Tall Islands Research Articles

Temperature Effects in the Initial Stages of Heteroepitaxial Film Growth

Kinetic Monte Carlo simulations of a model of thin film heteroepitaxy are performed to investigate the effects of the deposition temperature in the initial growth stages. Broad ranges of the rates of surface processes are used to model materials with several activation energies and several temperature changes, in conditions of larger diffusivity on the substrate in comparison with other film layers. When films with the same coverage are compared, the roughness increases with the deposition temperature in the regimes of island growth, coalescence, and initial formation of the continuous films. Concomitantly, the position of the minimum of the autocorrelation function is displaced to larger sizes. These apparently universal trends are consequences of the formation of wider and taller islands, and are observed with or without Ehrlich-Schwöebel barriers for adatom diffusion at step edges. The roughness increase with temperature qualitatively matches the observations of recent works on the deposition of inorganic and organic materials. In thicker films, simulations with some parameter sets show the decrease of roughness with temperature. In these cases, a re-entrance of roughness may be observed in the initial formation of the continuous films.

Alignment of morphology during high spatial frequency periodic structure formation in GaAs

The interaction between multiple intense ultrashort laser pulses and solids is known to produce a regular nanoscale surface corrugation. A coupled mechanism has been identified that operates in a specific range of fluences in GaAs that exhibits transient loss of the imaginary part of the dielectric function and Χ2, which produces a unique corrugation known as high spatial frequency laser induced periodic surface structures (HSFL). The final structures have 180 nm periods, and their alignment perpendicular to the laser polarization is first observed in an intermediate morphology with correlation distances of 150 ± 40 nm. Quantum molecular dynamics simulations suggest that HSFL self-assembly is initiated when the intense laser field softens the interatomic binding potential, which leads to an ultrafast generation of point defects. The morphological evolution begins as self-interstitial diffusion, driven by stress relaxation, to the surface producing 1–2 nm tall islands. An ab initio calculation of excited electron concentration combined with a Drude-Lorentz model of the excited GaAs dielectric function is used to determine that the conditions for SPP coupling at HSFL formation fluences are both satisfied and occur at wavelengths that are imprinted into the observed surface morphologies. The evolution of these morphologies is explained as the interplay between surface plasmon polaritons that localize defect generation within the structures present on the previous laser exposure and stress relaxation driven defect diffusion.

An unusual atmospheric vortex street

Atmospheric vortex streets quite often form in the wake of tall islands in the marine atmospheric boundary layer. Most satellite pictures of this phenomenon show that two rows of staggered, counter-rotating vortices are aligned more or less in a straight line downstream of the islands, like Karman vortex streets behind cylindrical obstacles. In this paper however, an unusual downstream behaviour of an atmospheric vortex street in the wake of Heard Island is discussed, where there is a sudden change in orientation of the vortices downwind of the island. The reason for this development can be traced to a special synoptic weather situation.

Growth mode of alumina atomic layer deposition on nanopowders

Alumina films were grown by atomic layer deposition in a rotary reactor on tungsten nanopowder with an average particle size of 54 nm. Films of various thicknesses were formed using trimethylaluminum and water at a reaction temperature of 110 °C by varying the number of deposition cycles from 2 to 78. The sharp contrast between the deposited alumina shell and the tungsten core in transmission electron microscopy allows for easy film thickness measurements and determination of the film thickness as a function of the deposition cycle. The growth curve shows that the rate of film thickness increase does not follow a single linear response but instead consists of three characteristically different growth phases. These phases occur in different deposition cycle regimes: phase I occurs in ≤5 cycles, phase II between 5 and ∼15 cycles, and phase III begins in ∼15 cycles and continues until at least 78 cycles. The average growth per cycle for phases I, II, and III is ∼4.5, <1, and 2 Å/cycle, respectively. The average growth per cycle for phase III is in excellent agreement with literature values for alumina growth on nanopowder and corresponds to ∼1 monolayer per cycle. The distinctly high/low average growth per cycle in phase I/phase II can be explained by the preferential contribution of added alumina in perpendicular/lateral growth of islands. Films grown from two deposition cycles include tall islands and regions which are ≤2 monolayer thick, similar to the dimpled surface of a golf ball. The amount of ≤2 monolayer thick regions decreases in areal density with an increase in the number of deposition cycles, resulting in an almost plateau followed by a linear increase in the growth curve and suggesting a transition of the growth mode. Such evolutions in growth are best described as the Stranski–Krastanov and Frank Vander Merwe type.

