Substrate Terraces Research Articles

Scaling behavior of island density in submonolayer growth of CaF 2 on vicinal Si(1 1 1)

We have studied the scaling behavior of two-dimensional island density during submonolayer growth of CaF 2 on vicinal Si(1 1 1) surfaces using scanning tunneling microscopy. We have analyzed the morphology of the Si(1 1 1) surfaces where CaF 2 partial monolayers with coverages of about 0.1 monolayer are deposited at ∼600 °C. The number density of terrace nucleated islands increases with substrate terrace width l as ∼ l 4 in a low island density regime. This scaling behavior is consistent with predictions for the case of the irreversible growth of islands.

Critical effects of substrate terraces and steps morphology on the growth mode of epitaxial SrRuO3 films

We report on the controlled fabrication of the arrays of self-organized fingerlike nanostructures of SrRuO3 that form on SrTiO3(001). We show that the size (width and height) of the one-dimensional nanometric units can be tuned or suppressed by using appropriate substrate vicinality θv and surface termination. Critical effects on the fingers height are observed when θv is below an angle as small as ∼0.1°. The value of θv also determines if the final two-dimensional growth is by a step flow (θv=0.5°), by layer by layer (θv=0.04°), or by coexistence of both mechanisms (θv=0.1°). The fingers are suppressed when TiO2-terminated low-miscut substrates are used, as film nucleation takes place on the terraces and then substrate steps are not active as templates for the one-dimensional structure formation. These findings shall contribute to progress toward lateral nanostructuration of complex oxide surfaces.

STM/STS Studies of Self-Organized Growth of Iron Silicide Nanocrystals on Vicinal Si(111) Surface

The evolution of iron silicide structure grown by solid state epitaxy on Si(111) vicinal surface was investigated by scanning tunnelling microscopy. The reactions, which occur on the surface, are compared for two various Fe coverages: 0.33 and 2 monolayers. The annealing at 250°C does not enable substantial recovery of the surface ordering, deteriorated by Fe deposition at room temperature. The onset of 2 x 2 surface reconstruction is observed upon annealing at 400°C. A three-dimensional growth tendency of iron silicide crystallites on a bare Si(111) 7 x 7 surface was found at 700°C. In the case of 2 monolayer coverage crystallites nucleate along the edges of substrate terraces forming a regular array of nanometer size dots. Basing on atomically resolved spectroscopic effects and statistical considerations, structure of iron silicide nanocrystallites as well as Schottky-like character of the barrier at the interface between metallic crystallite and semiconducting substrate is deduced.

Structural growth of strained Dy islands on W( [formula omitted

Growth of Dy on W(1 0 0) at room temperature produced a rough surface covered by rhombic-shaped islands. The islands were strained perpendicular to the direction of substrate terraces. Annealing to 820 K produced strained, but well-ordered islands of Dy with a c(8 × 2) structure, with the principal direction of strain the same as for the as-grown islands. Further investigation showed that Dy islands do not follow Gd in producing inverted-step islands on W(1 0 0).

Scanning tunnelling microscopy of MgO ultrathin films on Ag(001)

The morphology of ultrathin epitaxial MgO layers reactively grown on Ag(001) was investigated by using scanning tunnelling microscopy. In the initial deposition stage Ag atoms are partially removed from the substrate and form extended monoatomic islands leaving vacancy islands in the substrate itself. On individual substrate terraces Ag is thereafter found at three atomic levels. For submonolayer deposition MgO condensates in form of small islands of monoatomic height, located on the original substrate, on the protruding Ag islands and on the vacancy islands as well. The largest Ag(001) fractional coverage by monoatomic MgO islands is 70%. A limited amount of MgO bilayers or trilayers has also been detected (about 1% fractional coverage). At the nominal deposition of 1 ML flat, squared and connected MgO domains of about 10 nm in width form, with edges along the [110] directions. The actual substrate fractional coverage is about 85% and the occurrence of bilayers and multilayers becomes significant (about 30 and 5% fractional coverage, respectively), resulting in the formation of three-dimensional pyramidal MgO islands.

