Si 2p Core-level Spectra Research Articles

Void and Nanostructure Formations during Thermal Decomposition of 20-nm-Thick Silicon Oxide Layer on Si(100)

We investigate the thermal decomposition of 20-nm-thick silicon oxide layers on Si(100) at a temperature of 1050 °C in vacuum by X-ray photoelectron spectroscopy, scanning Auger electron microscopy, and atomic force microscopy. Time evolutions of chemical-shift components in Si 2p core-level spectra are analyzed with the SiO2-decomposition model based on void formation. The experimental data can be reproduced well if the void area is proportional to the square of the annealing time, being consistent with the model in which the reaction at the void periphery is the rate-limiting step for void growth. Microscopic images show that the void periphery is square with rounded corners, and many silicon nanostructures are formed inside the void. These observations reflect reaction processes of the thermal decomposition of the silicon oxide layer.

Near-Ideal Complete Coverage of CD3 onto Si(111) Surfaces Using One-Step Electrochemical Grafting: An IR Ellipsometry, Synchrotron XPS, and Photoluminescence Study

A one-step electrochemical grafting process using Grignard reagents has been performed to achieve a complete monolayer methyl-terminated Si(111) surfaces. Anodic treatment (0.5 mA/cm2 for 300 s) has been applied to atomically flat H-terminated Si(111) surfaces in methylmagnesium bromide (CH3MgBr), methylmagnesium iodide (CH3MgI), and methyl-d3-magnesium iodide (CD3MgI) to obtain methylated Si(111) surfaces. Infrared spectroscopic ellipsometry (IRSE) clearly reveals a vibrational shift of the symmetric “umbrella” mode characteristic for methyl groups (CH3 and CD3) with a preferential z-orientation. Additionally, X-ray photoelectron spectroscopy using synchrotron radiation (SXPS) shows a well-defined splitting of the Si 2p core level spectra from the methylated Si(111) surfaces. This splitting is more pronounced for the CD3-terminated Si(111) surfaces. Moreover, the C 1s spectra also confirm the well-defined structure of the CD3-terminated Si(111) surfaces by the presence of C–D vibrational stretching features. Photoluminescence (PL) measurements reveal better surface passivation in the case of CD3-terminated Si(111) surfaces. Finally, the quality of the surfaces depends strongly on the counterions. Grignard reagents containing iodine show the best performance in the formation of complete monolayer methylated Si(111) surfaces.

Changes in structural and electronic properties of graphene grown on 6H-SiC(0001) induced by Na deposition

The effects of Na deposited on monolayer graphene on SiC(001) were investigated by synchrotron-based photoelectron spectroscopy and angle resolved photoelectron spectroscopy. The experimental results show that Na prefers to adsorb on the graphene layer after deposition at room temperature. Nonetheless, part of the Na atoms are able to intercalate in between the graphene and the buffer layer and some go even further into the substrate interface as indicated by the shift of the bulk SiC component in the C 1s and Si 2p core level spectra. The ARPES spectrum exhibits a lowering of the Dirac point indicating increased n-type doping of the monolayer graphene induced by the deposited Na atoms. Upon subsequently heating the sample, we found that a slightly elevated temperature is essential in order to promote Na intercalation. A fully Na intercalation at the graphene-SiC interface is obtained after heating at a temperature of about 75 °C. The intercalated Na decouples the buffer layer and transforms it into a second graphene layer so two π-bands are observed in the ARPES spectra. Interestingly, the two bands show different locations of the Dirac point but both exhibit linear dispersion in the vicinity of the K¯ point and not the hyperbolic dispersion observed for AB stacked bi-layer graphene. When heating the sample to about 125 °C or higher, Na is found to leave the interface and the second graphene layer is transformed back to the carbon buffer layer.