Idealized simulations of sea breezes over mountainous islands

High‐resolution simulations of diurnally heated airflows over mountainous islands are conducted to study the impacts of island terrain on sea‐breeze circulations. As the simulated island terrain height increases, the sea‐breeze front (SBF) propagates inland faster but its frontal circulation weakens dramatically, as does the sea‐breeze flow behind it. This sensitivity is interpreted through the frontogenesis budget in a SBF‐relative reference frame. The dominant frontogenetic term (the cross‐frontal convergence) weakens over taller islands, but this trend is offset by a similar weakening of the dominant frontolytic term (the front‐relative advection) to yield minimal sensitivity of the SBF baroclinicity to island terrain height. At first glimpse, this finding appears inconsistent with the systematic reduction in SBF circulation strength over taller islands. However, as the island height is increased, baroclinicity at the SBF becomes increasingly associated not with the front itself but with the ‘background’ baroclinicity associated with elevated heating over steeper island slopes. When this latter contribution is removed, a clear weakening of the SBF baroclinicity is recovered over the taller islands. Analysis of the cross‐frontal convergence budget indicates that this reduction in cross‐frontal convergence, and hence the SBF baroclinicity, is caused by an increased slope‐parallel buoyancy gradient. As the SBF migrates inland, this gradient preferentially accelerates the air ahead of the SBF up the slope, which weakens the convergence along the front. Finally, as shown by a simple scaling of the slope‐parallel momentum equation, the weakened sea‐breeze flow over taller islands is associated with a weaker onshore perturbation pressure gradient force, owing to the protrusion of the convective boundary layer into the stable free troposphere.

Heteroepitaxial growth of cadmium carbonate at dolomite and calcite surfaces: Mechanisms and rates

The systematic variation of rates and the mechanism of cadmium uptake on the (104) surface of dolomite (CaMg(CO3)2) were investigated using in situ and ex situ atomic force microscopy (AFM), ex situ specular X-ray reflectivity (XR), and ex situ X-ray fluorescence (XRF). Selected experiments were performed on the calcite (CaCO3) (104) surface for comparison. Aqueous solutions of CdCl2, CaCl2, and NaHCO3, undersaturated with respect to calcite and supersaturated with respect to otavite (CdCO3) and the (CdxCa1−x)CO3 solid solution, were reacted with dolomite surfaces for minutes to days. Calcite substrates were reacted with solutions containing 1–50μM CdCl2, and with no added Ca or CO3. Thin carbonate films following the Stranski–Krastanov growth mode were observed on both substrates. Specular XR and XRF revealed the formation of nm-thick Cd-rich carbonate films that were structurally ordered with respect to the dolomite (104) plane. Epitaxial films adopted the calcite crystal structure with a d104-spacing (3.00Å) larger than those of pure dolomite (2.88Å) and otavite (2.95Å) indicating either a solid solution with x≈0.5, or a strained Cd-rich carbonate with a composition near that of otavite. The growth rate r of this phase increases with the initial supersaturation of the solution with respect to the solid solution, βmax, and follows the empirical relationship, as determined from XRF measurements, given by: r=10-4.88±0.42(βmax2.29±0.24-1),(in units of atoms of Cd/Å2/h).The morphology of the overgrowth also varied with βmax, as exemplified by AFM observations. Growth at step edges occurred over the entire βmax range considered, and additional growth features including 3Å high monolayer islands and ∼25Å high tall islands were observed when logβmax>1. On calcite, in situ XR indicated that this phase is similar to the Cd-rich overgrowth formed on dolomite and images obtained from X-ray reflection interface microscopy (XRIM) reveal the existence of laterally variable Cd-rich domains.

Real time investigation of the effect of thermal expansion coefficient mismatch on film-substrate strain partitioning in Ag/Si systems

We have used X-ray diffraction (XRD) to investigate strain partitioning between an epitaxial layer and the substrate as a function of temperature, where the substrate (Si) and the epilayer material (Ag) have large thermal expansion coefficient (α) mismatch. The Ag/Si(111) system undergoes morphological changes upon heating, and the larger and taller islands are formed exposing more substrate surfaces. Sample heating was carried out under nitrogen flow. At >300 °C, the Si(111) diffraction peak splits into three. One of these components conforms to the thermal expansion of bulk Si. The other two components correspond to a highly nonlinear decrease and increase of Si-d(111) planar spacing. The decreasing component has been associated with strained Si under Ag and the increasing component with strained Si under SiO2, which has been formed partly prior to the XRD experiment and partly during sample heating. The opposite trends of these two Si-d(111) components are because of the larger value of α for Ag (7 times) and smaller for SiO2 (1/5th) compared to Si. The out-of-plane strain partitioning has been such that at room temperature, the Si substrate is unstrained and the strain in Ag is ∼0.3%. At 800 °C, Ag is practically relaxed, while Si under Ag is ∼−0.7% strained. A temperature dependent strain partitioning factor has been introduced to fit the data.

