Si 1s Research Articles

Chemical-state analysis for low-dimensional Si and Ge films on graphite

Synchrotron radiation photoemission spectroscopy has been used to study the chemical interaction and the electronic structures of low-dimensional semiconducting Si and Ge on a graphite surface. The core level spectra, such as Si 1s, Ge 2p, and C 1s, show that no chemical interaction occurs between adatoms and the substrate atoms, and that the electronic structures of graphite remain intact upon deposition of the adatoms at room temperature. The Si 1s and Ge 2p photoemission spectra reveal that Si and Ge films on graphite remain in their elemental form. However, the electronic structures of both Si and Ge films are found to be thickness dependent. In the thicker films, such as Si 5.5 Å or Ge 4.2 Å, a single bulk-like component is detected. In the case of thinner films, such as Si 2.7 Å or Ge 0.3 Å, some additional components are observed at binding energies higher than the bulk-like component. We attribute these peaks to semiconductor nanostructures, for example, nanowires, and individual or polymerized chains consisting of nanoclusters.

Angle-Resolved Studies of SiO<sub>2</sub>/SiC Samples

Angle resolved studies of SiO2/SiC samples utilizing the Si 2p and Si 1s core levels and the Si KLL Auger transitions are reported. Samples with total oxide thicknesses from ca. 5 to 118 A are investigated. The data collected show that two oxidation states only, Si+1 and Si+4, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra. For all samples investigated the intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model when assuming that the sub-oxide, Si+1, is located at the interface. The SiO2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and moreover to depend on the thickness of the oxide layer.

High resolution-high energy x-ray photoelectron spectroscopy using third-generation synchrotron radiation source, and its application to Si-high k insulator systems

High-resolution x-ray photoelectron spectroscopy (XPS) at 6 keV photon energy has been realized utilizing high-flux-density x rays from the third generation high-energy synchrotron radiation facility, SPring-8. The method has been applied to analysis of high-k HfO2/interlayer/Si complementary metal–oxide–semiconductor gate-dielectric structures. With the high energy resolution and high throughput of our system, chemical-state differences were observed in the Si 1s, Hf 3d, and O 1s peaks for as-deposited and annealed samples. The results revealed that a SiOxNy interlayer is more effective in controlling the interface structure than SiO2. Our results show the wide applicability of high resolution XPS with hard x rays from a synchrotron source.

Valence band offset at silicon/silicon nitride and silicon nitride/silicon oxide interfaces

The valence band electronic structure of silicon nitride (Si 3N 4) is studied using the first principal quantum chemical calculation, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Assuming that the valence band is formed with N 2p and Si 3s, 3p and 3d electrons and based on the XPS and UPS results, the valance band offset at the Si/Si 3N 4 interface was estimated to be 1.5±0.2 eV. This hole barrier explains the hole dominating conduction in Si/Si 3N 4 structure when a positive potential is applied to the silicon substrate. In addition, the UPS study reveals that the valence band offset at the Si 3N 4/silicon oxide interface is 2.5±0.2 eV.

Kinetic Study for the Reactions of Si Atoms with SiH4

Rate constants for the reactions of Si (3P, 1D, and 1S) atoms with SiH4 have been determined using a combined laser photolysis/laser induced fluorescence technique. The reaction of Si(3P) with SiH4 is shown to proceed with a rate constant k(Si(3PJ) + SiH4) = (2.1±0.2) × 10-10 cm3 molecule-1 s-1, with no discernible pressure dependence over a pressure range of 5−20 Torr of N2 diluent. The rate constants of each triplet component of Si(3PJ=0,1,2) atoms exhibit no significant spin-orbit dependence within the experimental uncertainties. The results of the density functional theory calculations show that the reaction of Si(3P) atoms with SiH4 proceeds via a loose transition state. The Si(1D) + SiH4 reaction rate constant is determined to be (5.2 ± 0.3) × 10-10 cm3 molecule-1 s-1. The Si(1S) atom is less reactive with SiH4 than the Si(1D) atom, with an upper limit of k(Si(1S) + SiH4) = 4.7 × 10-13 cm3 molecule-1 s-1.

