Silicon carbide nanocrystals growth on Si(1 0 0) and Si(1 1 1) from a chemisorbed methanol layer

SiC nanocrystals are grown at high temperature on Si(1 0 0) and Si(1 1 1) surfaces starting from a chemisorbed layer of methanol. The decomposition of this layer allows to have a well defined amount of carbon to feed SiC growth. Nanocrystals ranging from 10 nm to 50 nm with density from 100 μm⁻² to 1500 μm⁻² are obtained, and the total volume of produced SiC corresponds to carbon provided by the chemisorbed organic layer. Large differences in nanocrystal size and density, as well as in surface roughness, are observed depending on substrate orientation. The internal structure, crystallinity and epitaxy of nanocrystals grown on Si(1 0 0) are studied using cross-sectional transmission electron microscopy (XTEM), methanol adsorption and surface evolution using scanning tunnelling microscopy (STM). The joint application of XTEM and STM techniques allows a complete characterization of the geometry and chemical composition of these nanostructures.     

Magnetic shape anisotropy calculations of Fe nanostructures at the Si/Fe(1 1 1) interface

The influence of Si capping layers on the magnetic properties of thin Fe films grown on Si(1 1 1) has been studied by means of shape anisotropy calculations. Fe surface morphology simulations are realized using experimental STM data. Surface modifications induced by the interaction between the Si overlayer and the Fe surface are performed in agreement with the model proposed in a previous work by Stephan et al. [J. Magn. Mater. 293 (2005) 746]. Calculations of the uniaxial anisotropy energy constant K_u are then performed on the modified Fe surface morphology for different Si deposition geometries as proposed in the model. The relevant data deduced by this method such as anisotropy constants and their related easy axis direction, are directly compared to the experimental ones obtained by ex situ magneto-optical Kerr effect (MOKE) measurements at room temperature using the transverse bias initial inverse susceptibility and torque (TBIIST) method. We show that a very good agreement between those results leads to a confirmation of the proposed model.     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

Following the oxidation of yttrium silicide epitaxially grown on Si(1 1 1) by core level photoemission spectroscopy

We have identified, by means of synchrotron radiation X-ray photoemission spectroscopy, several core-level shifted components in the Si-2p photoemission core level peak from a thin yttrium silicide layer epitaxially grown on a Si(1 1 1) surface. We have unequivocally assigned these components to different environments of the Si atoms in the silicide structure. This information has been used to monitor a surface oxidation process promoted by room temperature oxygen adsorption, identifying the final product of this reaction as a silicate-type ternary compound.     

Growth of Si nanostructures on Ag(0 0 1)

The first stages of the growth of silicon on Ag(0 0 1) at moderate temperatures start by the formation of a p(3 × 3) superstructure, which continuously evolves with increasing coverage toward a more complex superstructure. In this paper, the atomic arrangement of the p(3 × 3) and of the “complex” superstructure has been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 × 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(0 0 1) surface layer. For higher coverages, i.e., when the “complex” superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure.     

Shape variation in epitaxial microstructures of gold silicide grown on Br-passivated Si(1 1 1) surfaces

Kinetic Monte Carlo simulations for growth on substrates of three-fold symmetry predict the growth of islands of various shapes depending on the growth temperature [Phys. Rev. Lett. 71 (1993) 2967]. On Br-Si(1 1 1) substrates growth of epitaxial gold silicide islands of equilateral triangular and trapezoidal shapes have earlier been observed by annealing at the Au-Si eutectic temperature, 363 °C [Phys. Rev. B 51 (1995) 14330]. We carried out annealing with temperature variation within a small window--(363 ± 30) °C. This has led to island growth of additional shapes like regular hexagon, elongated hexagon, walled hexagon and dendrite. Some of the observed island shapes have not been predicted.     

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He⁺ scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe-Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe-Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.     

Effect of substrate doping concentration on quantum well states of Pb island grown on Si(1 1 1)

The interaction between the metallic film/island and the semiconductor substrate is important to the electronic properties of metallic nanostructure grown on semiconductor substrate. Here, we report a series of comparison experiments to investigate the effect of doping concentration of Si substrates on the quantum well state (QWS). Using scanning tunneling microscopy, we observed that the apparent QWS energy positions show a strong dependence on the substrate used and on the sample temperature. Further experimental results by varying the height of scanning tunneling microscope tip over the Pb island uncovered that the observed apparent QWS energy position changes mainly come from the partial bias voltage drop on the combined resistance of the Pb wetting layer and the substrate, which is comparable with the vacuum tunneling resistance at low temperatures.     

Crystal growth of SrTiO3 films on H-terminated Si(1 1 1) with SrO buffer layers

Growth of epitaxial SrTiO₃ (STO) films has been examined on H-terminated Si(1 1 1) with SrO buffer layers. The epitaxial SrO buffer layers have reduced stress on H-terminated Si substrates. On the SrO buffer layers, the STO films grow epitaxially with triple domains at low temperature. Each STO domain has equivalent epitaxial relationship to SrO buffer layers, STO(1 1 0)∥SrO(1 1 1) and .     

Thin PTCDA films on Si(0 0 1): 1. Growth mode

We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.5-2 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a Stranski-Krastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface.     

