Electron and ion beam analysis of composition and strain in Si-x Ge x /Si heterostructures

Si_1–x Ge _x heterostructures have been grown by molecular beam epitaxy, with nominal compositions of 10 and 15 at %. Analytical electron microscopy, Rutherford backscattering spectrometry and ion channeling have been used in order to determine film thickness, Ge molar fraction and tetragonal distortion. The actual Ge concentrations were found to be smaller than the nominal ones. For all the SiGe films a coherent growth was found, with a small deviation from the perfect tetragonal distortion. The good agreement found between the results obtained by each analytical technique demonstrate that these methods of characterization are powerful tools for the control of the epitaxial layer parameters.     

Critical review of the current status of thickness measurements for ultrathin SiO2 on Si Part V: Results of a CCQM pilot study

Results are reported from a pilot study under the Consultative Committee for Amount of Substance (CCQM) to compare measurements of and resolve any relevant measurement issues in the amount of thermal oxide on (100) and (111) orientation silicon wafer substrates in the thickness range 1.5–8 nm. As a result of the invitation to participate in this activity, 45 sets of measurements have been made in different laboratories using 10 analytical methods: medium—energy ion scattering spectrometry (MEIS), nuclear reaction analysis (NRA), RBS, elastic backscattering spectrometry (EBS), XPS, SIMS, ellipsometry, grazing—incidence x‐ray reflectometry (GIXRR), neutron reflectometry and transmission electron microscopy (TEM). The measurements are made on separate sets of 10 carefully prepared samples, all of which have been characterized by a combination of ellipsometry and XPS using carefully established reference conditions and reference parameters. The results have been assessed against the National Physical Laboratory (NPL) data and all show excellent linearity. The data sets correlate with the NPL data with average root‐mean‐square scatters of 0.15 nm, half being better than 0.1 nm and a few at or better than 0.05 nm. Each set of data allows a relative scaling constant and a zero thickness offset to be determined. Each method has an inherent zero thickness offset between 0 nm and 1 nm and it is these offsets, measured here for the first time, that have caused many problems in the past. There are three basic classes of offset: water and carbonaceous contamination equivalent to ～ 1 nm as seen by ellipsometry; adsorbed oxygen mainly from water at an equivalent thickness of 0.5 nm as seen by MEIS, NRA, RBS and possibly GIXRR; and no offset as seen by XPS using the Si 2p peaks. Each technique has a different uncertainty for the scaling constant and consistent results have been achieved. X‐ray photoelectron spectroscopy has large uncertainties for the scaling constant but a high precision and critically, if used correctly, has zero offset. Thus, a combination of XPS and the other methods allows the XPS scaling constant to be determined with low uncertainty, traceable via the other methods. The XPS laboratories returning results early were invited to test a new reference procedure. All showed very significant improvements. The reference attenuation lengths thus need scaling by 0.986 ± 0.009 (at an expansion factor of 2), deduced from the data for the other methods. Several other methods have small offsets and, to the extent that these can be shown to be constant or measurable, these methods will also show low uncertainty. Recommendations are provided for parameters for XPS, MEIS, RBS and NRA to improve their accuracy. Copyright © 2004 John Wiley & Sons, Ltd.     

Nanosynthesis routes to new tetrahedral crystalline solids: silicon-like Si3AlP

We introduce a synthetic strategy to access functional semiconductors with general formula A(3)XY (A = IV, X-Y = III-V) representing a new class within the long-sought family of group IV/III-V hybrid compounds. The method is based on molecular precursors that combine purposely designed polar/nonpolar bonding at the nanoscale, potentially allowing precise engineering of structural and optical properties, including lattice dimensions and band structure. In this Article, we demonstrate the feasibility of the proposed strategy by growing a new monocrystalline AlPSi(3) phase on Si substrates via tailored interactions of P(SiH(3))(3) and Al atoms using gas source (GS) MBE. In this case, the high affinity of Al for the P ligands leads to Si(3)AlP bonding arrangements, which then confer their structure and composition to form the corresponding Si(3)AlP target solid via complete elimination of H(2) at ～500 °C. First principle simulations at the molecular and solid-state level confirm that the Si(3)AlP building blocks can readily interlink with minimal distortion to produce diamond-like structures in which the P atoms are arranged on a common sublattice as third-nearest neighbors in a manner that excludes the formation of unfavorable Al-Al bonds. High-resolution XRD, XTEM, and RBS indicate that all films grown on Si(100) are tetragonally strained and fully coherent with the substrate and possess near-cubic symmetry. The Raman spectra are consistent with a growth mechanism that proceeds via full incorporation of preformed Si(3)AlP tetrahedra with residual orientational disorder. Collectively, the characterization data show that the structuro-chemical compatibility between the epilayer and substrate leads to flawless integration, as expected for pseudohomoepitaxy of an Si-like material grown on a bulk Si platform.     

