Si 2p Binding Energy Research Articles

XPS investigations of acid-base interactions in adhesion. Part 3. Evidence for orientation of carbonyl groups from poly(methylmethacrylate) (PMMA) at the PMMA-glass and PMMA-SiO 2 interfaces

The effect of the substrate acidity/basicity on the orientation of the carbonyl group (CO) from poly(methylmethacrylate) (PMMA) has been investigated by X-ray photoelectron spectroscopy (XPS). Analyses of the underside of PMMA films cast onto glass microscope slides, and silicon dioxide thermally grown on a silicon wafer, indicated an increasing intensity ratio O1s(CO)/O1s(OCH 3) with the substrate acidity. The XPS measure of substrate acidity was monitored by an O1s + Si2p binding energy parameter (∑BE) or an O1s - Si2p binding energy (ΔBE). Both ∑BE and ΔBE increase with the acidity of the pretreatment solutions (low pH). The PMMA XPS results suggest a preferential orientation of the carbonyl group at the interface when PMMA is solvent cast onto the substrate, a phenomenon which is most pronounced for the very acidic substrate. This orientation is most probably related to an interfacial acid-base interaction between the surface acidic sites of the substrate and the oxygen of the basic carbonyl group from PMMA.

Nonalloyed ohmic contacts to n-Si using a strained Si0.50Ge0.50 buffer layer

We have grown an 80-Å-thick strained Si0.50Ge0.50 layer on n-Si by molecular-beam epitaxy. The strained layer is used to lower the Schottky barrier height for making a nonalloyed shallow ohmic contact to the n-Si. X-ray photoelectron spectroscopy was employed to investigate the Si 2p and Ge 3d core-level binding energies of the strained and the relaxed Si0.50Ge0.50 and to determine their relative Fermi-level positions. Rutherford backscattering and Auger depth profiling were employed to determine the contact reactions using Ti, W, or Pt as contact metals. In the case of Pt, a 500-Å W diffusion barrier can protect the ohmic behavior up to 550 °C for 30 min. The specific contact resistance of the metal/Si0.50Ge0.50/n-Si contact extracted from the D-type cross-bridge Kelvin resistor was 3.5×10−5 Ω cm2.

Thermally grown Si3N4 thin films on Si(100): Surface and interfacial composition.

Synchrotron photoemission measurements of the Si(2p) and N(1s) levels have been made on ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ thin films grown in situ by high-temperature reaction of Si(100) with ${\mathrm{NH}}_{3}$. Surface sensitivity is enhanced in comparison to laboratory photoemission experiments by selecting photon energies that minimize the photoelectron mean free path in the nitride film. From the results, we are able to determine not only the types of chemical species present, but their approximate location within the film as well. Careful analysis of the Si(2p) photoemission spectra reveals the presence of a unique silicon species with a Si(2p) binding energy intermediate between elemental silicon and silicon in ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$. Furthermore, the persistence of this species with increasing nitride-film thickness supports its assignment to a monolayer of silicon at the outermost surface layer, on top of the growing stoichiometric ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ film. These surface silicon atoms can be distinguished from silicon atoms in intermediate oxidation states at the ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$/Si interface. The spectroscopic evidence for chemically distinct surface, nitride, and interfacial silicon is discussed in terms of the silicon nitridation mechanism.

XPS analysis of the interface of ceramic thin films for humidity sensors

MgAl 2O 4 thin films, deposited on Si/SiO 2 substrates, were studied as humidity sensors. This paper discusses the evaluation of the chemical composition at increasing depths, carried out by a combination of Ar + ion etching and XPS analysis. These analyses showed the simultaneous presence of Mg, Al and Si at the film/substrate interface. The thickness of the interfaces were calculated between 7 and 10 nm. The shift in the binding energies of the XPS peaks observed at the interface seems to demonstrate the occurrence of a chemical interaction between film and substrate. At the interface, Si 2p binding energy values are characteristic of a silicate, and this effect may be responsible for the good adhesive properties of MgAl 2O 4 films to silica, as demonstrated by peel tests with Scotch tape.

