Raman scattering study of dopant homogeneity in GaP and GaAs single crystals

Application of Raman scattering as a non-destructive method with high spatial resolution suitable for the determination of charge carrier profiles connected with dopant segregations in polar semiconductors is reported. Single crystals of GaP and GaAs are investigated. The results are in good agreement with electrical measurements.     

Surface enhanced Raman scattering phenomenon

Surface enhanced Raman scattering and surface enhanced fluorescence phenomena are the result of resonance excitation of localized surface plasmons in nanoparticles of some metals. By appropriate choice of the shape and size of metallic nanoparticles, the kind of adsorbed molecules and the wavelength of the exciting light, it is possible to achieve enhancement of the intensity of Raman scattering equal to 10¹²‐10¹⁴. This means that only some molecules inside the exciting light beam are sufficient to record the Raman spectrum. Electromagnetic and chemical mechanisms of enhancement of Raman scattered light and methods of roughness investigation of surfaces exhibiting this promising phenomenon are described in this paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface enhanced Raman scattering in an amorphous matrix for Raman amplification

In order to improve the efficiency of Raman Amplifiers, the Surface Enhanced Raman Scattering (SERS) effect of an amorphous matrix of TiO₂ was studied. First, optimisation of the amorphous layer quality was performed by depositing thin films on glass substrates at different temperatures. Then, thin films of amorphous TiO₂ were deposited on silicon commercial gold SERS substrates (Klarite®) by a dip-coating process. The SERS effect was demonstrated by the great difference of Raman intensities of the amorphous TiO₂ matrix dip-coated on active and inactive parts of Klarite® substrate under 633 nm and 780 nm laser excitations in the tail of the Surface Plasmon Resonance band of gold nanoparticles.     

Low frequency Raman scattering in chalcogenide glasses

Low frequency Raman spectra of chalcogenide glasses are analyzed in terms of matrix element effects and modes of a layered structure. The spectra of GeSe₂ at low temperature shows no peaks which can be assigned to layer modes. The reduced spectra indicates that the density of states exhibits nearly ω² dependence for ω < 60 cm^?1, and the coupling constant approaches ω² dependence at frequencies less than 20 cm^?1.     

Inelastic neutron and low-frequency Raman scattering in a niobium-phosphate glass for Raman gain applications

We present measurements of the vibrational spectrum of a binary niobium-phosphate glass in the THz frequency range using inelastic neutron and Raman scattering. The spectra of these glasses show a low-frequency enhancement of the vibrational density of states (“boson peak”). Using a recently developed theory of vibrational excitations in disordered solids we are able to reconcile the measured neutron and Raman spectra using fluctuating elastic and Pockels constants as a model concept. As the spontaneous Raman susceptibility is a key parameter for Raman amplification our results suggest a significant gain profile for application of niobium-phosphate glasses in Raman amplifiers.     

Raman scattering and structural order in liquid GeSe2

High temperature polarized and depolarized Raman spectra have been obtained in liquid GeSe₂ near the melting point of 770 C. The spectra indicate a number of similarities to amorphous GeSe₂, including the presence of the “companion” line which has been associated with intermediate range order in the amorphous state. The Raman spectra, as well as previous low angle diffraction studies, indicate that intermediate range structural order is also present in liquid GeSe₂.     

Diffuse scattering from crystals with point defects

The analytical expressions for calculating the intensities of X-ray diffuse scattering from a crystal of finite dimensions and monatomic substitutional, interstitial, or vacancy-type point defects have been derived. The method for the determination of the three-dimensional structure by experimental diffuse-scattering data from crystals with point defects having various concentrations is discussed and corresponding numerical algorithms are suggested.     

Raman spectroscopy of calcium aluminate glasses and crystals

Solar furnace melting and fast-quench techniques have been used to prepare calcium aluminate glasses from 75 mol% CaO to 82 mol% Al₂O₃, which have been studied by Raman spectroscopy. The CaAl₂O₄ glass spectrum may be interpreted in terms of a fully-polymerized network of tetrahedral aluminate units, which is depolymerized on addition of CaO component analogous to binary silicate systems. The spectra of glasses with higher alumina content than CaAl₂O₄ may not be simply interpreted and a structural model is proposed which would be consistent with the glass spectra and with observed crystal structures along the CaAl₂O₄Al₂O₃ join. This model suggests formation of highly condensed aluminate tetrahedral on initial addition of alumina, with the appearance of aluminate polyhedra of higher average coordination at higher alumina content. Similar high coordination polyhedral are also suggested for a limited composition range along the CaOCaAl₂O₄ join. These interpretations are compared with the results of a previous study in the SiO₂Al₂O₃ glass system.     

Low-frequency Raman scattering and small-angle X-ray scattering of glasses inclined to phase decomposition

Low-frequency (<1000 cm⁻¹) Raman scattering of lithium aluminosilicate (12Li₂O : 15Al₂O₃ : 73SiO₂ with 4 mol% TiO2) glasses with addition of titanium dioxide has been studied. With a heat treatment at temperatures 660°C, 700°C, 720°C and 820°C and for various times and sequences of temperature, our samples decompose into nanometer sized dispersed aluminotitanate particles. In Raman spectra of these glasses an evolution of a boson peak was observed. The width of the relatively broad boson band decreases as does the frequency of the band. From small-angle X-ray scattering data we conclude that the boson peak is connected with elastic vibrations of amorphous or crystalline regions of inhomogeneity with a dimension of ∼1.7 nm in initial glasses or larger depending on the heat treatment sequences.     

