Nickel diffusion and silicide island formation on silicon (111)

Crystalline nickel disilicide islands have been observed on the Si(111) surface by atomic force microscopy (AFM). The nickel disilicide islands coalesce following a high temperature anneal (≈1260K). The islands differ from those formed at lower temperature in both shape and orientation. To explain the differences, we discuss kinetically limited growth accompanying phase and surface segregation of Ni from the bulk silicon wafer, and condensation of a Ni-rich NiSi_2−x liquid phase at the surface. Condensation from the liquid phase to NiSi₂ is concluded to be responsible for the structure of the crystallites. High temperature growth conditions lead preferentially to A-type (non-twinned) silicide structures.     

Epitaxial growth and surface morphology of aluminum films deposited on mica studied by transmission electron microscopy and atomic force microscopy

The bulk structure and epitaxial growth of aluminum films deposited on mica substrates by thermal evaporation in a wide temperature range (16-550 °C) in high vacuum were investigated by transmission electron microscopy and transmission electron diffraction. The surface morphology of the films was observed and analyzed by atomic force microscopy. The films prepared at room temperature consist of single crystals having a diameter of 90 ± 40 nm with (111) planes. The surface of the films comprises spherical grains with morphology that is caused by self-shadowing during the deposition. The surface of the films becomes smoother as the temperature increases, and atomically-smooth surfaces with a root-mean-square roughness of about 0.45 nm over an area of 1 μm² are obtained at 250-350 °C. The crystals are oriented randomly along the [111] direction perpendicular to the substrate. The surface of the films consists of larger (> 300 nm) grains with terraces, and the surface becomes rough above 400 °C. Films with well-oriented single crystals along the [111] direction perpendicular to the substrate are obtained above 520 °C. The films grown epitaxially at 520-550 °C are characterized by the isolated grains with a diameter of 1220 ± 450 nm.     

In situ observations of endotaxial growth of CoSi2 nanowires on Si(110) using ultrahigh vacuum transmission electron microscopy

We report in situ observations of the growth of endotaxial CoSi(2) nanowires on Si(110) using an ultrahigh vacuum transmission electron microscope with a miniature electron-beam deposition system located above the pole-piece of the objective lens. Metal deposition at 750-850?°C results in formation of coherently strained silicide nanowires with a fixed length/width (L/W) aspect ratio that depends strongly on temperature. Both dimensions evolve with time as L, W ～ t(1/3). To explain this behavior, we propose a fixed-shape growth mode based on thermally activated facet-dependent reactions. A second growth mode is also observed at 850?°C, with dimensions that evolve as L ～ t and W ～ constant. This mode is accompanied by formation of an array of dislocations. We expect that other endotaxial nanowire systems will follow coherently strained growth modes with similar geometrical constraints, as well as dislocated growth modes with different growth kinetics.     

Sintering of silver and copper nanoparticles on (001) copper observed by in-situ ultrahigh vacuum transmission electron microscopy

The sintering of copper and silver nanoparticles with single crystal copper substrates has been studied using a novel in-situ ultrahigh vacuum transmission electron microscope (UHV TEM). The system is equipped with a UHV DC sputtering attachment enabling metal nanoparticles to be generated in-situ and transferred directly into the microscope in the gas phase. In both cases, we find the particles to be of initially random orientation on the substrate. Upon annealing, however, the particles reorient and assume the same orientation as the substrate. The process apparently occurs by a mechanism involving sintering and grain growth. In the case of silver on copper, grain growth cannot occur since the metals are immiscible. Our observations show that, upon annealing, the particles wet the substrate surface and form epitaxially oriented islands by surface diffusion and grain boundary migration. The post-anneal islands exhibit the orientation relationship (111)_Ag∥001)_Cu, [110]_Ag∥[110]_Cu.     

Effect of a ZnO buffer layer on the characteristics of MgZnO thin films grown on Si (100) substrates by radio-frequency magnetron sputtering

This is a report on the effect of a ZnO buffer layer on the microstructures and optical properties of MgZnO thin films grown on Si (100) substrates by radio frequency magnetron sputtering. For the sample without the ZnO buffer layer, the microstructural analyses carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the formation of Mg₂Si in the interface between the Si substrate and the MgZnO thin film. Mg₂Si induced the random oriented polycrystalline MgZnO thin film. For the sample with the ZnO buffer layer, a few Mg₂Si were observed. An epitaxial relationship between the Si substrate and the MgZnO thin film was formed. In both samples, the photoluminescence (PL) investigation showed a small blue shift of the emission peak, which was owing to the incorporation of Mg atoms in ZnO by co-sputtering the MgO and ZnO targets. In addition, the sample with the ZnO buffer layer showed the enhanced PL intensity, when compared with the sample without the buffer layer.     