Nonalloyed Ohmic Contacts to N-Si Using a Strained Si0.5Ge0.5 Buffer Layer

ABSTRACTNonalloyed shallow ohmic contacts to n-Si have been fabricated by using an 80 Å thick strained Si0.5Ge0.5 buffer layer grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was employed to investigate the Si 2p and Ge 3d core level binding energies of the strained and the relaxed Si0.5Ge0.5 and to determine their relative Fermi level positions. It was found that the surfaces of strained Si0.5Ge0.5 exhibit pinning very close to the conduction band. Rutherford backscattering and Auger depth profiling were employed to determine the contact reactions using Ti, W or Pt as contact metals. In the case of Pt, a 500 Å W diffusion barrier can protect the ohmic behavior up to 550 °C for 30 min. The specific contact resistance of the Pt/W/Si0.5Ge0.5/n-Si contact extracted from the D-type cross-bridge Kelvin resistor was 3.5x10-5 Ω·cm2.

Analysis of photoemission in amorphous SiOx and SiNx alloys in terms of a charge-transfer model.

Shifts of the Si 2p, O 1s, and N 1s core-level spectra with the O or N content x for amorphous (a-) ${\mathrm{SiO}}_{\mathit{x}}$ and ${\mathrm{SiN}}_{\mathit{x}}$ films were examined by means of the effective-charge-analysis (ECA) model, in which the averaged partial charge ${\mathit{P}}_{\mathit{M}}$(x) (M=Si, O, or N) on a given atom is expressed as a function of x, by using Sanderson's electronegativity results and the random-bonding model (RBM). The effective Si 2p binding energy ${\mathit{E}}_{\mathit{B}}$(Si 2p) was found to be linearly related to ${\mathit{P}}_{\mathrm{Si}}$(x) per single bond, ${\mathit{E}}_{\mathit{B}}$(Si 2p)=10.2${\mathit{P}}_{\mathrm{Si}}$(x)+99.3, independent of sample type. This equation predicts that addition of a unit of positive charge onto a Si atom shifts the Si 2p core level downward by 2.55 eV. Furthermore, the ECA model showed that spacing of the Si 2p lines due to five Si(${\mathrm{Si}}_{4\mathrm{\ensuremath{-}}\mathit{n}}$${\mathit{M}}_{\mathit{n}}$) (M=O or N) bonding units based on the RBM decreases with increasing n, in contrast to an assumption made by many workers. The evaluated negative partial charge, ${\mathit{P}}_{\mathrm{O}}$(x) or ${\mathit{P}}_{\mathrm{N}}$(x), on an O or N atom is found to decrease with increasing x, and the observed ${\mathit{E}}_{\mathit{B}}$(O 1s) and ${\mathit{E}}_{\mathit{B}}$(N 1s) are also linearly related to ${\mathit{P}}_{\mathrm{O}}$(x) and ${\mathit{P}}_{\mathrm{N}}$(x), respectively. Both the O 1s and N 1s core levels were shown to shift downward by around 1.2 eV as a unit negative charge is subtracted from the O and N atom.

Correlations between XPS binding energies and composition of aluminasilicate and phosphate molecular sieves

Silica alumina and silicoaluminophosphate-based molecular sieves were investigated by means of X-ray photoelectron spectroscopy. For all network elements, only one symmetrical peak of the binding energy was observed, indicating a very homogeneous distribution of the electron densities around the atoms throughout the lattice. Nevertheless, the strongest effects are related to the most nearest-neighbor environment. The binding energies of Si2p, O1s, and AI2p in silica alumina molecular sieves increased with increasing concentration of aluminum. Phosphorus decreases the binding energy of the oxygen without a large effect upon either aluminum or silicon. The implication of these observations for the polar properties of molecular sieve networks and their thermal stabilities are discussed.

Chemical interaction at the Tm/Si(111) interface

Interface investigations of Tm/Si(111) have been carried out using XPS, UPS and glancing incidence XRD at various temperatures. Room temperature interface evolution takes place in three stages. Initially there is no reaction between adsorbate and substrate and Tm clusters are formed. In the second regime cluster coalescence takes place and there is a strong interaction between Tm and Si to form TmSi 2. Finally, metal-rich Tm 5Si 3 phase formation takes place. High-temperature studies reveal that at 473 K, predominantly the Tm 5Si 3 phase is present with some TmSi 2, while the film is completely converted into a disilicide phase by annealing at 673 K. Binding energies of Si 2p corresponding to various Tm silicides are identified. At all temperatures, throughout the interface development, Tm was found to be trivalent in nature.

Initial stages of oxidation of Si(111) with condensed O2 and N2O at 20 K.