Methods for measuring light scattering in germanium and paratellurite crystals

Methods for studying the scattered light in germanium and paratellurite (α-TeO₂) crystals are considered. Investigations of the light scattering in Ge crystals were performed in the infrared wavelength range by the photometric-sphere method (in the range 2–3 μm) and by measuring the line-scattering functions (at 10.6 μm). In the visible range, the paratellurite single crystals were investigated by recording and analyzing images of laser beams transmitted through the samples. It is shown that small-angle Mie scattering is characteristic of both materials. Some conclusions about the sizes and the physical nature of scattering inhomogeneities are drawn. The effect of high-temperature annealing on the scattering intensity is studied.     

The structure of borosilicate glasses studied by Raman scattering

Raman spectra of alkali and alkaline earth borosilicate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of Al₂O₃ additions on the structure of borosilicate glass is also discussed. It is shown that the same type of groups are present in borosilicate glasses as in borate and silicate glasses. The presence of large borate groups such as tetraborate and metaborate groups is strongly suggested by the Raman spectra. It appears that boron ions are hardly taken up in the silicon-oxygen network. Our results suggest that the region of phase separation is larger than the region presently acknowledged.     

The structure of borate glasses studied by Raman scattering

Raman spectra of alkali and alkaline-earth borate glasses are reported. These spectra are used to discuss the molecular structure of the glasses. The influence of additions of Al₂O₃ on the structure of alkali borate glasses is also presented. The experimental evidence indicates that the same type of groups are present in borate glasses as in crystalline borates. The presence of tetraborate, metaborate, pyroborate and orthoborate groups in borate glasses is strongly suggested by the Raman spectra.     

Low-energy excitations in polyvinyl chloride: Raman scattering and thermal properties

The low-energy excitations of a very fragile glass-former (in the sense of Angell), poly(vinyl chloride), have been studied by low-frequency Raman scattering and by measurements of the low-temperature heat capacity and thermal conductivity. Two different samples were investigated : one with a crystallinity of about 15%, the other quenched and amorphous, as measured by small-angle neutron scattering. The boson peak in Raman scattering was observed in both samples even at room temperature. A clear correspondance between the Raman boson peak, the excess of heat capacity and the plateau of thermal conductivity was shown. It is confirmed that the boson peak or the heat capacity excess are relatively small in this very fragile glass-former. However it is deduced that the high concentration of tunnelling systems in the amorphous sample is the result of a rapid quenching rather than an intrinsic property of fragile glass-formers.     

X-ray and Raman studies of single crystals of CsSbF6

Crystals of CsSbF₆ belong to the rhombohedral space groupR¯3-C _3i ² witha=7.904(1)andc=8.261(1) Å,V=446.95 ų,Z=3,D _c=4.11 gcm^–3. The antimony atom is surrounded by six fluorine atoms in a nearly perfect octahedral configuration with Sb-F 1.875(9) Å, while 12 fluorine atoms surround the cesium atom with closest contact 3.116 Å. Polarized Raman spectra of single crystals of CsSbF₆ have been obtained, and it is shown that these results can be interpreted in terms of a unimolecular rhombohedral structure. The small distortion from an octahedral arrangement for the SbF ₆ ^– group is clearly reflected in the spectra. The Raman results are in better agreement with the space groupR¯3m-D _3d ⁵ than withR¯3-C _3i ² , but this conclusion must be regarded with caution since the two features in the vibrational spectra of CsSbF₆ which can be used to distinguish between the two structures are weak and ill-defined.     

Raman scattering and nonlinear refractive index measurements of optical glasses

The absolute Stokes-Raman scattering cross section is measured for optical glasses of varying composition, from which the nuclear contribution to the nonlinear polarizability is determined. The nonlinearities increase progressively from fluoroberyllate compositions to fluorophosphate to phosphate to silicate and finally to heavy-metal oxide glasses. Although values of the total nonlinear polarizability vary by nearly two orders-of-magnitude it is determined that the ratio of the nuclear to the electronic contribution is nearly constant and is independent of the glass composition. The electrostrictive contribution to the nonlinear polarizability is also calculated using previously measured elasto-optic constants. Correlations in the atomic structure were determined by measuring the position of the lowest-frequency Raman scattering peak and it is found that the correlation length for pure one-component glasses is larger than multi-component glasses. Examination of the low-frequency scattering region shows that there is an 0xcess light-scattering below 30 cm^?1 in all glasses tested. This excess scattering in LaSF-7 was fitted between 2 to 20 cm^?1 to a Lorentzian centered at zero frequency with a halfwidth of 2.5 ± 1 cm^?1, while the fit in pure silica between 5 and 30 cm^?1 yields a halfwidth of 8 ± 3 cm^?1.     

Raman scattering and electron spin resonance studies of fluorozirconate glasses

Electron spin resonance and Raman spectra are reported for fluorozirconate glasses which contain transition metal impurities. From the observed g values at 2.0 and a broad signal from 1 to 10, it is concluded that the dominant contribution to the ESR signal is due to Fe³⁺ in sites with rhombic distortion. The Raman spectra are shown to be superposed by luminescence emissions for paramagnetic impurity concentrations as low as ∼10¹⁶ spin/mol of ZrF₄. However, the amplitude and frequency of the dominant polarized Raman mode near 580 cm⁻¹ are observed to be relatively independent of impurities.     

Enhancement of Raman scattering in Subwave plasmonic nanostructures formed by ion-beam lithography

Subwave gratings with optimized architecture corresponding to the spectral position of resonances at desired wavelengths have been fabricated by ion-beam lithography. The structural and optical properties of the nanostructures have been studied. It is shown that experimental transmission (reflection) spectra are in good agreement with the theoretical spectra calculated within the grating-eigenmode model. An analysis of the Raman scattering of molecules adsorbed on the surface of gratings with optimized architecture demonstrates an increase in the Raman signal intensity from these molecules by four orders of magnitude. Highly sensitive sensors and various elements of optoelectronics can be developed based on these nanostructures.