Oxide formation on the silicon (111) surface studied by Auger electron spectroscopy and by low energy electron loss spectroscopy

Core electron and valence electron excitation spectra measured using low energy electron loss spectroscopy in combination with Auger electron spectroscopy were used to study oxide formation on clean crystalline silicon. The chemical bonds formed in the various oxidation stages are described by localized molecular states. SiO double bonds, Si—O bonds of the type found in SiO₄ tetrahedra, Si—Si bonds and broken Si—O bonds were detected.     

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.     

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.     

In situ oxygen conditioning of (001) MgO thin film substrates for film growth studies by electron microscopy

It was found that the in situ treatment of (001) single-crystal films of MgO (prepared by epitaxial growth from the vapor phase) at high temperatures with a jet of oxygen will produce a surface that is almost equivalent, for epitaxial studies, to surfaces with the same orientation prepared by vacuum cleavage of bulk single crystals. The effectiveness of the process is demonstrated by its impact on the epitaxy of silver.     

High resolution transmission electron microscopy study of iron-silicide nanodot structures grown on faintly oxidized Si (111) surfaces

Epitaxial iron-silicide (α-FeSi₂, ε-FeSi and β-FeSi₂) nanodots were grown on Si(111) substrates with SiO₂ ultrathin films by Fe deposition on Si nanodots. The nanodots were characterized in-situ by reflection high-energy electron diffraction and scanning tunneling microscopy, and ex-situ high resolution transmission electron microscopy (HRTEM). For the β-FeSi₂ nanodots, the HRTEM images and the corresponding fast Fourier transform patterns analyses revealed that the coherent β-FeSi₂ nanodots with a relation of β-FeSi₂(110)/Si(111) had a compressive strain of ~ 0.8% in the [001]_β-FeSi2 direction and a tensile strain of ~ 2.6% in the direction normal to (110)_β-FeSi2 plane.     

The effect of oxygen adsorption on the electronic properties of the polar GaAs(111) surface after thermal cleaning in ultrahigh vacuum

The influence of oxygen adsorption on the electronic properties of clean, polar n-type GaAs(111)As surfaces prepared by prolonged ultrahigh vacuum heating at 770 K have been investigated by means of photoemission yield spectroscopy combined with Auger electron spectroscopy control. For the clean surface the evaluation of arsenic content strongly depends on the theoretical models used whereas the work function φ and absolute band bending eVs were 4.05±0.03 eV and −0.21±0.07 eV respectively. Moreover, two stable filled electronic surface state bands localized in the band gap tailing up to Fermi level E_F and above the top of the valence band at the surface were observed. After exposure of this surface to 10³ L of oxygen the work function φ and absolute band bending eVs increased by 0.13 eV and the filled surface state band close to the Fermi level E_F drastically decreased in amplitude. For oxygen exposures of 10⁴ L and 10⁵ L the above parameters increased only by 0.09 eV and 0.04 eV respectively whereas the filled surface state band above the top of the valence band at the surface was stable.     

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 × 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 × 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (111)-7 × 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures.     

Investigations of surface structure for thermally evaporated silicon on a Cu(111) surface

The surface structure and composition of semiconductor/Cu(111) films prepared by thermal evaporation in an ultrahigh vacuum condition have been investigated. As Si atoms were deposited on a Cu(111) surface, diffused spots were observed up to 2 monolayers while 1 × 1 spots become dimmer as revealed using low-energy electron diffraction technique. Because of a larger electron affinity of Si than that of Cu, the Cu L₃M₄₅M₄₅ Auger line shifts to a lower kinetic energy. Annealing treatments at 425 K causes a splitting of the Cu L₃M₄₅M₄₅ line. This shows the interdiffusion at the Si/Cu interface and the formation of a Cu-rich surface layer. After annealing treatments, the domains grow and aggregate to form larger domains as revealed by the decreasing full-width at half maximum of diffraction spots. Ge/Cu(111) shows 1 × 1 structure as annealing up to 500 K. Lack of a dominant structure and a large valence diameter of Ge result in different structures as compared to Si/Cu(111).     

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.     