In situ STM study of electrodeposition and anodic dissolution of Ni on Ag(111)

A study of the electrodeposition and electrochemical dissolution of ultrathin Ni films on Ag(111) electrodes in Watts electrolyte by in situ scanning tunneling microscopy (STM), electrochemical quartz microbalance (EQCM), and cyclic voltammetry (CV) is presented. Ni deposition starts at potentials negative of − 0.72 V s. SCE (i.e., overpotentials η160 mV), where an incommensurate, (111)-oriented film with an in-plane lattice rotation of 0.5° relative to the Ag substrate lattice is formed. The lateral nearest neighbor spacing is as in bulk Ni (2.49 A) for a film thickness 3 ML and expanded for monolayer (2.54 A) and bilayer (2.52 A) films. Depending on the deposition potential, three growth regimes, resulting in different deposit morphologies, are observed: At low overpotentials (160⩽η/mV⩽200) a smooth Ni film is formed ia a 2D step-flow growth process, commencing at steps of the Ag substrate. At medium overpotentials (200η/mV300) a transformation from 2D step-flow to 3D growth occurs, resulting in the selective formation of Ni multilayer islands along the Ag steps. At even higher overpotentials (η300 mV) 3D islands are formed at the steps and on the substrate terraces. The size of the Ni multilayer islands is independent of the terrace widths, indicating that Ni growth proceeds ia direct discharge at step sites (“direct deposition”). The transition from 2D to 3D growth as well as the change in island shape with overpotential can be rationalized by a different potential dependence of the various microscopic nucleation and growth processes. The multilayer growth at steps is attributed to next-layer nucleation at the structural defect induced by the Ag–Ni boundary and can be described quantitatively as a function of deposition time by a simple 2D model. In addition, place-exchange of Ni with Ag surface atoms and encapsulation of Ni islands by Ag is observed. Dissolution of the electrodeposited multilayer Ni films proceeds ia step-flow etching, with a higher dissolution rate for the Ni monolayer as compared to higher Ni layers.

Growth and structure of ultrathin vanadium oxide layers on Pd(111)

The growth of thin vanadium oxide films on Pd(111) prepared by reactive evaporation of vanadium in an oxygen atmosphere has been studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and ab initio density-functional-theory (DFT) calculations. Two-dimensional (2D) oxide growth is observed at coverages below one-half of a monolayer (ML), displaying both random island and step-flow growth modes. Above the critical coverage of 0.5 ML, three-dimensional oxide island growth is initiated. The morphology of the low-coverage 2D oxide phase depends strongly on the oxide preparation conditions, as a result of the varying balance of the mobilities of adspecies on the substrate terraces and at the edges of the growing oxide islands. Under typical V oxide evaporation conditions of $p({\mathrm{O}}_{2})=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\mathrm{mbar},$ $T(\mathrm{substrate})=523\mathrm{K},$ the 2D oxide film exhibits a porous fractal-type network structure with atomic-scale ordered branches, showing a $p(2\ifmmode\times\else\texttimes\fi{}2)$ honeycomb structure. Ab initio DFT total-energy calculations reveal that a surface oxide model with a formal ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ stoichiometry is energetically the most stable configuration. The simulated STM images show a $(2\ifmmode\times\else\texttimes\fi{}2)$ honeycomb structure in agreement with experimental observation. This ${\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\ensuremath{-}\mathrm{V}}_{2}{\mathrm{O}}_{3}$ layer is very different from bulk ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and represents an interface stabilized oxide structure. The V oxide layers decompose on annealing above 673 K and 2D island structures of V/Pd surface alloy and metallic V are then formed on the Pd(111) surface.

Electrochemical adsorbate-induced substrate restructuring: gold(110) in aqueous bromide electrolytes

