Modes Of Si Research Articles

Optical and material properties of sandwiched Si/SiGe/Si heterostructures

Si 1− x Ge x layers sandwiched between Si were grown at low temperature of 450 °C by molecular beam epitaxy. A comprehensive characterization has been performed on these heterostructures by multiple techniques, including X-ray diffraction (XRD), photoluminescence, Raman scattering, Fourier transform infrared (FTIR) spectroscopy, Rutherford backscattering spectrometry (RBS), ion channeling and secondary ion mass spectroscopy (SIMS). XRD confirmed the single crystallinity and the (100) orientation of the Si 1− x Ge x layer. The Ge compositions and layer thicknesses were precisely determined by RBS. FTIR measurements revealed the vibration modes of Si–O–Si from the oxidation on surface and Si–H due to the hydrogenization during growth.

Annealing crystallization of a‐Ge/Al/Si and a‐Ge/Si thin films

AbstractThis work describes the temperature‐induced crystallization of amorphous Ge (a‐Ge) as a function of the thickness of the a‐Ge films (in the 12–2600 nm range), which were deposited both onto c‐Si substrates and c‐Si substrates covered with aluminium. After deposition, the samples were submitted to cumulative thermal annealing treatments. It is shown that the temperature of crystallization depends on the thickness of the a‐Ge films and to the presence (or not) of the Al layer. For an annealing temperature (Ta) of ∼700 °C, for example, the Raman spectra of films thinner than ∼1000 nm and deposited onto c‐Si substrates are completely dominated by the sharp phonon mode of crystalline Si. Films with thicknesses equal to 300, 1000 and 2600 nm, deposited onto Al/c‐Si, and treated at Ta = 600 °C, on the other hand, clearly display two additional peaks at 405 and 490 cm–1. They correspond to the Raman modes of Si–Ge and Si–Si modes, suggesting the formation of a SiGe alloy during the thermal anneal of the films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly sensitive strain detection in strained silicon by surface-enhanced Raman spectroscopy

We used surface-enhanced Raman spectroscopy to observe selective amplification of the vibration mode of Si–Si in strained silicon. By covering the silver island film on a strained silicon layer, the Raman signal from the strained silicon can be detected with a high sensitivity compared with the overwhelming background signal from the underlying silicon layer. This technique allowed us to carry out micro-Raman spectroscopy on strained silicon. This technique can be a precursor for nano-Raman spectroscopy using a tip-enhanced Raman microscope with a sharpened metallic probe tip.

OH-related infrared absorption bands in oxide glasses

We report the infrared activity, in the spectral region of the OH stretching modes, of different composite silicate glasses whose chemical composition is established by X-ray fluorescence measurements. The analysis of the absorption line profiles is made in terms of different spectral contributions, Gaussian in shape. The comparison with analogous spectra obtained in vitreous silica samples with impurity concentrations <100 part per million moles is evidence of the effects of the different oxides on the vibrational properties of the OH groups. In particular, for oxide glasses a red shift of the composite band at about 3670 cm −1, assigned to the OH stretching modes of free Si–OH groups and of water molecules, is found with respect to those silica samples with the smallest impurity concentrations. Moreover, for oxide glasses the absorption activity in the 2500–3200 cm −1 region shows several spectral contributions that depend on the chemical composition of each sample. Also in view of the fact that in this zone no significant absorption activity is revealed in the case of silica samples, these spectral components are assigned to OH stretching modes of Si–OH groups in interaction with the different atoms present (Na, Ca, K, Al, etc.) and/or in modified sites of the vitreous structure.

Si nanostripe formation on vicinal Ge(1 0 0) surfaces

Kinetic growth modes of Si in the submonolayer regime on vicinal Ge(1 0 0) surfaces miscut by 2.7° and 5.4° towards the [0 1 1] direction were investigated using low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS, UPS). At a Si coverage around 0.5 ML and temperatures between 470 and 600 K Si nanostripes separated by Ge nanostripes are formed due to preferential nucleation of Si adatoms on the (1 × 2)-reconstructed Ge(1 0 0) domains. At room temperature both Ge(1 0 0) domains are covered by pseudomorphic Si islands. The stability of the Ge–Si stripe structure is limited to 600 K above which alloy formation was found by photoelectron spectroscopy.

