Search

Corrole molecules deposited from aqueous solution on Au(111) arrange flat forming mesa-like clusters (islands), as evidenced by in situ scanning tunneling microscopy (STM). A morphology quantitative assessment of the entire image is given by STM data analysis, evaluating the islands’ fractal dimension as a whole. To this end, two methods are employed: the first exploits the power law that binds the perimeter and the area of all the islands; the second is simply a value of an appropriate average of the fractal dimensions of the islands. In fact the two methods return very close results.

In this work we shed light on the early stage of the chemical vapor deposition of graphene on Ge(001) surfaces. By a combined use of m-Raman and x-ray photoelectron spectroscopies, and scanning tunneling microscopy and spectroscopy, we were able to individuate a carbon precursor phase to graphene nucleation which coexists with small graphene domains. This precursor phase is made of C aggregates with different size, shape and local ordering which are not fully sp(2) hybridized. In some atomic size regions these aggregates show a linear arrangement of atoms as well as the first signature of the hexagonal structure of graphene. The carbon precursor phase evolves in graphene domains through an ordering process, associated to a re-arrangement of the Ge surface morphology. This surface structuring represents the embryo stage of the hills-and-valleys faceting featured by the Ge(001) surface for longer deposition times, when the graphene domains coalesce to form a single layer graphene film. (C) 2018 Elsevier Ltd. All rights reserved.

We systematically investigate the chemical vapor deposition growth of graphene on Ge(110) as a function of the deposition temperature close to the Ge melting point. By merging spectroscopic and morphological information, we find that the quality of graphene films depends critically on the growth temperature improving significantly by increasing this temperature in the 910–930 °C range. We correlate the abrupt improvement of the graphene quality to the formation of a quasi-liquid Ge surface occurring in the same temperature range, which determines increased atom diffusivity and sublimation rate. Being observed for diverse Ge orientations, this process is of general relevance for graphene synthesis on Ge.

By combining scanning probe microscopy with Raman and x-ray photoelectron spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a function of the deposition temperature in close proximity to the Ge melting point, highlighting an abrupt change of the graphene's quality, morphology, electronic properties and growth mode at 930 degrees. We attribute this discontinuity to the incomplete surface melting of the Ge substrate and show how incomplete melting explains a variety of diverse and long-debated peculiar features of the graphene/Ge(001), including the characteristic nanostructuring of the Ge substrate induced by graphene overgrowth. We find that the quasi-liquid Ge layer formed close to 930 degrees is fundamental to obtain high-quality graphene, while a temperature decrease of 10 degrees already results in a wrinkled and defective graphene film., in press

Using high-resolution scanning tunneling microscopy, we investigate the periodicity of mixed arrays of single and triple steps and the atomic structure of triple steps on three vicinal Si(111) samples with off-angles 1.5 ∘ ≤ θ ≤ 9.45 ∘ towards the [ 1 ¯ 1 ¯ 2 ] direction. The comparison between the (556) and (557) triple-step structure demonstrates that only the number of 7x7 terraces is affected by miscut. At variance with the singular Si(111) case, Ge epitaxy is strongly correlated to step morphology, resulting, at the critical coverage and small misorientation, in the nucleation of holed nanoislands from step-edge protrusions. During step-flow growth, stress-driven bunching of atomic steps (ripples) is observed on the (557) surface, indicating that this mechanism of stress relief of Si(001) vicinal surfaces works also on Si(111) surfaces.

Graphene, the wonder material of the 21st century, has not yet achieved the expected outcomes in terms of applications in nanoelectronics. This is not surprising, as large-scale graphene growth is still mostly limited to CVD on metallic foils, followed by graphene transfer to the semiconductor substrate required for electronic devices, which is cumbersome and difficult to automatize. Moreover, graphene is gapless, and this is still seriously limiting its applications. This book is an attempt to dispel this pessimistic outlook, summarizing the latest achievements in the direct growth of graphene on semiconductors.SiC is the ideal semiconductor for graphene growth, which is typically achieved by thermal graphitization. Through high-temperature annealing in a controlled environment, it is possible to decompose the topmost SiC layers, obtaining quasi-ideal graphene by Si sublimation. Graphene on SiC with record electronic mobilities has been demonstrated, opening the way for applications in nanoelectronics, by exploiting selective growth on patterned structures and gap opening by quantum confinement. The book opens with a chapter on the significance and challenges of graphene growth on semiconductors, by Mishra et al., drawing a picture of the perspectives of this technology.The three following chapters, by Aristov et al., Gupta et al., and Tomellini et al., respectively, are dedicated to an up-to-date analysis of the synthesis of graphene on SiC in ultrahigh vacuum. The fifth chapter, by Forti, Starke, and Coletti, is a review of the effect of atomic intercalation at the SiC–graphene interface, with an in-depth discussion of the doping effects and of the electronic properties of lateral superlattices. The sixth chapter, written by the de Heer and Berger group, reporting graphene growth on SiC by sublimation, summarizes the whole history of graphene growth on SiC by confined controlled sublimation, up to the latest developments in the growth of templated graphene nanostructures. We hope that this book can be of inspiration to the many scientists striving to improve the growth of graphene on semiconductors and to the young researchers from industry and academia approaching this fascinating world. The developments sketched here show a promising outlook for the future, with many exciting outcomes on the horizon, from the artificial opening of a gap to the creation of 2D field-effect transistors with nanodimensions.

