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In this study, strontium titanate bicrystals with a twist grain boundary were successfully formed using a spark plasma sintering apparatus. Diffusion bonding was accomplished at high pressure and fast heating, and provides a novel method to form bicrystals at lower temperatures and accelerated time scales compared to more established techniques. Scanning electron microscopy was used to quantitatively evaluate the degree of successful interfacial bonding. Atomic resolution imaging using transmission election microscopy techniques combined with spatially resolved electron energy-loss spectroscopy measurements confirm a clean and atomically abrupt interface free of secondary phases.

The formation of a pre-silicide layer below Ni1-xPtxSi films is reported with structure and composition distinctly different from previously observed diffusion layers. It was found that during two-step rapid thermal annealing Ni interstitial diffusion can kinetically dominate over the formation of Ni silicide, which results in a metastable pre-silicide layer. Aberration corrected scanning transmission electron microscopy experiments have revealed Ni to occupy interstitial and substitutional sites in the pre-silicide layer. Rapid thermal annealing and Pt alloying determines the stoichiometry and thickness of the layer, while the point defect configurations give rise to lowering of the associated Schottky barrier heights. The pre-silicide layer effectively limits diffusion of Ni into the substrate and therefore allows for the low-temperature growth of Ni2Si and NiSi. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Spray-pyrolysis of a ferrocene–xylene–triethylborane mixture at 900 °C results in a novel “sea-cucumber”-like structure containing carbon and boron. SEM studies show that these structures are hollow with diameter between 100 and 500 nm and lengths varying from 30 to 40 μm. HRTEM and EELS studies reveal that the hollow structures are partly filled with iron. They are formed by a more graphitic internal core, which hardly contains any B. This core is coated by a more disordered B-containing C material. The amount of B in this coating is around 3 at.%. In addition, at this pyrolysis temperature, the growth of open-ended tubular nanostructures (30–40 nm diameter; 100–200 nm length) on the surface of the sea-cucumber-like structure is observed. These tubular nanostructures are not very graphitic and exhibit a similar composition to the coating of the sea-cucumber-like structure (around 3 at.% of B). The study of material produced at different pyrolysis temperatures show that the growth of these tubular nanostructures and the boron content are temperature dependent. Finally, preliminary results corresponding to oxidation resistance studies are presented.

Functional properties of oxide ceramics are often controlled by the addition of dopant elements and the resulting alteration of oxygen vacancy concentrations within grain boundary core structures. A challenge in designing nanoscale ceramic microstructures is forming stable grain boundary networks, while minimizing unwanted impurity concentrations. In this study, it was discovered that the application of electrostatic fields during diffusion bonding of undoped SrTiO3 bicrystals leads to modifications of grain boundary core structures while misorientation angles remained unchanged. The applied electric field not only changes atomic and electronic interface structures, but also causes modifications of ensuing dielectric properties by altering local oxygen vacancy concentrations. The observations for this model system demonstrate the potential to control and modify the microscopic degrees of freedom of grain boundaries in the absence of dopant elements. Field-assisted modifications of grain boundary networks may become a disruptive technology in designing oxide microstructures for a wide range of applications.Functional properties of oxide ceramics are often controlled by the addition of dopant elements and the resulting alteration of oxygen vacancy concentrations within grain boundary core structures. A challenge in designing nanoscale ceramic microstructures is forming stable grain boundary networks, while minimizing unwanted impurity concentrations. In this study, it was discovered that the application of electrostatic fields during diffusion bonding of undoped SrTiO3 bicrystals leads to modifications of grain boundary core structures while misorientation angles remained unchanged. The applied electric field not only changes atomic and electronic interface structures, but also causes modifications of ensuing dielectric properties by altering local oxygen vacancy concentrations. The observations for this model system demonstrate the potential to control and modify the microscopic degrees of freedom of grain boundaries in the absence of dopant elements. Field-assisted modifications of grain boundary networks ...

