Si Signal Research Articles

Tris(trimethylsilyl)methane as an internal13C NMR chemical shift thermometer

Tris(trimethylsilyl)methane has excellent properties for an internal chemical shift thermometer to measure temperatures in 13C DNMR spectroscopy [Δδ between the 13C signals of Si(CH3)3 and CH carbons]. It is chemically inert, soluble in most organic solvents and has an almost linear dependence of Δδ on temperature. The sensitivity (approximately 1°C Hz-1 at 90 MHz) allows the measurement of temperature to within 1°C. The chemical shift is solvent dependent, so each solvent mixture must be calibrated. Calibrations are presented for seven solvents. © 1998 John Wiley & Sons, Ltd.

Tris(trimethylsilyl)methane as an internal 13C NMR chemical shift thermometer

Tris(trimethylsilyl)methane has excellent properties for an internal chemical shift thermometer to measure temperatures in 13C DNMR spectroscopy [Δδ between the 13C signals of Si(CH3)3 and CH carbons]. It is chemically inert, soluble in most organic solvents and has an almost linear dependence of Δδ on temperature. The sensitivity (approximately 1°C Hz-1 at 90 MHz) allows the measurement of temperature to within 1°C. The chemical shift is solvent dependent, so each solvent mixture must be calibrated. Calibrations are presented for seven solvents. © 1998 John Wiley & Sons, Ltd.

Mass and energy dispersive elastic recoil detection studies of low temperature Si/Pd/GaAs and Si/Pd/Al xGa (1 − x) As interfacial reactions

Interface reactions between bi-layer films of Si (75 nm), Pd (50 nm) and GaAs or substrates of Al x Ga (1 − x) As have been studied using mass and energy dispersive elastic recoil detection (ERD) to identify the formation and thermal stability of Pd-silicides on GaAs-based systems. Samples were prepared by sequential vacuum evaporation and subsequently annealed in a vacuum furnace at temperatures up to 700°C. Pd and Si signals indicate that silicidation has started at 250°C and developed further as the temperature increased for both systems. The results indicate PdSi was formed in the surface layer, and PdSi 2 in a region deeper than 2.5 × 10 17at. cm −2 without significant reaction with the underlying substrate at temperatures up to 500°C. In the GaAs system at 700°C, all the Pd was bound into the substrate together with excess Si, whilst volatile As was lost from the surface. The results indicate that the same reactions occurred during the heat treatments of both GaAs and Al x Ga (1 − x) As systems.

Backscattering/channeling study of high-dose rare-earth implants into Si

The implantation of high doses of rare-earth metals into Si single crystals results in amorphization of the implanted rigion, even when the target is held at temperatures exceeding 500°C during the implantation. The situation changes drastically when the ion implantation is performed under channeling conditions, i.e., when the ion beam is aligned with a major crystal axis during implantation. A target temperature of 450°C is sufficient for recovery of a majority of implantation-related defects, resulting in an RBS-channeling minimum yield of 13% after implantation. MARLOWE simulations have been performed to explain the difference between non-channeled and channeled implantation. High temperature annealing of such samples results in the formation of continuous rare-earth silicide layers of high structural quality. Polar axial scans around the perpendicular axis result in a minimum yield of 1.5% in the Er signal. In the Si signal, however, a value of 20% is found. This high value is shown to be related to the large difference in atomic number between the rare-earth atoms and Si, which, in the case of planar channeling, can even result in missing channeling dips. The experimental backscattering and channeling results are compared to theoretical predictions for channeling in polyatomic crystals, and to Monte Carlo simulations performed with the FLUX code.

ON THE EXISTENCE AND HYDROLYTIC STABILITY OF TITANOSILOXANE BONDS IN THE SYSTEM : GLYCIDOXYPROPYLTRIMETHOXYSILANE-WATER-TITANIUMTETRAETHOXIDE

Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for optical applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes, their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR spectroscopy has been used to investigate sols with molar ratios Si : Ti = 1 and H2O : OR (H) = 0.5 − 2.0 and their gels after heat treatment at 130°C. The presence of Si—O—Ti bonds in sols with a low water content (H < 0.2) and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up to 2–3 ppm compared to corresponding signals of homo-condensed Si—O—Si bonds. The existence of Si—O—Ti bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage of the Si—O—Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds and the building up of homo-condensed species leads to a separation into areas with predominantly Ti—O—Ti and Si—O—Si bonds resulting in a decreased molecular homogeneity of the materials.

