Initial formation of contact layers on Ni/SiC samples studied by XPS

This study deals with the quantitative assessment of the coverage and thickness of Ni silicide films formed during annealing of SiC substrates with sputtered thin films of Ni. The analytical approach involves the use of XPS and depth profiling by means of successive ion etchings and XPS analyses. For either 3 or 6 nm initial Ni film thickness, a 10 nm Ni₂Si product is formed. On top of this product, the C released is accumulated in a very thin (1–2 nm) film. In neither case, the Ni₂Si covers the whole surface, although the coverage is almost complete (～90%) in the latter case. For the greater initial Ni‐film thickness of 17 nm, the thickness of the Ni₂Si product corresponds well to the value of 25 nm expected from the Ni/Ni₂Si stoichiometric relationship. This thickness is significantly greater than a critical level and the film covers the whole surface. Carbon is similarly accumulated in a very thin layer on the top surface, although the major part of C (～70%) is found inside the main reaction product layer. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface characterisation of laser irradiated SiC ceramics by AES and XPS

Samples of sintered silicon carbide (SSiC) were irradiated with a KrF excimer laser (λ = 248 nm) at energy densities of 10, 15 and 25 J/cm² in He atmosphere. The composition of the near surface region was investigated by Auger electron spectroscopy (AES) and photoelectron spectroscopy (XPS) after lapping, laser irradiation and tribological treatment, respectively. By laser irradiation a surface layer is formed which contains about 30% oxygen. The existence of different bonding states of Si, C and O was established by factor analysis of the AES depth profiles and by XPS. By laser irradiation SiC is decomposed and a siliconoxycarbide with the average composition SiC_3.5O_1.5 is formed. Beneath the oxidised surface layer the nominal elemental composition SiC is found but the sample represents a mixture of Si, graphite and siliconoxycarbide with a small amount of SiC only. Obviously, the decomposition zone exceeds in a depth > 300 nm.     

Vacuum annealing phenomena in ultrathin TiD y /Pd bi-layer films evaporated on Si(100) as studied by TEM and XPS

Using a combination of TEM and XPS, we made an analysis of the complex high-temperature annealing effect on ultrathin titanium deuteride (TiD _y ) films evaporated on a Si(100) substrate and covered by an ultrathin palladium layer. Both the preparation and annealing of the TiD _y /Pd bi-layer films were performed in situ under UHV conditions. It was found that the surface and bulk morphology of the bi-layer film as well as that of the Si substrate material undergo a microstructural and chemical conversion after annealing and annealing-induced deuterium evolution from the TiD _y phase. Energy-filtered TEM (EFTEM) mapping of cross-section images and argon ion sputter depth profiling XPS analysis revealed both a broad intermixing between the Ti and Pd layers and an extensive inter-diffusion of Si from the substrate into the film bulk area. Segregation of Ti at the Pd top layer surface was found to occur by means of angle-resolved XPS (ARXPS) and the EFTEM analyses. Selected area diffraction (SAD) and XPS provided evidence for the formation of a new PdTi₂ bimetallic phase within the top region of the annealed film. Moreover, these techniques allowed to detect the initial stages of TiSi phase formation within the film–substrate interlayer.     

NMR investigations on the lithiation and delithiation of nanosilicon-based anodes for Li-ion batteries

Lithiation and delithiation of nanosilicon anodes of 100–200 nm diameter have been probed by ex situ solid-state high-resolution ⁷Li nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM) methods. Samples were charged within pouch cells up to capacities of 1,500 mAh/g at 0.1 C, and subsequently discharged at the same rate. The NMR spectra reveal important quantitative information on the local lithium environments during the various stages of the charging/discharging process. The TEM experiments show that the electrochemical lithiation of nanosilicon particles results in core-shell materials, consisting of Li_xSi shells surrounding a core of residual silicon. The NMR spectra yield approximate Li/Si ratios of the lithium silicides present in the shells, based on the distinct local environments of the various types of ⁷Li nuclei present. The combination of NMR with TEM gives important quantitative conclusions about the nature of the electrochemical lithiation process: Following the initial formation of the solid electrolyte interphase layer, which accounts for an irreversible capacity of 240 mAh/g, lithium silicide environments with intermediate Li concentrations (Li₁₂Si₇, Li₇Si₃, and Li₁₃Si₄) are formed at the 500 to 1,000 mAh/g range during the charging process. At a certain penetration depth, further lithiation does not progress any further toward the interior of the silicon particles but rather leads to the formation of increasing amounts of the lithium-richest silicide, Li₁₅Si₄-type environments. Delithiation does not result in the reappearance of the intermediate-stage phases but rather only changes the amount of Li₁₅Si₄ present, indicating no microscopic reversibility. Based on these results, a detailed quantitative model of nanophase composition versus penetration depth has been developed. The results indicate the power and potential of solid-state NMR spectroscopy for elucidating the charging/discharging mechanism of nano-Si anodes.     

