layer to Stranski-Krastanov between 4 ML and 6 ML. Annealing at around 850 K results in alloying of submonolayer films with the substrate, while for higher coverages the Ni agglomerates into nanowedge islands. Ni films grow pseudomorphically on W(1 0 0) up to a coverage of around 2 ML at 300 K, above which there is a structural change from bcc to hcp Ni with the epitaxial relationship . This is accompanied by the formation of orthogonal domains of uniaxial strain-relieving dislocations from the third layer of the film. For coverages up to 1 ML the growth proceeds by formation of two-dimensional islands, but shifts to three-dimensional growth by 2 ML with rectangular islands aligned along the 〈0 1 1〉 substrate directions. Annealing at around 550 K results in agglomeration of Ni into larger islands and increasing film roughness.     

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.     

Growth of well-aligned Bi nanowire on Ag(1 1 1)

We report the fabrication of one-dimensional (1D) Bi nanowires grown on Ag(1 1 1) with average lateral width from 9 to 20 nm by physical vapor deposition in ultra high vacuum conditions. In situ low-temperature scanning tunneling microscopy analyses reveal that the preferred growth of 1D Bi nanostructures is driven by the highly anisotropic bonding in the crystallographic structure of the Bi(1 1 0) plane. The Bi nanowires grow along direction and align with the directions on Ag(1 1 1). The growth of the Bi nanowires proceeds in a bilayer growth mode resulting from the layer pairing in Bi(1 1 0) which saturates the dangling bonds and lowers the total energy.     

Heteroepitaxial praseodymium sesquioxide films on Si(1 1 1): A new model catalyst system for praseodymium oxide based catalysts

The structure, growth and stoichiometry of heteroepitaxial Pr₂O₃ films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD confirm the growth of a (0 0 0 1) oriented hexagonal Pr₂O₃ phase on Si(1 1 1), matching the surface symmetry by aligning the oxide in-plane direction along the Si azimuth. After an initial nucleation stage RHEED growth oscillation studies point to a Frank-van der Merwe growth mode up to a thickness of approximately 12 nm. XPS and UPS prove that the initial growth of the Pr₂O₃ layer on Si up to ∼1 nm thickness is characterized by an interface reaction with Si. Nevertheless stoichiometric Pr₂O₃ films of high crystalline quality form on top of these Pr-silicate containing interlayers.     

Oxidation of epitaxial Y(0 0 0 1) films

We have investigated the oxidation behavior of MBE grown epitaxial Y(0 0 0 1)/Nb(1 1 0) films on sapphire substrates at elevated temperatures under atmospheric conditions with a combination of experimental methods. At room temperature X-ray diffraction (XRD) reveals the formation of a 25 Å thick YO_xH_x layer at the surface, while simultaneously oxide growth proceeds along defect lines normal to the film plane, resulting in the formation of a single crystalline cubic Y₂O₃ (2 2 2) phase. Furthermore, nuclear resonance analysis (NRA) reveals that hydrogen penetrates into the sample and transforms the entire Y film into the hydride YH₂ phase. Additional annealing in air leads to further oxidation radially out from the already existing oxide channels. Finally material transport during oxidation results in the formation of conically shaped oxide precipitations at the surface above the oxide channels as observed by atomic force microscopy (AFM).     

Co-induced nano-structures on Si(1 1 1) surface

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.     

Sapphire (0 0 0 1) surface modifications induced by long-pulse 1054 nm laser irradiation

We have investigated modifications of sapphire (0 0 0 1) surface with and without coating, induced by a single laser pulse with a 1054 nm wavelength, 2.2 s duration, 7.75 mm spot and energy of 20-110 J. A holographic optical element was used for smoothing the drive beam spatially, but it induced small hotspots which initiated damage on the uncoated and coated surfaces. The individual damage effects of hotspots became less pronounced at high fluences. Due to high temperature and elevated non-hydrostatic stresses upon laser irradiation, damage occurred as fracture, spallation, basal and rhombohedral twinning, melting, vitrification, the formation of nanocrystalline phases, and solid-solid phase transition. The extent of damage increased with laser fluences. The formation of regular linear patterns with three-fold symmetry ( directions) upon fracture was due to rhombohedral twinning. Nanocrystalline -Al₂O₃ formed possibly from vapor deposition on the coated surface and manifested linear, triangular and spiral growth patterns. Glass and minor amounts of -Al₂O₃ also formed from rapid quenching of the melt on this side. The - to -Al₂O₃ transition was observed on the uncoated surface in some partially spalled alumina, presumably caused by shearing. The nominal threshold for laser-induced damage is about 47 J cm⁻² for these laser pulses, and it is about 94 J cm⁻² at the hotspots.     