Study of the adsorption of hydrogen and oxygen on Si(100) using positron-annihilation induced Auger electron spectroscopy (PAES)

The adsorption of hydrogen and oxygen on a Si(100) surface is investigated by positron-annihilation induced Auger electron spectroscopy (PAES), as well as conventional electron-induced Auger electron spectroscopy (EAES) and low-energy electron diffraction (LEED). The Si L 2,3VV Auger intensities were observed to decrease exponentially with increasing H 2 (O 2) exposure. The sticking coefficients of H 2 and O 2 were found from fits to plots of the attenuation of the Si Auger signal versus gas exposure to have values of 4.4 ± 1.8 × 10 −5 and 2.6 ± 0.6 × 10 −4 for hydrogen and oxygen, respectively. After 9000 L O 2 (4500 L H 2) exposure, the Si LVV signal measured using PAES was found to decrease by 71% (60%) as compared to a 25% (< 10%) decrease of the Si signal found using EAES. This is consistent with the higher degree of surface selectivity expected in PAES. This attenuation of the Si PAES intensity is consistent with previous studies of H 2S adsorption on Cu, which indicated that the positron surface-state wave function is displaced away from substrate atoms by an adsorbed overlayer, resulting in a decrease in the annihilation rate of positrons with core electrons of the substrate. Our results indicate that PAES could be useful in other H adsorption measurements due to its high sensitivity to the presence of a H adlayer.

Raman spectroscopy investigation of cubic boron nitride single crystals and layers on Si(100)

Micro-Raman spectroscopy (μ-RS) measurements were performed on cubic boron nitride (c-BN) single crystals and c-BN thin films deposited on Si(100) by magnetron sputtering of a h-BN target. The 482.5 nm laser line of a Kr+ laser as well as the 488.0 nm laser line of an Ar+ laser were found to be most sensitive to the detection of c-BN. For single crystals of different sizes (< 100 nm, 1 μm, 100 μm, 1 mm) and nanocrystalline c-BN layers on Si(100) substrates the influence of the crystal size on the frequency position and the lineshape of the longitudinal (LO) and transversal optical (TO) phonon features was investigated. With decreasing crystal size a shift towards lower frequencies as well as a broadening and increasing asymmetry of the LO and TO phonon is observed. The comparison of these results with a phonon-confinement model is discussed. For the c-BN layers on Si(100) substrates, additional two-dimensional Raman mappings of the Si signal were performed revealing the influence of the transparent hard coating on the substrate.

XAFS Study of the Main Constituent Elements of Layered Aluminosilicates

Changes in the Si and Al K-edges XANES signals for a series of well characterized phyllosilicates have been examined. Results obtained for the different specimens have been interpreted in accordance with the changes in the structural compositions of the minerals and with the results of previous theoretical and experimental studies of related compounds. In both edges appear typical features that can be associated with dioctahedral and trioctahedral series.

NMR study in the superconducting silicon clathrate compound NaxBaySi46.

Si, Na, Ba, and Ba NMR experiments were carried out in the superconducting silicon clathrate compound NaxBaySi46 to study the electronic states above the superconducting transition temperature. We observed three distinct Si signals with different Knight shift ~2036, 862, and 720 ppm at 90 K with a small temperature dependence of ;10%!, which were ascribed to inequivalent Si sites in the Si46 structure. The results indicate that the conduction-electron spin and/or charge density is strongly site-dependent. At all the atomic sites, the Korringa relation in NMR relaxation was observed, indicative of metallic electronic structure with moderate electronic correlation. The Na and Ba atoms are partially ionized in NaxBaySi46 , which shows the noticeable difference from the system of alkali-metal-doped fullerides. The comparison with band calculations is discussed. @S0163-1829~96!06042-0#