Interaction of Co nanoparticles with SiO2: silicide formation

The interaction of 1 and 4 nm thick Co films with SiO₂ support in vacuum at high temperature has been investigated by TEM, SAED and HRTEM methods. It was found that annealing in vacuum at 800 °C caused the transformation of the smallest Co particles into Co₂Si silicide.     

Characterization of Laser-Arc deposited multilayer systems by means of AES, Factor Analysis and XPS

In a vacuum chamber at 5 · 10^–4 Pa, multilayer systems (single layer thickness 20 nm) consisting of Ti/C and Al/C, respectively, have been deposited on Si (111) disks by the laser assisted coating (Laser-Arc). Structure and composition have been investigated by means of Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES) in conjunction with Factor Analysis. AES depth profile measurements through the outermost part of the layers show for both the Ti/C and the Al/C samples a regular structure of the layer sequence metal/ carbon with a constant distance along the sample normal and sharply formed interfaces. In the metallic layers an oxygen enrichment was found, which is more intensive in the Ti/C deposit than in the Al/C one. By means of Factor Analysis in the evaluation of the differentiated spectra as a function of sputtering time, the formation of carbides at the metal/carbon interfaces has been detected. However, in the present state of the investigations it can not be decided, whether the observed carbide formation is the result of the energy impact due to ion sputtering or the coating fabrication process itself.     

Effects of TriSilanolIsobutyl-POSS on thermal stability of methylsilicone resin

Methylsilicone resin/polyhedral oligomeric silsesquioxane (POSS) composites with various proportions of POSS monomer were synthesized by the reaction of functionalized TriSilanolIsobutyl-POSS macromonomer with hydroxyl-terminated methylsilicone resin. The structures of the obtained hybrid polymers were characterized with Fourier-transformed infrared (FT-IR) and transmission electron microscopy (TEM). The FT-IR spectra suggested successful bonding of TriSilanolIsobutyl-POSS and methylsilicone resin. TEM analysis showed that POSS can dissolve in methylsilicone resin at the molecular level. The influences of TriSilanolIsobutyl-POSS on the thermal stability and degradation behavior of methylsilicone resin were studied by thermogravimetric analysis (TGA), solid-state ²⁹Si NMR and X-ray photoelectron spectroscopy (XPS). All these techniques showed that TriSilanolIsobutyl-POSS incorporation results in increased decomposition temperatures and oxidation resistance, primarily by reducing the effect of silanol end groups on the thermolysis through condensation reaction of Si-OH groups and partial loss of isobutyl followed by the formation of an inorganic SiO₂ layer to prevent methylsilicone from further degradation.     

XPS Depth Profile Analysis of a Thin Non-Conducting Titanate Superlattice

 New BaTiO₃-SrTiO₃ (BTO-STO)-superlattices which may be interesting for future electronic applications have been investigated by X-ray photoelectron spectroscopy (XPS) depth profiling. At first XPS measuring conditions were optimized for that non-conducting and thin layer systems (21 nm double layer thickness) considering the practical instrumental limitations. Second a simulation of the sputtering process for the concrete experimental conditions were done by a dynamic T-DYN code. By comparison of experimental and simulated depth profiles the maximum sample roughness could be estimated to be in the range of 2 nm.     

Interfacial bonding of gold nanoparticles on a H-terminated Si(100) substrate obtained by electro- and electroless deposition

Dome-shaped gold nanoparticles (with an average diameter of 10.5 nm) are grown on H-terminated Si(100) substrates by simple techniques involving electro- and electroless deposition from a 0.05 mM AuCl3 and 0.1 M NaClO4 solution. XPS depth profiling data (involving Au 4f core-level and valence band spectra) reveal for the first time the formation of gold silicide at the interface between the Au nanoparticles and Si substrate. UV-visible diffuse reflectance spectra indicate that both samples have surface plasmon resonance maxima at 558 nm, characteristic of an uniform distribution of Au nanoscale particles of sufficiently small size. Glancing-incidence XRD patterns clearly show that the deposited Au nanoparticles belong to the fcc phase, with the relative intensity of the (220) plane for Au nanoparticles obtained by electroless deposition found to be notably larger than that by electrodeposition.     