One of the problems that graphene is facing in electronic applications is its quality, which is still far from the level required to obtain an industrial scale production of reliable nanoscale devices, and it is ultimately related to the growth method. Understanding the kinetics of the graphene growth is an outstanding problem in physics, whose solution is the key to achieve full control on the graphene properties . This chapter deals with the microscopic phenomena occurring during epitaxial graphene growth on SiC by high temperature annealing. We discuss recent experimental and theoretical findings on the Si diffusion in the SiC matrix and on the kinetics of graphene growth as a function of temperature and of time. The theoretical framework developed in this chapter will help to understand the physical basis of the graphene formation, which is essential for a perfect control of the graphene quality, helping to define the optimal conditions for the growth of a continuous monolayer or bilayer graphene on SiC.

We have applied reflectance anisotropy spectroscopy (RAS) to investigate the rebonded-step (RS) reconstructed strained Ge/Si(105) surface. RAS spectra display a marked peak at 2.1 eV that is completely cancelled after exposure to about 1200 L of molecular oxygen. Ab initio calculations of the RAS signal explain the experimental spectra in terms of optical transitions between filled bulk-like states and empty surface bands. The early stages of exposure of the clean surface to molecular oxygen have been monitored by RAS and scanning tunneling microscopy (STM), correlating the evolution of the optical anisotropy to the bonding configuration of the adsorbed gas. Oxygen atoms are proposed to first adsorb onto the arms and later on the head dimers of the “horseshoe” motifs that comprise the RS reconstruction, thus gradually destroying the local order.

International audience; We fabricated flat, two-dimensional germanium sheets showing a honeycomb lattice that matches that of germanene by depositing submonolayers of Ge on graphite at room temperature and subsequent annealing to 350 degrees C. Scanning tunneling microscopy shows that the germanene islands have a small buckling with no atomic reconstruction and does not give any hints for alloy formation and hybridization with the substrate. Our density functional theory calculations of the structural properties agree well with our experimental findings and indicate that the germanene sheet interacts only weakly with the substrate underneath. Our band structure calculations confirm that the Dirac cone of free-standing germanene is preserved for layers supported on graphite. The germanene islands show a small but characteristic charge transfer with the graphite substrate which is predicted by our ab initio simulations in excellent agreement with scanning tunneling spectroscopy measurements.

© 2015 Elsevier Ltd. All rights reserved. We followed by X-ray Photoelectron Spectroscopy (XPS) the time evolution of graphene layers obtained by annealing 3C SiC(111)/Si(111) crystals at different temperatures. The intensity of the carbon signal provides a quantification of the graphene thickness as a function of the annealing time, which follows a power law with exponent 0.5. We show that a kinetic model, based on a bottom-up growth mechanism, provides a full explanation to the evolution of the graphene thickness as a function of time, allowing to calculate the effective activation energy of the process and the energy barriers, in excellent agreement with previous theoretical results. Our study provides a complete and exhaustive picture of Si diffusion into the SiC matrix, establishing the conditions for a perfect control of the graphene growth by Si sublimation.

A few layers of functionalized graphene (FLG) have been obtained by the oxidative unzipping reaction of single-walled carbon nanotubes. A detailed Scanning Tunneling Microscopy and Transmission Electron Microscopy study shows that several different morphologies are observed, depending on the substrate upon which they are deposited, thus providing useful hints for a real world application of these carbon nanostructures. In addition, a combined Scanning Tunneling Spectroscopy investigation has shown that the electronic properties of these FLG are affected by their functionalization degree, opening interesting perspectives for their use in sensor and biosensor devices. © Taylor & Francis Group, LLC.