Sensitivity of deep convection to terrain forcing over Mediterranean islands

AbstractThe influence of mountainous islands on the initiation of deep convection is investigated using the Consortium for Small‐scale Modeling (COSMO) model. A case study and sensitivity analysis is performed for 26 August 2009, when moist convection occurred over both Corsica and Sardinia islands in the western Mediterranean Sea. Sensitivity runs with systematically modified topography are explored to evaluate the relative importance of the land–sea contrast and the terrain height for convection initiation. Whereas no island precipitation is simulated when the islands are completely removed, all simulations that represent these land surfaces develop convective precipitation. Although convection initiates progressively earlier in the day over taller islands, the precipitation rates and accumulations do not show a fixed relationship with terrain height. This is due to the competing effects of different physical processes. Firstly, whereas the forcing for low‐level ascent increases over taller islands the boundary‐layer moisture supply decreases, which diminishes the conditional instability and precipitable water. Secondly, whereas taller islands enhance the inland propagation speeds of sea‐breeze fronts they also block these fronts mechanically and prevent them from reaching the island interior. As a result, the island precipitation is rather insensitive to island terrain height except for one particular case in which the island precipitation increases considerably due to an optimal superposition of the sea breeze and upslope flow. Comparing the two islands, convective precipitation develops earlier over Corsica due to its increased terrain height and reduced convective inhibition. However, precipitation amounts are substantially larger over Sardinia, where a stronger land–sea contrast gives rise to a more intense and organized line of sea‐breeze convergence. These results demonstrate the complexity of interactions between sea breezes and orography and reinforce that an adequate representation of detailed topographic features is necessary to account for thermally induced wind systems that initiate deep convection.

NC-AFM Study of the Adsorption of Hexamethoxytriphenylene on KBr(001)

Molecular adsorption studies on atomically clean, well-defined surfaces of bulk insulators are still very scarce and have been restricted to a small number of molecules. In this study, the adsorption of 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP) on KBr(001) was investigated by atomic force microscopy in noncontact mode (NC-AFM) under ultrahigh vacuum at room temperature from submonolayer to multilayer coverage. This molecule is the first member of a new family of molecules, designed and synthesized for adsorption studies on alkali halide surfaces. They were built around a flat aromatic triphenylene core equipped with peripheral polar groups to enhance the interaction of the molecule with the surface in order to limit molecular diffusion at room temperature. Constant-frequency-shift NC-AFM imaging established the following adsorption sequence: The molecules decorate the KBr steps before forming two-molecular-layer-thick islands with a distorted hexagonal arrangement. At higher coverage, a second generation of much taller islands with the structure of the bulk HMTP crystal appears as a consequence of a dewetting transition, driven by the deformation energy accumulated in the first-generation islands during growth. High-resolution images obtained on top of these islands show submolecular contrast. These results will be useful in optimizing the next generation of molecules of this new family.

Structural Evolution of Perfluoro-Pentacene Films on Ag(111): Transition from 2D to 3D Growth

The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the <110> azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along <110> azimuth directions.

Molecular orientation and film structure of gramicidin on highly oriented pyrolitic graphite

Gramicidin molecules were deposited on HOPG surfaces to characterize molecular orientation and film structure as a function of surface coverage and temperature. At low coverage (0.35 ML), the molecules adopted a flat-lying orientation and formed dendritic islands. At higher coverage (0.86 ML), molecules adopted an upright orientation and circular holes formed in the films. The upright film exhibited higher adhesion in force spectroscopy measurements, supporting our molecular orientation assignments. At elevated deposition temperatures (50 °C) on the higher coverage films, the holes were still present, but partially filled in. At 60 °C the film structure was quite different, forming tall irregular islands without the circular holes observed at lower temperatures. These results demonstrate that gramicidin molecular orientation and film structure on HOPG can be controlled by tuning the surface coverage and deposition temperature.