A Low Dielectric Study on Hybrid Plasma-Polymer Thin Films of Different Ratio Between Toluene and TEOS

Organic-inorganic hybrid co-polymer thin films were deposited on silicon(100) substrates under the several ratio of TEOS (tetraethoxysilane) against toluene by plasma enhanced chemical vapour deposition (PECVD) method. Toluene and TEOS were utilized as organic and inorganic precursors each, and hydrogen and argon were also used as a bubbler and carrier gases, respectively. In order to compare the difference of the electrical and the mechanical properties of the plasma polymerized thin films, we grew the hybrid co-polymer thin films under the conditions of various ratio between toluene and TEOS with fixed RF (radio frequency using 13.56 MHz) powers at 30 W. The as-grown polymerized thin films were in first analyzed by FT-IR and XPS. The result of FT-IR showed that the co-polymer thin films were polymerized with fragmented each precursor. Also, XPS results showed the chemical species and binding energies of each species. Si 2p core-level spectra from the hybrid polymer thin film showed the status of Si oxidation number. Impedance analyzer was utilized for the measurements of I-V curves and capacitance values. Also, the thin films were analyzed for hardness and Young's modulus by nano-indenter.

Synchrotron photoemission studies on reconstructed strained surfaces

Recently, based on scanning tunneling microscopy studies of the reconstructed Si(5 5 12)−2×1 surface, it has been suggested that its unit cell simply consists of four kinds of one-dimensional (1D) structures: π-bonded (π) chain, honeycomb (H) chain, tetramer (T) row, and dimer-adatom (D-A) row. In the present study, by angle-resolved ultraviolet photoelectron spectroscopy, it has been found out that the Si(5 5 12)−2×1 surface has two kinds of surface states, one with a negligible dispersion originating from row structures (T/D-A) and the other with a strong dispersion originating from chain structures (π/H). Also, the Si 2p core-level spectrum shows at least two kinds of surface components, one with 0.23 eV higher binding energy originating from upward-relaxed surface atoms and subsurface atoms, and the other with 0.52 eV lower binding energy originating from downward-relaxed surface atoms. It can be realized that these spectroscopic results quantitively match with the structural model of Si(5 5 12)−2×1 having four kinds of 1D structures.

Thermal stability of TiN/HfSiON gate stack structures studied by synchrotron-radiation photoemission spectroscopy

We have investigated the thermal stability of TiN/HfSiON gate stack structures using synchrotron-radiation photoemission spectroscopy. Spectral intensities of the Si-oxide components in Si 2p core-level spectra systematically increase with annealing temperature, which strongly depends on the thickness of the TiN metal gate layer. Changes brought by annealing procedures in depth profiles of atomic concentration indicate segregation of Si-atoms at the TiN surface. Furthermore, chemical-state-resolved depth analyses by angle-resolved photoemission spectroscopy suggest formation of TiSix and HfNy components due to chemical bond breaking in the HfSiON layer during TiN film growth. This can be related to the degradation of thermal stability.

Annealing effects of in-depth profile and band discontinuity in TiN/LaO/HfSiO/SiO2/Si gate stack structure studied by angle-resolved photoemission spectroscopy from backside

We have investigated annealing effects on in-depth profile and band discontinuity for a metal gate/high-k gate stack structure on a Si substrate using backside angle-resolved photoemission spectroscopy with synchrotron radiation. In-depth profiles analyzed from angle-resolved photoemission spectroscopy show that La atoms diffuse through the HfSiO layer and reach interfacial SiO2 layers by rapid thermal annealing. Chemical shift of Si 2p core-level spectra suggests that there are changes in the band discontinuity at the high-k/SiO2 interface, which is well related to the Vth shift based on the interface dipole model.

