Si K-edge Research Articles

Effect of substrates on the molecular orientation of silicon phthalocyanine dichloride thinfilms

Molecular orientations of silicon phthalocyanine dichloride(SiPcCl2) thin films deposited on three different substrates have been measured by near-edge x-rayabsorption fine structure (NEXAFS) spectroscopy using linearly polarized synchrotron radiation.The substrates investigated were highly oriented pyrolitic graphite (HOPG), polycrystallinegold and indium tin oxide (ITO). For thin films of about five monolayers, the polarizationdependences of the Si K-edge NEXAFS spectra showed that the molecular planes ofSiPcCl2 on three substrates were nearly parallel to the surface. Quantitative analyses ofthe polarization dependences revealed that the tilted angle on HOPG was only2°, whichis interpreted by the perfect flatness of the HOPG surface. On the other hand, the tilted angle on ITOwas 26°. Atomic force microscopy (AFM) observation of the ITO surface showed that the periodicity of thehorizontal roughness is of the order of a few nanometres, which is larger than the molecular size ofSiPcCl2. It isconcluded that the morphology of the top surface layer of the substrate affects the molecular orientationof SiPcCl2 molecules not only for mono-layered adsorbates but also for multi-layered thinfilms.

X-ray absorption spectroscopy study at the Si K-edge of tungsten carbide–silicon carbide thin films

X-ray absorption spectroscopy (XAS) has been used to determine the compositional and structural nature of the phases formed in sputter-deposited W–Si–C films. The Si K-edge was examined to determine the presence and crystallinity of the SiC phase. For a film composition of 22% SiC, the XAS results show that there is at best only a minor amount of crystallization of SiC when deposited at 350 °C, but crystallization becomes more definitive at 600 °C. For a film with 29% SiC deposited at 350 °C, no crystalline SiC was detected.

Determination of the local environment of silicon and the microstructure of quaternary CrAl(Si)N films

Nanocomposite CrAlSiN compounds prepared by the cathodic arc evaporation technique were subjected to structural and mechanical characterization tests. X-ray diffraction, X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) were employed to investigate the effects of Si addition on the structure and phase development of the metastable NaCl structure of high aluminum CrAlN films. TEM studies revealed that partial substitution of the metal component by Si in CrAlN results in the nucleation of a wurtzite h-AlN phase even for amounts of silicon as low as ∼2–3 at.%. XAS measurements at the Cr and Si K-edges indicated that the local environment of Cr atoms is strongly affected by the Si addition, and that silicon may also be part of the crystalline phase. These results indicate the formation of complex Cr–Si–X compounds, where X can be N, Al or both, and the formation of composite nanocrystalline CrAlSiN films.

Characterization of heterogeneous SiO2 materials by scanning electron microscope and micro fluorescence XAS techniques

Micro X-ray absorption near edge structure XANES and micro fluorescence experiments have been carried out using X-ray microbeam from synchrotron radiation source with high brightness to investigate the local structural evolutions of heterogeneous and natural SiO2 submitted to alkali-silica reaction ASR process. Compared to elemental maps obtained by Environmental Scanning Electron Microscope ESEM, micro fluorescence X maps showed the diffusion of potassium cations inside the grains with higher accuracy. Si K-edge spectra show the disorder induced by the dissolution of the grain from the outside to the inside. Potassium K-edge spectra do not show significant changes around K cations. The breaking of Si–O–Si bonds and the disorder of the (SiO4)n network may be affected to potassium cations.

Photooxidation of plasma polymerized polydimethylsiloxane film by 172nm vacuum ultraviolet light irradiation in dilute oxygen

Plasma polymerized polydimethylsiloxane films irradiated under different partial pressures of oxygen with a 172nm vacuum ultraviolet light were investigated in order to clarify the roles of molecular oxygen and photons in photooxidation. The thickness, densities, surface roughness, elemental compositions, and molecular structures of the irradiated and unirradiated films were examined by using glazing incidence x-ray reflectivity, Rutherford backscattering, infrared, and x-ray absorption (XAS) spectroscopies. Photooxidation is hardly promoted by irradiation in a high vacuum of 1×10−4Pa, though photodesorption of the methyl group and formation of Si–H bonds were observed. Silica films thicker than 140nm were formed at room temperature by irradiating them in low pressure oxygen gases. The degree of oxidation was smaller for the oxygen pressure of 10kPa than for 83Pa. Si K-edge XAS was performed to clarify the change of coordination environment of silicon by photooxidation in dilute oxygen flow containing less than 5ppm of molecular oxygen.