Characterization of the stoichiometry of coevaporated FeSix films by AES, EDX, RBS, and electron microscopy

The composition of FeSi_x films on Si coevaporated from separate sources of Fe and Si was analyzed comparatively by AES, EDX, RBS, and electron microscopy. Cross-checks between EDX and RBS reveal systematic errors originating from the spectra background subtraction in RBS and from the thickness correction of the EDX signals. PCA (principal component analysis) assisted AES was successfully applied to the characterization of different Si bonding states in nonstoichiometric FeSi_x films. For the growth of β-FeSi₂ films by means of molecular beam epitaxy (MBE) the adjustment of the atomic beam intensities is reported in order to illustrate the capabilities of the various techniques.     

Spectroscopic studies of the Met182Thr mutant of nitrite reductase: role of the axial ligand in the geometric and electronic structure of blue and green copper sites

A combination of spectroscopic methods and density functional calculations has been used to describe the electronic structure of the axial mutant (Met182Thr) of Rhodobacter sphaeroides nitrite reductase in which the axial methionine has been changed to a threonine. This mutation results in a dramatic change in the geometric and electronic structure of the copper site. The electronic absorption data imply that the type 1 site in the mutant is like a typical blue copper site in contrast to the wild-type site, which is green. Similar ligand field strength in the mutant and the wild type (from MCD spectra) explains the similar EPR parameters for very different electronic structures. Resonance Raman shows that the Cu-S(Cys) bond is stronger in the mutant relative to the wild type. From a combination of absorption, CD, MCD, and EPR data, the loss of the strong axial thioether (present in the wild-type site) results in an increase of the equatorial thiolate-Cu interaction and the site becomes less tetragonal. Spectroscopically calibrated density functional calculations were used to provide additional insight into the role of the axial ligand. The calculations reproduce well the experimental ground-state bonding and the changes in going from a green to a blue site along this coupled distortion coordinate. Geometry optimizations at the weak and strong axial ligand limits show that the bonding of the axial thioether is the key factor in determining the structure of the ground state. A comparison of plastocyanin (blue), wild-type nitrite reductase (green), and the Met182Thr mutant (blue) sites enables evaluation of the role of the axial ligand in the geometric and electronic structure of type 1 copper sites, which can affect the electron-transfer properties of these sites.     

Comparative composition analysis of SiOx and SiNx thin films by AES,EDX and RBS

The chemical composition of thin films in SiO_x and SiN_x systems (“SIPOS”) have been analyzed by comparative AES, EDX, and RBS of the same samples to assess the extent of possible errors introduced by the “weak points” of each method. These exist for EDX in the necessity of a thickness correction, for AES in a possible composition dependence of the sensitivity factors, and for RBS in the low sensitivity in detecting light elements. Linear correlations between the data obtained by RBS and EDX have been revealed over the whole range of x. Therefore, EDX can be calibrated by appropriate standards of pure SiO₂ or Si₃N₄ films. For AES analysis a calibration curve is needed. Using RBS data for calibration the content of the light components O and N might be slightly overestimated by this method.     

Electric field standing wave effects in FT-IR transflection spectra of biological tissue sections: Simulated models of experimental variability

The so-called electric field standing wave effect (EFSW) has recently been demonstrated to significantly distort FT-IR spectra acquired in a transflection mode, both experimentally and in simulated models, bringing into question the appropriateness of the technique for sample characterization, particularly in the field of spectroscopy of biological materials. The predicted effects are most notable in the regime where the sample thickness is comparable to the source wavelength. In this work, the model is extended to sample thicknesses more representative of biological tissue sections and to include typical experimental factors which are demonstrated to reduce the predicted effects. These include integration over the range of incidence angles, varying degrees of coherence of the source and inhomogeneities in sample thickness. The latter was found to have the strongest effect on the spectral distortions and, with inhomogeneities as low as 10% of the sample thickness, the predicted distortions due to the standing wave effect are almost completely averaged out. As the majority of samples for biospectroscopy are prepared by cutting a cross section of tissue resulting in a high degree of thickness variation, this finding suggests that the standing wave effect should be a minor distortion in FT-IR spectroscopy of tissues. The study has important implications not only in optimization of protocols for future studies, but notably for the validity of the extensive studies which have been performed to date on tissue samples in the transflection geometry.     