Band offset energies in zirconium silicate Si alloys

Transition metal silicates, (ZrO 2) x (SiO 2) 1− x , with dielectric constants, k>10 have been proposed as alternative dielectrics for advanced Si devices. Studies by X-ray absorption, X-ray photoelectron and Auger electron spectroscopy are combined to identify the compositional variation of the valence and conduction band offset energies with respect to Si in Zr silicate alloys. The minimum conduction band offset energy, associated with localized Zr 4d ∗-states, is ∼1.4 eV, and is independent of alloy composition, while valence band offsets decrease monotonically with increasing ZrO 2 content. Differences between the coupling of tunneling electrons to localized Zr 4d ∗ and extended Si 3s ∗ states, characterized by respective tunneling masses of ∼0.5 m o and ∼0.2 m o, combine to contribute to a minimum in the direct tunneling current in the mid-silicate-alloy composition range, x∼0.4–0.6.

Tetrakis(trimethylsilyl)cyclodisilthiane and -selenane

Reactions of 2,2-dichlorohexamethyltrisilane ( 1) with lithium sulfide or selenide furnishes (Me 3Si) 2Si(E) 2Si(SiMe 3) 2 (E=S ( 2), Se ( 3)). Both compounds were characterized by multinuclear NMR spectroscopy ( 1 H , 13 C , 29 Si , 77 Se ). The molecular structure of 2 is reported. 2 crystallizes in monoclinic space group P2 1/ n ( Z=2). The four membered ring Si 2S 2 is planar with an angle Si–S–Si of 83.32° resulting in a transannular Si–Si distance of 2.886 Å.

Properties of the SiO2/SiC interface investigated by angle resolved studies of the Si 2p and Si 1s levels and the Si KLL Auger transitions

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL2,3L2,3 Auger transitions from SiO2/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered √3×√3 reconstructed 4H–SiC(0001) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si+1 and Si+4, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra.The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si2O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å.The SiO2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO2 shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined.The relative final state relaxation energy ΔR(2p) is determined from the modified Auger parameter. This yields a value of ΔR(2p)=−1.7 eV and implies, for SiO2/SiC, a “true” chemical shift in the Si 2p level of only ≈0.4 eV for oxide layers of up to 10 Å thick.

Electronic structures of semiconducting alkaline-earth metal silicides

Electronic structures and densities of states of the following alkaline-earth metal (AEM) silicides have been calculated using the first-principle pseudopotential method, Mg 2Si, BaSi 2, Ca 2Si, and Sr 2Si. Mg 2Si and BaSi 2 were predicted to be indirect semiconductors while Ca 2Si and Sr 2Si were direct semiconductors. Estimated band gaps are about 50% or less of the observed values. The valence bands of these are composed of Si 3s, 3p as well as of Mg 3s, 3p, Ca 4s, 4p, 3d, Sr 5s, 5p, 4d or Ba 6s, 6p, 5d. The conduction bands are mainly composed of AEM. s, p, d states and the contributions of Si 3s, 3p are relatively small. Formation energy of stoichiometric Mg 2Si was rather successfully predicted. Energetics of the non-stoichiometry and the atomic site of dopants for Mg 2Si are also discussed.

Information depth and the mean escape depth in Auger electron spectroscopy and x-ray photoelectron spectroscopy

The information depth (ID) is a measure of the sampling depth for the detected signal in Auger-electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) while the mean escape depth (MED) is a measure of surface sensitivity. We report ID and MED calculations for Si 2s, Si 2p3/2, Cu 2s, Cu 2p3/2, Au 4s, and Au 4f7/2 photoelectrons excited by Mg Kα x rays. These calculations were made for various electron emission angles and for a common XPS configuration. Similar calculations were made for Si L3VV, Si KL23L23, Cu M3VV, Cu L3VV, Au N7VV, and Au M5N67N67 Auger transitions. The IDs and MEDs were derived from an analytical expression for the signal-electron depth distribution function obtained from a solution of the kinetic Boltzmann equation within the transport approximation. The ratios of the IDs and the MEDs to the corresponding values found if elastic-electron scattering were assumed to be negligible, RID and RMED, were less than unity and varied slowly with electron emission angle α for emission angles less than 50°. For larger emission angles, these ratios increased rapidly with α. For α⩽50°, average values of RID and RMED varied linearly with the single-scattering albedo, ω, a simple function of the electron inelastic mean-free path and transport mean-free path. For α=70° and α=80°, RID also varied linearly with ω but RMED showed a quadratic variation. The albedo is thus a useful measure of the magnitude of elastic-scattering effects on the ratios RID and RMED. As a result of the elastic scattering of the signal electrons, AES and XPS measurements at α=80° are less surface sensitive than would be expected if elastic scattering had been neglected. Conversely, AES and XPS measurements made for α⩽50° are more surface sensitive as a result of elastic-scattering effects.