Pressure tuning of localized and extended vibrational modes in Si:O

AbstractInterstitial oxygen in silicon (Si:O) is a model system for the study of vibrational modes in semiconductors. Hydrostatic pressure has been used to probe the vibrational properties of this defect. In this paper, the results of infrared (IR) spectroscopy experiments on Si:16O and Si:18O are reviewed. As pressure is applied, the oxygen atom buckles outward. This structural change results in a qualitative change in low‐frequency vibrational motion in the [111] plane, as the system transforms from a harmonic oscillator to a rotor. A particularly interesting phenomenon occurs when the Si:18O local vibrational mode (LVM) approaches the two‐phonon continuum. First, the LVM exhibits an avoided crossing with certain combination modes of the defect. Second, when the LVM enters the two‐phonon continuum, the linewidth abruptly broadens, due to a decrease in the lifetime. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

VIBRATIONAL STUDIES AND RESONANT INTERACTION BETWEEN LOCALIZED AND EXTENDED MODES IN Si:O UNDER PRESSURE

The relaxation mechanisms of local vibrational modes in solids are of central importance in understanding how such modes decay into phonons, the diffusion of impurities, and the loss of hydrogen passivation in technologically important materials. Interstitial oxygen in silicon is a model system for studying the relaxation of local vibrational modes into extended vibrational modes by measuring the interactions between the two. We have used hydrostatic pressure to bring the antisymmetric stretch mode of Si :18 O i into resonance with the second harmonic of the 18 O i resonant mode and observed an anticrossing between the two vibrational modes near pressures of 4 GPa. A theoretical model of this interaction produced excellent agreement with the experimentally observed frequencies and linewidths.

On the origin of strain fluctuation in strained-Si grown on SiGe-on-insulator and SiGe virtual substrates

We investigated the microscopic strain fluctuation in strained-Si grown on SiGe-on-insulator (SGOI) and SiGe virtual substrates, and clarified the origins of the strain fluctuation in the strained-Si film. A periodic strain fluctuation, which reflects a cross-hatch pattern of the substrate, was observed in the sample on the virtual substrate. On the other hand, a featureless strain fluctuation with suppressed amplitude was observed in the sample on SGOI substrate. By analyzing the correlation of the Raman peak positions of the Si–Si modes in strained-Si and SiGe, the dominant mechanism of the strain fluctuation in the strained Si film was found to be the compositional fluctuation in underlying SiGe for the sample on SGOI, and the strain fluctuation reflecting the cross-hatch pattern for the sample on the virtual substrate, respectively.

Assessment of the use of Raman spectroscopy for the determination of amphibole asbestos

AbstractRaman spectroscopy was assessed for its ability to rapidly identify asbestos phases by submitting the amphiboles anthophyllite, amosite, crocidolite and tremolite to spectroscopic analysis. All the phases were characterized by using XRD, SEM, TEM (CTEM, SAED), AEM and EDS techniques. This study demonstrates that accurate identification of the mineralogical phase can be attained by analysing the position in the Raman spectrum of the bands corresponding to the symmetric stretching modes (vs) and to the antisymmetric stretching modes (vas) of the different Si—O linkages. Raman spectroscopy thus proves to be an effective technique for rapidly distinguishing the different fibrous minerals examined.

Mechanical Property and Network Structure of Porous Silica Films

We investigated the relationship between Young's elastic modulus of a series of spin-on sol-gel silica films and the shift of the longitudinal-optical component of the asymmetric stretching vibration mode of Si–O–Si (LO4), by nanoindentation measurements and Fourier transform infrared reflection spectroscopy. With the increase of the annealing temperature from 673–1073 K in air, the position of the LO4 mode reflection peak shifted from 1203 to 1253 cm-1, in parallel with the increase of Young's elastic modulus. The correlation between the LO4 peak position and Young's elastic modulus was also confirmed among a set of silica films in which the preparation conditions and/or the postpreparation treatment conditions were varied. It was concluded that the shift of the LO4 mode peak position is a good measure of the skeletal silica mechanical property of porous silica films.

CHARACTERIZATION OF SILICON OXIDE NANOWIRES DIRECTLY GROWN FROM NiO/Si

Amorphous silicon oxide nanowires ( a-SiONW's ) were synthesized on a NiO -catalyzed silicon substrate through a simple thermal annealing at 1000–1100°C without any additional silicon source materials. The synthesized nanowires were highly pure (no metal catalyst contamination) and very long (hundreds of micrometers). Most nanowires had a diameter in the range of 50–100 nm. The nickel oxide nanoparticles play a peculiar role in the synthesis of a-SiONW's . The growth of a-SiONW's is most likely controlled by the solid–liquid–solid (SLS) mechanism. IR absorption characteristics clearly showed bending (796 cm-1) and asymmetric stretching (1075 cm-1) modes of Si – O – Si bonds in a-SiONW's . An extraordinary UV emission at 370 nm was observed from a-SiONW's , which could be attributed to defect centers of oxygen excess in the nanowires.

