High-resolution analytical electron microscopy of catalytically etched silicon nanowires

We report on the characterization of hexagonally ordered, vertically aligned silicon nanowires (SiNW) by means of analytical transmission electron microscopy. Combining colloidal lithography, plasma etching, and catalytic wet etching arrays of SiNW of a sub-50 nm diameter with an aspect ratio of up to 10 could be fabricated. Scanning transmission electron microscopy has been applied in order to investigate the morphology, the internal structure, and the composition of the catalytically etched SiNW. The analysis yielded a single-crystalline porous structure composed of crystalline silicon, amorphous silicon, and SiO _x with x≤2.     

Structural and chemical analysis of a model Si–SiO2 interface using spatially resolved electron-energy-loss spectroscopy

This work reports an experimental electron-energy-loss spectroscopy study carried out on a model thermal Si–SiO₂ interface. Valence-loss spectra and core-loss spectra (Si L_2,3 and O K edges) were recorded across the interface in line-spectrum mode with a high spatial resolution in a field emission gun scanning transmission electron microscope. From the analysis of the line spectra and on the basis of high-resolution electron microscopy and high-angle annular dark-field experiments, it is concluded that the interface is not sharp but extends over about three atomic planes consisting of Si and O atoms arranged in a structure evolving between crystalline SiO and SiO₂ before growing as an amorphous SiO₂ layer. In addition, from the analysis of the valence-loss spectra in terms of energy-loss function or dielectric function ε, we show that valence-electron-energy-loss spectroscopy could be a relevant alternative method for determining the electron properties, for example the bandgap, and the dielectric constant of dielectric gates on a nanometre scale.     

A New Method to Measure Small Amounts of Solute Atoms on Planar Defects and Application to Inversion Domain Boundaries in Doped Zinc Oxide

We demonstrate the application of a new method of analytical transmission electron microscopy for measuring very accurately small amounts of solute atoms within a well-defined planar defect such as a stacking fault, grain boundary or an interface. The method is based on acquiring several spectra with different electron beam diameters from the same position centred on the defect. It can be applied to energy-dispersive X-ray microanalysis (EDXS) or electron energy-loss spectroscopy (EELS) and does not necessitate a scanning unit. The accuracy has been tested numerically under different conditions using simulations for a specific geometry and has been found to be substantially better than that of any other current standard technique. Our calculations suggest an extremely high accuracy theoretically achievable in the determination of e.g. the Gibbsian solute excess or the doping level of a grain boundary down to about ±1% of an effective monolayer, i.e. ±0.1 atoms/nm² under typical experimental conditions. The method has been applied to zinc oxide, which forms inversion domain boundaries (IDBs) when doped with different transition metal oxides such as SnO₂ or Sb₂O₃. We obtained an experimental precision of ±0.4 atoms/nm², which has allowed us to solve the atomic structure of the IDBs.     

SnO2 / In2O3 one-dimensional nano-core-shell structures: Synthesis, characterization and photoluminescence properties

SnO₂/In₂O₃ one-dimensional nano-core-shell structures have been synthesized at 1350 ^°C by thermal evaporation of the mixture of metal Sn, Fe(NO₃)₃ powders and In particles. The as-synthesized products have been characterized by energy-dispersive X-ray spectroscopy, selected-area electron diffraction and high-resolution transmission electron microscopy. Microstructure characterization indicates the orientation relationship between core and shell is , . The formation mechanism of this nano-core-shell structure can be attributed to the cover of In₂O₃ on the surface of SnO₂ nanochains. The photoluminescence properties of the nano-core-shell structures have been measured. The PL spectrum shows some difference with the result from pure SnO₂ and In₂O₃ nanostructure that be deemed to relate to interface defects in SnO₂/In₂O₃ nano-core-shell structure.     

Electron energy loss spectra of the silicon 2p, 2s, carbon 1s and valence shells of tetramethylsilane

Inner shell excitation spectra of tetramethylsilane, (CH₃)₄Si, have been measured in the silicon 2s, 2p (L_I,II,III-shell) and carbon is (K-shell) regions using electron energy-loss spectroscopy at an impact energy of 2.5 keV and a scattering angle of ~1°. The high-resolution valence shell spectrum has also been observed at an impact energy of 3 keV and a zero degree scattering angle. The silicon 2p spectra are compared and contrasted with published photoabsorption spectra of SiF₄, SiH₄, and other related Si-containing molecules with varying ligands.     