The electrode potential-dependent atomic structure of ordered Au(110) in bromide electrolytes is examined by in situ scanning tunneling microscopy, focusing on the ordered adlayer arrangements formed on the (1×1) substrate terraces, and especially the adsorbate-induced nanoscale restructuring which is observed to occur at higher potentials. Comparison is made with the previously examined behavior of Au(110) in iodide electrolytes. Several ordered bromine structures were identified, all featuring ‘close-packed’ Br rows along the (001) direction, i.e. perpendicular to the (110) ‘rails’. At lower coverages, θ Br<0.75, rectangular packing was commonly observed, featuring also Br rows parallel to the (110) direction. The Br binding sites were determined unambiguously from ‘potentiodynamic’ STM data, featuring juxtaposed substrate and adlayer domains created by potential-step perturbations during image acquisition. The rectangular unit cells contain Br atoms lying within the (110) troughs, and exhibit periodic variations from ‘long twofold’ to fourfold-hollow binding sites. These include (3×1) and (4×1) unit cells, corresponding to θ Br=0.66 and 0.75, respectively. At similar potentials to the latter, an alternative (4×1) structure can be formed having the same coverage, yet with quasi-hexagonal Br packing. Increasing the potential further yields erosion of terrace edges, especially along the (001) direction, and the appearance of arrays of single atomic-layer gold ‘nanorods’, 1.8 nm wide, also oriented along (001). These strings eventually self-assemble into ordered arrays, with rod separations of 0.9–1.2 nm. The restructuring can be reversed rapidly by returning the potential to lower values. The ordered restructuring is markedly different from that observed previously in the Au(110)–I system, which features ‘miniterrace stripes’ largely paralleling the (1 1 ̄ 0) direction. These structural differences can be understood in terms of the known dissimilarities in the close-packed Br and I adlattices; the strict (001) orientation of the nanorods in the Au(110)–Br system is apparently triggered by the similarly oriented Br strings. The widths of the restructured gold miniterraces in both electrolytes are surmised to be determined by adsorbate-induced dipole repulsion forces.

Surface Structure of Nickel in Acid Solution Studied by In Situ Scanning Tunneling Microscopy

The structure of Ni(111) single‐crystal surfaces in (pH 3) has been investigated in situ by scanning tunneling microscopy after cathodic reduction and after passivation at potentials ranging from 0.45 to 1.15 V/SHE. The metallic surface has monoatomic and diatomic step edges and an unreconstructed Ni(111)‐(1 × 1) atomic lattice. The passivated surface has two different structures corresponding to the two chemical layers of the passive film. The hydroxide outer layer has a granular and amorphous structure. The lack of crystalline order is assigned to the incorporation of water molecules. Aging under polarization favors the hydration of this layer and the increase of the passivation potential favors its thickening. The oxide inner layer of the passive film has a crystalline structure with a stepped surface independent of the passivation potential. The average tilt angle between the terraces of the oxide layer and the substrate terraces is 3 ± 1°. The lattice parameters correspond to NiO(111) in parallel or antiparallel epitaxy with the substrate. The (111)‐(1 × 1) unreconstructed orientation of the oxide layer is stabilized by surface hydroxylation. The local orientation of the step edges of the oxide is assigned to its dissolution in the passive state. © 2000 The Electrochemical Society. All rights reserved.

Structure and stability of the platinum/aluminium interface: alloying and substrate vacancy formation on Pt{111}/Al

The structure and stability of Al overgrowths on platinum have been studied by LEED, STM, Auger spectroscopy and photoemission. Annealing dendritic Al islands on Pt{111} to 500 K promotes intralayer Al adatom diffusion across the substrate terraces. Evaporation from smaller Al dendrites results in island densification and coalescence, accompanied by Al/Pt intermixing at ascending steps. Simultaneously, triangular vacancy islands appear that decorate the step edges of the platinum surface. This process plays an integral part in the intermixing of Pt and Al at step sites. Metastable compact islands of the Al overgrowth persist up to 800 K, above which temperature interlayer transport commences, resulting in the formation of an ordered (2×2) Pt 3Al surface alloy. Thus, a low-temperature quasi-one-dimensional alloying mechanism is succeeded by a place exchange mechanism at higher temperatures. The (2×2) alloy, formed for Al coverages above 0.5 ML, remains stable up to ∼1000 K, at which point, complete Al dissolution occurs, though some Pt vacancy islands persist at the surface.