Depth profiling of Co/Ti – silicide films using total reflection X-ray fluorescence (TXRF) spectrometry combined with low energy ion beam etching (IBE) for sample preparation

Solid state phase epitaxy (SSPE) by rapid thermal processing (RTP) of Co/Ti double layers deposited on (100)-Si substrates is a common technique for the production of buried CoSi₂-silicide conducting layers for microelectronics technology. The understanding of the processes during the SSPE silicide formation on the atomic scale needs the study of the elemental depth distributions with nanometer scale depth resolution of all multi-layer elemental constituents at different RTP conditions. A new experimental technique, the laterally resolved TXRF analysis line scan method across the bevelled section of the sample prepared by ex-situ ion beam sputter etching, was used to obtain the multi-element depth profiles. First results on the as evaporated Co/Ti (30 nm thick) double layer system prior to the RTP and on the final CoSi₂/Ti_xCo_ySi_z-system (160 nm thickness) after the RTP were obtained.     

Round‐robin test for the measurement of layer thickness of multilayer films by secondary ion mass spectrometry depth profiling

An international round‐robin test (RRT) was performed to investigate a method to determine the interface location and the layer thickness of multilayer films by secondary ion mass spectrometry (SIMS) depth profiling as a preliminary study to develop a new work item proposal in ISO/TC‐201. Two types of reference materials were used in this RRT. A SiGe alloy (Si_52.4Ge_47.6) reference film was used to determine the relative sensitivity factors of Si and Ge. A Si/Ge multilayer reference film was used to determine the relative sputtering rates of the Si and Ge layers. The layer thicknesses were measured from the interfaces determined by a 50 atomic percent definition. Seven laboratories from 5 countries participated in this international RRT. The RRT reference expanded uncertainties for Si and Ge layers in a Si/Ge multilayer with similar thicknesses as the reference film were 0.76 and 1.17 nm, respectively. However, those in a thinner Si/Ge multilayer film were slightly larger at 1.04 and 1.59 nm, respectively. Most of the thickness ratios in the 2 Si/Ge multilayer films were consistent with the RRT reference value within their expanded uncertainties.     

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 _R of the interfaces between the amorphous Si- and MoSi_x-layers was determined by image analysis. On the average a level _R 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.     

Effects of post-deposition annealing on chemical composition of SiNx film in SiO2/SiNx/SiO2/Si stacked structure

To systematically evaluate the quality of SiN_x films in multi-stacked structures, we investigated the effects of post-deposition annealing (PDA) on the film properties of SiN_x within the SiO₂/SiN_x/SiO₂/Si stacked structure by performing X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), Fourier transform infrared (FT-IR) spectroscopy, and scanning transmission electron microscope–electron energy loss spectroscopy (STEM-EELS) analyses. The XPS results showed that PDA induces the oxidation of the SiN_x layer. In particular, new finding is that Si-rich SiN_x in the SiN_x layer is preferentially oxidized by PDA even in multi-stacked structure. The XRR results showed that the SiN_x layer becomes thinner, whereas the interface layer between the SiN_x layer and Si becomes thicker. It is concluded by STEM-EELS and XPS that this interface layer is SiON layer. The density of N–H and Si–H bonding within the stacked structure strongly depends on the PDA temperature. Our study helps elucidate the properties of SiN_x films in stacked structures from various perspectives.     

Nitrogen depth distribution, interface and structure analysis of SiNx layers produced by low-energy ion implantation

Thin silicon nitride (SiN _x ) layers with the stoichiometric N/Si ratio of 1.33 in the maximum of the concentration depth distributions of nitrogen were produced by implanting 10 keV¹⁵N ₂ ⁺ in 100 silicon at room temperature under high vacuum conditions. The depth distribution of the implanted isotope was measured by resonance nuclear reaction analysis (NRA), whereas the layer structure of the implanted region and the geometrical thickness of the layers were characterised by high resolution transmission electron microscopy (TEM). SiN _x layers with a thickness of about 30 nm were determined by NRA. Channeling Rutherford backscattering spectrometry was used to determine the disorder in the silicon substrate. Sharp interfaces of a few nanometers between the highly disordered implanted region and the crystalline structure of the substrate thickness were observed by TEM. The high thermal stability of SiN _x layers with N/Si ratios from under to over stoichiometric could be shown by electron beam rapid thermal annealing (1100 °C for 15 s, ramping up and down 5 °C/s) and NRA.     

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.     

On the identification of interface oxides and interface serration by ARXPS

Summary Oxidized surfaces show smeared out XPS lines which cannot be fitted by bulk compounds and by lateral growth. By simultaneously fitting XPS spectra obtained for take-off angles between 5 ° and 80 ° the resolution is enhanced. Thus small amounts (<0.5 nm) of interface oxides are identified in level shift, stoichiometry and spatial distribution, i.e., serration and protrusion of interfaces are revealed. The ARXPS-analysis of oxidized (<100 °C) NbN, NbC and Nb shows that the metal surface is serrated by metallic ( 1 nm) and dielectric ( 1 nm) interface oxides, where Nb₂O₅ forms the outermost layer. The serration is deeper for Nb than for NbN, because Nb is weaker and dissolves more O. In contrast, in high temperature oxidation (900 °C) of a clean Si single crystal (100) surface, less interface oxides (<1 nm) have been found which grow laterally.