Controlled synthesis of carbon nanotubes (CNTs) is highly desirable for nanoelectronic applications. To date, metallic catalyst particles have been deemed unavoidable for the nucleation and growth of any kind of CNTs. Ordered arrays of nanotubes have been obtained by controlled deposition of the metallic catalyst particles. However, the presence of metal species mixed with the CNTs represents a shortcoming for most electronic applications, as metal particles are incompatible with silicon semiconductor technology. In the present paper we report on a metal-catalyst-free synthesis of CNTs, obtained through Ge nanoparticles on a Si(001) surface patterned by nanoindentation. By using acetylene as the carbon feed gas in a low-pressure Chemical Vapor Deposition (CVD) system, multi-walled carbon nanotubes (MWNT) have been observed to arise from the smallest Ge islands. The CNTs and the Ge three-dimensional structures have been analysed by SEM, EDX and AFM in order to assess their elemental features and properties. EDX and SEM results allow confirmation of the absence of any metallic contamination on the surface, indicating that the origin of the CNT growth is due to the Ge nanocrystals.

The aim of this work is to understand the surface interactions of Reduced Graphene Oxide (RGO) with a water soluble corrole using graphene as a versatile substrate for sensor devices. Scanning Tunneling Microscopy (STM) and UV-Vis Spectroscopy are employed for understanding the morphology and microstructure of the deposited corrole molecules on RGO at atomic level. Other surface characterization technique like X-Ray Photoelectron Spectroscopy (XPS) is also used for better understanding of the chemical distribution, the molecular environment and the hybridization of the corrole molecules on RGO.

Here, we present a reliable process to deposit transparent conductive films on silicon oxide, quartz, and sapphire using a solid carbon source. This layer consists of partially ordered graphene flakes with a lateral dimension of about 5 nm. The process does not require any catalytic metal and exploits a high current arc evaporation (Φ-HCA) to homogeneously deposit a layer of carbon on heated substrates. A gas atmosphere consisting of Argon or Argon/Hydrogen blend acting as a buffer influences the morphology of the growing film. scanning tunneling microscopy, transmission electron microscopy, and Raman spectra were used for a thorough characterization of the samples in order to optimize the growth parameters. The best carbon layers have a surface resistance of 5.7 × 103 Ω◻ whereas the optical transparency of the coatings is 88% with an excellent homogeneity over areas of several cm2. Such results are compatible with most semiconductor fabrication processes and make this method very promising for various industrial applications.

Nanostructured substrates are an interesting path towards the production of quantum dots devoted to microelectronic applications. In this work we report results on the effects of different nanopatterning methods to obtain lateral ordering of Ge islands grown on Si. By using Scanning Tunneling Microscopy we have studied in real-time the wetting layer growth and islands formation on nanopatterned Si substrates at 500 °C. We compare results obtained on Si substrates nanopatterned by using two different techniques: STM lithography and natural patterning induced by surface instabilities such as step bunching. Different issues on both cases have been addressed: substrate preparation, Ge dots placement and growth mode. We have observed that, on Si(001), the Ge islands nucleate near the holes and on Si(111) step bunching can guide the growth of aligned rows of islands.

The first stages of acetylene reaction with the Si(1 1 1)7 × 7 reconstructed surface kept at 600 °C are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C2H2 pressure of 2 × 10−4 Pa (2 × 10−2 mbar). We observed the progressive substitution of the 7 × 7 reconstruction with a carbon induced Si(1 1 1)√3×√3R30° reconstruction characterized by an atomic distance of 0.75 ± 0.02 nm, very close to that of the silicon 7 × 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si–C phase different from the √3×√3R30° reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T4 and/or S5 sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 × 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer.

The growth mode of sapphyrin deposited on a gold substrate by the Langmuir-Blodgett technique has been studied by scanning tunneling microscopy. The results suggest that the efficiency of the LB deposition is low during the first dippings and it increases after the completion of the first monolayer. The tilt angle formed by the molecules with the substrate has been evaluated as a function of coverage. Scanning tunneling spectroscopy characterization suggests an electronic interaction with the substrate which is important for potential applications.

We have followed by scanning tunneling microscopy (STM) the growth of thin Ge films obtained by reactive deposition epitaxy on Si(111) substrates kept at 500°C. For a Ge coverage less than 0.45 ML, STM images show large 7×7 flat areas without any protrusion. For increasing coverage, flat, triangular 5×5 islands start nucleating while the Si substrate retains the 7×7 reconstruction. The islands' evolution, up to the completion of the wetting layer, is described in the framework of a statistical model of growth. At 3 ML, the composition and ordering of the wetting layer are investigated by Current Imaging Tunneling Spectroscopy (CITS) measurements, revealing small differences in the atomic corrugations. The analysis of topographic and current images supports an elemental composition for the topmost layer. At coverages larger than 3 ML, thick Ge islands nucleate according to the Stranski–Krastanov mechanism of growth. We analyze the evolution of the lattice strain up to 15 ML coverage. A clear expansion of the lattice parameter as a function of coverage is found both on the islands' top and on the wetting layer.