The oxidation induced by soft-x-ray illumination of condensed multilayers of ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$O on cleaved Si(111) at 20 K has been investigated with high-resolution core-level spectroscopy. The results for ${\mathrm{O}}_{2}$/Si(111) show that a ${\mathrm{SiO}}_{2}$ phase is produced by illumination with as few as 4.3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ photons ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ of energy 130 eV. This ${\mathrm{Si}}^{4+}$ oxide is characterized by a Si 2p binding energy that is shifted 3.4 eV relative to bulk Si, and its effective thickness increases with illumination. Intermediate oxides having ${\mathrm{Si}}^{1+}$, ${\mathrm{Si}}^{2+}$, and ${\mathrm{Si}}^{3+}$ bonding configurations are also observed, and their total thickness also increases. The enhanced formation of these intermediate oxides is due primarily to the growth of ${\mathrm{Si}}^{3+}$ bonding configurations throughout the ${\mathrm{SiO}}_{2}$ layer. Exposure of the oxidized Si surface to white light from the synchrotron-radiation source or annealing to 300 K produces structural changes as the ${\mathrm{SiO}}_{2}$ layer thickens at the expense of the intermediate oxides. Hence, the defectlike ${\mathrm{Si}}^{3+}$ configuration converts to a ${\mathrm{SiO}}_{2}$ configuration and a sharper interface develops. Studies of ${\mathrm{N}}_{2}$O/Si(111) interactions during photon irradiation show that the initial oxidation rate is slower than with ${\mathrm{O}}_{2}$. After more extended illumination, the oxidation rate with ${\mathrm{N}}_{2}$O approximates that with ${\mathrm{O}}_{2}$ as ${\mathrm{N}}_{2}$O molecules are dissociated. The intermediate oxide formed with ${\mathrm{N}}_{2}$O is thicker than with ${\mathrm{O}}_{2}$, and extended growth of the intermediate oxide reflects the formation of both ${\mathrm{Si}}^{2+}$ and ${\mathrm{Si}}^{3+}$. From these ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$O results, we conclude that the oxidation rate for the submonolayer oxide is determined mainly by the availability of surface reaction sites and the cross section for electron capture by the condensed species. Further oxide growth is also limited by the existing oxide layer through which atomic oxygen must diffuse in order to react at the interface.

X‐ray Photoelectron Spectroscopy as a Tool to Differentiate Silicon‐Bonding State in Amorphous Iron Oxides

AbstractAn x‐ray photoelectron spectroscopy (XPS) study was conducted to: (i) compare the Si (2p) and O (1s) peaks of selected Si‐containing minerals, and (ii) evaluate the application of XPS in identifying probable Si‐bonding environments in Si‐containing ferrihydrites. For silica gel, the Si (2p) and O (1s) XPS peaks occurred at 104.0 and 534.2 eV, respectively; with biotite, these peaks occurred at 102.8 and 532.4 eV. The difference in the Si (2p) binding energies between silica gel and biotite was attributed to differences in the Si‐bonding environment and the number of ‐Si‐O‐Si‐ linkages at corners of the SiO4 tetrahedron, and the charge deficit in the tetrahedral layer of biotite. The Si (2p) XPS peak for silicate adsorbed on ferrihydrite occurred at 100.9 eV, which indicated the probable presence of ligand‐bound silicate or small units of polymerized silica at the ferrihydrite surface. For ferrihydrite samples treated with an excess of Si (≥75 g kg−1), peaks attributable to a separate Si‐rich phase were identified on decomposition of the Si (2p) and O (1s) XPS peaks. In the case of ferrihydrite coprecipitated with silicate at Si/Fe molar ratios ≥0.10, the Si (2p) peak occurred at approximately 102.8 eV, which is a position similar to that observed for layer silicates, indicating the possible presence of Si, i.e., the presence of ‐Fe‐O‐Sl‐ bonds, within the ferrihydrite structure.