Silicon has been the semiconductor of choice for microelectronics largely because of the unique properties of its native oxide (SiO2) and the Si/SiO2 interface. For high-temperature and/or high-power applications, however, one needs a semiconductor with a wider energy gap and higher thermal conductivity. Silicon carbide has the right properties and the same native oxide as Si. However, in the late 1990’s it was found that the SiC/SiO2 interface had high interface trap densities, resulting in poor electron mobilities. Annealing in hydrogen, which is key to the quality of Si/SiO2 interfaces, proved ineffective. This paper presents a synthesis of theoretical and experimental work by the authors in the last six years and parallel work in the literature. High-quality SiC/SiO2 interfaces were achieved by annealing in NO gas and monatomic H. The key elements that lead to highquality Si/SiO2 interfaces and low-quality SiC/SiO2 interfaces are identified and the role of N and H treatments is described. More specifically, optimal Si and SiC surfaces for oxidation are identified and the atomic-scale processes of oxidation and resulting interface defects are described. In the case of SiC, we conclude that excess carbon at the SiC/SiO2 interface leads to a bonded Si-C-O interlayer with a mix of fourfold- and threefold-coordinated C and Si atoms. The threefold coordinated atoms are responsible for the high interface trap density and can be eliminated either by H-passivation or replacement by N. Residual Si-Si bonds, which are partially passivated by H and N remain the main limitation. Perspectives for the future for both Si- and SiC-based MOSFETs are discussed.

We show that individual Hf atoms may get incorporated into the SiO2 interlayer which is formed between the HfO2 dielectric film and the Si substrate during rapid thermal annealing. We report atomically-resolved Z-contrast images of a Si/SiO2/HfO2 structure together with first-principles calculations which demonstrate that single Hf atoms are in fact present in the interlayer. The location of individual Hf atoms within the interlayer oxide is closely related to the structure of the amorphous oxide near the Si/SiO2 interface. The Hf defects may affect channel mobility and leakage currents in the HfO2/Si electronic devices.

Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3. Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states. The experimentally determined indirect band gap energy is 3.25 eV, while the direct band gap energy is 3.75 eV. The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV. The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV. The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 3.25 eV up to 15 eV. The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV. The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV. Interband transitions from the O 2s band to the conduction bands appear at 26 eV. A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons.

Valence electron energy loss spectroscopy in a dedicated scanning transmission electron microscope has been used to obtain the interband transition strength of a S13 tilt grain boundary in SrTiO3. In a first step the electronic structure of bulk SrTiO3 has been analysed quantitatively by comparing VEELS spectra with vacuum ultraviolet spectra and with ab initio density of states calculations. The electronic structure of a near S13 grain boundary and the corresponding dispersion forces were then determined by spatially resolved VEELS. Also the effects of delocalization of the inelastic scattering processes were estimated and compared with results from the literature. # 2001 Elsevier Science B.V. All rights reserved.

The energy loss near-edge structures in electron energy-loss spectra contain information about bonding characteristics, the electronic structure and coordinations of the excited atoms. We have calculated sets of reference spectra for the normal and for the inverse Mg–Al-spinel using a full multiple scattering approach. By a quantitative comparison of these reference spectra with experimental data ELNES-quantification becomes possible. We characterized the degree of inversion λ by the analysis of the relative peak-intensities and the relative peak-positions within 35 eV beyond the edge onset. The results demonstrate that by using the provided methods ELNES-quantification will become possible when uncertainties in the experiment are reduced and a better fit of the simulations to the experiment is achieved.

We use first-principles calculations and experimental data to demonstrate that impurity segregation at heterointerfaces is governed by several factors. In particular, Hf impurities avoid the ${\text{Si-SiO}}_{2}$ interface when present in the ${\text{SiO}}_{2}$ side, might segregate if present in the Si side, but do not cross into ${\text{SiO}}_{2}$. Substitutional Hf impurities in ${\text{SiO}}_{2}$, as revealed by a through-focal series of $Z$-contrast images, act as markers for Si sites, suggesting ordering in the first two Si planes of the amorphous ${\text{SiO}}_{2}$. Finally, we show that dopants in Si segregate at the interface by adopting several distinct configurations and also do not cross into ${\text{SiO}}_{2}$.

The Scanning Transmission Electron Microscope1(STEM) is one of the most useful tools in many areas of atomic-scale materials science and nanocharacterisation. A STEM has the ability to generate local maps of the chemical composition and electronic structure at atomic resolution, even in complex or u...