Error quantification in strain mapping methods.

In this article a method for determining errors of the strain values when applying strain mapping techniques has been devised. This methodology starts with the generation of a thickness/defocus series of simulated high-resolution transmission electron microscopy images of InAsxP1-x/InP heterostructures and the application of geometric phase. To obtain optimal defocusing conditions, a comparison of different defocus values is carried out by the calculation of the strain profile standard deviations among different specimen thicknesses. Finally, based on the analogy of real state strain to a step response, a characterization of strain mapping error near an interface is proposed.     

Synthesis and characterization of single-layer silver-decanethiolate lamellar crystals

We report the synthesis of silver-decanethiolate (AgSC10) lamellar crystals. Nanometer-sized Ag clusters grown on inert substrates react with decanethiol vapor to form multilayer AgSC10 lamellar crystals with both layer-by-layer and in-plane ordering. The crystals have strong (010) texture with the layers parallel to the substrates. The synthesis method allows for a precise control of the number of layers. The thickness of the lamellae can be manipulated and systematically reduced to a single layer by decreasing the amount of Ag and lowering the annealing temperature. The single-layer AgSC10 lamellae are two-dimensional crystals and have uniform thickness and in-plane ordering. These samples were characterized with nanocalorimetry, atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Fourier transform infrared spectroscopy (FTIR), and Rutherford backscattering spectroscopy (RBS).     

Electronic structure, chemical bonding, and spin polarization in ferromagnetic MnAl

The electronic structure of ferromagnetic τ-MnAl has been calculated using density-functional techniques (TB-LMTO-ASA, FLAPW) and quantum-chemically analyzed by means of the crystal orbital Hamilton population tool. While all observable quantities are in good agreement with experiment, the tetragonal structure of ferromagnetic MnAl is interpreted to arise from a nonmagnetic cubic structure by two subsequent steps, namely (a) an electronic distortion due to spin polarization followed by (b) a structural distortion into the tetragonal system. The various strengths of interatomic bonding have been calculated in order to elucidate the competition between electronic and structural distortion.     

Quantitative analysis of the Ge concentration in a SiGe quantum well: comparison of low-energy RBS and SIMS measurements

The germanium concentration and the position and thickness of the quantum well in molecular beam epitaxy (MBE)-grown SiGe were quantitatively analyzed via low-energy Rutherford backscattering (RBS) and secondary ion mass spectrometry (SIMS). In these samples, the concentrations of Si and Ge were assumed to be constant, except for the quantum well, where the germanium concentration was lower. The thickness of the analyzed quantum well was about 12 nm and it was situated at a depth of about 60 nm below the surface. A dip showed up in the RBS spectra due to the lower germanium concentration in the quantum well, and this was evaluated. Good depth resolution was required in order to obtain quantitative results, and this was obtained by choosing a primary energy of 500 keV and a tilt angle of 51° with respect to the surface normal. Quantitative information was deduced from the raw data by comparing it with SIMNRA simulated spectra. The SIMS measurements were performed with oxygen primary ions. Given the response function of the SIMS instrument (the SIMS depth profile of the germanium delta (δ) layer), and using the forward convolution (point-to-point convolution) model, it is possible to determine the germanium concentration and the thickness of the analyzed quantum well from the raw SIMS data. The aim of this work was to compare the results obtained via RBS and SIMS and to show their potential for use in the semiconductor and microelectronics industry. The detection of trace elements (here the doping element antimony) that could not be evaluated with RBS in low-energy mode is also demonstrated using SIMS instead.     