Self-assembly of 1-nitronaphthalene on Au(111)

We report low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigations of the organic molecule 1-nitronaphthalene adsorbed on the reconstructed Au(111) surface. Depending on the coverage, molecular structures of distinct dimensionality are observed. After saturation of the step edges with molecules, clusters form on the substrate terraces at coverages up to 0.15 monolayers (ML). The smallest clusters consist of three molecules and are stable at specific surface reconstruction sites only. In contrast, pinwheel-shaped clusters comprised of ten molecules are observed on various inequivalent sites of the reconstructed Au surface. For coverages larger than 0.2ML one-dimensional molecular double chains prevail. The structure of the decamers and the chains is determined by intermolecular electrostatic forces, whereas their position and orientation are affected by the surface reconstruction. At saturation coverage a periodic two-dimensional close-packed arrangement of molecules that is independent of the surface reconstruction occurs simultaneously with one-dimensional structures along the uniaxial reconstruction domains. The structural features are discussed in terms of competing effects of intermolecular forces and the interaction with the substrate.

Interaction of oxygen with Ni 3Al(111) at 300 K and 1000 K

We have investigated the oxidation of Ni 3Al(111) at 300 K and 1000 K with STM and work function change measurements. Oxygen exposure at 300 K initially increases the work function, indicating oxygen chemisorption. Exposures above 5 L lead, however, to a rapid decrease of the work function, indicative of oxygen incorporation. This process is accompanied by a strong surface roughening which leads to a disordered surface. Annealing this surface at 1000 K results in the formation of “strawberry”-shaped oxide islands on the substrate terraces. Adsorption of oxygen at 1000 K produces a well-ordered oxide phase that gives rise to a Moiré pattern with a periodicity of 2.5 nm. This pattern disappears for bias voltages below ∼3 V, accompanied by a strong decrease in the apparent height of the oxide layer. This effect can be explained in terms of the band gap of the alumina layer.

Scanning tunnelling microscopy on the growth and structure of NiO(100) and CoO(100) thin films

We have prepared ordered thin films of NiO and CoO in (100) orientation by evaporating Ni (Co) in an O2 atmosphere onto Ag(100). The films have been analysed by scanning tunnelling microscopy and low-energy electron diffraction. In the initial stage (coverage up to a few monolayers), growth and structure of the grown films drastically depend on the preparation conditions (in particular, on the temperature of the substrate during deposition and post-annealing). In this case we also observe strong interactions with the substrate. Ag atoms are partially removed from the substrate terraces and form islands or migrate to step edges. No indications for incorporation in the oxide thin films are seen. The oxidic features grow on top of the substrate or in the vacancy islands within the first layer of the substrate left behind by the removed Ag atoms. At low substrate temperatures (near room temperature) an essential part of the oxidic features corresponds to a precursor state rather than to the fully developed (100) oxide film which only develops after post-annealing to higher temperatures (typically around 500 K). I/U characteristics and the sample bias dependency of the contrast of the islands grown have been utilised for identification of whether an oxide reaction had taken place or not. The surfaces of the oxide precursor show a typical defect structure similar to those found on cleaved NiO(100) (M. R. Castell etal., Phys. Rev. B:Condens. Matter, 1997, 55, 7859). This feature shows ‘random walk’ at room temperature.

Local decomposition of NiO ultra-thin films formed on Ni(111)

AES, LEED and STM have been combined to study the decomposition of NiO(111) and NiO(100) thin films formed on Ni(111) after annealing between 500 and 700K in UHV. The annealing of the two to four layer thick films leads to their partial decomposition and produces stable NiO(111) crystallites and the Ni(111)-(2×2) phase of adsorbed oxygen. The decomposition of the NiO(111) thin films produces an intermediate (3×2)rect. phase assigned to 2D arrangements of residual NiO rows on reduced Ni(111) planes, before the formation of the (2×2) adsorbed phase. The loss of oxygen of the (2×2) adsorbed phase leads to the uncovering of the substrate terraces preferentially on the upper side of the steps, indicating a preferential removal of the oxygen atoms by adsorption in the metal or reactive desorption at the ledges of the superstructure. The formation of a surface monolayer of nickel hydroxide on the NiO(111) thin film shifts the observation of the early stages of decomposition from 550 to 600K with respect to the NiO(111) surface covered by a monolayer of hydroxyl groups. This is attributed to the increase of the film thickness resulting from the surface hydroxidation. The NiO(111) orientation has a higher resistance to decomposition than the NiO(100) one, which is already totally decomposed into the adsorbed phase at 550K. This is assigned to the higher stability of the NiO(111)/Ni(111) interface.