In-depth Profile Analysis in Metal/high-k Gate Stack Structures Studied by Synchrotron-radiation Photoelectron Spectroscopy

We have investigated the thermal stability of TiN/HfSiON gate stack structures using synchrotron-radiation photoemission spectroscopy. Spectral intensities of the Si-oxide components in Si 2p core-level spectra systematically increase with annealing temperature, which strongly depends on thickness of the TiN metal gate layer. Changes brought by annealing procedures in in-depth profiles indicate segregation of Si atoms at the TiN surface. Furthermore, chemical-states-resolved in-depth analyses by angle-resolved photoemission spectroscopy suggest formation of TiSix and HfNy components, due to chemical bond breaking in the HfSiON layer during TiN film growth. This can be related to the degradation of thermal stability.

Selective Reactions and Adsorption Geometries of a Multifunctional Molecule: cis-2-Butene-1,4-diol on Si(100)-2 × 1

The adsorption geometry of cis-2-butene-1,4-diol (BEDO, HOCH(2)CH=CHCH(2)OH) on Si(100)-2 x 1 was studied using scanning tunneling microscopy (STM), high resolution X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Bias-voltage-dependent STM images exhibited features characteristic of two distinct BEDO adsorption geometries. One feature was a bright protrusion located on the center of a single dimer, indicating an on-top configuration. The low bias-voltage STM image clearly showed dark features indicative of the formation of Si-H bonds on adjacent dimers in the same dimer row. The other feature was a bright protrusion bridged on end between two adjacent dimers in the same dimer row, indicating an end-bridge configuration. Accompanying this feature, two dark features attributed to Si-H bonds were observed on opposite positions to the bridged protrusion. From the XPS results, the Si 2p core level spectra revealed that the dimer atoms are involved in the formation of Si-O and Si-H bonds. On the other hand, carbon K-edge NEXAFS spectra showed that the C=C bond does not participate in the adsorption reaction and remains as an unreacted group. Collectively, the experimental results indicate that the adsorption of BEDO on Si(100)-2 x 1 occurs through the formation of two Si-O bonds via nucleophilic reaction between the two OH groups of BEDO and two Si-Si dimers. Importantly, the maintenance of the C=C bond means that the C=C functional group can be utilized as a new reaction site for further surface chemical reactions.

High resolution photoemission study of SiOx/Si(111) interface disruption following in situ HfO2 deposition

We report on an in situ high resolution core level photoemission study of the early stages of interface formation between an ultrathin SiOx layer (∼0.3 nm) grown on the atomically clean Si(111) surface and a HfO2 dielectric layer. Si 2p core level spectra acquired at 130 eV photon energy reveal evidence of a chemically shifted component on the lower binding energy side of the substrate peak which is attributed to interface defect states resulting from the incorporation of silicon atoms from the substrate into the interfacial oxide at room temperature. This evidence of Si/SiOx interface disruption would be expected to increase charge carrier scattering mechanisms in the silicon and contribute to the generally observed mobility degradation in high-k stacks with ultrathin silicon oxide interface layers.

Effects of thermal annealing on charge density and N chemical states in HfSiON films

We have investigated the charge density and N chemical states in HfSiON films annealed at various oxygen gas pressures by time-dependent photoemission spectroscopy. The Si 2p core-level spectra and the sample current reveal that annealing these films at low oxygen partial pressures affects the number of inherent fixed charges and annealing at high oxygen partial pressures results in a decrease in the number of trapped charges. The N 1s spectra of the annealed HfSiON samples indicate a decrease in Hf–N bonds due to the substitution of N by O along with a decrease in the number of trapped charges.

Dependence of energy band offsets at Ge2Sb2Te5/SiO2 interface on nitrogen concentration

The electronic property for a series of nitrogen-doped Ge2Sb2Te5 phase change material was characterized using high-resolution x-ray photoelectron spectroscopy. The Te 3d5/2 and Si 2p core-level spectra as well as valence band spectra were used in the analysis. As the nitrogen content increases, the valence band offset also decreases, while that of the conduction band increases. Our results show that the valence band and conduction band offsets of nitrogen-doped Ge2Sb2Te5 on silicon oxide exhibit a linear dependence on nitrogen content in the film, for nitrogen content of up to 8.4 at. %.