An XPS study of amorphous MoO3/SiO films deposited by co-evaporation

X-ray photoelectron spectroscopic (XPS) core-level spectra of MoO3/SiO thin films are presented. The effects of changes in composition, substrate temperature during deposition and annealing on the binding energy of Mo(3d) and Si(2p) core lines in mixed films are compared with those of MoO3 and SiO. Appreciable changes in Mo(3d) peak positions and slight changes in Si(2p) peak positions are observed. The change in binding energy of the Mo(3d) doublet may be attributed to the effective incorporation of silicon ions in an MoO3 lattice which may cause the molybdenum orbital to be a little less tightly bound. This helps in the internal electron transfer from the oxygen (2p) to the molybdenum (4d) level as a result of which the molybdenum is readily changed to lower oxidation states during heat treatment. XPS spectra show that the position of the Si(2p) core state shifts monotonically with increasing oxygen concentration from the value of 101.8 to 102.6 eV.

Ion-assisted deposition of mixed TiO2-SiO2 films

Mixed thin films of TiO2 and SiO2 were produced by coevaporation from separate electron-beam sources and simultaneous bombardment of the growing film with oxygen ions. The optical properties of the films were determined during growth by in situ ellipsometry and the surface composition of the deposited films studied by in situ ion scattering spectroscopy, ex situ x-ray photoelectron spectroscopy, and energy filtered electron diffraction. The correlation between the optical and surface characterization is presented. There is evidence of local variations in the relative concentrations of TiO2 and SiO2. The position of the Si 2p binding energy depends on the TiO2 content in the film, indicating the possible formation of an intimate mixture.

Electron spectroscopic studies of the Dy/Si(111) interface

Low-energy electron diffraction (LEED), x-ray photoelectron (XPS), ultraviolet photoelectron (UPS), and Auger electron (AES) surface spectroscopic techniques were used to study the chemical interaction between Dy atoms and Si atoms at the Dy–Si interface at room temperature, as well as Dy silicides formed after annealing at 400 °C for 30 min. Experiment shows that at low coverages [≤4 monolayers (ML)] atom intermixing takes place, causing chemical shifts of Dy 4f and Dy 3d. However, no significant chemical shift of Si 2p has been observed, suggesting a lack of strong chemical bonds of the Dy–Si type. With coverage increasing, a (7×7) LEED pattern from a clean Si(111) surface turns into a (1×1), and finally becomes ambiguous. After annealing for 30 min at 400 °C, the Dy silicides formed with an atomic concentration ratio of Dy to Si of ∼1:2. The binding energies of Dy 4f (Dy Si2), Dy 3d(Dy Si2), and Si 2p(Dy Si2) are 5.5, 1295.2, and 98.65 eV, respectively.

X-ray photoelectron spectroscopy study of Si-C film growth by chemical vapor deposition of ethylene on Si(100)

The growth of a thin film of SiC grown by chemical vapor deposition (CVD) of ethylene on Si(100) at 970 K was studied by x-ray photoelectron spectroscopy (XPS). The growth of the film was observed through the behavior of the Si(2p) and C(1s) core levels and their plasmon losses. A 1.2-eV (towards higher binding energy) shift is observed for the Si(2p) binding energy between silicon in Si(100) and silicon in SiC. The plasmon loss energies measured as a function of film thickness below the C(1s) emission indicate that the C/Si ratio of the Si-C film throughout the CVD process is fairly constant.

Alloyed interface formation in the AuSi(111)2 × 1 system studied by photoemission spectroscopy

Room temperature deposition of gold (Au) on the Si(111)2×1 cleaved surface has been studied by photoelectron spectroscopy using synchrotron radiation. Binding energies of Si(2p) and Au(4f) and the splitting of the Au(5d) signal are changed with increasing coverage in a different manner below and above one monolayer coverage and reach to saturation values at 20–100 monolayers. The Au(4f) binding energy and the Au(5d) splitting at saturation are clearly different from those of pure Au metal. The origin of alloying in the AuSi(111)2×1 system is discussed from these results. A new model, “chemical bonding model” is proposed to explain the initial stage of the metallic overlayer formation in the AuSi(111) system.

