Changes In Phonon Frequencies Research Articles

Raman study of element doping effects on the superconductivity ofMgB2

The influences of phonon frequency and unit cell volume on the superconductivity of element-doped ${\mathrm{MgB}}_{2}$ are discussed with reference to a Raman study on $\mathrm{SiC}$, C, Mn, and $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Ag}$-doped $\mathrm{Mg}\text{\ensuremath{-}}\mathrm{B}$ materials. A phenomenon has been found in the doped samples, in that the phonon frequency changes to counteract the crystal lattice variation to keep the system stable within a Gr\uneisen parameter of 2.0\char21{}4.0. The chemical doping effects on phonon frequency and unit cell volume can be explained by the harmonicity-anharmonicity competition in the compounds. A decreased electronic density of states is responsible for the depression of superconductivity that is seen in doped ${\mathrm{MgB}}_{2}$. The possibility of a high critical temperature, ${T}_{c}$, in the $\mathrm{Mg}\text{\ensuremath{-}}\mathrm{B}$ system exists if the material can possess both a high phonon frequency and a big unit cell volume at the same time, as indicated by the isotope effect and hydrogenation experiments.

Raman scattering on silicon nanowires: The thermal conductivity of the environment determines the optical phonon frequency

We studied the Raman spectra of silicon nanowires as a function of excitation power for various ambient gases. For a given excitation power, we find that the gas thermal conductivity determines the wire temperature, which can be detected by a change in phonon frequency. This shows that the redshift of the optical phonon in silicon nanowires compared to bulk silicon is mainly due to the lower thermal conductivity of nanowires and an increase in laser heating. The spectra of nanowires allow distinguishing gases on the basis of their thermal conductivity.

Structural phase transition in indium tungstate

Structural phase transition in In2W3O12 has been studied using differential scanning calorimetry, dilatometry, and Raman spectroscopy. The phase transition at 523 K has been found to be of first order with a heat of transition of 1.22 kJ/mole. In contrast to the earlier reports, the present dilatometric studies suggest that high temperature phase could have a small positive thermal expansion. Phonon frequencies in the monoclinic phase of this network structure were obtained from the Raman spectrum at 10 K. Large discontinuous changes in phonon frequencies are not found across the phase transition; however, temperature coefficients of phonon frequencies exhibit changes. Present results indicate that the local environment of tungstate ions in the high temperature phase is not significantly different from that in the monoclinic phase. Furthermore, weak temperature dependence of the phonon frequencies suggests that their anharmonicities are small in the high temperature phase and this could be responsible for the low value of thermal expansion.

In-plane anisotropy and temperature dependence of oxygen phonon modes in YBa2Cu3O6.6

The dispersion of the Cu–O bond-stretching and bond-bending vibrations in YBa2Cu3O6.6 has been studied by inelastic neutron scattering. While the behavior of the bond-bending vibrations can be well accounted for by a simple potential model, the bond-stretching vibrations show a highly anomalous behavior. The displacement pattern of the most anomalous phonons is in principle consistent with dynamic charge stripe formation. However, the pattern is rotated by 90° to what was expected from the magnetic fluctuations reported in the literature. Temperature dependent measurements revealed only moderate changes of phonon frequencies between 12 and 300 K.

Optical investigation of CdSe/ZnSe quantum nanostructures

We have used resonant and non-resonant Raman scattering as well as photoluminescence and cathodoluminescence experiments to study the structure and composition properties of CdxZn1−xSe formed by migration enhanced epitaxy of CdSe layers on ZnSe buffers. The spectral change of the photoluminescence maximum correlates with the increase of the Cd content, depending on the nominal CdSe thickness in the 1.5–3.0 ML range. The inhomogeneous broadening of the photoluminescence band is caused by the composition difference between the two-dimensional mixed CdxZn1−xSe layer and the inserted islands with larger Cd concentration. This is confirmed by phonon frequency changes in resonant Raman scattering for samples with different nominal CdSe thicknesses as well as in Stokes and anti-Stokes frequency changes observed in the 1.5 ML sample. Attention is paid to the role of defects on Raman scattering and photoluminescence for the 3.15 ML sample.