Doping effects in single-layered La0.5Sr1.5MnO4 manganite

In this paper we report the results of the synthesis and structural, transport, and magnetic characterization of pure La(0.5)Sr(1.5)MnO(4) and B-site lightly doped samples, i.e. La(0.5)Sr(1.5)Mn(0.95)B(0.05)O(4), where B = Ru, Co, and Ni. The choice was made in order to probe the charge ordering/orbital ordering ground state of the monolayered La(0.5)Sr(1.5)MnO(4) manganite as a consequence of the cation doping. It is shown that even a light doping is successful in suppressing the charge and orbital order found in pure La(0.5)Sr(1.5)MnO(4). No long-range magnetic order has been detected in any of the doped samples but the setup of a spin-glass state with a common freezing temperature ( approximately 22 K). Structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the orbital character of the e(g) electrons along with the overall cation size.     

Characterization of ultra smooth interfaces in Mo/Si-multilayers

The interface structure of Mo/Si-multilayers prepared by Pulsed Laser Deposition (PLD) on Si substrates at room temperature has been investigated. Already the in-situ ellipsometer data acquired during film growth indicate a particular behaviour of this material system that is caused by reaction/diffusion processes of the condensing atoms. MoSi(x) interlayers are formed both at the Mo on Si- and at the Si on Mo-interfaces. The results of multilayer characterization carried out by SNMS and RBS show similar concentration profiles for both types of the interlayers. More detailed information about interface structure and morphology can be provided by HREM investigations. In the TEM micrographs of various multilayers prepared for different laser light wavelengths an improvement of layer stack quality, i.e. formation of abrupt interfaces, with increasing photon energy is observed. Layer stacks having almost ideally smooth interfaces were synthesized by UV-photon ablation. HREM micrographs of these multilayers show a pronounced separation of spacer and absorber layers. The roughness sigma(R) of the interfaces between the amorphous Si- and MoSi(x)-layers was determined by image analysis. On the average a level sigma(R) approximately 0.1 nm is found. There is no indication for roughness replication or amplification from interface to interface as it is known from the appropriate products of conventional thin film technologies.     

Synthesis of Mn3O4 nanoparticles by thermal decomposition of a [bis(salicylidiminato)manganese(II)] complex

The present investigation reports on the novel synthesis of Mn₃O₄ nanoparticles using thermal decomposition and their physicochemical characterization. The Mn₃O₄ nanoparticle powder has been prepared using [bis(salicylidiminato)manganese(II)] as a precursor. The effect of oleyl amine and triphenylphosphine on the particle morphology has been investigated. Transmission electron microscopy (TEM) analysis demonstrated Mn₃O₄ nanoparticles with an average diameter of about 25 nm. The structural study by X-ray diffraction (XRD) indicates that these nanoparticles have a pure tetragonal phase. The phase pure samples were characterized using X-ray photoelectron spectroscopy (XPS) for both Mn 2p and Mn 3s levels. The values of binding energies are consistent with related values reported in the literature.     

Advanced surface characterization of silver nanocluster segregation in Ag–TiCN bioactive coatings by RBS, GDOES, and ARXPS

Surface modification by means of wear protective and antibacterial coatings represents, nowadays, a crucial challenge in the biomaterials field in order to enhance the lifetime of bio-devices. It is possible to tailor the properties of the material by using an appropriate combination of high wear resistance (e.g., nitride or carbide coatings) and biocide agents (e.g., noble metals as silver) to fulfill its final application. This behavior is controlled at last by the outmost surface of the coating. Therefore, the analytical characterization of these new materials requires high-resolution analytical techniques able to provide information about surface and depth composition down to the nanometric level. Among these techniques are Rutherford backscattering spectrometry (RBS), glow discharge optical emission spectroscopy (GDOES), and angle resolved X-ray photoelectron spectroscopy (ARXPS). In this work, we present a comparative RBS–GDOES–ARXPS study of the surface characterization of Ag–TiCN coatings with Ag/Ti atomic ratios varying from 0 to 1.49, deposited at room temperature and 200 °C. RBS analysis allowed a precise quantification of the silver content along the coating with a non-uniform Ag depth distribution for the samples with higher Ag content. GDOES surface profiling revealed that the samples with higher Ag content as well as the samples deposited at 200 °C showed an ultrathin (1–10 nm) Ag-rich layer on the coating surface followed by a silver depletion zone (20–30 nm), being the thickness of both layers enhanced with Ag content and deposition temperature. ARXPS analysis confirmed these observations after applying general algorithm involving regularization in addition to singular value decomposition techniques to obtain the concentration depth profiles. Finally, ARXPS measurements were used to provide further information on the surface morphology of the samples obtaining an excellent agreement with SEM observations when a growth model of silver islands with a height d?=?1.5 nm and coverage θ?=?0.20 was applied to the sample with Ag/Ti?=?1.49 and deposited at room temperature.