Kinetic and X-ray Photoelectron Spectroscopic Studies of the Thermal Nitridation of Si(100)

ABSTRACTThe thermal nitridation of Si(100) by NH3 and N2H4 has been studied by X-ray photoelectron (XPS) and Auger electron (AES) spectroscopies. The pressure dependence of the rates as a function of reaction time has been measured. It has been found that the growth of the first monolayer (ML) of nitride is mediated by a surface reaction step. For subsequent growth, diffusion of one or more of the reacting species becomes an important process in determining the rate of reaction. Such species may be substrate Si diffusing to the vacuum/Si3N4 interface, or possibly network nitrogen diffusing into the Si substrate. The independence of the reaction rate on NH3 or N2H4 pressure at long reaction times rules out a mechanism involving molecular diffusion of the nitriding gas to the Si3N4/Si interface in a manner similar to the oxidation of Si by O2 or H2O. Careful analysis of the Si(2p) XPS spectra reveals the presence of a unique Si species with a Si(2p) binding energy intermediate between elemental Si and Si in Si3N4. Further, the relative intensity of the Si(2p) features due to this species, and the angular dependence of the XPS peaks indicate that they result from a ML of Si at the outermost surface layer, on top of the growing Si3N4 film.

Effects of Ion Implantation on Protein Adsorption onto Silicone Rubber

AbstractA study has been made on the surface wettability, atomic ratio, chemical structure, chemical bonding states and plasma protein adsorption of ion implanted silicone rubbers. C+-, N2+-, 02+-, Ar+- and Na+-ion implantations were performed at an energy of 150 keV at room temperature. The doses ranged from 1x1012 to 1x1017 ions/cm2. Ion implantation caused the surface roughness to increase by 1–5 times. Surface wettability was estimated by means of a sessile drop method using water. With increasing ion dose, the contact angle of water decreased from 98.9° to 48.5°. However, if the sample was in the air, the contact angle of water returned to its initial valve in time elapsed. The results of XPS measurements showed that implanted elements were incorporated in a gaussian like distribution and host elements were redistributed in the polymer matrix. No change of binding energies of 01s, CIS, Si2p and Si2p can be observed upon ion implantation. Results of FT-IR-ATR showed that C+-, N2+-, 02+-, and Ar+-ion implantation decomposed the original chemical bonds to form the new radicals. The amounts of new radicals are related to doses of implanted ions. In contrast, Na+-ion implantation hardly formed new radicals. The amount of albumin adsorbed onto 02+-, Ar+-, N2+-, and C+- ion implanted silicone is less than that on unimplanted specimens, but more fibrinogen is adsorbed. Na+-ion implantation produced an increase in the amount of adsorbed albumin as the dose increased. In summary, Na+ion implantation produces effects that are attributable to the additional implanted constituent in the surface of the silicone, and 02+-, N2+-, C+-, and Ar+-ion implantations primarily cause radiation effects on silicone rubber.

Structure of Sol-Gel Silic

AbstractCombined Raman, 29Si NMR and reactivity studies of silica gels indicate that dehydroxylation of the silica surface results in cyclotrisiloxane species with reduced Si-O-Si bond angles and altered acid/base characteristics. XPS experiments indicate that the expected 0.35 eV shifts in Si2p and O1s binding energies due to the reduced bond angles are hidden within broad peaks due to the remaining hydroxyls. Additional Si2P, OlS, and ClS peaks are also observed and are postulated to result from preferential adsorption of extrinsic C-containing species on sites with enhanced acid/base properties.

XPS, Auger study of Cu 3Si and its reaction with oxygen

The copper silicon alloy Cu 3Si (η phase) was investigated by XPS and Auger analysis. XPS binding energies of Si2p 3 2 = 99.4 eV and Cu2p 3 2 = 932.7 eV were found, identical to their values in pure elemental Si or Cu, respectively. The X-ray induced Cu LMM Auger peak was observed at kinetic energy of 914.1 eV, again similar to that found for pure copper metal. In the electron excited N( E) Auger spectrum the Si LVV for η phase displays a shoulder at 92.3 eV on the main Si peak (89.0 eV). This shoulder causes the derivative spectrum to split into two negative excursions at 90.3 and 94.3 eV. Cu 3Si was found to react readily with oxygen. The oxidation state of Cu in η phase was stable with respect to O 2 exposure. Silicon however, was preferentially oxidized to SiO 2 with a decrease in the surface Cu/Si ratio with exposure to O 2 and air. Exposure of Cu 3Si to air for two weeks produced oxide layers 3600 Å deep.

Compositionally Dependent Si 2p Binding Energy Shifts in Silicon Oxynitride Thin Films

The Si 2p binding energies measured from a series of silicon oxynitride thin films are related to the nitrogen contents of the films using a simple Pauling charge distribution model. The linear relation found between the binding energy and the calculated charge is expected if Si–N bonds replace Si–O bonds as the nitrogen content of the films is increased.