CO oxidation on Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces studied by infrared chemiluminescence spectroscopy

The infrared (IR) chemiluminescence studies of CO₂ formed during steady-state CO oxidation over Pd(1 1 1), Pt(1 1 1), and Rh(1 1 1) surfaces were carried out. Analysis of their emission spectra indicates that the order of the average vibrational temperature () values of CO₂ formed during CO oxidation was as follows: Pd(1 1 1) > Pt(1 1 1) > Rh(1 1 1), and the order is coincident with the potential energy in the transition state expected by the theoretical calculations. Furthermore, it is suggested that the bending vibrational temperature () can also be influenced by the angle of O-C-O (∠_OCO) of the activated complex in the transition state, which has also been proposed by the theoretical calculations.     

Edge-dimer row - The reason of three-bilayer steps and islands stability on Si(1 1 1)-7 × 7

A hypothesis of perpendicular dimer row formation along three-bilayer (3 BL) step was suggested. The hypothesis, explains the stability of 3 BL steps on the vicinal Si(1 1 1) surface deflected in direction as well as the limitation of Ge and Si island height by 3 BL at the initial nucleation stages on Si(1 1 1) surface. The detailed examinations of STM images of 3 BL steps were carried out. New peculiarities of atomic structure of 3 BL single step on Si(1 1 1) and 3 BL steps on Si(5 5 7) surfaces were revealed. The results of STM images examination verify the hypothesis of perpendicular dimer row formation along the boundary of the 3 BL step.     

Adsorption structure of germanium on the Ru(0 0 0 1) surface

Coverage-dependent adsorption energy of the Ge/Ru(0 0 0 1) growth system and the geometrical distortions of the most stable adsorption structure are investigated through first-principles calculations within density functional theory. A local minimum in adsorption energy is found to be at a Ge coverage of 1/7 monolayer with a Ru(0 0 0 1)- symmetry. Based on this stale superstructure, the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) images are simulated by means of surface local-density of states (LDOS). The results are consistent well with the STM measurements on the phase for Ge overlayer on Ru(0 0 0 1). From this stimulation, the relations between the STM images and the lattice distortion are also clarified.     

Reactivity to sulphur of clean and pre-oxidised Cu(1 1 1) surfaces

The reactivity of clean and pre-oxidised Cu(1 1 1) surfaces exposed to sulphur (H₂S) has been studied at room temperature by Auger electron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. On the clean surface, the sulphur-saturated surface structure is dominated by the or so-called “zigzag” superstructure. It is shown that a single orientation domain is favoured by the slight misorientation (∼2°) of the surface with respect to the (1 1 1) plane. Scanning tunneling microscopy measurements also revealed two minority structures. Pre-oxidation was performed by exposure to 1.5 × 10⁴ L of O₂ at 300 °C. Under exposure to H₂S (1 × 10⁻⁷ mbar) at room temperature, the oxygen is totally substituted by sulphur. Once initiated, sulphur adsorption seems to propagate to cover the whole surface on the O-covered surface faster than on the clean Cu(1 1 1). At saturation by adsorbed sulphur, the surface is completely covered by the superstructure of highest coverage. This enhanced uptake of sulphur is assigned to the surface reconstruction of the copper surface induced by the pre-oxidation, causing a stronger reactivity of the Cu atoms released by the decomposition of the oxide.     

Atomic structure of thin dysprosium-silicide layers on Si(1 1 1)

We report on scanning tunneling microscopy results of thin dysprosium-silicide layers formed on Si(1 1 1). In the submonolayer regime, both a and a 5 × 2 superstructure were found. Based on images taken at different tunneling conditions, a structure model could be developed for the superstructure. For one monolayer, a 1 × 1 superstructure based on hexagonal DySi₂ was observed, while several monolayers thick films are characterized by a superstructure from Dy₃Si₅.