The interaction of Cu atoms with the [formula omitted] surface

The room-temperature deposition of Cu onto the Si(111)- 3 × 3 - Ag surface has been studied by means of LEED-AES-RBS techniques. As the Cu deposition increases, the 3 × 3 LEED pattern is changed into a 1 × 1 pattern at a thickness of about 4 ML, which disappears at about 10 ML. An ordered structure of (1 × 1)R30° appears at 18 ML. In AES spectra, the intensity of the Si signal decreases with increasing Cu deposition, while the Ag signal does not change at all after an initial small decrease at 1 ML. The RBS-channeling measurement shows that Cu atoms are situated at an interstitial site at a Cu coverage of 1.5 ML. The Si surface peak in the RBS-channeling spectra increases linearly with the Cu coverage in a proportion of Cu Si = 3 1 . These results indicate that Cu atoms deposited onto the Si(111)- 3 × 3 - Ag surface at the initial stage up to 4 ML are buried at an interstitial site in subsurface layers of the Si substrate and that a film of Cu 3Si is formed at higher coverages.

Investigation of GaN deposition on Si, Al2O3, and GaAs using in situ mass spectroscopy of recoiled ions and reflection high-energy electron diffraction

In situ mass spectroscopy of recoiled ions (MSRI) and reflection high-energy electron diffraction (RHEED) are used to monitor the surface structure, and the near surface layer composition during the pre-growth preparation step and the deposition of GaN thin films on Si (001), Al2O3 (0001), and GaAs (001) substrates by electron-cyclotron-resonance (ECR)-assisted molecular beam epitaxy. Due to the high lattice mismatch between GaN thin films and these substrates, the initial nucleation step is found to be critical on the resulting thin film properties. Nitridation of the clean Al2O3 (0001) and GaAs(001) surface is observed by RHEED, and the formation of an AlN/GaN overlayer is confirmed by MSRI. During GaN deposition on Si, Al2O3, and GaAs, MSRI and RHEED are used to follow the growth mode and coverage. If a RHEED pattern from the nucleating GaN overlayer has a cubic or wurtzitic symmetry and the MSRI spectra—which is sensitive to the first few monolayers—shows the presence of Ga and N, only then the growth is layer by layer. Scanning electron microscopy micrographs also show a smooth and continuous surface. If an amorphous RHEED pattern is observed and the presence of the Si (Al, O in the case of Al2O3 and As in the case of GaAs) signal is detected in addition to Ga and N, the growth is columnar. In conclusion, we show that RHEED and MSRI form a powerful combination of thin film surface characterization tools ideally suited for real-time optimization of substrate preparation and thin film growth parameters for the nitride materials system.

Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route

Despite the fact that H-terminated, HF-etched Si crystals are the starting point for construction of most contemporary electronic devices,1 little is known about the chemical reactions of H-terminated Si surfaces under ambient temperature and pressure.2,3 Functionalization of Si without partial oxidation and/or formation of electrical defects is potentially important in fabricating improved electronic devices4,5 as well as in measurement of charge transfer rate constants at semiconductor/ liquid contacts.6 One recently described approach involves the reaction of HF-etched Si(111) with olefins and organic diacyl peroxides, in which formation of a self-assembled (near)monolayer of Si-alkyls was hypothesized.2 We report here an alternative strategy to functionalize HF-etched Si surfaces involving halogenation and subsequent reaction with alkyl Grignard or alkyl lithium reagents. We report vibrational spectroscopic and temperature programmed desorption data which confirm that the alkyl groups are bonded covalently to the Si surface, and we demonstrate that such overlayer formation can impede the undesirable oxidation of Si in a variety of environments while providing surfaces of high electrical quality. The H-terminated Si surface7 was first exposed to PCl5 for 20-60 min at 80-100 °C, in chlorobenzene with benzoyl peroxide as the radical initiator.8,9 Upon chlorination, the XP survey spectra (Figure 1) showed peaks at 270.2 ( 0.4 binding electron volts, BeV, (Cl 2s) and 199.3 ( 0.4 BeV (Cl 2p), indicating that this procedure yielded Cl on the surface. The high-resolution XP spectrum of the Si 2p peak of this surface displayed, in addition to the substrate Si signal, an additional peak located at 0.98 ( 0.12 BeV higher in binding energy (Figure 2) whose position and intensity was consistent with the formation of a surface Si-Cl bond.10 Auger electron spectra