Local bonding in PECVD-SiOxNy films

In this work, we report studies on the Si, N and O local bonding in silicon oxynitride films deposited by PECVD technique, with chemical compositions varying from SiO2 to Si3N4, utilizing N2O+N2+SiH4 and N2O+NH3+SiH4 as precursor gaseous. The films are characterized by X-ray absorption near edge structure (XANES) at the Si-K, N-K and O-K edges, in order to determine the local structure and by Rutherford backscattering (RBS) spectroscopy to obtain their chemical composition. The results at the Si-K edge show, for the samples with lower nitrogen content, a homogeneous structure compatible with a random distribution of Si–N and Si–O bonds (random bonding model) and for the samples with higher nitrogen incorporation a structure composed by two phases compatible with the random mixture model (RMM). On the other hand, the analysis of the results at the N-K edge demonstrates two features, which are attributed to two different nitrogen sites, a trigonal Si3N4-like site and SiO2-like sites, corresponding to nitrogen substituting O in Si–O–Si bridges, which originate a resonant line (RL) in the XANES spectra at approximately 399.1eV. Finally, the results at the O-K edge show only one edge, attributed to O–Si bonds, independently of the N and O concentration, for both gaseous mixtures.

Photon stimulated ion desorption from condensed chloroform on Si(100) at the Cl and Si K-edges

Photon stimulated ion desorption (PSID) studies have been performed in chloroform condensed on Si(100) at ∼85 K using synchrotron radiation at the chlorine and silicon 1s-edges. Data acquisition was performed at the Brazilian synchrotron light source (LNLS), during a single-bunch operation mode of the storage ring. It was found that hydrogen ion is the main ionic species to desorb after photon excitation followed by other fragments with lower intensities. Desorption ion yields for H + and Cl + were obtained as a function of the photon energy. A pronounced enhancement of the Cl + yield was observed at the first Cl 1s resonance, which may be related to a spectator Auger process. In order to verify the influence of the substrate in the ionic desorption of condensed chloroform, PSID spectra were also measured at the silicon 1s-edge. The indirect X-ray induced electron stimulated desorption (XESD) mechanism seems to dominate.

Study of the Effects of Si Addition on the Properties of Hard Nanocomposite Thin Films

AbstractIn the present study, hard TiSiN coating compounds with different Si content have been deposited by the magnetron sputtering technique. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) have been employed to investigate the effects of Si addition on the composition, structure and phase development of the cubic NaCl-type structure. To get some insight on these changes, X-ray absorption spectra have been measured at the Ti and Si K-edges in fluorescence yield mode. The collected data showed that the interaction between Ti and Si was weak and the formation of a-Si3N4 can be assumed. Hardness measurements were also consistent with the formation of hard nanocomposite films. A maximum hardness of about 36 GPa was obtained for the sample with 11 at. % Si.

Organosulfur-Functionalized Au, Pd, and Au−Pd Nanoparticles on 1D Silicon Nanowire Substrates: Preparation and XAFS Studies

A hybrid preparative method was developed to prepare organosulfur-functionalized Au nanoparticles (NPs) on silicon nanowires (SiNWs) by reacting HAuCl(4) with SiNW in the presence of thiol. A number of organosulfur molecules-dodecanethiol, hexanethiol, 1,6-hexanedithiol, and tiopronin-were used to functionalize the Au surface. Size-selected NPs ranging from 1.6 to 7.5 nm were obtained by varying the S/Au ratio and the concentration of HAuCl(4). This method was further extended to the preparation Pd and Pd-Au bimetallic NPs on SiNWs. The morphology of the metal nanostructures was examined by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The local structure and bonding of the SiNW-supported metal nanostructures were studied using X-ray absorption fine structures (XAFS) [including both X-ray near-edge structures (XANES) and extended X-ray absorption fine structures (EXAFS)] at the Au L(3)-, Pd K-, S K-, and Si K-edges. It was also found that the annealing of the thiol-capped Au NPs up to 500 degrees C transforms the surface of the thiol-capped NPs to gold sulfide, as identified using Au L(3)- and S K-edge XANES. We also illustrate that this preparative approach can be used to form size-controllable Au NPs on carbon nanotubes.

