STM observation of Ag adatom interactions on the Si(1 1 1)-(7×7) surface

STM imaging of Ag atoms adsorbed on the Si(1 1 1)-(7×7) surface is studied. Appearance of a single Ag adatom on perfect surface is compared with images of adatoms interacting with surface defects and adsorbates. Importance of real-time observation of surface processes for image interpretation is demonstrated on imaging Ag adatoms at various situations. Influence of tunnelling conditions (voltage between a tip and surface) on imaging surface objects is studied and visibility of single Ag adatoms and clusters is discussed.     

Adsorption/desorption kinetics of Na atoms on reconstructed Si (111)-7 × 7 surface

Self-assembled nanostructures on a periodic template are fundamentally and technologically important as they put forward the possibility to fabricate and pattern micro/nano-electronics for sensors, ultra high-density memories and nanocatalysts. Alkali-metal (AM) nanostructure grown on a semiconductor surface has received considerable attention because of their simple hydrogen like electronic structure. However, little efforts have been made to understand the fundamental aspects of the growth mechanism of self-assembled nanostructures of AM on semiconductor surfaces. In this paper, we report organized investigation of kinetically controlled room-temperature (RT) adsorption/desorption of sodium (Na) metal atoms on clean reconstructed Si (111)-7 × 7 surface, by X-ray photoelectron spectroscopy (XPS). The RT uptake curve shows a layer-by-layer growth (Frank-vander Merve growth) mode of Na on Si (111)-7 × 7 surfaces and a shift is observed in the binding energy position of Na (1s) spectra. The thermal stability of the Na/Si (111) system was inspected by annealing the system to higher substrate temperatures. Within a temperature range from RT to 350 °C, the temperature induced mobility to the excess Na atoms sitting on top of the bilayer, allowing to arrange themselves. Na atoms desorbed over a wide temperature range of 370 °C, before depleting the Si (111) surface at temperature 720 °C. The acquired valence-band (VB) spectra during Na growth revealed the development of new electronic-states near the Fermi level and desorption leads the termination of these. For Na adsorption up to 2 monolayers, decrease in work function (−1.35 eV) was observed, whereas work function of the system monotonically increases with Na desorption from the Si surface as observed by other studies also. This kinetic and thermodynamic study of Na adsorbed Si (111)-7 × 7 system can be utilized in fabrication of sensors used in night vision devices.     

Thermal evolution of the morphology of Ni/Ag/Si(111)-√3 × √3 surface

The temperature-driven changes in morphology of the interface formed by room temperature (RT) deposition of Ni atoms onto an Ag/Si(111)-√3 × √3 surface were investigated by scanning tunneling microscopy. Roughly 70% of Ni deposition diffused into bulk substrate within the temperature range between RT and 573 K. The images as obtained after annealing up to 670 K correspond to the formation of nano-sized islands of nickel silicides. Two types of islands, large triangular islands typical of the whole range of applied coverage, and smaller islands of different shapes, coexist at Ni coverage higher than 1 monolayer. Annealing above 870 K led to the formation of a 7 × 7 phase in coexistence with small 5 × 5 domains at the expense of a complete disappearance of the √3 × √3 phase. Also, formation of Ni,Si alloy was observed at the temperature, along with segregation of bulk-dissolved Ni species onto the surface.     

Adsorption and decomposition of 1,3-disilabutane on Si (111)-7×7

The adsorption and decomposition of 1,3-disilabutane (DSB) was studied on Si (111)-7×7 in the temperature range 100-1200 K by Cs⁺ reactive ion scattering and X-ray photoelectron spectroscopy (XPS). By combining the results of these two techniques, adspecies in the intermediate states during the decomposition of DSB were qualitatively identified and an adsorption model was proposed. At 100-150 K, DSB was found to adsorb on the surface as the C₂H₈Si₂ species as well as CH₄Si and to condense molecularly on a monolayer of C₂H₈Si₂ adspecies. XPS indicates that the molecular species desorbs mostly at 200 K and completely at 300 K. Up to 600 K, the C₂H₈Si₂ adspecies are converted to CH₄Si with increasing temperature and then above this temperature the CH₄Si species decomposes to form the SiC film. The intensity variations of Si (2p) and total C (1s) peaks and the analysis by curve fitting of the C (1s) peaks suggest that one CH₄Si species leaves the system by cleavage of C-Si bonds in C₂H₈Si₂ adspecies rather than to form two CH₄Si adspecies, and the breakage probably occurs within the extremity ones in accordance with the double-bonded chemisorption character.     