Total reflection x-ray excited photoelectron spectra of copper phthalocyanine thin layer on Si wafer

Photoelectron spectra of a Si wafer, on which copper phthalocyanine was evaporated with a thickness of 50 Å, were measured using grazing incidence x rays under a total reflection condition. It was observed that the backgrounds owing to inelastic electron scattering in solids were reduced. It was also observed that the substrate Si signal was removed and that surface signal was enhanced due to the total x-ray reflection. Oxygen depth was determined using the angle dependence of the x-ray photoelectron spectral intensity.

Multi‐Nuclear Solid‐State NMR Study of the Local Structure of SiOHAl Groups and their Interaction with Probe‐Molecules in Dehydrated Faujasite, Mordenite and Zeolite ZSM‐5

AbstractStructural variations of Si–O–Al bridges in dehydrated zeolites have been studied by 29Si CP/MAS NMR and 27Al NMR spectroscopy. After deammoniation of zeolites Y (nSi/nAl = 2.6 and 8.0), mordenite (nSi/nAl = 8.0) and ZSM‐5 (nSi/nAl = 22.0) 29Si NMR high‐field shifts of Si(1 Al) signals of maximum 0.8 ppm and of about 2.7 ppm and 4.0 ppm, respectively, were observed. This is explained by a protonation‐induced increase in the mean Si–O–T bond angles of SiOHAl groups. Simultaneously, an increase in the aluminium quadrupole coupling constant from about 5.0 MHz to about 16.0 MHz was determined indicating a strong protonation‐induced shear strain of framework AlO4 tetrahedra. On the other hand, the proton transfer from Si–O–Al bridges to basic probe‐molecules causes a decrease in the aluminium quadrupole coupling constant which corresponds to a higher AlO4 symmetry. The high symmetry of AlO4 tetrahedra in zeolite HZSM‐5 loaded with one methanol molecule per SiOHAl group, the mobility of the hydroxyl protons existing in this sample and their chemical shift value observed at the temperature of T = 295 K by 27Al, 2H and 1H NMR, respectively, indicate a rapid exchange of hydroxyl protons between bridging OH groups, hydrogen‐bonded methanol molecules and methoxonium ions. At the temperature of T = 85 K methanol molecules are fixed at the zeolite framework by hydrogen bonds.

Electron cyclotron resonance plasma deposition as a promising technique for low-temperature hard coatings

Si 3N 4 and SiC films have been deposited by electron cyclotron resonance (ECR) plasma at low substrate temperature (T s ⩽150°C). Films prepared by this new technique exhibited better properties than those deposited by other plasma methods. Different gas ratios and microwave powers have been used. The films have been analysed by infrared spectrometry and spectroscopic ellipsometry. Si 3N 4 and SiC films show in general a good thickness homogeneity (< 4% in 3 in. wafers) and reproducibility. Depending on the process parameters we have found growth rates up to 3500 Åmin −1 for Si 3N 4 and 400 Åmin −1 for SiC. The infrared spectra of these layers have shown no signal of SiH and NH bonds in the Si 3N 4 films and low traces of SiH and CH n bonds in the SiC films.

An AES study of intrinsic and ion-induced structure in the SiO 2-Si system

Detailed changes in the Si LVV N( E) spectrum across the SiO 2-Si system are evaluated in relation with ion beam damage (argon ions of energy 90–500 eV). Typical samples are: thin or thick SiO 2 films, the depth-profiled SiO 2/Si interface and SiO x compounds. The width of the main oxide peak and the shape of the elemental Si signal are useful indicators of modifications affecting the oxide: structural distortion and Si clustering induced by the ion beam, and structural distortion due to the proximity of the Si phase. These effects depend critically on the oxide thickness in the range 0–2.0 nm.