Transmission electron microscopy study of the initial growth stage of GaSb grown on Si (001) substrate by molecular beam epitaxy method

The microstructural properties at the initial growth stage of the GaSb heteroepitaxial growth on a silicon (Si) substrate were investigated using transmission electron microscopy. Well-separated and tall GaSb islands were observed when GaSb was directly grown on a Si substrate (sample A). On the other hand, GaSb was grown to the coalesced and flat islands when a low-temperature AlSb buffer (sample B) was introduced. The different morphologies of the GaSb islands were related to the microstructural properties of the interface between the GaSb and the Si substrate. The GaSb/Si interface was rough, and disordered atomic arrangements were observed at the interface in sample A. On the other hand, the GaSb/Si interface was flat, and well-ordered atomic arrangements appeared at the interface in sample B. Entirely different mechanisms for the relaxation of a misfit strain were demonstrated from a microstructural viewpoint.

Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)

The endemic Hawaiian lobeliads are exceptionally species rich and exhibit striking diversity in habitat, growth form, pollination biology and seed dispersal, but their origins and pattern of diversification remain shrouded in mystery. Up to five independent colonizations have been proposed based on morphological differences among extant taxa. We present a molecular phylogeny showing that the Hawaiian lobeliads are the product of one immigration event; that they are the largest plant clade on any single oceanic island or archipelago; that their ancestor arrived roughly 13 Myr ago; and that this ancestor was most likely woody, wind-dispersed, bird-pollinated, and adapted to open habitats at mid-elevations. Invasion of closed tropical forests is associated with evolution of fleshy fruits. Limited dispersal of such fruits in wet-forest understoreys appears to have accelerated speciation and led to a series of parallel adaptive radiations in Cyanea, with most species restricted to single islands. Consistency of Cyanea diversity across all tall islands except Hawai ;i suggests that diversification of Cyanea saturates in less than 1.5 Myr. Lobeliad diversity appears to reflect a hierarchical adaptive radiation in habitat, then elevation and flower-tube length, and provides important insights into the pattern and tempo of diversification in a species-rich clade of tropical plants.

Hydrogen induced roughening and smoothing in surface morphology during synchrotron-radiation-excited GeH4-source homoepitaxy on Ge(001)

The kinetics of roughening and smoothing in Ge(001) surface morphology during synchrotron-radiation-excited GeH4-source homoepitaxy has been investigated. At high GeH4 pressures and low substrate temperatures, two-dimensionally flat surface morphology is maintained while passivating the epitaxially growing surface with a Ge hydride layer. After exceeding the critical temperature (330°C), the delivery of GeHx species to the partially dangling bond terminated surface and the accelerated migration of Ge atoms result in roughening from the initial stage of growth, the onset of which can be detected in terms of a small loop in real-time ψ-Δ trajectories of ellipsometric monitoring. The roughening tendency is enhanced with increasing synchroton radiation beam intensity and substrate temperature and with decreasing GeH4 pressure. Submicrometer tall islands consisting of a (001) top surface surrounded by (113) sidewalls are produced. Roughening is terminated when all dangling bonds are tied up with H atoms, i.e., when the surface free energy is minimized. Then, preferential embedding into valleys between islands yields sinusoidal-like ripple morphology, which is eventually converted into a flat Ge epilayer with no inner defects.

Comparison between surfactant-mediated Bi/Ge/Si(111) epitaxy and Ge/Si(111) epitaxy

Using scanning tunneling microscopy (STM) the Ge epitaxy on a Bi terminated Si(1 1 1) surface is compared to the growth without surfactant. During the growth of the Ge wetting layer in surfactant mediated epitaxy (SME), Si and Ge can be distinguished by different apparent heights in the STM images due to the Bi termination of the surface. As soon as the two bilayer high wetting layer is completed, Ge islands with a flat top and an underlying dislocation network occur. Elastic distortions due to the dislocation network result in periodic sub-angstrom height undulations measured by the STM. In this case the Ge islands have the form of a mesa. With increasing Ge coverage, these mesas spread laterally. Beyond a Ge coverage of 10 bilayers, the Ge mesas have coalesced and further Ge deposition leads to a 2D layer-by-layer growth of Ge on Si(1 1 1). In epitaxy without the use of a surfactant as well, the formation of Ge islands with an underlying dislocation network is observed. However, in this case the Ge islands are much higher and show no tendency to coalesce. The partially relaxed islands coexist with another type of tall islands.

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-( 7 x7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si6H12 with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si6H12 cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands.