Ge/Si alloys were prepared in UHV by solid phase epitaxy on Si(111) substrates. The alloy formation, as a function of the evaporation rate and the Ge layer thickness has been followed in situ by RHEED and scanning tunneling microscopy. The 5 × 5 surface reconstruction appeared after annealing at 450°C Ge layers (up to 10 A thick), obtained from a low rate Knudsen cell evaporator. In this case a nearly flat and uniform layer of reconstructed alloy was observed. When using an e-gun high rate evaporator we needed to anneal the Ge layer up to 780°C to obtain a 5 × 5 reconstruction. The grown layer was not flat, with many steps and Ge clusters; at high coverages (10 A and more) large Ge islands appeared. Moreover, we then succeeded in visualizing at atomic resolution the top of some of these Ge islands which displayed a 2 × 1 reconstruction, probably induced from the high compressive strain due to the lattice mismatch with the substrate. We suggest that this unusual behavior could be connected to the high evaporation rate, which helped the direct formation of Ge microcrystals on the Si substrate during the deposition process.

By using step-bunched Si(111) surfaces as templates, we demonstrate the self-assembly of an ordered distribution of Ge islands without lithographic patterning. Initially, islands nucleate and evolve at step edges, up to complete ripening, forming long ribbons. Subsequently, island nucleation takes place at the center of flat terraces. Ge islands appear to be regularly spaced in scanning tunneling microscope images. The exploitation of this effect provides a possible route to grow ordered arrays of semiconducting nanostructures.

The strain-induced self-assembly of suitable semiconductor pairs is an attractive natural route to nanofabrication. To bring to fruition their full potential for actual applications, individual nanostructures need to be combined into ordered patterns in which the location of each single unit is coupled with others and the surrounding environment. Within the Ge/Si model system, we analyze a number of examples of bottom-up strategies in which the shape, positioning, and actual growth mode of epitaxial nanostructures are tailored by manipulating the intrinsic physical processes of heteroepitaxy. The possibility of controlling elastic interactions and, hence, the configuration of self-assembled quantum dots by modulating surface orientation with the miscut angle is discussed. We focus on the use of atomic steps and step bunching as natural templates for nanodot clustering. Then, we consider several different patterning techniques which allow to harness the natural self-organization dynamics of the system, such as: scanning tunneling nanolithography, focused ion beam and nanoindentation patterning. By analyzing the evolution of the dot assembly by scanning probe microscopy, we follow the pathway which leads to lateral ordering, discussing the thermodynamic and kinetic effects involved in selective nucleation on patterned substrates.

A morphological model of the Volmer-Weber type is developed for the reactive interface formed between Fe and a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single-crystal surface which takes into account microscopic aspects of the growth such as clustering, metal oxidation via the superconductor substrate, and diffusion through the cluster of elemental species released in the metal-surface redox reaction. The experimental findings of a local chemical analysis by core emission spectroscopy and an atomic-scale structural characterization by scanning tunneling microscopy are in good quantitative agreement with the proposed model indicating that the basic microscopic assumptions are correct.

We present a novel approach to engineer the growth of strained epitaxial films based on tailoring the elastic-interaction potential between nanostructures with substrate vicinality. By modeling the island-island interaction energy surface within continuum elasticity theory, we find that its fourfold symmetry is broken at high miscuts, producing directions of reduced elastic-interaction energy. As a consequence, it is possible to direct the Ge island growth on highly misoriented Si(001) substrates towards desired pathways. © 2011 American Physical Society.

The pair interaction between Ge islands on vicinal Si(001) substrates is investigated by scanning tunneling microscopy measurements as a function of the miscut angle. By the analysis of the nearest-neighbor island distributions, we assess the dependence of the local strain field on the substrate misorientation. We support our results by modeling elastic relaxation for different shapes and arrangements of islands with finite element calculations. © 2010 The American Physical Society.