EXAFS study on dynamic structural property of porous morph-genetic SiC

Novel porous morph-genetic silicon carbide has been fabricated through sintering treatment, after infiltrating the methyl organic silicone resin to the bio-template. Its dynamic transition of structure during sintering process is investigated by extended X-ray absorption fine structure (EXAFS) for the first time. By analyzing Si K-edge EXAFS, it is found that the coordination number of the nearest C shell remains almost unchanged while that of the nearest Si shell dramatically changes when the structure is transformed from amorphous into crystalline state.

Preparation and characterization of pulsed laser deposition (PLD) SiC films

Si K-edge XAFS was used to characterize a stoichiometric SiC film prepared by pulsed KrF laser deposition. The film was deposited on a p-type Si(1 0 0) wafer at a substrate temperature of 250 °C in high vacuum with a laser fluence of ∼5 J/cm2. The results reveal that the film contains mainly a SiC phase with an amorphous structure in which the Si atoms are bonded to C atoms in its first shell similar to that of crystalline SiC powder but with significant disorder.

XANES and XPS Study of Silica-Modified Titanias Prepared by the Glycothermal Method

Silica-modified titania with the anatase structure having high thermal stabilities was prepared by the glycothermal method, and XANES and XPS techniques were applied for investigating the local structure of Si atoms in these samples. For comparison, two silica-modified titania samples were prepared by impregnation of TEOS or silica sol on titania. The surface concentration of Si atoms in the glycothermal product, as determined by XPS, was smaller than those of the reference samples prepared by the impregnation method, suggesting that in the glycothermal product silicon atoms are highly dispersed in the anatase matrix. The Si K-edge XANES spectra of the glycothermal products exhibited a main peak at 1847.1 eV due to the tetrahedrally coordinated Si atoms. However, a shoulder peak at 1849 eV was detected, which was attributed to the octahedrally coordinated Si atoms. The proportion of the latter component in the glycothermal product was larger than those in the reference samples and became large as the Si content decreased. These results clearly indicate the formation of the silica−titania solid solution. The thermal stabilization effects of Si atoms are discussed in detail.

Measurement of luminescence from silica glasses: an optical x-ray absorption fine structures study at Si K-edge

Soft x-ray excited optical luminescence (XEOL) of silica glasses was studied with respect to the excitation x-ray energy (near the Si K-edge) and irradiation time. The luminescence spectra showed emission bands at 3.1 eV and 2.7 eV, assigned to B2β and B2α oxygen deficient centers in silica glasses, and the luminescence yields of these bands changed drastically before and above the Si K-edge. Si K-edge photoluminescence yield (PLY) spectrum of the band at 3.1 eV was similar to Si K-edge XANES spectrum recorded in the photocurrent mode for the same silica sample, except for the negative edge jump in the PLY spectrum.The time evolutions of the band intensity at 3.1 eV were also investigated under the excitation by soft x-ray below and above Si K-edge. The band intensity in both cases decreased slightly with the irradiation. These results indicate that a part of B2β centers in the silica glass probably changed to other types of oxygen deficiencies by the electron excitation effect. In the early stage of the irradiation, a rapid increase in the intensity of the 3.1 eV band was also observed, indicating that soft x-ray irradiation produced luminescence sites such as B2β centers.

Local structure of six-coordinated silicon in Li2O-SiO2-P2O5 glasses by transmission XAFS experiments

Six-coordinated silicon [6]Si was observed by Si and P K-edge XAFS measurements in xLi2O-ySiO2-(100-y)P2O5 glasses (x = 25 and 33, y = 40, 45, 50, 55, and 60). Si K-edge EXAFS spectra of the glasses indicated the co-existence of [6]Si and four-coordinated silicon [4]Si. The proportion of octahedral [SiO66-] to [6]Si depends upon the alkali content and the [SiO66-]/[SiO44-] tetrahedral ratios were estimated by Si K-edge XANES spectra. The interatomic distances were 1.88–1.90 Å for Si-O and 1.55–1.61 Å for P-O and the amounts of [SiO66-] were 11.8–46.9%.

Soft x-ray XAFS experiments of SiO2-P2O5 sol-gel film

Structural evolutions in a heat-treatment process have been studied using Si and P K-edge XAFS in SiO2-P2O5 sol-gel layers on silicon substrates. The Si K-edge EXAFS results indicate that structural changes occur between 1173 K and 1273 K. In the case of heat-treated samples at 1273 K, the Si-O interatomic distance is 1.63–1.64 Å. On the other hand, the distances are 1.66–1.68 below 1273 K. Furthermore, structural properties around Si and P in the sol-gel process have been studied. In the XAFS spectra, the ratio of P to Si shows 84% decrease after heat-treatment above 1073 K. For the Si-O and P-O distances, wet-gel has shorter interatomic distance than sol-gel raw solution, TEOS and TEP, respectively.