In situ X-ray analysis of solid/electrolyte interfaces: electrodeposition of Cu and Co on Si(1 1 1):H and GaAs(0 0 1) and corrosion of Cu3Au(1 1 1)

Electrochemical deposition of Cu and Co in monolayer amounts on hydrogen terminated Si(1 1 1) was studied ex situ and in situ by X-ray techniques. The X-ray beam was found to have a strong effect on the deposit causing desorption under the beam. Cu deposition on GaAs(0 0 1) from UHV is compared with electrodeposited Cu on the same surface, elucidating similarities and differences of electrochemical and UHV deposition. Roughening due to corrosion of Cu₃Au(1 1 1) is observed by crystal truncation scattering. The observed behaviour of passivation of this surface is explained by the formation of Au clusters, increasingly covering the surface at higher oxidation potential.     

RT growth of acetonitrile and acrylonitrile on Si(0 0 1)-2 × 1 studied by XPS and LEED

In this work we show the adsorption of acetonitrile (CH₃CN) and acrylonitrile (CH₂CHCN) on Si(0 0 1)-2 × 1 at room temperature by increasing the molecular doses. Especially, by means of XPS and LEED data, we stress the action of these molecules on the silicon surface locating the dangling-bonds quasi-saturation within 10 L. The shortage of nitrogen XPS signal and some anomalies in carbon spectra point to an invading action from a traditional X-ray source (Al-K_α line) against chemisorbed molecules. In particular, we think that a long exposure to this radiation could break carbon-silicon bonds changing some adsorption geometries and making desorb molecular fragments.     

Surface composition and structure of palladium ultra-thin films deposited on Ni(1 1 1)

Ultra-thin palladium films deposited on the Ni(1 1 1) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). For low coverage, LEED shows a (1 × 1) pattern similar to that of the substrate. For intermediate coverage, the LEED pattern displays extra spots around the main (1 × 1) spots, resembling a Moiré coincidence pattern, probably associated with the formation of Pd bi-dimensional islands oriented in different directions on the Ni(1 1 1) surface. The results obtained by XPS and XPD corroborate this finding. The LEED pattern displays this structure up to 500 °C. Annealing at 650 °C brings back the (1 × 1) pattern, which is associated with a Pd island coalescence and alloy formation by Pd diffusion in the first atomic layers of the Ni(1 1 1). In this paper we present a detailed study of this surface structure via a comparison between XPD experiment and theory.     

Degeneracy pressure of electrons: a new effect for Pb islands grown on Si(1 1 1) substrate

We have found that the degeneracy pressure of electrons (DPE) inside Pb islands grown on a silicon substrate plays a crucial role in stabilizing the islands. In most cases, at a metal-semiconductor interface charge spilling takes place due to the difference of Fermi energies between the two materials, which makes DPE decrease along with the energy of the system. Based on this new effect, calculations of energy as a function of height are carried out for Pb islands grown on Si(1 1 1)-() and -(7 × 7) phases, which have most stable heights of 5 and 7 monolayers (ML), respectively. Our results explain why these most stable heights are observed. Using this new effect supplemented with experimental data, all the preferred heights of the Pb islands on Si(1 1 1)-(7 × 7) can be explained too.     

Thin PTCDA films on Si(0 0 1): 2. Electronic structure

We have studied the thin film formation and the electronic structure of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA), on clean and on hydrogen-passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), angle-resolved photoelectron spectroscopy (ARPES), near edge X-ray absorption fine structure (NEXAFS) and low energy electron diffraction (LEED). On the H passivated surface the changes in the electronic structure of the substrate and the molecules with increasing film thickness are very small. The molecular orbitals show a dispersive behavior, indicating that the PTCDA layers are ordered. On the reactive clean surface the anhydride groups of the molecule interact with the substrate as indicated by changes in the core level binding energies. This results in a much lower ordering in the film compared to PTCDA on a passivated silicon surface. There is no sign of decomposition of the molecule because of the more reactive substrate.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.     

Vanadium nanoclusters on Si(1 1 1) 7 × 7 surface studied by scanning tunneling microscopy

The size distribution and shape transition of self-assembled vanadium silicide clusters on Si(1 1 1) 7 × 7 have been investigated by scanning tunneling microscopy. Nanoclusters were formed by submonolayer vanadium deposition at room temperature followed by subsequent annealing (solid phase epitaxy - SPE). At room temperature, initially V-nanoclusters are formed which occupy sites avoiding the corner hole parts of the unit cells in the Si(1 1 1) 7 × 7 surface. Upon annealing, strong metal-silicon reaction occur leading to the formation of vanadium silicide nanoclusters. As a function of temperature, both, flat (2D) and three dimensional (3D) clusters have been obtained. After annealing at temperatures around 900 K many faceted clusters are created, whereas at higher annealing temperature, around 1300 K, predominantly 3D clusters are formed. The size distribution of SPE grown clusters could be well controlled in the range of 3-10 nm. The cluster size depends on the annealing temperature as well as on the initial vanadium coverage. Based on high resolution STM images a structure model for one kind of vanadium disilicide clusters exposing atomically flat surfaces was proposed.     

X-ray characterization of as-deposited, epitaxial films of Bi(0 1 2) on Au(1 1 1)

Electrodeposition is used to produce epitaxial single-crystal films on Au(1 1 1) substrates without annealing or other post-deposition modification. X-ray techniques show that the Bi(0 1 2) plane is parallel to the underlying Au(1 1 1) surface, and the azimuthal orientation of the films is determined. Combination of the X-ray data with in situ scanning tunneling microscopy (STM) images suggests a common growth mode from the first few layers up to thick films.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.