Interaction of iron atoms with the oxidized silicon surface

The room-temperature interaction of iron atoms with the oxidized Si(100)2×1 surface at a coverage from a submonolayer to four monolayers is studied by core-level photoelectron spectroscopy using synchrotron radiation. Computer simulation of the Si 2p core electron spectra demonstrates that iron atoms penetrate beneath the silicon oxide even at room temperature. This process causes the initial silicon phases at the SiO_x/Si interface to disappear; gives rise to a complex ternary phase involving Fe, O, and Si atoms; and favors the formation of a Fe-Si solid solution at the interface.     

EELS characterization of TiN grown by the DC sputtering technique

Titanium nitride thin films were deposited on monocrystalline silicon (mc-Si) substrates by direct current reactive magnetron sputtering. Auger electron spectra (AES) of deposited films at different nitrogen partial pressures, show the typical N KL₂₃L₂₃ and Ti L₃M₂₃M₂₃ Auger transition overlapping. Also, changes in the Ti L₃M₂₃M₄₅ Auger transition peak are observed. X-ray diffraction and high resolution electron microscopy (HRTEM) of a golden color TiN/mc-Si sample, reveal a preferential polycrystalline columnar growth in the 〈111〉 orientation. This sample was also analyzed by electron energy-loss spectroscopy (EELS). The N/Ti elemental ratio is slightly different to the value determined by AES. Atomic distribution around the N atoms is in agreement with that expected from the N atom in the fcc unit cell of TiN. This distribution was obtained via an extended energy-loss fine structure (EXELFS) analysis from EELS spectra.     

Effect of thermal annealing on microstructural properties of Ti/Ge2Sb2Te5/Ti thin films deposited on SiO2/Si substrates by a sputtering method

Ti/Ge₂Sb₂Te₅/Ti thin films deposited by a sputtering method on SiO₂/Si substrates were annealed at 400 °C in N₂ atmosphere and characterized by using transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) in order to investigate the inter-diffusion of the Ti/Ge₂Sb₂Te₅/Ti system due to annealing. The TEM and AES results showed that the interface between the Ti and the Ge₂Sb₂Te₅ layers was unstable and Ti atoms were incorporated into the Ge₂Sb₂Te₅ thin film upon annealing. The Te and Sb atoms of the Ge₂Sb₂Te₅ layer diffused into the Ti layer. The intermixing layers between the Ge₂Sb₂Te₅ layer and two Ti layers were formed. These results indicate that the microstructural properties of the Ti/Ge₂Sb₂Te₅/Ti systems can be degraded by the postgrowth thermal annealing.     

A photoelectron and energy-loss spectroscopy study of Ti and its interaction with H2, O2, N2 and NH3

A unified electron spectroscopic study of polycrystalline Ti and its interaction with H₂, O₂, N₂, and NH₃ are described. Auger electron spectroscopy (AES), electron energy-loss spectroscopy (ELS), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) are combined to provide detailed information about the electronic structure of the titanium surface and its interaction with these adsorbates. X-ray and ultraviolet photoelectron spectra and electron energy-loss spectra are presented for the clean titanium surface, and following exposure to H₂, O₂, N₂ and NH₃. Spectral assignments are provided in each case. The electron spectra of oxygen exposed Ti and nitrogen sputtered Ti are quite similar, and are interpreted with reference to band structure calculations for TiO and TiN. Electron spectroscopy indicates essentially complete dissociative adsorption of NH₃ on the clean titanium surface.     

The microstructure of SiO thin films: from nanoclusters to nanocrystals

The microstructure and electronic structure of silicon-rich oxide (SRO) films were investigated using transmission electron microscopy and electron energy loss spectroscopy as the main analytical techniques. The as-deposited SRO film was found to be a single phase SiO_1.0, as suggested by its electronic structure characteristics determined by the valence electron energy loss spectrum. This single phase undergoes a continuous but incomplete phase decomposition to Si and SiO₂ for films annealed between 300 and 1100°C. The resulting Si phase first appears as ～2?nm-diameter amorphous clusters which grow to larger sizes at higher annealing temperatures, but only crystallize at a critical temperature between 800 and 900°C. This cluster/matrix configuration of the SRO films is consistent with the appearance of the interface plasmon and its oscillator strength as a function of the nanoparticle size. Three separate stages were identified in the sequence of annealed films that were characterized by the presence of single-phase SiO, amorphous silicon nanoclusters, and silicon nanocrystals, respectively. The presence of amorphous silicon nanoclusters in the intermediate stage, the mean size of which can be controlled via annealing, may offer an alternative to silicon nanocrystal composites for optical applications.     