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Several samples of Pb₃O₄ have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase (T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb₃O₄ exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).     

Surfactant (bi)layers on gold nanorods

Gold nanorods in aqueous solution are generally surrounded by surfactants or capping agents. This is crucial for anisotropic growth during synthesis and for their final stability in solution. When CTAB is used, a bilayer has been evidenced from analytical methods even though no direct morphological characterization of the precise thickness and compactness has been reported. The type of surfactant layer is also relevant to understand the marked difference in further self-assembling properties of gold nanorods as experienced using 16-EO(1)-16 gemini surfactant instead of CTAB. To obtain a direct measure of the thickness of the surfactant layer on gold nanorods synthesized by the seeded growth method, we coupled TEM, SAXS, and SANS experiments for the two different cases, CTAB and gemini 16-EO(1)-16. Despite the strong residual signal from micelles in excess, it can be concluded that the thickness is imposed by the chain length of the surfactant and corresponds to a bilayer with partial interdigitation.     

The diagnetic circular dichroism of KBr: Sn2+

The magnetic circular dichroism (MCD) of thoroughly-quenched KBr: Sn²⁺ crystals has been measured over the temperature range 10–230 K. Theoretical calculations of the MCD line shape have verified that such crystals contain Sn²⁺ ions at centres that are principally of either cubic or tetragonal symmetry. The possibility that a small fraction of rhombic Sn²⁺ centres is also present is examined. The calculated spectra are in reasonable agreement with experiment over a wide temperature range, apart from the spectral region above 4.85 eV. The factors that might contribute to this discrepancy are discussed.     

On the “martensitic” transformation in the A-15 superconductors compounds

Although a great number of studies have been reported on the “martensitic” transformation in A-15 superconductors, there are still fundamental questions concerning the nature of the transformation which remain unsolved. After a critical analysis of previous experimental work on this transformation in V₃Si and Nb₃Sn, including our own results, we propose a model for the origin and mechanism of the tetragonal distortion based on the consequences of the strong structural relation between the A-15 cubic structure and the W₅Si₃ tetragonal one. As a matter of fact since the analysis of the previous results leads to the conclusion that the transforming A₃B sample has a composition richer in B than stoichiometric, we propose that the distortion is due to a number of defects (antisite type which is larger in transforming than in nontransforming crystals). The presence of these point defects leads to the formation of local strain of tetragonal symmetrical resulting from repulsive interactions between the B atoms, in equilibrium with the cubic symmetry of the three-dimensional network of strongly bonded A atoms. At low temperature, as soon as the lattice softening becomes important enough, a new equilibrium is reached, namely the distortion of the lattice.     

Superhard coatings — Production and analysis

In the development of diamond and c-BN products the analytical methods for characterizing the surface, bulk and interface of the diamond coatings are very important. SEM, Raman, XRD and IR are the methods used for characterization and SIMS, TEM, AES, NRA, RBS, XPS, STM, etc. are used for the investigation of special problems. The techniques for diamond and c-BN production are briefly summarized to give an idea of the complex interactions between production, application and analytical characterization. The analytical methods for diamond characterization and many relevant results are summarized in this paper; some physical properties (e.g. thermal conductivity, transparency, etc.) and their interaction with applications are also discussed.Abbreviations AES Auger electron spectroscopy - AFM atomic force microscopy - c-BN cubic boron nitride - CL cathodoluminescence - CVD chemical vapour deposition - EELS electron energy loss spectroscopy - EPMA electron probe microanalysis - ERDA elastic recoil detection analysis - h-BN hexagonal boron nitride - HP-HT high-pressure high-temperature - HF hot-filament - IR infra-red - LEED low energy electron diffraction - MW microwave - NAA neutron activation analysis - NRA nuclear reaction analysis - PL photoluminescence - PVD physical vapour deposition - RBS Rutherford backscattering spectrometry - RHEED reeflected high energy electron diffraction - SAD selected area diffraction - SEM scanning electron microscopy - SIMS secondary ion mass spectrometry - STM secondary ion mass spectrometry - TEM transmission electron microscopy - TMB trimethylborate - XPS X-ray photoelectron spectroscopy - XRD X-ray diffraction Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday