Phonon Mode Frequencies Research Articles

Raman studies of BEDT-TTF molecule-based organic superconductors

The temperature dependence of the frequencies of the Raman features due to ${\ensuremath{\nu}}_{9}$ ${(A}_{g})$ and ${\ensuremath{\nu}}_{60}$ ${(B}_{3g})$ modes has been measured in the BEDT-TTF (ET) molecule-based organic superconductors $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br},$ $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl},$ ${\ensuremath{\alpha}}_{t}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{I}}_{3},$ and $\ensuremath{\beta}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{AuI}}_{2}.$ The frequency of the ${\ensuremath{\nu}}_{9}$ ${(A}_{g})$ mode was observed to soften in $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$ and $\ensuremath{\kappa}\ensuremath{-}(\mathrm{ET}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}$ at low temperatures where NMR data indicated antiferromagnetic fluctuations, providing evidence of spin-phonon interactions. The result is also compared with a very recent theory which deals with the effect of electronic correlations on the frequencies of the molecular phonon modes. We observed a peak splitting for the ${\ensuremath{\nu}}_{60}$ ${(B}_{3g})$ mode in ${\ensuremath{\alpha}}_{t}\ensuremath{-}(\mathrm{ET}{)}_{2}{\mathrm{I}}_{3}$ below 190 K, where an incommensurate superstructure was reported, and a frequency decrease of $1.7\ifmmode\pm\else\textpm\fi{}0.4 {\mathrm{cm}}^{\ensuremath{-}1}$ for the upper component of the ${\ensuremath{\nu}}_{60}$ ${(B}_{3g})$ mode in the superconducting compound, demonstrating further the unusual properties of this mode.

Raman scattering studies on low-energy nitrogen-implanted semi-insulating GaAs

Liquid-encapsulated Czochralski (LEC) grown undoped semi-insulating (SI) gallium arsenide (GaAs) single crystals have been implanted with 120 keV nitrogen ion (N +) of fluences 1×10 14, 1×10 15 and 1×10 16 cm −2 . Raman scattering studies of the as-grown and implanted samples have been carried out at room temperature and analyzed. It is observed that the A 1(LO) phonon mode frequency decreases with increase of fluence, whereas the full width at half maximum (FWHM) increases and the area under the peak decreases. All these effects are due to the lattice disorder induced by the implantation. The implanted samples have been annealed at 673 K. They show a partial recovery of the A 1(LO) phonon mode frequency and the FWHM which tend towards the unimplanted values due to partial annealing of the implantation-induced lattice damage. The post-implantation annealed samples show an increase of the area under the peak due to the enhanced Raman scattering caused by the surface roughness which was caused by the implantation. This increase in area was less at high fluences because of the residual lattice damage induced by the implantation, which caused the decrease of the area.

Strain and composition dependence of the E1(TO) mode in hexagonal Al1−xInxN thin films

Infrared spectroscopic ellipsometry is used to study the influence of strain and composition on the transverse-optical phonon mode of E1 symmetry in hexagonal Al1−xInxN films for 0.12⩽x⩽0.21. The 0.1–0.2-μm thick films were grown on slightly compressively strained hexagonal GaN buffer layers, or directly on [0001] sapphire by metalorganic vapor phase epitaxy. The Al1−xInxN E1(TO) phonon shows a one-mode behavior in contrast to recent theoretical predictions [H. Grille, C. Schnittler, and F. Bechstedt, Phys. Rev. B 61, 6091 (2000)]. Films grown on GaN reveal the influence of strain on the phonon mode frequencies due to pseudomorphic film growth. Al1−xInxN deposited directly on sapphire possesses phonon modes which indicate fully relaxed film growth.

Infrared and Ultraviolet Raman Spectra of AlN Thin Films Grown on Si(111)

AbstractInfrared (IR) spectroscopy and ultraviolet near-resonance enhanced Raman scattering have been used as alternative techniques to characterize the AlN films grown on Si(111). Epitaxial AlN films have been prepared by plasma source molecular beam epitaxy (PSMBE) at growth temperatures of 400 and 650 oC. The AlN/Si grown at 650°C shows distinct reflection high-energy electron diffraction (RHEED) patterns with the following epitaxial relations: AlN [0001] ‖ Si [111], and AlN < 011 0 > ‖ Si < 112 >. This is in sharp contrast with the observation of a spotty RHEED pattern for AlN/Si sample grown at 400°C, which appeared the same when viewed along any Si azimuth. Furthermore, the X-ray diffraction (XRD) rocking curve width showed a significant reduction from 5.7ofor sample grown at 400°C to 2° for the one grown at 650°C. The observation of Raman active A1(LO) and E2, and IR active E1(TO) zone center phonons, in both the films, is in accordance with the c-axis orientation of the films. However, the observed phonon mode frequencies and line widths indicate that the AlN samples grown at 400°C have less strain but more disorder compared to the ones grown at 650°C in agreement with RHEED and XRD data.

Raman spectroscopy studies in InGaN/GaN wurtzite epitaxial films

ABSTRACTIn this work we studied a set of nominally undoped epitaxial InxGa1−xN wurtzite films grown on (0001) sapphire substrates. In order to separate the contribution of the strain and indium content in the phonon mode frequency, indium mole fraction was determined using a strain insensitive method, Rutherford backscattering spectrometry (RBS). Strain was evaluated by comparing the lattice constants measured by X-ray diffraction (XRD) with the relaxed lattice parameters given by Vegard's law. Samples with comparable indium content, but under different states of strain were used as reference. This allowed the behaviour of different Raman shift modes for both, strain and composition to be independently established. We also assess the potentiality of Raman spectroscopy for the evaluation of crystalline quality by comparing the results obtained with the ones provided by other well-established methods such as XRD and RBS.

IR-VUV Dielectric Function of Al1−xInxN determined by Spectroscopic Ellipsometry

ABSTRACTSpectroscopic Ellipsometry from the mid-infrared (mid-ir) to the vacuum-ultraviolet (vuv) spectral range (350 cm−1 … 8.8 eV) is used to study the optical properties of hexagonal MOVPE-grown Al1−xInxN films for 0.11 ≤ × ≤ 0.21. The AlInN E1(TO) phonon shows a onemode behavior in contrast to recent theoretical predictions [H. Grille, Ch. Schnittler, and F. Bechstedt, Phys. Rev. B 61, 6091 (2000)]. Approximately 120 nm thick Al1−xInxN films grown on slightly compressively strained hexagonal GaN buffer layers reveal the influence of in-plane strain on the E1(TO) phonon mode frequencies. Al1−xInxN deposited directly on [0001] sapphire substrate possesses E1(TO) mode frequency which indicate fully relaxed film growth. For highquality Al0.890In0.110N one A1(LO) phonon mode was observed. Furthermore, we present the complex dielectric function of hexagonal Al0.872In0.128N from the mid-ir to vuv spectral range.

Observation of Thermal Phonon Resonance in Cylindrical Microcavities

Brillouin scattering measurements were made in small capillary tubes filled with water with 1450 µm and 920 µm in inner diameters. Hyper-resolution spectroscopy using an optical beating technique revealed thermal phonon resonance in the cavity around 8 MHz and 30 MHz. The observed spectra have a complicated structure with periodical main peaks and other subpeaks accompanying each main peak. The resonance frequencies of the cylindrical phonon modes were theoretically calculated and given through a series of n-th order Bessel functions. Frequencies of the fundamental (n=0,1) modes were in good agreement with those of the observed main peaks, and the subpeaks were associated with higher modes with n≥2. The mode assignment was successful. As the frequency increased, all the sub-peaks gathered towards the relevant main peak and were degenerated. This phenomenon was applied to measure the phonon velocity of liquid confined in a small cavity. Measurement was made for water over a temperature range from 20 to 28°C, and the result demonstrated a high sensitivity for detecting velocity changes smaller than 0.1%.

Micro-Raman investigation of stress variations in lead titanate films on sapphire

Using the sol–gel method, PbTiO3 films of 21, 64, 128, 210, 310, and 420 nm thicknesses were obtained on (0001) sapphire. Raman scattering and x-ray diffraction techniques were used to study the correlation between the film thickness and the structural changes on these films. The Raman and x-ray intensities in 21 nm film were too weak to reveal any structural information, while all other films showed tetragonal structure. At room temperature, the variation of lowest E(1TO) phonon mode frequency with film thickness was observed due to compressive stresses in the films. The lattice parameters and the degree of a axis orientation values of tetragonal PbTiO3 have been evaluated as a function of film thickness. The changes in lattice parameters thus obtained were used to estimate the stress at each thickness. An excellent agreement was found between the stress values obtained using Raman and x-ray results. An exponential decrease in stress with increasing film thickness was observed because of the structural changes in the lattice. Accordingly, a downshift in the tetragonal–cubic transition temperature with decreasing film thickness has been interpreted using the conventional Landau–Devonshire approach.