Crystal and electronic structure of solid C76

The crystal and electronic structure of solidC ₇₆ has been studied using transmission electron microscopy and electron energy-loss spectroscopy in transmission. C₇₆ forms a close packed structure with an average facecentred cubic symmetry. From valence band and core electron excitations information on the dielectric function and the unoccupied density of states has been obtained.     

Nanoscale laser processing and diagnostics

The article summarizes research activities of the Laser Thermal Laboratory on pulsed nanosecond and femtosecond laser-based processing of materials and diagnostics at the nanoscale using optical-near-field processing. Both apertureless and apertured near-field probes can deliver highly confined irradiation at sufficiently high intensities to impart morphological and structural changes in materials at the nanometric level. Processing examples include nanoscale selective subtractive (ablation), additive (chemical vapor deposition), crystallization, and electric, magnetic activation. In the context of nanoscale diagnostics, optical-near-field-ablation-induced plasma emission was utilized for chemical species analysis by laser-induced breakdown spectroscopy. Furthermore, optical-near-field irradiation greatly improved sensitivity and reliability of electrical conductance atomic force microscopy enabling characterization of electron tunneling through the oxide shell on silicon nanowires. Efficient in-situ monitoring greatly benefits optical-near-field processing. Due to close proximity of the probe tip with respect to the sample under processing, frequent degradation of the probe end occurs leading to unstable processing conditions. Optical-fiber-based probes have been coupled to a dual-beam (scanning electron microscopy and focused ion beam) system in order to achieve in-situ monitoring and probe repair.     

Electron energy-loss spectroscopy in the low-loss region as a characterization tool of electrode materials

Energy losses below 100 eV are by far less exploited than higher losses in electron energy-loss spectroscopy. Two new examples are given to illustrate the characterization possibilities offered by spectra in this energy range. Typical materials that could be used as electrodes in electrochemical cells were chosen as application cases. Through the use of calculations based on density functional theory, we first demonstrate that the first peak present at the lithium K edge in Li _x TiP₄ can give access to the precise localisation of inserted Li atoms in the f.c.c. structure. In particular, different tetrahedral sites could be differentiated according to their distance to the Ti site. Secondly, calculations of valence electron energy-loss spectra of perovskite materials indicate that a characteristic peak for regular perovskite (Pm m space group) exists in the 10–15 eV range. The high sensitivity of this peak to the distortion of the octahedron arrangements is also demonstrated. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Manipulating Bonding at a Cu/(0001)Al2O3 Interface by Different Substrate Cleaning Processes

Growth of Cu films on (0001)Al₂O₃ substrates can result in metallic Cu—Al or ionic-covalent Cu—O bonds at atomically abrupt interfaces. The type of bonding depends on the substrate cleaning procedure prior to film growth. Cu films deposited on Ar⁺-ion sputter-cleaned substrates exhibit interfacial Cu-L_2,3, Al-L_2,3 and O-K energy-loss near-edge structures that indicate the formation of metallic Cu—Al bonds at the Cu/Al₂O₃ interface. In contrast, growth on chemically cleaned -Al₂O₃ substrates results in interfacial energy-loss near-edge structures that suggest Cu—O bonding at the interface. The experimental electron energy-loss spectroscopic results are compared to calculated spectra, and the mechanisms causing the changes in the atomic and electronic structure are addressed.     

The interaction of H2S with Si(111) 7 × 7 surfaces by energy loss and Auger electron spectroscopies

Low energy electron loss spectroscopy (ELS) and Auger electron spectroscopy (AES) have been applied for the studies of the interaction of H₂S molecules with Si(111)7 × 7 surfaces. The observations are consistent with the interpretation that the room temperature non-dissociative adsorption state of H₂S molecules changes substantially after annealing at 550°C, resulting in the desorption of hydrogen and the covalent bond formation between silicon and sulfur atoms. The silicon disulfide films formed on Si(111) surfaces have been identified by the characteristic loss peaks in comparison with those of silicon dioxide.     