Pinhole formation in solid phase epitaxial film of CoSi2 on Si(111)

The long-standing pinhole problem in solid phase epitaxial growth of a CoSi2 film on Si(111) has been revisited with in situ scanning tunneling microscopy. While the as-deposited film with 5 Å of Co at room temperature shows a smooth granular texture with original substrate terraces remaining intact, annealing at 580 °C produces an epitaxial CoSi2 film with large pinholes enclosed by a thin ring CoSi2, exhibiting a volcano feature. Quantitative analysis shows that the formation of pinholes is a result of rapid Si outward diffusion from bulk to surface, and of the subsequent Si reaction with Co on the outer surface. Evidence suggests that inhibiting the Si diffusion channels during the thermal annealing process is the key to solving the pinhole problem.

Nucleation Mechanism ofYBa2Cu3O7−δonSrTiO3(001)

Using UHV-scanning-tunneling microscopy and spectroscopy we show that submonolayer coverages of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ deposited on ${\mathrm{SrTiO}}_{3}(001)$ nucleate as cubic, semiconducting $(\mathrm{Y},\mathrm{Ba}){\mathrm{CuO}}_{3\ensuremath{-}x}$ phase as proven by the height of the nuclei and their density of states. Nuclei are observed at upper step edges and on the substrate terraces exclusively. Minimization of surface and interface energies forces this nucleation behavior. At about one monolayer, the film transforms into the orthorhombic, metallic ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ phase indicating that the formation enthalpy of the bulk phases now dominates the growth process.

The role of antiphase domain boundaries in Si epitaxy by ultrahigh vacuum chemical vapor deposition from SiH4 or SiH2Cl2 on Si(100)-(2×1)

and SiH2Cl2 at intermediate temperatures is illustrated. Under these conditions multilayer growth is governed by heterogeneous nucleation of dimer strings at antiphase domain boundaries of the underlying layer which represent deep potential minima for diffusing species. This is in contrast to the formation of the first epitaxial layer, which nucleates homogeneously on the flat substrate terraces.

Corrugation reversal in scanning tunneling microscope images of organic molecules

Submonolayer coverages of perylene-tetracarboxylic-dianhydride on Ag(110) were investigated with scanning tunneling microscopy (STM) at 295 and 50 K. The molecules form highly ordered rhombic islands which at 295 K coexist with molecules diffusing on the substrate terraces. At 50 K the molecular islands appear 1.4 \AA{} higher than the Ag substrate in STM images, independent of the tunneling voltages and currents. At 295 K this apparent height is observed only at high tunneling resistances. Lowering of the resistance decreases the height difference until finally the contrast between islands and the surrounding substrate is reversed. At the same time, vacancies within islands show no contrast reversal. We explain the contrast reversal by trapping of molecules below the tip apex.

Inhomogeneous growth of CaF2 adlayers on Si(111) at intermediate temperatures

The growth morphology of CaF2 adlayers deposited in the temperature range 500–650°C on Si(111) samples with 0.9° miscut in [112¯] direction has been studied by atomic force microscopy (AFM). After transferring the samples from the MBE chamber to ambient air we found an inhomogeneous film morphology: on one hand there are smooth islands wetting substrate terraces and on the other hand different substrate terraces show small clusters. The cluster formation is attributed to the reaction of the first unstable CaF2 layer after contact with ambient air while the smooth areas consist of thickness stabilized CaF2 islands. The smooth islands grow preferentially in the [11¯0] direction since the substrate steps block the growth in [112¯] direction.

Inter-layer mass transport in a low-coverage, low island-density regime

We use Monte Carlo simulations to study inter-layer mass transport during submonolayer epitaxial growth in systems where the ratio of the diffusion coefficient for intra-layer mobility to the deposition flux D F is very high, such that the average diffusion length is comparable to the average terrace length. Under such growth conditions, there exists a low island-density regime where the scaling relation between the island density N and the ratio D F breaks down. We employ realistic terrace boundaries in our simulations (i.e. atom capture at ascending steps and a step edge barrier E s between adjacent substrate terraces at descending steps) to investigate how inter-layer diffusion depends on the D F ratio in this low island-density regime. Information about the magnitude of E s can be extracted by analyzing the distribution of deposited atoms among the three possible capture sites: ascending steps, descending steps and nucleated islands. We apply the results of these simulations to the growth of Ag Ag(111) .