Structures of clean and oxygen-adsorbed SiC(0 0 0 1)-(3 × 3) surfaces

The structures of clean and oxygen-adsorbed SiC(0 0 0 1)-(3 × 3) surfaces were investigated by high-resolution medium energy ion scattering (MEIS) and photoelectron spectroscopy (PES). The high-resolution MEIS directly evidenced the Si-adatom/trimer/adlayer structure proposed by Starke et al. [U. Starke, J. Schardt, J. Bernhardt, M. Franke, K. Reuter, H. Wedler, K. Heinz, J. Furthmüller, P. Köchkel, F. Bechstedt, Phys. Rev. Lett. 80 (1998) 758]. For the surfaces exposed to dry O 2 gas at room temperature, only Si 4+ and Si + states were observed in the Si 2p core level spectra at an exposure above 10 L (1 L = 1.0 × 10 –6 Torr s) and the ratio of Si 4+ atoms to Si + atoms is almost constant and estimated to be ∼1/3. The MEIS analysis demonstrated that the amount of oxygen adsorbed is increased steeply with increasing O 2-exposure up to 10 L and saturated at ∼50 L with an oxygen coverage of 0.40 ± 0.05 ML (1 ML = 1.22 × 10 15 atoms/cm 2). The above results indicate that the Si-tetramer(adatom/trimer) making the (3 × 3) periodicity is preferentially oxidized to form a SiO 4-like structure. Such an initial oxidation is intimately related to the reconstructed surface structure and different from those of the SiC(0 0 0 1)- 3 × 3 and Si-rich SiC( 0 0 0 1 ¯ )-3 × 3 surfaces, for which all the oxidation states emerge in the former case and Si +, Si 2+ and Si 3+ states in the latter.

Electronic structure and surface chemistry of alkyl-passivated Si nanoparticles

We have synthesized the n-butyl-passivated Si nanoparticles with mean diameter of 1.4 nm by the solution routes, and have carried out the various spectroscopic studies in order to investigate their optical properties, electronic structures, and surface chemistry. It is found that photoluminescence (PL) spectra of these Si nanoparticles strongly depend on the surface conditions. Moreover, it is found their electronic features obtained from the synchrotron-radiation valence-band photoemission and PL spectra are reproduced by the recent quantum Monte Carlo calculation. From the synchrotron-radiation Si 2p core-level photoemission spectra, it is found that their interfacial electronic structure of the present Si nanoparticles is significantly different with that of alkyl-terminated bulk Si surface. From these results, we discuss the detailed electronic and surface chemical properties of alkyl-passivated Si nanoparticles.

In situ observation of wet oxidation kinetics on Si(100) via ambient pressure x-ray photoemission spectroscopy

The initial stages of wet thermal oxidation of Si(100)−(2×1) have been investigated by in situ ambient pressure x-ray photoemission spectroscopy, including chemical-state resolution via Si 2p core-level spectra. Real-time growth rates of silicon dioxide have been monitored at 100 mTorr of water vapor. This pressure is considerably higher than in any prior study using x-ray photoemission spectroscopy. Substrate temperatures have been varied between 250 and 500 °C. Above a temperature of ∼400 °C, two distinct regimes, a rapid and a quasisaturated one, are identified, and growth rates show a strong temperature dependence which cannot be explained by the conventional Deal-Grove model.