In the present paper, we report on a metal-catalyst-free synthesis of carbon nanotubes (CNTs) on a pre-patterned Si(001) surface. Arrays of triangular-shaped holes were created by nanoindentation in specific sites of the sample. After germanium deposition and chemical vapor deposition (CVD) of acetylene, a few CNTs nucleated and grew from germanium nanoparticles. This result illustrate how it is possible to control the growth of CNTs without the use of a metal catalyst, by leading the assembly of Ge nanoparticles with a patterning technique.

A complete description of Ge growth on vicinal Si(001) surfaces in the angular miscut range 0\ifmmode^\circ\else\textdegree\fi{}--8\ifmmode^\circ\else\textdegree\fi{} is presented. The key role of substrate vicinality is clarified from the very early stages of Ge deposition up to the nucleation of 3D islands. By a systematic scanning tunneling microscopy investigation we are able to explain the competition between step-flow growth and 2D nucleation and the progressive elongation of the 3D islands along the miscut direction [110]. Using finite element calculations, we find a strict correlation between the morphological evolution and the energetic factors which govern the ${105}$ faceting at atomic scale.

We present a detailed scanning tunneling microscopy study which describes the morphological transition from ripple-to-dome islands during the growth of Ge on the vicinal Si(1 1 10) surface. Our experimental results show that the shape evolution of Ge islands on this surface is markedly different from that on the flat Si(001) substrate and is accomplished by agglomeration and coalescence of several ripples. By using published data of surface energy and finite-element analysis, we provide a meaningful explanation of our experimental observations. © 2010 The American Physical Society.

We report on measurements of step-step interaction on a flat Si (111) - (7×7) surface and on vicinal Si(001) surfaces with miscut angles ranging between 0.2 and 8°. Starting from scanning tunneling microscopy images of these surfaces and describing steps profile and interactions by the continuum step model, we measured the self-correlation function of single steps and the distribution of terrace widths. Empirical parameters, such as step stiffness and step-step interaction strength, were evaluated from the images. The present experiment allows to assess the dependence of the step-step repulsion on miscut angle, showing how parameters drawn from tunneling images can be used to interpolate between continuum mesoscopic models and atomistic calculations of vicinal surfaces. © 2009 The American Physical Society.

The work function of solid layers of increasing thicknesses of E2M8− sapphyrin, deposited on a gold substrate by the Langmuir–Blodgett method, has been measured by the Kelvin-probe technique. The results show that the contact-potential-difference values depend upon the layer thickness, reaching saturation after a certain amount of deposited sapphyrin. Scanning tunneling microscope images taken at the same coverages show that corresponding with this threshold, sapphyrin forms a true continuous layer on gold, completely covering the substrate. Evolution of the layer towards its completion is accompanied by a continuous variation of the work-function value, consistent with an increasing dipole term due to the interaction of sapphyrin with the metal substrate.

Surface patterning is expected to influence the nucleation site of deposited nanostructures. In the present study, clean Si and SiO2 surfaces were patterned by a nanolithographic process using a Focused Ion Beam (FIB). Ge was evaporated in ultra high vacuum at 873 K on these substrates, resulting in the formation of island arrays. Based on scanning tunneling microscopy and atomic force microscopy images, a statistical analysis was performed in order to highlight the effect of patterning on the size distribution of islands compared to a non-patterned surface. We find that the self-organization mechanism on patterned substrates results in a very good arrangement and positioning of Ge nanostructures, depending on growth conditions and holes distance, both on Si and SiO2 surfaces.

We report a detailed investigation of interdiffusion processes that occur during the growth of germanium nanostructures on the (111)-oriented surface of silicon. In particular, X-ray Absorption Fine Structure (XAFS) measurements performed ex situ show that a Ge(1-x)Si(x) alloy forms during deposition, with average composition x varying between 0.25 and 0.50, depending on substrate temperature and total coverage. By fitting the Si nearest-neighbor numbers around Ge as a function of the deposited thickness with a simple model, the effective vertical composition profile in the growth direction has been estimated. The latter has been described with a static effective diffusion length of (10.0 +/- 1.5) nm at 530 degrees C and (5 +/- 1) nm at 450 degrees C, which is interpreted as the dominance of surface transport processes in the intermixing dynamics. The analysis of the data on Ge-Ge bond length indicates a decrease of the Ge-Ge atomic distances with increasing Ge fraction, confirming previous theoretical predictions for strained epilayers. The XAFS results are compared to morphological information obtained by scanning tunneling microscopy investigations carried out in situ, yielding a satisfactory description for the epitaxy of this system.