Phonon modes in Si [111] nanowires

We solve a long standing question concerning the boundary conditions for optical phonons in nanowires. The controversy between the so-called clamped and free conditions is well documented in standard literature. We answer this question of the boundary conditions by presenting a spring-and-mass model of the Stillinger and Weber type to calculate optical and acoustic phonons in Si [111] nanowires. Our results show that this model, in parallel to the bulk case, is sufficient to describe important features of the phonon spectrum. Furthermore, our results predict that optical phonons in nanowires are subject to clamped boundary conditions.

Near-surface sensitive infrared reflection spectroscopy on SiO 2 implanted with high-flux negative ions

Implantation of Cu negative ions of 60 keV induces frequency shifts of infrared (IR) vibration modes of SiO 2. Since the penetration depth of IR light is much larger than the projectile range of Cu − ions, conventional IR reflection spectroscopy detects little shifts. To evaluate the true shifts induced by the implantation, the near-surface sensitive IR reflection spectroscopy (NSS-IRRS) was applied using a suitable sample structure. Distinct shifts of −5, +6 and −44 cm −1, respectively, are observed in rocking, bending and stretching modes of Si–O–Si bonding of SiO 2. Ion-flux dependence was examined in the flux range of 0.95–100 μA/cm 2 with keeping the total fluence 3×10 16 ions/cm 2. Almost no ion-flux dependence is observed in the shifts of these modes, which is in contrast with the strong flux dependences of size- and depth-distributions of Cu nanoparticles in SiO 2.

Degenerate four wave mixing spectroscopy of oxygen vibrations in amorphous silicon

We report on DFWM spectroscopy on the asymmetric stretching mode in a-SiO x with x from 0.1 to 1.11. For a-SiO 0.1, a 10 K lifetime of 2.8 ps is obtained using laser-induced transient grating spectroscopy. Two-pulse photon echo measurements reveal phase dynamics having a temperature dependent component due to elastic phonon scattering and an excitation dependent component suggesting the presence of non-equilibrium Si–Si modes. At high excitation densities, the DFWM signal is dominated by a quasi-thermal grating whose population decay is extremely long lived (⪢1 ns) . This effect is enhanced as the oxygen concentration is increased.

Complex dynamics of the Si(111)-7×7surface: Total-energy calculations

We studied the dynamics of a semi-infinite Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ system, using the force-constant matrix obtained from the total-energy calculation based on a nonorthogonal tight-binding Hamiltonian. We modeled the semi-infinite system by merging the optimized surface slab seamlessly onto the bulk to underscore the importance of the system size in obtaining all the relevant surface phonon modes of Si(111)-$7\ifmmode\times\else\texttimes\fi{}7.$ Our study has uncovered modes that have hitherto not been reported by ab initio molecular-dynamics studies and, also, provides detailed explanations of the nature of all important surface dynamical features (e.g., modes associated with dimers, adatoms, stacking faults, and rest atoms), using an analysis of the site density of states and pair density of states that identifies directly coupled vibrations.

Form of fluorine in Na2O-NaF-SiO2 slags determined by infrared spectroscopy

The form of fluorine in Na2O-NaF-SiO2 slags has been investigated using Fourier transform infrared spectroscopy. Additions of NaF shifted absorption peaks for stretching modes of Si O bonds to higher wavenumbers, where the basicity of (mol-%Na2O)/ (mol-%SiO2 ) was kept constant, and the shift became larger with an increase in the concentration of NaF. This shift can be explained by the change from Si-O to Si-F bonds, with reference to harmonic oscillation theory of diatomic molecules. It is likely that added NaF depolymerises the silicate network to form Si-F bonds in Na2O-NaF-SiO2 slags.