Temperature-dependent structural stability and optical properties of ultrathin Hf–Al–O films grown by facing-target reactive sputtering

The structural stability and optical properties of ultrathin HfAlOx films grown by facing-target reactive sputtering, depending on the postannealing temperature, have been determined via x-ray photoelectron spectroscopy and spectroscopic ellipsometry (SE). By virtue of the chemical shifts of Hf4f, Al2p, and Si2p core-level spectra, it has been found that the structural stability of HfAlOx∕Si system sustains up to 800°C. However, the breaking of the Hf–Al–O bond and the phase separation take place drastically at the annealing temperature of 900°C. In particular, the information of an interfacial Si–O–Si bond as the dominant reaction during the postannealing treatment has been observed, confirmed by Fourier transform infrared spectroscopy. Analysis by SE, based on the Tauc-Lorentz model, has indicated that increase in the refractive index and reduction in thickness has been observed as a function of annealing temperature, originating from the annealing-induced higher packing density. The change of the complex dielectric functions and bandgap Eg correlated with the annealing temperature are discussed in detail.

Formation and Characterization of Manganese Silicide on Si(111) and Si(100) Substrates

The electronic structure of the interface formation at Mn/Si at room temperature (RT) and the growth of manganese silicide were studied by using photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) with synchrotron radiation. The Mn coverage dependence of the Si 2p and the Mn 3p core-level spectra revealed that the growth of Mn on Si(111)-7 × 7 at RT started in a layer-by-layer growth at a coverage of less than 0.5 ML (monolayer). Mn 2p XAS spectra also showed no reaction between Mn and Si at RT. Three thin manganese silicide samples were prepared by using solid phase epitaxy at 610 ◦C (MSS), reactive deposition epitaxy at 290 ◦C for Si(111) and at 350 ◦C for Si(100) (MSR) and post annealing the MSR sample at 610 ◦C (MSP) on the Si(111) and the Si(100) substrates. The LOW ENERGY ELECTRON DIFFRACTION (LEED) pattern and the fine structure in the Mn 2p XAS spectra showed that the phases of the MSS, the MSR, and the MSP samples were the same. Emissions at the Fermi energy in the valence-band (VB) PES spectra and the line shape of Si 2p core-level photoemission spectra also showed the formation of metallic MnSi.

Size dependence of Si 2p core-level shift at Si nanocrystal/SiO2 interfaces

Synchrotron-radiation x-ray photoelectron spectroscopy (XPS) has been used to analyze size-dependent Si 2p core-level spectra of Si nanocrystals (NCs) embedded in SiO2. The Si0 and suboxide XPS peaks of Si NCs shift to higher binding energies with decreasing NC size, which is based on the resolved spectra fitted by using Gaussian-Lorentzian lines for the Si oxidation states. It is also found that the shell region around Si NC bordered by SiO2 consists of the three Si suboxide states, Si1+, Si2+, and Si3+, whose densities are also strongly dependent on NC size. These results suggest that the analysis of the Si 2p core-level shift by XPS is useful for characterizing the size effect of Si NC at the Si NC∕SiO2 interfaces.

Dopant diffusion and surface morphology of vanadium implanted 4H-silicon carbide

The diffusion behaviours of vanadium implanted p- and n-type 4H-SiC are investigated by using the secondary ion mass spectrometry (SIMS). Significant redistribution, especially out-diffusion of vanadium towards the sample surface is not observed after 1650°C annealing for both p- and n-type samples. Atomic force microscopy (AFM) is applied to the characterization of surface morphology, indicating the formation of continuous long furrows running in one direction across the wafer surface after 1650°C annealing. The surface roughness results from the evaporation and re-deposition of Si species on the surface during annealing. The chemical compositions of sample surface are investigated using x-ray photoelectron spectroscopy (XPS). The results of C 1s and Si 2p core-level spectra are presented in detail to demonstrate the evaporation of Si from the wafer and the deposition of SiO2 on the sample surface during annealing.

X-Ray Photoelectron Spectroscopy and Reflection High EnergyElectron Diffraction of Epitaxial Growth SiC on Si(100) Using C60 and Si

The formation of silicon carbide upon deposition of C60 and Si onSi(100) surface at 850°C is studied via x-ray photoelectronspectroscopy and reflection high energy electron diffraction (RHEED).The C 1s, O 1s and Si 2p core-level spectra and theRHEED patterns indicate the formation of 3C–SiC.