Structure and impedance spectroscopy of Pr1? x Sr x Fe0.8Co0.2O3? δ ( x =0.1, 0.2, 0.3) thin films grown by laser ablation

Polycrystalline samples of Pr_1−x Sr _x Fe_0.8Co_0.2 O_3−δ (x=0.1, 0.2, 0.3) (PSFC) were prepared by the combustion synthesis route at 1200°C. The structure of the polycrystalline powders was analysed with X-ray powder diffraction data. The X-ray diffraction (XRD) patterns were indexed as the orthoferrite similar to that of PrFeO₃ having a single-phase orthorhombic perovskite structure (Pbnm). Pr_1−x Sr _x Fe_0.8Co_0.2O_3−δ (x=0.1, 0.2, 0.3) films have been deposited on yttria-stabilized zirconia (YSZ) single-crystal substrates at 700°C by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM). All films are polycrystalline with a marked texture and present pyramidal grains in the surface with different size distributions. Electrochemical impedance spectroscopy (EIS) measurements of PSFC/YSZ single crystal/PSFC test cells were conducted. The Pr_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ film at 850°C presents a lower area specific resistance (ASR) value, 1.65 Ω cm², followed by the Pr_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (2.29 Ω cm² at 850°C) and the Pr_0.9Sr_0.1Fe_0.8Co_0.2O_3−δ films (5.45 Ω cm² at 850°C).     

Hydrogen-related defect creation at the Si(100)–SiO2interface of metal-oxide-semiconductor field effect transistors during hot electron stress

We explore the hydrogen-related microstructures involved in hot electron defect creation at the Si(100)–SiO₂interface of metal-oxide-semiconductor field effect transistors. Based on the energetics of hydrogen desorption from the interface between silicon and silicon-dioxide, we argue that the hard threshold for hydrogen-related degradation may be considerably lower than the previously assumed value of 3.6 eV. Also, hydrogen atoms released from Si–H bonds at the interface by hot electron stress are trapped in bulk silicon near the interface.     

On the influence of the surface roughness onto the ultrathin SiO2/Si structure properties

The surface roughness of the semiconductor substrate substantially influences properties of the whole semiconductor/oxide structure. SiO₂/Si structures were prepared by using low temperature nitric acid oxidation of silicon (NAOS) method and then the whole structure was passivated by the cyanidization procedure. The influence of the surface morphology of the silicon substrate onto the electrical properties of ultrathin NAOS SiO₂ layer was investigated. Surface height function properties were studied by the AFM method and electrical properties were studied by the STM method. The complexity of analyzed surface structure was sensitive to the oxidation and passivation steps. For describing changes in the oxide layer structure, several fractal measures in an analysis of the STM images were used. This fractal geometry approach enables quantifying the fine spatial changes in the tunneling current spectra.     

Fabrication and characterization of amorphous silica nanostructures

Large-scale amorphous silica nanostructures, including nanowires, nanotubes and flowerlike nanowire bunches depending on the position, have been fabricated on silicon wafer through a cheap route under the assistance of gold and germanium. Accompanying the observation of blue-green light emission, comprehensive micro-structural characterization reveals that the growth of nanostructures is catalyzed only by gold whereas the final morphology of nanostructures depends on the location to germanium ball. Au₂Si, a compound of gold and silicon, is also disclosed as an intermediate state during the catalysis. Correspondingly, a growth scheme is proposed based on the experimental results and the vapor-liquid-solid mechanism.     

Disorder-order phase transition induced by size effect in bulk La2/3Sr1/3MnO3

Bulk La_2/3Sr_1/3MnO₃ ceramic samples prepared by thermal decomposition are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). An abnormal phenomenon, where three kinds of La_2/3Sr_1/3MnO₃ phases with different structures and the same composition coexist in the same grain, has been observed. Besides the stable rhombohedral majority phase, the two other phases are a simple cubic structure with a=0.389 nm and a new hexagonal structure with a=0.544 nm, c=0.668 nm. The simple cubic phase is a residual phase of high-temperature due to the size effect and bondage of twin boundary. Image simulations have suggested that the new hexagonal phase is the La-Sr ordered structure with space group , which is converted from the disordered simple cubic phase. The formation mechanism of the ordered phase is explained from volume energy and interface energy considerations.