Dopant segregation in a ${\mathrm{Mn}}_{\mathrm{x}}{\mathrm{Ge}}_{1\ensuremath{-}x}$ dilute magnetic semiconductor leads to a remarkable self-assembly of Mn-rich nanocolumns, embedded in a fully compensated Ge matrix. Samples grown at $80\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ display a giant positive magnetoresistance that correlates directly with the distribution of magnetic impurities. Annealing at $200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ increases Mn substitution in the host matrix above the threshold for the insulator-metal transition, while maintaining the columnar morphology, and results in global ferromagnetism with conventional negative magnetoresistance. The qualitative features of magnetism and transport in this nanophase material are thus extremely sensitive to the precise location and distribution of the magnetic dopants.

Recent and ongoing improvements in aberration correction have opened up the possibility of depth sectioning samples using the scanning transmission electron microscope in a fashion similar to the confocal scanning optical microscope. We explore questions of principle relating to image interpretability in the depth sectioning of samples using electron energy loss spectroscopy. We show that provided electron microscope probes are sufficiently fine and detector collection semi-angles are sufficiently large we can expect to locate dopant atoms inside a crystal. Furthermore, unlike high angle annular dark field imaging, electron energy loss spectroscopy can resolve dopants of smaller atomic mass than the supporting crystalline matrix.

Aberration correction in scanning transmission electron microscopy represents a major breakthrough in transmission electron microscopy, enabling the formation of sub‐Angstrom probe sizes. Thus, electron microscopy achieved single atom sensitivity. Here, we show how this technique with its unique spatial resolution in combination with high‐resolution electron energy‐loss spectroscopy can be used to investigate atomic and electronic structures of semiconductor interfaces with single atom sensitivity. We employ a Si/HfO2/SiO2/Si high‐k dielectric interface to show the presence of single Hf atoms in the SiO2 interlayer. Furthermore, we demonstrate how local dielectric properties and local band structure information can be obtained by electron energy‐loss spectroscopy.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Near-edge structures of absorption edges in electron energy-loss spectra (ELNES) of SrTiO 3 were calculated and compared to experimental inelastic electron scattering data. The goal of this study was to investigate final-state effects on the electronic structure. Two theoretical approaches were applied: density-functional theory with a band-structure supercell method and a real-space multiple-scattering cluster approach. Within both techniques, the Z +1 approximation was used to model the core hole generated by the inelastic scattering process. For the band-structure calculations, supercells of ( SrTiO 3 ) n (n=1,4,8,16) composition with three-dimensional periodic boundary conditions were applied. The influence of supercell size and shape on calculated site- and symmetry-projected local densities of unoccupied states is assessed quantitatively. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole, and the interaction energy of neighbouring core hole centres. For a sufficiently large supercell size, the Z +1 approximation yields a reasonable description of the local densities of unoccupied states probed by the energy losses of inelastically scattered electrons of the Ti L 3 -, O K- and Sr L 3 -absorption edges. The quantitative equivalence of ELNES information extracted from the multiple-scattering cluster calculations and the band-structure supercell calculations is demonstrated. Discrepancies between theoretical and experimental results are discussed.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Temperature dependent current-voltage measurements show that the addition of only 10% Pt to NiSi causes an increase of Schottky barrier height (SBH) from 0.65 eV for NiSi to 0.78 eV for the 10% Pt alloy. Internal photoemission measurements resolve two SBHs in all alloyed samples with ≥5% Pt incorporation corresponding to NiSi and PtSi (∼0.68 eV and ∼0.80 eV), proving that each contributes independently to junction current. High angle annular dark field imaging with scanning transmission electron microscopy confirms Pt segregation to the Ni(Pt)Si/Si interface. The resulting increased SBH may therefore be detrimental to contact resistivity in future technology nodes.

The influence of Hf-based dielectrics on the underlying SiO2 interfacial layer (IL) in high-k gate stacks is investigated. An increase in the IL dielectric constant, which correlates to an increase of the positive fixed charge density in the IL, is found to depend on the starting, pre-high-k deposition thickness of the IL. Electron energy-loss spectroscopy and electron spin resonance spectra exhibit signatures of the high-k-induced oxygen deficiency in the IL consistent with the electrical data. It is concluded that high temperature processing generates oxygen vacancies in the IL responsible for the observed trend in transistor performance.