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Über die Identifizierung von Interface-Oxiden und der Interface-Verzahnung durch ARXPS

     

Measuring the electronic structure of disordered overlayers by electron momentum spectroscopy: the Cu/Si interface

The Cu? Si interface was studied by electron momentum spectroscopy. A thick disordered interface is formed if one material is deposited on the other. Electron momentum spectroscopy measures intensity as a function of binding energy and target electron momentum. Momentum resolution is demonstrated to be very helpful in interpreting the data, even for these disordered interfaces. The interface layer has a well‐defined electronic structure, different from either Si or Cu, and consistent with silicide formation. Information is obtained about the total bandwidth of the interface compound, effective Brillouin zone size and Fermi radius. No clear differences are observed in the electronic structure of the interface layer for Si deposited on Cu or Cu deposited on Si. Copyright © 2006 John Wiley & Sons, Ltd.     

A simple route to nanocrystalline silicon carbide

Nanocrystalline silicon carbide has been prepared via reacting magnesium silicide (Mg₂Si) with carbon tetrachloride (CCl₄) in an autoclave at 450-600°C. X-ray diffraction patterns of the products can be indexed as the cubic cell of SiC with the lattice constant, a=4.352 Å, in good agreement with a=4.349 Å (JCPDS card No. 75-0254). The transmission electron microscopy images show that the sample mainly consists of nanoparticles with an average size from 30 to 80 nm co-existing with a small fraction of nanorods and nanowires. Typically the nanorods range from 20 to 40 nm in diameter and the nanowires have diameters of 20 nm and lengths up to 10 μm. The Raman spectrum shows a characteristic sharp peak at 790 cm⁻¹. X-ray photoelectron spectra (XPS) gives an atomic ratio of Si to C as 1.08:1.00 from the quantification of the peak intensities. Photoluminescence spectrum reveals that the SiC sample emits ultraviolet light of 328 nm. A possible mechanism and the influence of temperature on the formation of crystalline SiC are proposed.     

Effect of ZnO on the interfacial bonding between Na2O–B2O3–SiO2 vitrified bond and diamond

Diamond composites were prepared by sintering diamond grains with low melting Na₂O–B₂O₃–SiO₂ vitrified bonds in air. The influence of ZnO on the wettability and flowing ability of Na₂O–B₂O₃–SiO₂ vitrified bonds was characterized by wetting angle, the interfacial bonding states between diamond grains and the vitrified bonds were observed by scanning electron microscope (SEM), and the micro-scale bonding mechanism in the interfaces was investigated by means of energy-dispersive spectrometer (EDS), Fourier transform infrared (FTIR) spectrometer and X-ray photoelectron spectroscopy (XPS). The experimental results showed that ZnO facilitated the dissociation of boron/silicon–oxygen polyhedra and the formation of larger amount of non-bridging oxygen in the glass network, which resulted in the increase of the vitrified bonds' wettability and the formation of –CO, –O–H and –C–H bonds on the surface of diamond grains. B and Si diffused from the vitrified bonds to the interface, and C–C, C–O, CO and C–B bond formed on the surface of sintered diamond grains during sintering process, by which the interfacial bonding between diamond grains and the vitrified bonds was strengthened.     

Structural and chemical characterisation of titanium deuteride films covered by nanoscale evaporated palladium layers

Thin titanium deuteride (TiD_y) films, covered by an ultra-thin palladium layer, have been compared with the corresponding titanium and palladium films using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TiD_y layers were prepared under ultra-high vacuum (UHV) conditions by precisely controlled deuterium sorption at 298 K on a Ti film evaporated onto a Si(100) substrate. Both Ti and TiD_y films were then covered in situ by a nanoscale Pd layer. It was found that a 10- to 12-nm-thick Pd layer protects the TiD_y films efficiently against extensive air interaction. The morphology of both the surface and bulk Pd/TiD_y (Ti) films have been observed using SEM and cross-sectional TEM analysis, respectively. A polycrystalline bulk morphology in both Ti and TiD_y films accompanied by a fine-grained Pd surface was observed. High-magnification cross-sectional TEM images reveal the TiD_y film to be plastically deformed leading to an increase in the roughness of the top Pd layer. Complex structures, including Moiré patterns, have been identified within the Pd/TiD_y interface. The chemical nature of this interface has been analysed after partial sputtering of the Pd top layer using XPS. Besides TiD_y and Pd, TiO and PdO were found to be the main chemical species in the interface region of the Pd/TiH_y film. The XPS valence-band spectra of the Pd/TiD_y interface reveal electronic features characteristic of a Pd–Ti bimetallic structure.