Comparisons of practical attenuation lengths obtained from different algorithms for application in XPS

Practical effective attenuation lengths (EALs) are needed for the determination of overlayer-film thicknesses by X-ray photoelectron spectroscopy (XPS). We have investigated the reliability of calculations of practical EALs from four analytical formulae, three proposed by Nefedov and Federova and one proposed by Tilinin et al. for the signal–electron depth distribution function in XPS. Comparisons were made between practical EALs from these formulae and practical EALs obtained from Monte Carlo (MC) simulations for Si 2s, Si 2p 3/2, Cu 2s, Cu 2p 3/2, Au 4s, and Au 4f 7/2 photoelectrons excited by Mg K α X-rays. For a common XPS configuration in which the angle between the X-ray source and the analyzer axis was 54°, practical EALs were calculated for electron emission angles between 0° and 80° (with respect to the surface normal) and for overlayer-film thicknesses such that the substrate signal intensities were reduced to 10% of their values for an uncovered substrate at each emission angle. The average deviations between practical EALs from the four formulae and from MC simulations were 4.97% for the Tilinin et al. algorithm and 8.99%, 7.02%, and 5.79% for the three Nefedov and Federova algorithms. Practical EALs from the Tilinin et al. formula are thus generally more reliable that those from the Nefedov and Federova formulae. For a less-common XPS configuration in which the angle between the X-ray source and the analyzer axis was less than 10° and the emission angle is 0°, practical EALs from one of the Nefedov and Federova formulae were found to be more reliable than those from the Tilinin et al. formula.

Surface electronic states of Yb silicide ultrathin films studied with He metastable deexcitation spectroscopy

We studied the growth of Yb silicide films on Si(100) with Metastable Deexcitation Spectroscopy (MDS) assisted by Low Energy Electron Diffraction (LEED). Different surface structure phases are observed at progressively high annealing temperature. Annealing at T<550 °C produces Yb-rich disordered compounds, with metallic character shown on MDS. A 3×1 ordered phase is observed by low energy electron diffraction (LEED) after annealing of the films at about 640 °C and it is associated with the formation of a stable silicide. Correspondingly, electronic states assigned to the Yb 6s–5d hybridised band and to Si 3s and 3p bands in the compound are observed in MDS. A further increase in the annealing temperature causes the system to evolve rapidly. Above 700 °C, the films present a character similar to that of the Si substrate.

Laser spectroscopic study of the SiAr van der Waals complex

Laser fluorescence excitation spectra of the SiAr van der Waals complex, in the vicinity of the Si D°3←3P atomic resonance transition near 220.7 nm are reported. At low resolution, a single excited-state (v′,0) progression of bands terminating in a dissociation continuum is observed. Several weaker bands associated with many of these strong bands are found in scans at higher resolution. A transition to an excited Σ−3 state which correlates with the excited Si(3D°)+Ar asymptote was assigned, and a rotational and vibrational analysis of the observed bands was carried out. The dissociation energies of the Ω=0+ components of the ground X 3Σ− and excited Σ−3 states were determined [D0″=178.8±0.4 and D0′=122.5±0.4 cm−1]. Ab initio calculations of SiAr X 3Σ− and A 3Π electronic states correlating with the ground-state Si(3s23p2 3P)+Ar asymptote were also carried out. The potential energy curves of the definite-Ω states were computed and used to estimate the dissociation energy, rotational constant, and phenomenological spin–spin interaction in the X 3Σ− state. These parameters were found to be in reasonable agreement with the experimental determinations.

The role of the Si 3s3d states in the bonding of iridium silicides (IrSi, Ir3Si5 and IrSi3)

Si L2,3 soft-x-ray emission spectra of IrSi,Ir3Si5 and IrSi3 compounds have been takenand the valence band densities of states associated with themhave been analysed as functions of the concentration of Siatoms, the coordination number of Si-Ir in the iridiumsilicide compounds and the average distances of the Si-Si bondsin those compounds. The role of silicon 3s3d states in thebonding of the different iridium silicides is discussed. Allthe spectra show two regions, one associated with thelow-energy Si 3s states and another one associated withhigh-energy Si 3s3d states. The effects of the interaction between the Si and Ir atoms in the silicides are: (a) thedistribution of silicon states, even those associated with theleast energetic s electrons, are modified, suggesting that thes electrons play a role in the formation of the iridiumsilicides; (b) the energy bandwidth corresponding to the Si 3s3dstates increases up to 2.5 eV as the number of Si atoms thatinteract with the Ir atoms increases; and (c) there is asignificant contribution from the d electrons, the fraction ofthe states associated with them being at least 20% of thetotal of states associated with the 3s3d states.

Local electronic and geometric structures of silicon atoms implanted in graphite

Low-energy Si + ions were implanted in highly oriented pyrolitic graphite (HOPG) up to 1% of surface atomic concentration, and the local electronic and geometric structures around the silicon atoms were in situ investigated by means of the Si K-edge X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy using linearly polarized synchrotron radiation. The resonance peak appeared at 1839.5 eV in the Si K-edge XANES spectra for Si +-implanted HOPG. This energy is lower than those of the Si 1s→σ * resonance peaks in any other Si-containing materials. The intensity of the resonance peak showed strong polarization dependence, which suggests that the final state orbitals around the implanted Si atoms have π *-like character. It is concluded that the σ-type Si–C bonds produced by the Si +-ion implantation are nearly parallel to the graphite plane, and Si x C phase forms two-dimensionally spread graphite-like layer with sp 2 bonds.

Electron effective attenuation lengths for applications in Auger electron spectroscopy and x‐ray photoelectron spectroscopy

AbstractThe electron effective attenuation length (EAL) can be defined for the measurement of overlayer film thicknesses (the most common application) and for the measurement of the depth of a thin marker layer by Auger electron spectroscopy (AES) and x‐ray photoelectron spectroscopy (XPS). For each of these applications, a local EAL can be defined for a narrow range of thicknesses or depths and a practical EAL can be defined for specified larger ranges of thicknesses or depths. We show the results of illustrative calculations of these different EALs for XPS of Si 2p3/2 photoelectrons with Mg Kα x‐rays, and point out situations where numerical differences occur. We compare practical EALs (with the overlayer film definition) for Si 2s, Si 2p3/2, Au 4s and Au 4f7/2 photoelectrons that were obtained from Monte Carlo simulations and from an analytical expression derived from solution of the Boltzmann equation within the transport approximation. The average of the percentage deviations between the EALs from the two approaches for various overlayer thicknesses and emission angles was 5.28%. This average percentage deviation was considered to be acceptably small and justifies the use of the more approximate analytical expression in calculations of practical EALs for the measurement of overlayer thicknesses by AES and XPS. This computational approach is much faster than Monte Carlo simulations. We report calculations of practical EALs for the principal photoelectron and Auger electron lines of Si, Cu, Ag and W. For overlayer thicknesses of practical relevance, the practical EALs do not change appreciably with film thickness for electron emission angles (with respect to the surface normal) of &lt;60°. Under these conditions, average practical EALs can be used to determine overlayer film thicknesses. For emission angles of &gt;60°, the practical EALs generally change rapidly with film thickness and emission angle and, for these conditions, an EAL should be determined for the particular conditions. We have compared our practical EALs for photoelectron and Auger electron lines of Si, Cu, Ag and W with similar EALs derived from Monte Carlo simulations by Cumpson and Seah. At an emission angle of 45°, we find a root‐mean‐square deviation of 1.6% between the EALs from the two approaches. Published in 2002 John Wiley &amp; Sons, Ltd.