Manifestations of strain-relaxation in the structure of nano-sized Co-2 × 2 islands grown on Ag/Ge(111)-√3 × √3 surface

We have examined strain-relaxation of Co-2 × 2 islands grown on the Ag/Ge(111)-√3 × √3 surface by analyzing scanning tunneling microscopy images. We have found that the Co-2 × 2 islands commonly adopt a more compact arrangement as compared to that of the Ge(111) substrate, however they differ in a degree of an atomic compactness. We have not found a distinct relation between strain-relaxation and the island height. Three groups of islands have been identified upon analyzing a correspondence between strain-relaxation and the island size: (i) small islands (not bigger than 80 nm²) with a high atomic compactness, displaying fixed inter-row distances, (ii) small islands with unfixed distances between atomic rows, and (iii) big islands (bigger than 80 nm²) with fixed inter-row distances, but with a less compact atomic arrangement compared to that of the first two groups. We propose a model to account for the relation between the relaxation and the island size.     

Thickness dependent reactivity and coercivity for ultrathin Co/Si(111) films

For Co/Si(111) films thinner than 15 ML, the thickness dependent reactivity and magnetic properties have been systematically studied. As the Co coverage increases, Co adatoms on the Si(111) surface show enhanced chemical reactivity for oxidation due to the change of the chemical state. After the saturation oxygen exposure, oxygen atoms interact with a thick Co layer to form a rougher interface. Complex adsorption kinetics of oxygen in the Co layer is observed. From the depth-profiling measurements for Co layers close to the Co-Si interface, the sputtering rate is enhanced due to that the solid surfaces of Si and Co-Si compounds are resistive against oxidation. The descending of the Kerr intensity by saturation oxygen exposure shows the limited diffusion length of oxygen atoms into the films. The inertness of the Co-Si interface, the reduction of pure cobalt and imperfection introduced by oxygen influence the coercivity of O/Co/Si(111).     

Photoemission spectroscopy study of Na-induced phase transitions on Si(1 1 3)

We reinvestigate the Si 2p spectrum of clean to testify two competitive structure models, i.e. adatom-dimer-interstitials and oppositely puckered models, in comparison with recent theoretical calculations. This reveals that only the adatom-dimer-interstitials model reproduces the surface components of the Si 2p spectrum. After decorating Na atoms on clean at room temperature, the 3×2 phase was found to transit into the 3×1 phase by low-energy electron diffraction and photoemission spectroscopy experiments. We will discuss the (3×2)-(3×1) phase transition on the adatom-dimer-interstitials model.     

Surface phase transitions upon reduction of epitaxial WO3(1 0 0) thin films

The surface structure and morphology of WO₃(1 0 0) thin films were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. The films experienced a net-reducing environment when annealed in oxygen at 800 K leading to surface phase transitions from p(2×2) to p(4×2), and from p(4×2) to a mix p(4×2) and p(3×2). Increasing the annealing temperature to 830 K in ultra-high-vacuum (UHV) led to a fully p(3×2) reconstructed surface. Continued UHV annealing above 800 K caused (1×1) islands to appear on the p(3×2) surface and the film color to darken. Eventually, prolonged UHV annealing led to a (1×1)-terminated surface with straight steps oriented in [0 0 1] or [0 1 0] directions due to crystallographic shear planes. The randomly spaced steps on the (1×1) surface indicated variations in the local stoichiometry in the film. An added row model proposed for the p(4×2) structure is also shown to be consistent with the p(3×2) structure. The formation of the p(4×2) structure from the p(2×2) structure was attributed to W⁵⁺ migration into the bulk to form the troughs between the added rows. Reduction of the p(4×2) structure caused the troughs to narrow rather than deepen, suggesting that W⁵⁺ or added row surface diffusion competes with migration of reduced W ions into the bulk when the p(3×2) structure forms. The STM images also show evidence that the (1×1) structure forms through coalescence of the added rows.     

Structure of Pt3Co(1 1 0)c(2×4)-O surface studied by scanning tunneling microscopy

A Pt₃Co(1 1 0)c(2×4)-O surface has been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. At a very initial oxidation stage exposed at 500°C, creation of missing and/or added row structures running to the [0 0 1] direction on clean Pt₃Co(1 1 0)2×1 surface was imaged from the steps. The surface is fully covered by a well-ordered c(2×4) structure at 2 L oxygen exposure. Atomic-resolution STM image shows the added row-type anti-phase Co-O zigzag chains along the [0 0 1] direction. Based on the images, the structure model for the c(2×4) surface was suggested as a first oxidized layer, which comes from the chemical reaction forming stoichiometric Co monoxide. Further oxygen exposure above 5 L, Co-O clusters imaged to large dots were formed on the surface with the size of about 5-7 Å.     

Adsorption of Ni on Si(1 0 0) surface

The chemisorption of one monolayer Ni atoms on ideal Si(1 0 0) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of adsorption systems of Ni atoms on different sites are calculated. It is found that Ni atoms can adsorb at fourfold site above the surface and bridge site below the surface. The adsorption of Ni atoms can readily diffuse and penetrate into the subsurface. A Ni, Si mixed layer might exist at the Ni-Si(1 0 0) interface. The layer projected density states are calculated and compared with that of the clean surface. The charge transfers are also investigated.     

Two-dimensional Mn–Co surface alloy on Co(0 0 1)

Surface alloy formation of Mn on Co(0 0 1) was detected by low-energy electron diffraction and in situ magneto-optic Kerr effect experiments. Here we discuss the electronic and magnetic properties of an equivalent Mn monolayer (ML) at the surface of Co(0 0 1) using the tight-binding linear muffin-tin orbitals method. Magnetic configurations and stability of ordered 2D Mn–Co alloy 2 ML thick vs Mn/Co(0 0 1) and Co/Mn/Co(0 0 1) have been considered. Multiple magnetic configurations have been always obtained but a comparison between the ground states display an instability of the perfect Mn ML on Co(0 0 1). Total energy differences are considerably reduced in the case of spin-polarization calculations as compared with the non-magnetic one.     

Characterization of bimetallic Au-Pt(111) surfaces

Structural and electronic properties of ultrathin Au films deposited on Pt(111) and annealed at different temperatures have been studied by ultraviolet photoelectron spectroscopy (UPS), photoemission of adsorbed xenon (PAX) and low energy electron diffraction (LEED). The LEED measurements indicate an initial pseudomorphic growth of the Au films. The UPS and PAX experiments show a strong temperature dependence of the surface morphology. The surface covered with Au at 150 K is quite rough but smoothens significantly above room temperature. At a temperature of 750 K intermixing and the formation of an Au-Pt surface alloy start at the interface. The electronic properties of this surface alloy seem to be nearly independent from the originally deposited Au amount in the investigated range of 1-10 monolayers. The removal of Au from the surface regions has also been verified by scanning tunneling microscopy. Adsorption experiments with CO as a titration agent show a significantly lower affinity of the Au-Pt surface alloy in comparison with the clean Pt surface.     

Fractal pattern formation in thermal grooving at grain boundaries in Ag films on Si(111) surfaces

Growth of Ag films on Br- and H-passivated Si(111) surfaces and the annealing behaviour have been investigated by Rutherford backscattering spectrometry, scanning electron microscopy and photoemission electron microscopy techniques. Upon annealing the phenomenon of thermal grooving was observed in the Ag films. Depending on the annealing temperature, at an intermediate annealing time Ag depletion (evaporation) from the grain boundaries produces fractal patterns of Ag-depleted regions. Continued annealing eventually produces a percolated network of Ag-depleted regions (thermal grooves) along the grain boundaries and isolated Ag grains appear as the depth of the grooves reaches the substrate. For the fractal structures produced by thermal grooving, the fractal dimension has been estimated to be 1.60 ± 0.04. Observation of a fractal pattern in thermal grooving was not hitherto reported. A thorough analysis of the experimental results has been carried out in the context of current theories. These theories are inadequate to describe the experimental results.     

Polycrystalline silicon films on glass grown by amorphous-liquid-crystalline transition at temperatures below 330 °C

An approach to deposit polycrystalline silicon layers on amorphous substrates is presented. It is shown that metastable amorphous silicon can be transformed into its more stable microcrystalline structure at a temperature below 330 °C via an intermediate liquid solution stage. In particular, the interaction of liquid indium nanodroplets in contact with amorphous silicon is shown to lead to the formation of circular polycrystalline domains. Crystallinity of these domains is analyzed by micro-Raman spectroscopy. The droplet size necessary for the onset of crystallization is related to the temperature of the film. Full coverage of the substrate with microcrystalline silicon has been obtained at 320 °C within less than one hour. These films might act as seeding layers for further enlargement by steady-state liquid phase epitaxy.     