Correlation betweenTcand oxygen arrangement of the charge reservoir block in(Cu,C)Ba2Ca2Cu3O9−δ:A Raman study

The polarized Raman spectra of Cu0.5C0.5Ba2Ca2Cu3O92d @(Cu,C)1223# and their variations upon removal of oxygen were studied. The observed Raman lines at 117, 144, 160, 265, 381, and 507 cm for the as-prepared samples were assigned to definite atomic vibrations. It was found that the increase of Tc with the withdrawal of oxygen correlates with changes of the phonon mode frequencies and with appearance and enhancement of a Raman band near 600 cm. The latter band decreases again with further reduction of the oxygen content. The results are discussed within a model accounting for the oxygen out-diffusion from the chain sites in the (Cu,C)-O22d charge reservoir layer in close comparison with earlier reports on the variations with laser annealing of the Raman spectra of YBa2Cu3O72d . @S0163-1829~99!12813-3#

Raman probing of thermal damage depth profile in annealed GaAs

Raman scattering has been used to get information on the depth profile of both lattice damage and solid arsenic formation in gallium arsenide annealed at different temperatures. Measurements of Raman peak intensity ratio of GaAs transverse to longitudinal optical phonons as a function of exciting wavelength with a different penetration depth of the light gave information on the depth profile of the chemical and structural disorder in the gallium arsenide lattice. Moreover, measurements of the arsenic A1g and Eg phonon mode frequencies as a function of temperature showed that the arsenic formed in the vicinity of the surface is crystalline and submitted to very high temperature-dependent tensile stresses, with values ranging from 1.5 GPa at 300 K until 2.8 GPa at 10 K. On the other hand, the analysis of selection rules, phonon frequency, and linewidth indicates a strain-free and misoriented GaAs matrix.

Adsorbate vibrations and substrate surface phonons ofp(2×2)O/Mo(110)

We investigated the system $p(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{O}/\mathrm{M}\mathrm{o}(110)$ by electron energy-loss spectroscopy. The specular spectra of the ordered $p(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure reveal an intense loss peak at $519{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ which corresponds to the perpendicular vibration of the O atoms adsorbed in the long bridge site. Increasing O coverage or hydrogen coadsorption causes a site conversion towards the triply coordinated hollow site. Measurements of the dispersion curves of the molybdenum Rayleigh wave and the longitudinal mode do not show any significant change compared to the clean surface. This fact is surprising because the experimentally observed Fermi surface of the $p(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure should favor Fermi-surface nesting and hence an anomalous softening of the frequencies of the surface phonon modes, as previously observed for the hydrogen-saturated Mo(110) surface.

Substrate surface phonons in the cases of saturated (1×1)H/Mo(110) and p(2×2)O/Mo(110): a critical comparison

We investigated the dispersion of substrate surface phonons of p(2×2)O/Mo(110) with electron energy loss spectroscopy (EELS). As a result, we obtained no significant change compared to the clean surface. This fact is surprising because the shape of the Fermi surface should favour Fermi surface nesting and hence an anomalous softening of the frequencies of the surface phonon modes. Such a softening had been observed for the hydrogen-saturated Mo(110) surface.

Lattice Modes in PbHPO4 and H2C4O4

A microscopic model has been proposed which gives an explanation of the observed anomalies in the phonon spectra of hydrogen-bonded ferroelectrics. Using a Green's function technique we have obtained a nonlinear temperature dependence of the frequencies of the phonon modes in PbHPO 4 and H 2 C 2 O 4 in agreement with the experimental data. The importance of the proton ordering effects, i.e. the pseudo-spin-phonon interaction on the lattice modes is shown.