Surface alteration of plagioclase during acid dissolution

To examine the dissolution mechanism of silicate minerals in acidic solution, XPS was applied to plagioclase samples (Na 0.5 Ca 0.5 Al 1.5 Si 2.5 O 8 ) treated with 5 × 10 -2 , 5 x 10 -4 or 5 x 10 -6 mol l -1 H 2 SO 4 for 3 h, 1 day and 1 week. The atomic abundances of Na, Ca and Al relative to Si in the surface layer of acid-leached (5 x 10 -2 mol l -1 H 2 SO 4 ) plagioclase decreased with increasing reaction time. The Si 2s and O is photoelectron spectra were shifted to higher binding energies during dissolution, and their binding energies were comparable to those of quartz after treatment for 1 week. From the preferential leaching of Na, Ca and Al, and the chemical shifts of the Si 2s and O is spectra, the formation of hydrous silicon dioxide on the mineral surface was deduced. The surface chemical composition of plagioclase changed in the same manner during dissolution by H 2 SO 4 at a low concentration (5 × 10 -4 mol l -1 ), although the magnitude of the decrease in surface concentrations of Na, Ca and Al was lower, suggesting the formation of a thinner altered surface layer. In addition, Al was less depleted than Na or Ca. Thus Na and Ca are more susceptible to acid leaching than Al. However, after dissolution in 5 x 10 -6 mol l -1 H 2 SO 4 , the relative surface abundance of Al was slightly higher than that of the untreated sample. Surface enrichment in Al is probably associated with the formation of hydrous oxides of Si and Al.

In situ analysis using synchrotron radiation photoemission spectroscopy for initial oxidation of oxygen preadsorbed Si(001) surfaces induced by supersonic O2 molecular beams

By means of synchrotron radiation photoemission spectroscopy we have performed in situ analyses for initial oxidation of oxidized Si(001) surfaces induced by supersonic O2 molecular beams. In the Si 2p, Si 2s and valence band photoemission spectra with an angle integrated mode, the O2 chemisorption processes induced by the O2 translational energy of 3.0 eV were clarified as a function of the exposure time. The Si2+ oxidation states, as well as Si1+, rapidly increased and then gradually decreased as a function of the O2 dose. However, Si4+ continued increasing gradually from the initial exposure. We found that the O2 incident energy of 3.0 eV directly induced oxidation in the backbond of the second Si layer. Copyright © 2002 John Wiley & Sons, Ltd.

Plasmon energy shift in porous silicon measured by x-ray photoelectron spectroscopy

In order to provide experimental support for quantum confinement models describing electronic effects in porous silicon (p-Si), the Si 2s and Si 2p plasmon losses have been studied by x-ray photoelectron spectroscopy. The p-Si plasmon energy was found at a value 0.8÷1.6 eV higher than that of bulk Si (17.4 eV), as measured on the cleaned Si substrate as a reference. The magnitude of these shifts suggests possible quantum confinement effects ascribed to the p-Si nanostructures.

Silicon-lead chalcogenides of the types Me 4Si 2(E) 2PbPh 2 and Ph 2Pb(E) 2Si 2Me 2(E) 2PbPh 2 (E=S, Se) and related compounds containing tin and antimony

The reaction of a 1:1 mixture of Ph 2PbCl 2 and ClSiMe 2–SiMe 2Cl with H 2S/NEt 3 yielded the mixed silicon–lead sulfide Me 4Si 2(S) 2PbPh 2 ( 1a), a bicyclic silicon lead sulfide, Ph 2Pb(S) 2Si 2Me 2(S) 2PbPh 2 ( 1b) was obtained by similar treatment of a 2:1 mixture of Ph 2PbCl 2 and Cl 2SiMe–SiMeCl 2. The corresponding selenium compounds ( 2a– b) were obtained by reactions of mixtures of Ph 2PbCl 2 and methylchlorodisilanes with Li 2Se in THF. All products were characterized by multinuclear ( 1H, 13C, 29Si, 77Se and 207Pb) NMR spectroscopy. The molecular structure of 1a is reported revealing a central five membered ring Si 2S 2Pb in envelope conformation with one sulfur atom (S1) above the plane defined by the atoms Pb1S2Si2Si1. For comparison, the tin compounds Me 4Si 2(Se) 2SnPh 2 ( 4a), and Ph 2Sn(Se) 2Si 2Me 2(Se) 2SnPh 2 ( 4b) have also been prepared essentially applying the same procedure as for compounds 2a– b. A plot of δ ( 207Pb) of 1a– 2b versus δ ( 119Sn) of the corresponding tin compounds 3a– 4b exhibits a linear correlation with a slope of 4.11 (±0.17). Attempts to build related cycles containing a Group 15 element led to the isolation of the antimony compounds Me 4Si 2(E) 2SbPh ( 5a: E=S, 5b: E=Se) starting from ClSiMe 2–SiMe 2Cl, PhSbCl 2 and either H 2S/NEt 3 or Li 2Se.