Simple method for coating Si (1 0 0) surfaces with ferritin monolayers—Iron oxide quantum dots

With the goal to develop iron oxide quantum dots we developed a simple method to spread horse spleen ferritin monolayers on a Si (1 0 0) surface. Application of atomic force microscopy and spectroscopic ellipsometry showed the existence of regions with dense ferritin monolayers. Application of transmission electron microscopy identified the core of the spread ferritin as FeO nanocrystals.     

Theoretical investigation of atomic structure and electronic properties of Ca/Si(110)-(2 × 1) reconstruction

We have presented first-principles total-energy calculations for the adsorption of Ca metals onto a Si(110) surface. The density functional method was employed within its local density approximation to study the atomic and electronic properties of the Ca/Si(110) structure. We considered the (1 × 1) and (2 × 1) structural models for Ca coverages of 0.5 monolayer (ML) and 0.25 ML, respectively. Our total-energy calculations indicate that the (1 × 1) phase is not expected to occur. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The Ca/Si(110)-(2 × 1) produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. One filled and two empty surface states were observed in the gap; these empty surface states originate from the uppermost Si dangling bond states and the Ca 4 s states. It is found that the Ca-Si bonds have an ionic nature and complete charge being transferred from Ca to the surface Si atoms. Finally, the key structural parameters of the equilibrium geometry are detailed and compared with the available results for metal-adsorbed Si(110) surface, Ca/Si(001), and Ca/Si(111) structures.     

Assembling of redox proteins on Au(111) surfaces: A scanning probe microscopy investigation for application in bio-nanodevices

The morphology and conductive properties of azurin molecules, chemically attached to sulfhydryl terminated alkanethiol monolayer assembled on Au(111) surface, are mapped at single molecule level and compared with those observed for the same molecule immobilised on bare Au(111). High-resolution Tapping Mode Atomic Force Microscopy shows that the protein molecules immobilised on modified gold, better reproduces the crystallographic height of the protein, than that immobilised on bare gold. Such a height recovering is also found in the Scanning Tunnelling Microscopy images. Consistently, a good tunnelling conduction of azurins on the modified gold electrode is demonstrated by Tunnelling Spectroscopy. Cyclic voltammetry measurements show, in addition, that the redox activity of azurin molecules covalently immobilised on sulfhydryl functionalised Au(111) surface is retained. These results are discussed in connection with possible use of this linker in the assembling of nano-hybrid systems.     

Study of step instability on Si surfaces

Step instabilities on Si(1 1 1) vicinal surface and Si(1 1 1) vicinal surface induced by Au adsorption were observed by reflection electron microscopy. On the Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and surface steps are bunching. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they are permeable at higher temperature. On the Si(1 1 1) vicinal surface the permeability of steps depends on the heating current direction. The steps are permeable for step-up current heating and are impermeable for step-down current heating. Above a critical coverage of Au (≈0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands is formed for the step-down current heating.     

Formation of silicon dioxide layers at low temperatures (150-400 °C) by atmospheric pressure plasma oxidation of silicon

The formation of silicon dioxide (SiO₂) layers at low temperatures (150-400 °C) by atmospheric pressure plasma oxidation of Si(0 0 1) wafers have been studied using a gas mixture containing He and O₂. A 150 MHz very high frequency (VHF) power supply was used to generate high-density atomic oxygen in the atmospheric pressure plasma. Oxidation rate, structure, and thickness and refractive index profiles of the oxidized layers were investigated by ellipsometry and infrared absorption spectroscopy. Atomic force microscopy was also employed to observe atomic-scale morphologies of the layer surface and wafer Si surface, after chemical removal of the oxidized layers. It was found that stoichiometric SiO₂ layers were obtained at higher oxidation rates than conventional dry O₂ thermal oxidation and radical oxidation processes, even at a very low substrate temperature of 150 °C. Although thickness variations were observed in the plasma region, the refractive index was independent of both substrate temperature and VHF power. In addition, the SiO₂ surface and SiO₂/Si interface roughnesses were comparable to those obtained in conventional dry oxidation at high temperatures.