Anharmonic effects in LHP- and LDP-type ferroelectrics, investigated with the Green‘s function technique

A Green‘s function technique is used to study the anharmonic pseudo-spin-phonon and phonon-phonon interaction effects in LHP and LDP-type ferroelectrics on optical phonon mode frequency, damping and transverse dynamical structure factor below and above Tc. The coupling between the transverse soft-mode and the optical phonon mode produces a central peak in the transverse dynamical structure factor. The temperature dependence of these quantities is discussed and it is in good agreement with the experimental data. At low temperatures the contribution of the anharmonicity is very small, but at temperatures near the critical temperature Tc and above Tc it increases strongly.

Concentration dependence of phonon mode frequencies and the Grüneisen coefficients in BaxSr1−x TiO3 solid solutions

A study of the Raman spectra of BaTiO3-SrTiO3 solid solutions has yielded information on the dependence of phonon mode frequencies on component concentration. The contribution to the dependences by the change in lattice volume through the Gruneisen coefficients, and the term connected directly with the solid solution composition are discussed.

Lattice dynamics study of Tl 2Ba 2CaCu 2O 8 high-temperature superconductor

We present the frequencies of the Brillouin zone center phonon modes for Tl 2Ba 2CaCu 2O 8, and give the phonon mode displacement patterns. The calculations are carried out in the framework of a simple force constant model, which includes first neighbor interactions and second neighbor interactions. The ten adjustable parameters are obtained by fitting the experimental data. Our calculated results are in good agreement with the experimental data available from Raman and infrared measurements, and provide new assignments for some zone center phonon modes.

Intrinsic and Thermal Stress in Gallium Nitride Epitaxial Films

ABSTRACTStrain in GaN epitaxial layers at room temperature is measured with three complementary methods: Raman spectroscopy (via shifts of phonon frequencies), low temperature photoluminescence (via shifts of band-edge luminescence), and X-ray diffraction (via shifts in lattice spacings). GaN films grown on the c-plane of sapphire tend to be in compression. Increasing the Si-dopant concentration (up to 1019 cm−3) is observed to add compressive strain to the layer. Axially resolved measurements obtained by micro-Raman in 4 μm thick Si-doped films reveal strain relaxation toward the sample surface at Si concentrations above 1018 cm−3. Mg- and Si-doped GaN films on SiC substrates are found to be in tension. An experimental methodology is presented that separates two contributions to the room temperature residual stress in GaN epilayers: (1) the thermal stress due to differences in the thermal expansion coefficients of the epilayer and substrate and (2) the intrinsic stress, which is influenced by the growth conditions. We measure stress as a function of temperature up to 325 C, about one-third of the growth temperature, by monitoring the frequency of the E2 phonon mode by Raman spectroscopy. A high-quality bulk single crystal of GaN is used as a strain-free standard. Over this temperature range, most layers behave elastically; the observed stress trends are well-fit by a thermal expansion model using previous reported values of the thermal expansion coefficients of GaN and the substrates. The intrinsic stress states at the growth temperature for films grown on sapphire and SiC are predicted to be tensile and compressive, respectively, in agreement with the a-plane lattice coefficient mismatch.

Superconductive phonon anomalies in high- Tc cuprates

We consider the effects of spin-phonon coupling within the slave-boson mean-field treatment of the extended t- J model. With no additional assumptions the theory gives a semi-quantitative account of the frequency and linewidth anomalies of certain phonon modes in YBa 2Cu 3O 7 at the superconducting transition. It contains also a good correspondence with observed spin-gap behaviour and isotope effect measurements.

Symmetry properties of chiral carbon nanotubes.

The method of zone folding is applied to the calculation of the phonon mode frequencies in carbon nanotubules. The Raman and infrared-active mode frequencies are determined for nanotubules of different diameters and chiralities.

Stark Effects on Band Gap and Surface Phonons of Semiconductor Quantum Dots in Dielectric Hosts

AbstractWe have investigated quantum-confined Stark effect (QCSE) on GaAs and CdSe nanocrystals and the electric field effect on surface phonons of GaAs nanocrystals isolated in sapphire substrates. For a strongly quantum-confined system, GaAs quantum dots illustrated no exciton energy shift. When the excitons are weakly confined in CdSe, a ∼ 2 meV red-shift was observed. On the other hand, the results of the electric field effect on surface phonon are dramatic both phonon oscilator strength and freqnency. As the strength of the electric field increases, the total intensity of the surface phonon decreases. At the same time, an additional peak was also observed at 277 cm-1, which is about 3 cm-1 above the center frequency of the surface phonon mode of GaAs nanocrystals embedded in a sapphire host.