Determination of growth mechanisms by X-ray photoemission and ion scattering spectroscopies: application to thin iron oxide films deposited on SiO 2

We have studied the formation of iron oxides on SiO 2 under varying growth conditions by X-ray photoemission spectroscopy (XPS) and ion scattering spectroscopy (ISS). Three different procedures for quantitative analysis of the experimental data were used: (1) traditional analysis of the XPS peak intensity by assuming a layer formation that covers the surface completely; (2) combination of the XPS peak and ISS intensities assuming a Poisson distribution of island heights; and (3) Tougaard method to determine in-depth profiles from analysis of the peak and background. The difference between the three methods lies mainly in the a priori assumptions made on the surface morphology. In contrast to methods (1) and (2), the Tougaard method is free from assumptions about the growth structure of the overlayer. By a critical comparison of the results, it is concluded that the Tougaard method gives the most complete and reliable result. Iron oxides on SiO 2 grow with strong island formation, whose height depends on the preparation conditions of the deposit. The growth mechanisms are proposed. Thus, for example, if iron oxide is deposited on SiO 2 at room temperature in P O 2 =2×10 −6 mbar, islands of ∼22 Å homogeneous thickness are formed for surface coverages below 20%. If the samples are annealed at 773 K in P O 2 ≈4×10 −5 mbar, higher islands are formed (∼32 Å). For iron oxide treated by a plasma of oxygen, ∼40 Å tall islands are formed for surface coverages below 30%. Besides, even ∼35 Å of iron oxide does not fully cover the SiO 2 substrate surface. Strong shadowing effects are observed in the ISS signal from the iron oxide deposits on SiO 2 due to the tall island formation. As expected, the shadowing effects are stronger when taller islands have been formed.

Effect of hydrogen on Cu formation on Si(111)

Cu formation processes on clean Si(111)-(7 × 7) and hydrogen-terminated Si(111) surfaces have been investigated in situ by means of a medium-energy ion scattering (MEIS), RHEED, AES and observed ex situ with a field emission-scanning electron microscope (FE-SEM). The non-uniform Cu films including Si atoms are formed on the Si(111)-(7 × 7) surface at room temperature and the “5 × 5” incommensurate structure can be observed on the Cu Si(111) at high temperature (130–600°C). On the hydrogen-terminated Si(111) surface at room temperature, the growth processes of Cu are not so different from those on the clean Si(111)-(7 × 7) surface. However, the formation processes of Cu on the hydrogen-terminated Si(111) surface at high temperature (∼300–500°C) are quite different from those on the bare Si(111)-(7 × 7) surface. Cu atoms can easily migrate on the hydrogen-terminated Si(111) surfaces and form tall islands. The hydrogen-terminated surfaces remain, and can be observed by means of RHEED and SEM.

Interface roughness effects on transport in tunnel structures

Direct simulations of interface roughness effects on transport properties of tunnel structures are performed using the planar supercell stack method. The method allows for the inclusion of realistic three-dimensional rough interfacial geometries in transport calculations. For double barrier resonant tunneling structures, we used our method to analyze the effect of roughness at each of the four interfaces, and to test for sensitivity of transport properties to island size and height. Our simulations yields the following conclusions: (1) We find that scattering of off-resonance states into on-resonance states provides the dominant contribution to interface roughness assisted tunneling. Analyses of scattering strength sensitivity to interface layer configurations reveals preferential scattering into Δk∥≊2π/λ states, where λ is the island size. (2) We find that roughness at interfaces adjacent to the quantum well can cause lateral localization of wave functions, which increases with island size and depth. Lateral localization can result in the broadening and shifting of transmission resonances, and the introduction of preferential transmission paths. In structures with wide and tall islands, it is possible to find localization over ‘‘islands’’ as well as localization over ‘‘oceans.’’ (3) The leading rough interface is the strongest off-resonance scatterer, while rough interfaces adjacent to quantum well are the strongest on-resonance scatterers. The trailing interface is the weakest scatterer.

Growth and structure of thin Co films on Cu(111) studied by full-solid-angle x-ray photoelectron distributions.

Photoelectron spectroscopy at kinetic energies of ≃570 eV has been performed on a Cu(111) surface with Co coverages of up to 50 monolayers. The angle-integrated data indicate that tall islands develop with Cu atoms in the top layers. The angular distributions of the photoelectrons were measured using a display-type analyzer with an opening angle of 88 o . The Co emission patterns always show some threefold symmetry corresponding to the fcc structure of the clean Cu(111) surface. Most of the Co atoms are in the hcp structure and this faction increases with coverage