Growth mode of Ge films on Si(100) substrate deposited by ion beam sputtering

Germanium thin films have been grown on silicon(100) substrates by ion beam sputter deposition (IBSD) in an ultrahigh vacuum system. The growth mode of the films as a function of the growth temperature and the ion incidence angle was investigated by in situ Auger electron spectrometry. The morphology of the film surface was examined by scanning electron microscopy. The effect of the deposition temperature on the interface roughness was investigated after selectively etching the Ge film. We have found that the growth mode of IBSD-Ge on silicon does not agree with a layer-by-layer growth. The decay of the Si signal as the growth proceeds may be interpreted in terms of either a Stransky-Krastanov growth mode with island formation after two-dimensional growth or Si segregation to the surface. We showed that IBSD leads to more abrupt interfaces than molecular beam epitaxy. This last result may be related to the energetic bombardment of the film during deposition. To support this model more quantitatively, computer calculations have been carried out by using an advanced version of the Monte Carlo “transport of ions in matter” code ( trim).

Vacuum ultraviolet photolysis of supersonic free jets of SiH 4

We have measured the relative abundance of neutral photofragments in pulsed supersonic free jets of SiH 4 excited in the wavelength range 115–170 nm using synchrotron radiation and quadropole mass spectrometry (QMS). Using a sub-ionization QMS threshold electron-impact energy of 11 V and taking the difference between VUV irradiated and non-irradiated SiH 4 jet pulses, the relative abundance of the neutral species Si, SiH, SiH 2 and SiH 3 was determined. The neutral Si signal exhibited a threshold wavelength of ≈ 135 nm, reaching its maximum value at ≈ 115 nm, the short wavelength limit of the current measurements. The SiH signal rose rapidly to a peak at ≈ 158 nm, falling off rapidly to ≈ 145 nm. An additional shoulder on the SiH signal was observed at ≈ 138 nm that gradually diminished for Ω < 125 nm. The SiH 2 and SiH 3 radicals exhibited a maximum near 145 nm with the SiH 2 peak exhibiting a secondary maximum at ≈128 nm.

Determination of silicon in high siliconnickel alloys by ICP-AES after dissolving in nitric-hydrofluoric acids

Silicon was determined by ICP-AES in high Si-Ni alloys dissolved in HNO3-HF. The interference by Si signals originating from the corrosion of the torch by HF was eliminated by keeping it almost constant. The relative standard deviation was within ±1%.

Heterogeneity of OH Groups in Faujasites Studied by IR Spectroscopy

Heterogeneity of OH groups in faujasites was studied by analysis of the IR absorption band of free hydroxyls and hydroxyls engaged in hydrogen-bonding with chlorobenzene. The Si-O 1H-Al groups were found to be homogeneous in NaHX zeolite (Si/Al = 1.06) and heterogeneous in NaHY zeolite (Si/Al = 2.56). Experiments of pyridine sorption and desorption have shown that the more acidic hydroxyls desorb pyridine at higher temperatures than the less acidic hydroxyls. The IR spectroscopic results agree well with previously published 29Si MAS NMR results. Only one Si(4A1) signal is present in the NMR spectrum of X zeolite (Si/Al ≅ 1), but there are four signals (Si(3Al), Si(2Al), Si(1Al), and Si(OAl)) in Y zeolite. As Si(OAl) cannot create bridging hydroxyls, this implies three kinds of hydroxyls of various acid strengths in NaHY zeolite and one kind in NaHX, as actually observed in the present IR study. The hydroxyl groups in NaHX zeolite can be assigned to (AlO) 3Si-OH-Al(SiO) 3, and the three kinds of hydroxyls in NaHY to (AlO) 2(SiO)Si-OH-Al(SiO) 3, (AlO)(SiO) 2Si-OH-Al(SiO) 3, and (SiO) 3Si-OH-Al(SiO) 3, respectively. The elimination of a small amount of the most acidic S-O 1H-Al by pyridine significantly reduces the catalytic activity of NaHY zeolite in butene-1 isomerization.