We investigate the surface structure of Ge(0 0 1) during the (2 × 1)-(1 × 1) phase transition occurring at T > 1130 K by high-resolution scanning tunnelling microscopy. We find a drastic size reduction of dimerized domains in line with substantial dimer breakup accompanied by surface roughening. Completing the picture provided by previous spectroscopic observations, probing with high spatial resolution reveals the nucleation of several nanodomains with distinct vicinal orientations and reconstructions. The structural transformation is irreversible and is not observed for other singular faces of Ge.

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era like nanotransistors and nanomemories. Novel approaches to form ordered patterns of homogeneous nanostructures consist of natural patterning induced by surface instabilities, as step bunching of Si(111) or misoriented Si(001) surfaces, and of nanolithographic techniques, as focused ion beam (FIB) or patterning by scanning tunneling microscopy (STM). Based on the analysis of STM images and movies we report on growth and arrangement of Ge islands on nanopatterned silicon. Several issues are discussed: substrate nanostructuring using different techniques, wetting layer growth, transition up to 3D islands formation and arrangement of QDs to form nanomemories.

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era. New frontiers in nanodevices technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. In this paper we will review some recent studies regarding the control of the nucleation process on semiconductor surfaces. After a brief review of the theory of free nucleation on surfaces and of the role of steps and defects, a few novel approaches to form ordered patterns on surfaces are explored: natural patterning induced by surface instabilities (as step bunching or step meandering), in situ substrate patterning by Scanning Tunneling Microscopy (STM), high resolution patterning by Focused Ion Beam (FIB). Growth of epitaxial layers of semiconductors (Ge/Si(100) or InAs/GaAs(100)) on these patterned surfaces has been studied by STM or Atomic Force Microscopy (AFM) unveiling the way in which the first atoms start to aggregate and identifying their exact nucleation site. Control of the dot size to match the patterning typical wavelength has been achieved by using surfactants on misoriented substrates. STM images acquired in real time allowed to identify the mechanism of Ge cluster formation on patterned Si(100), and to follow the island transition from pre-pyramid to pyramid. Nucleation of Ge islands on SiO2 layers has been obtained thanks to FIB tight patterning, achieving island densities of 3.5x10^10/cm^2.

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era. New frontiers in nanodevice technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. Novel approaches to form ordered patterns of homogeneous nanostructures are explored: natural patterning induced by surface instabilities (as step bunching of Si(111) or misoriented Si(001) surfaces), standard patterning with high resolution lithographic techniques, implantation of Ga+ ions by Focused Ion Beam (FIB), or in situ substrate patterning by Scanning Tunneling Microscopy (STM). Based on the analysis of STM images we report on growth and arrangement of Ge islands on Si(001) substrates nanopatterned using several different approaches. The first is a natural method based on the regular step bunching that occurs on Si(111) surfaces with different annealing treatments. The second is based on the self organization of a Si(001) misoriented surface covered by a thin layer of a GeSi alloy. The third exploit an array of holes produced by STM lithography. The forth is a tight pattern created by FIB. We analyze the resulting distribution of islands resulting from all these approaches.

To investigate the effect of surface patterning on island growth, a real-time study by scanning tunneling microscopy (STM) of Ge deposition on nanostructured Si(001) surfaces is presented. The substrate is nanopatterned by the STM tip and the subsequent evolution of a Ge layer deposited at 500 \ifmmode^\circ\else\textdegree\fi{}C is recorded. The formation of the wetting layer, a transition stage and the growth of three-dimensional (3D) Ge huts are examined dynamically. The 2D-3D transition is described in terms of the nucleation and evolution of pre-pyramids consisting of (001) oriented terraces, which eventually transform into pyramids by successive introduction of {105} facets. Substrate patterning strongly affects the positioning of 3D islands, and represents a route toward ordering of Ge islands.

The morphology of the first stages of acetylene reaction with Si(1 1 1)7 x 7 reconstructed surface has been investigated by scanning tunneling microscopy (STM). We studied the surface modification for two substrate temperatures during reaction (650 and 700 degreesC) and as a function of acetylene doses. Both these parameters have been observed to play a crucial role in the carbonization reaction, the formation of nanostructures and their development. For low acetylene exposure, almost the entire surface appears root3 x root3-R30degrees reconstructed but islands, mainly decorating the step edges, are generally surrounded by 7 x 7 reconstructed areas, located deeper with respect to the plane of root3 x root3-R30degrees reconstruction. For highest acetylene dose, the surface is characterized by nanostructures and large holes (as deep as 3-5 nm) that occupy one or more terraces between the original silicon surface step edges with their inner side constituted of islands itself. For intermediate acetylene exposure, the small increase (from 650 to 700 degreesC) of substrate temperature during the reaction is sufficient to make the resulting carbonization mechanism switch towards the highest exposure behaviour. (C) 2004 Elsevier B.V. All rights reserved.