Photoemission studies of the initial interface formation of ultrathin MgO dielectric layers on the Si(111) surface

This study investigates the interface formation between a magnesium oxide dielectric overlayer and an ultrathin SiO₂ layer (∼ 0.3 nm) grown on the atomically clean p-type Si(111) surface in ultra high vacuum. Both soft X-ray synchrotron radiation based photoemission and conventional X-ray photoelectron spectroscopy have been used to characterise the evolution of the interface and monitor the change in the interfacial oxide thickness. As the MgO film grows, there is an increase in the intensity of the silicon oxide features indicating the growth of the interfacial oxide which saturates at a thickness of approximately 0.7 nm. Spectra acquired at the surface sensitive 130 eV photon energy, reveal the emergence of a chemically shifted component on the low binding energy side of the substrate peak which is attributed to atomic displacement of silicon atoms from the substrate to the interfacial oxide at room temperature. This evidence of atomic disruption at the high dielectric constant material (high-κ) and silicon interface would be expected to contribute to charge carrier scattering mechanisms in the silicon and could account for the generally observed mobility degradation in high-κ stacks. Thermal annealing studies of deposited MgO films show that dissociation begins to occur above 600 °C with desorption of Mg and the growth of a silicon oxide.     

Nanoheteroepitaxy of GaN on a nanopore array of Si(111) surface

Nanoheteroepitaxial (NHE) growth of GaN using AlN/AlGaN as a graded buffer layer by metalorganic chemical vapor deposition has been demonstrated on the nanoporous patterned Si(111) substrates. The nanopore array on Si(111) has been fabricated by using anodized aluminum oxide membrane as an induced couple plasma dry etching mask. The reduction of the threading dislocation density and relaxation of the tensile stress in NHE GaN are revealed by transmission electron microscopy (TEM), micro-Raman spectrum and photoluminescence spectrum, respectively. Cross-sectional TEM analysis shows that dislocations nucleated at the interface are forced to bend into (0001) basal plane. Red shift in the E₂ (TO) phonon peak of micro-Raman spectrum indicates the relaxation of tensile stress in the nanoheteroepitaxial lateral overgrowth of GaN. A single step ELO without mask on nanopatterned Si(111) substrates is a simple and promising way for the improvement of the quality of GaN on Si substrates.     

Assembly behavior and monolayer characteristics of OH-terminated alkanethiol on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Self-assembled monolayers (SAMs) of 6-mercapto-1-hexanol (MHO) on an Au(111) electrode were prepared in an electrochemical system. The adsorption behavior of MHO and the time-dependent organization of the SAM were investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). The results show that a potential higher than 0.28?V (relative to RHE) is required to induce the adsorption of MHO. At 0.28?V, the MHO molecules adsorb in a flat-lying orientation, forming an ordered striped phase with a molecular arrangement of ([Formula: see text]). However, the adlayer is not stable at this potential. The adsorbed striped phase may recover to the herringbone feature of the gold substrate due to the desorption of adsorbed MHO. At a higher potential (0.35?V), the adlayer becomes stable and can undergo a phase evolution from the striped phase to a condensed structure, identified as c([Formula: see text]). This structure can also be described as a c(4 × 2) superlattice of a [Formula: see text] hexagonal adlattice. The surface coverage of the MHO SAM is identical to the saturated structure of an 11-mercapto-1-undecanol (MUO) SAM reported in a previous work, [Formula: see text]. However, the STM image of MHO adlayer shows a modulation in intensity, reflecting the presence of various conformations of adsorbed molecules. This result is attributed to the shorter chain length of MHO, which gives a weaker?van der Waals interaction between adsorbed molecules. This effect also results in a higher charge permeability across the adlayer and a lower striping potential to an MHO SAM.     

Surfactant-induced layered growth in homoepitaxy of Fe on Fe(100)-c(2 × 2)O reconstruction surface

In this study, the effects of several surfactants (Pb, Bi, and Ag) on the homoepitaxial growth of Fe(100) were studied and compared. The reflection high-energy electron diffraction measurements clearly reveal that these surfactants enhance the layer-by-layer growth of Fe on an Fe(100)-c(2 × 2)O reconstruction surface. The dependence of growth on the surfactant layer thickness suggests that there exists a suitable amount of surfactant layer that induces a smoother layer-by-layer growth. Comparisons between the atomic force microscopy images reveal that the root-mean-square surface roughness of Fe films mediated by Pb and Bi surfactants are considerably smaller than those of the films mediated by Ag surfactant. The Auger electron spectra show that Pb and Bi segregate at the top of the surface. It has been concluded that Pb and Bi are effective surfactants for enhancing layer-by-layer growth in Fe homoepitaxy. Ag has the same effect, but it is less efficient due to the weak surface segregation of Ag.