XAFS study on silica glasses irradiated by high-energy particles

X-ray absorption technique was applied to analyze the local structures of fused and synthesized silica glasses before and after irradiation in a nuclear reactor. Si K-edge EXAFS revealed changes in the SiO4 tetrahedron structure by the irradiation, i.e. a distorted tetrahedron in the silica glass returned to more regular one but the connection between neighboring tetrahedrons became looser. In addition, the detailed analysis of EXAFS spectra gave evidence for the formation of direct Si-Si bonding after prolonged irradiation, suggesting the appearance of Si precipitates in silica glass.

Na-, Al-, and Si K-edge XANES study of sodium silicate and sodium aluminosilicate glasses: influence of the glass surface

X-ray Absorption Near Edge Structure (XANES) experiments were performed at the O-, Na-, Al-, and Si K-edges in sodium silicate and aluminosilicate glasses with a NBO/T (non-bridging oxygen per tetrahedron) ratio ranging from 0 to 1. In order to better understand how the sample surface can affect the measured XANES spectra, special attention was paid to the glass surface (i.e., fresh vs. “old” surfaces but also surfaces vs. powders) and to the detection mode (fluorescence vs. electron yield). We observe that, at the Na K-edge, XANES experiments on glass compositions with high Na/Al ratios must be performed on highly “fresh” surfaces (i.e., at most a couple of hours old). In these compositions, the collected XANES may show features that are relevant to major structural reorganization near the surface (carbonate-like re-organizations). In contrast, highly polymerized glasses (such as albitic glass, Na 2O–Al 2O 3–6SiO 2) are much less sensitive to long-term exposure to atmospheric agents (namely water and CO 2). A study of a 30-year-old albite glass and a natural rhyolitic glass confirms these observations, suggesting that these glasses are highly durable.

A Si K-edge XANES study of Ti containing alkali/alkaline-earth silicate glasses

Si K-edge X-ray absorption near edge structure (XANES) spectra were collected from Ti-bearing alkali and alkaline-earth silicate glasses with metasilicate (R 2O/SiO 2) compositions where R=Na 2O, K 2O and CaO. The Si K-edges of the Na 2SiO 3, K 2SiO 3, and CaSiO 3 glasses containing TiO 2 are observed at ∼0.8, ∼0.9, and ∼1.0 eV lower energy than that of silica glass (a-SiO 2), respectively, and shift to higher energy with added TiO 2. This indicates that the glasses are less polymerised than a-SiO 2 but become more polymerised with increasing TiO 2. Ti is predominantly acting as a network former and this is reflected at the local Si environment as an increase in polymerisation. A broad resonance near ∼1864 eV is related to the average Si–O–Si angle and/or the Si–O bond strength. In addition, a resonance observed at ∼2–5 eV above the absorption edge is located at a similar energy to one assigned to [6]Si in silicophosphate glasses and may interfere with resonance area calculations used to quantify [6]Si in those glasses. The presence of this resonance indicates changes in the median range structure; in particular, changes in the oxygen environment surrounding the Si atoms.

Evidence of clusters size-dependent photoluminescence on silicon-rich silicon oxynitride films

This work reports on the growth of silicon clusters within silicon oxynitride (SiO x N y ) matrices, at low temperatures, by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The films were characterized by Fourier transform infrared spectroscopy (FTIR), soft X-ray absorption spectroscopy (XANES) measurements at the Si K-edge and photoluminescence, before and after thermal annealing at various temperatures. The XANES spectra demonstrate the presence of silicon aggregates in as-deposited and annealed films. A correlation between the presence of silicon clusters in the as-deposited samples and the intensity of the photoluminescence emission is observed.

Structures of sub-monolayered silicon carbide films

The electronic and geometrical structures of silicon carbide thin films are presented. The films were deposited on graphite by ion-beam deposition using tetramethylsilane (TMS) as an ion source. In the Si K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra for sub-monolayered film, sharp peaks due to the resonance from Si 1s to π*-like orbitals were observed, suggesting the existence of Si=C double bonds. On the basis of the polarization dependencies of the Si 1s → π* peak intensities, it is elucidated that the direction of the π*-like orbitals is just perpendicular to the surface. We conclude that the sub-monolayered SiCx film has a flat-lying hexagonal structure of which configuration is analogous to the single sheet of graphite.