The initial growth of Mn on Pt(1 1 1) at room temperature has been followed by auger electron spectroscopy and scanning tunneling microscopy. In the interface formation, two distinct regimes have been observed as a function of manganese deposit, with a sharp change in the growth mode at about 1.6 ML. A model has been developed for the earlier stage of growth, suggesting the formation of an intermixed phase extending to about 14 layers, with an average composition Pt 0.89 Mn 0.11 .

Quantum dots (QDs) are actually easily produced by self-assembling during heteroepitaxial growth of semiconductors. In order to exploit the unique electronic properties of semiconductor QDs in novel quantum effect devices, the lateral dimensions of these structures have to be reduced to the order of tens of nanometers, which is the range of the De Broglie wavelength of electrons inside these materials. Moreover, millions of QDs must be arranged in dense ordered arrays to achieve the necessary active volume for optoelectronic applications. Nowadays it is possible to control size and shape of the nanocrystals, but it is still difficult to decide their nucleation site. Many approaches have been undertaken to overcome this problem, like using regular dislocation networks, lithographically and Atomic Force Microscopy (AFM) patterned substrates, naturally patterned surfaces. We present results obtained by some of these methods, visualized by Scanning Tunnelling Microscopy (STM) or AFM microscopy. STM measurements at high temperature during the epitaxial growth are of great help in these studies. Images and movies of the growth of Ge on Si help to identify the real nucleation sites of the islands and to follow their evolution. The influence of the ‘step bunching’ on the self-organization of Ge islands on Si(111) surfaces will be analysed, as an example of growth on self-nanostructured surfaces.

We have investigated by atomic force microscopy and scanning tunneling microscopy subsequent stages of the heteroepitaxy of InAs on GaAs(001) from the initial formation of the strained two-dimensional wetting layer up to the development of three-dimensional quantum dots. We provide evidence of structural features that play a crucial role in the two- to three-dimensional transition and discuss their contribution to the final morphology of the self-assembled nanoparticles. A model is suggested for the strained phase at the critical thickness consisting of an intermixed ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ surface layer of composition $x=0.82$ and InAs ``floating'' on top. Such ``floating'' phase participate to the large mass transport along the surface during the two- to three-dimensional transition that accounts quantitatively for the total volume of dots.

In this contribution we present a study of the Ge-Si intermixing process that arises during the growth of Ge/Si(111) self-assembled islands. The samples, grown by means of a molecular beam epitaxy (MBE)-like technique, have been characterized by in-situ scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by ex-situ X-ray absorption fine structure (XAFS). We have observed by STM a change in the island morphological evolution, from truncated tetrahedra to atoll-like islands, that can be related to the misfit the reduction effect induced by the intermixing process. We have evaluated the intermixing by measuring the average coordination numbers of Si and Ge around a Ge atom by XAFS. We show that the Si content in the nominally pure Ge wetting layer reaches 50% while in the three-dimensional (3-D) islands it is about 25%, and that the intermixing increases with increasing deposition temperature in the 450-530 degreesC range.

We study by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in situ the evolution of Ge islands grown by physical vapor deposition (PVD) on 7/spl times/7 Si[111] reconstructed surfaces. On the 5/spl times/5 reconstructed wetting layer, large 3D islands form whose average lateral dimension is about 200-500 nm. The statistical distribution of the island shapes has been analyzed, showing that three types of shapes coexist under certain conditions: strained, relaxed and ripened (atoll-like) islands. By scanning probe microscopy (SPM) it was possible to measure the contact angles of the island facets and to observe the depletion of the substrate around the ripened islands. All these features are attributed to the misfit strain. These findings successfully compare with a recent theoretical model.

The segregation of Sn in Cu and the formation of a Sn–Cu surface alloy has been studied by Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). By annealing Cu(Sn) samples at T=500°C, we have found a Sn0.33Cu0.66 p ( 3 × 3 ) R 30° reconstructed surface layer. By increasing T to 750°C, the tin concentration decreases up to 0.20, while the reconstruction is maintained. Atomic-resolution STM images of this phase show no rippling, but several steps. The surface lattice parameters are 0.47±0.02 and 0.51±0.02 nm, respectively, along and across the steps.