Silicon–germanium films prepared from SiH 4 and GeF 4 by low frequency plasma deposition

The properties of silicon–germanium films (a-Si 1− x Ge x :H,F) deposited by low frequency plasma enhanced chemical vapor deposition (LF PECVD) from SiH 4 and GeF 4 have been studied. The films were deposited in capacitive discharge at frequency f=110 kHz, substrate temperature T s=300 °C, pressure P=0.6 Torr and power W=350 W. Ge gas mixture composition, Xg, determined as [GeF 4]/[SiH 4 + GeH 4] was varied from Xg=0.1 to 1. The films were deposited from (1−Xg) SiH 4 + XgGeF 4 mixture without dilution, with Ar and with H 2 dilution. Composition of the films was characterized by SIMS. The chemical bonding structure was studied by IR spectroscopy. It was found that the modes of Si–H, Ge–H, Si–Ge bonding depended on Ge content and gas dilutor. Transport of carriers was studied by the measurement of conductivity dependence on temperature. The reduction of activation energy from 0.86 to 0.39 eV and the increase of room temperature conductivity from 1 × 10 −9 to 2.1 × 10 −4 Ω −1 cm −1 were observed with the changes of Ge gas mixture Xg, from 0 to 1. No correlation between pre-exponent factor of conductivity σ 0 and activation energy E a was observed in the samples studied, in contrast to that in amorphous silicon (Meyer–Neldel rule). The data obtained are discussed in comparison with information available in literature on similar films obtained by alternative techniques.

LO−TO Splittings in Plasma-Deposited Siloxane Films

The present work presents LO and TO functions in the mid-infrared region for thin films deposited from glow discharge plasmas of tetramethylsilane (TMS) diluted either in Ar or O2 or in mixtures of these two gases. These functions were calculated through the Kramers−Krönig analysis of transmittance spectra of the films supported on KBr disks. To correlate structural aspects of the films with the observed LO−TO splittings, a group frequency analysis based on the literature was made. Such an analysis indicated that the films deposited from the TMS−Ar mixture were formed mainly by a polycarbosilane skeleton, whereas those deposited from TMS−O2 and TMS−O2−Ar were formed by a random network of four types of distorted tetrahedra: (CH3)3SiO0.5, (CH3)2SiO, (CH3SiO1.5), and SiO2. From the LO−TO splitting for the asymmetrical stretching mode of Si−O−Si groups, the density and the presence of defects in samples obtained from TMS−O2 and TMS−O2−Ar mixtures were evaluated. The number of defects increased as the Ar-to-O2 flow rate decreased. We also report for the first time LO−TO splittings for bands related to the bending of CH3 and to the stretching of the Si−C bond in Si(CH3)x groups. The knowledge of such splittings is very important for a correct evaluation of the infrared reflection−absorption spectra taken at oblique incidence of thin films containing Si−O bonds and Si(CH3)x groups deposited on metals.

High-frequency FTIR absorption of SiO 2/Si nanowires

An IR absorption measurement of SiO 2/Si nanowires made by thermal evaporation was conducted. In comparison with SiO 2 nanoparticles, enhancement absorption of SiO 2/Si nanowires around 1130 cm −1 was observed. This enhancement was considered to result from: (1) the interface effect of the open structure of chainlike SiO 2/Si nanowires; (2) the vibration of an interstitial oxygen atom in a silicon single-crystalline core of nanowire; and. The longitudinal optical (LO) modes of Si–O–Si stretching in an amorphous SiO 2 outer shell of SiO 2/Si nanowires were also discussed.

Alloy effects on the Raman spectra of Si1−xGex and calibration protocols for alloy compositions based on polarization measurements

The Raman spectra of bulk and epitaxial Si1−xGex alloys (0 < x < 1) were studied with the aim of establishing a metrology for the determination of alloy compositions from the first-order Si–Si, Ge–Ge and Si–Ge optical phonon Raman modes. However, the lineshapes and intensities of these modes are distorted by various subsidiary features including (i) a broad quasi-amorphous feature on the low-energy side of the Ge–Ge mode and (ii) peaks in the range of 340–500 cm−1, very close to the main modes and believed to arise from various types of Si–Ge clusters. Standard curve-fitting procedures cannot cope with such background features and fail to give reliable values for the intensities of the main lines. We demonstrate that the background features can be minimized by protocols based on polarization measurements. One of these methods that reveals less distorted Ge–Ge and Si–Si modes involves subtraction of the first-order-forbidden spectrum from the allowed spectrum in a parallel polarization configuration. The other involves measurements in a cross-polarization geometry in which the background is suppressed. These protocols allow better estimates of the integrated phonon intensities for a wide range of alloy compositions by usual curve-fitting, and improvements in the ratios of the IGe–Ge/ISi–Si integrated intensities by factors as large as 8 were obtained for Si-rich samples. The simpler procedure for calibration based on phonon frequency positions, sufficient for bulk alloys, may be incorrect for heterostructures due to the additional strain dependence of the frequencies superimposed on the alloy compositional dependence. Therefore, a protocol based on the integrated phonon intensities is applicable and performs better in both strained-layer devices and bulk alloys.