A study of the adsorption of ethylene ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$ on Si(111)7\ifmmode\times\else\texttimes\fi{}7 at room temperature by scanning tunneling microscopy and scanning tunneling spectroscopy is reported. No significant change in the surface reconstruction is observed. The spatial distribution of the surface reaction has been analyzed on an atom-by-atom basis. Reacted and unreacted sites have been selectively imaged. A higher reactivity of the adatoms in the faulted subunits with respect to those in the unfaulted subunits is reported. Furthermore, the center adatoms are more reactive than the corner adatoms. On the basis of this distribution of reacted atoms, the adsorption site is assigned. The proposed bond model implies that an ethylene molecule is bonded to a pair of adjacent Si adatom--rest atom. It is also shown that the adsorption process affects not only the surface atoms directly involved in the chemical bond with the ethylene molecules, but also some of the neighboring adatoms.

The growth of Fe on Si(111)(7 × 7) has been studied by scanning tunneling microscopy as a function of the deposited thickness using both solid phase epitaxy and reactive deposition epitaxy. At submonolayer deposition of Fe and subsequent annealing at 600°C, flat islands of silicide with hexagonal symmetry become visible above the Si(7 × 7) reconstructed surface which supplies a bond length reference scale for the alloy. The islands tend to grow at the expense of the silicon substrate, creating a large number of surface steps. Atomically resolved images of the islands show that in all cases a cubic (2 × 2) phase is formed. The interplanar (111) distance is compatible more with the FeSi (CsCl structure) than with the FeSi2 (CaF2) epitaxy. The orthorhombic β phase does not form under the present growth conditions.

The spontaneous formation of nanometric and highly dense (∼3×1012cm−2) Ge droplets on thin SiO2 film on Si(001) is investigated by scanning tunneling microscopy and spectroscopy. Ge dots have been grown by depositing Ge on the clean SiO2 surface at room temperature and then annealing the sample at 500°C. Ge dots appear to be free of germanium oxides and characterized by a flat surface with the onset of {113} faceting. I-V curves show that they have an energy gap of approximately 1.8eV, well above that of bulk Ge. Fabrication of nanometer-sized, highly dense pure Ge droplets is very promising for nanoelectronics applications.

An electron energy loss investigation up to 40 eV of YBa 2 Cu 3 O 7−δ (δ≈0.2) is presented. All structures are identified and the e 2 spectrum in the region 0–10 eV is accounted for by a simple convolution of occupied and empty densities of states. A sizeable temperature dependence of e 2 , below and above T c , involving Cu 3d-O 2p valence states is observed.

We have studied by Scanning Tunneling Microscopy (STM) the effect of step bunching on Ge/Si(111) epitaxy. We have verified that self-organization of Ge islands is greatly influenced by “step bunching” which arises from the flash-annealing procedure used to reconstruct the Si surface. Two different growth regimes arise: initially islands nucleate and evolve only at steps, up to complete ripening; subsequently the same evolution is observed on flat areas of the sample. The average distance between islands and steps is nearly constant, originating a single row of equally spaced islands, followed by other rows of islands in between. The exploitation of this phenomenon, which is governed by the surface diffusion length of Ge on Si (estimated from our data) and by the terrace width, constitutes one possible path to achieve self-organization of quantum dots.

Through high-resolution electron energy loss spectroscopy, the Ag/GaAs(001) 2×4 and Ag/GaAs(001) 4×2 interfaces are investigated. The results are compared both with each other and with those previously obtained for the Ag/GaAs(110) interface. X-ray photo emission spectroscopy and scanning tunneling microscopy have also been employed. As the Ag film grows, it forms 3D islands (Volmer–Weber growth mode) that, with respect to those of the (110) surface, are more in number and flatter. Moreover, at odds with the (110) surface, a continuum of states in the energy range of the gap forms at the early stage of junction formation. However, the band-gap losses come from the perimeter of the islands as in the (110) case.

We have followed by scanning tunneling microscopy (STM) the growth of thin Ge films obtained by reactive deposition epitaxy on the Si(111) surface kept at 500 °C. For Ge thickness smaller than 0.45 monolayers (ML), STM images show large 7×7 flat regions without protrusions while at higher coverages flat, triangular 5×5 islands start nucleating. We have followed the evolution of this wetting layer up to its completion and investigated its surface composition at 3 ML by current imaging tunneling spectroscopy measurements. At larger coverages thick Ge islands (quantum dots) start to nucleate according to the Stranski-Krastanov mechanism. We analyze the evolution of the lattice strain both on the wetting layer and on the islands up to 15 ML coverage. A clear expansion of the lattice parameter as a function of the coverage is evidenced both on the islands’ top and on the wetting layer. The luminescence yield measured at 10 K on samples covered by 40 A of Ge and capped with 10 A of Si evidences a structure that...