Forward Raman scattering by quasilongitudinal optical phonons in GaN

Quasilongitudinal optical phonons in wurtzite GaN have been studied by Raman spectroscopy in the forward scattering geometry with annular apertures. It is demonstrated that the geometry is simple and powerful for determining frequency variation of quasilongitudinal optical phonons in a relatively wide range of their propagation directions, over which the quasilongitudinal optical phonon frequency changes remarkably. Good agreement is obtained between experimental and theoretical results. Discussion is also given on selection rules for quasilongitudinal optical phonons, which are used to interpret polarized Raman signals.

Curious Variations in Energy Gap of Polytypic CdI2 and PbI2 Crystals on Laser Irradiation

Crystals of CdI2 and PbI2 were grown by zone melting. They were subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied. The crystals of CdI2 were of type 4H while those of PbI2 were of type 2H. Their energy gaps were determined by UV spectrophotometry. The CdI2 crystals showed a linear variation of energy gap with intensity of radiation (0 to 125 mW), with the energy gap value decreasing from 3.28 to 3.11 eV, when the exposure time was kept fixed (7 min). When the time of exposure was varied, with the intensity of radiation fixed at 50 mW, the energy gap initially decreased from 3.28 to 3.07 eV and then gradually rose to be restored to the original value, thus showing a nonlinear variation. For the PbI2 crystals, the energy gap steadily decreased from 2.53 to 2.02 eV, when the intensity of exposure was varied. However, the value drastically decreased from 2.53 to 1.04 eV for the case of variation of exposure time. The results have been explained in terms of changes in phonon frequency, amount of absorption and anisotropy of thermal conductivity of the material.

Ductile and brittle modes in single-point-diamond-turning of silicon probed by Raman scattering

In recent years, considerable progress has been made on the study of the machinability of fragile materials as crystalline semiconductors because of the demand for faster fabrication processes of complex surface shapes for optoelectronic applications. The most-studied machined semiconductors are silicon and germanium [1– 3]. Some studies indicate that ductility could be related to the high-pressure metallization (brittle-to-ductile) transformation that occurs when these materials are subjected to high hydrostatic pressures [4–7]. Because of this, most of the studies reported previously are devoted mainly to microindented or microcut Si and Ge. Among the experimental techniques used to probe their effects, Raman scattering has been successfully employed, mainly to exploit the presence of residual stresses around the indentations and grooves made by indenters [8]. It is well known that, due to the machining (or to the polishing or lapping) process, semiconductor surfaces can undergo structural damage [9]. Raman scattering is a powerful characterization technique in these cases because the vibrational spectrum of the material is greatly influenced by disorder and residual strains: These lead to changes in phonon frequencies, broadening of Raman peaks and breakdown of Raman selection rules [10]. For bulk crystalline Si (c-Si), the triple degenerate optical phonons display in the first-order Raman spectrum one sharp peak at 521 cm−1. Due to the positive phonon deformation potentials of Si, compressive (tensile) strains lead to positive (or negative) frequency shifts. On the other hand, due to the loss of phonon correlation length and the consequent breakdown of the q = 0 Raman selection rules (q is the phonon wave vector), disorder effects can lead to an asymmetric broadening and shifting of Raman peaks compatible to the dispersion relation of the material [11]. In the silicon case, the frequency and asymmetry point to lower values because the dispersion relationship presents decreasing optical frequencies, increasing phonon wave vectors. At maximum disorder (amorphous material, aSi), the first-order Raman spectrum reflects the phonon density of states: It presents two broad bands centered at about 100 cm−1 (acoustic band) and 470 cm−1 (optical band) [12]. In this letter, an original (macro-) Raman investigation of single-point-diamond-turned silicon samples machined in ductile and brittle modes is presented. To probe the depth profile of disorder effects, the 457.9, 488.0 and 514.5 nm lines of an argon ion laser were used as exciting light. For these lines, the penetration depth of the light is about 140, 270 and 340 nm for c-Si, respectively. For a-Si, the optical absorption coefficient can reach one order of magnitude higher, leading to penetration depths of about tenths of nanometers, depending on the degree of amorphization [13, 14]. All measurements were performed at room temperature, with special care taken taken to avoid overheating the samples. Cutting tests were performed on Si samples on the (1 0 0) surface. The samples were single-pointdiamond-turned using a facing operation on a RankPneumo ASG 2500 diamond-turning machine. Alkalisol 900 coolant cutting fluid was continuously sprayed onto the workpiece during machining to avoid overheating the sample. A 0 (−25) degree rake angle diamond tool with nose radius R= 1.52 mm and a clearance angle of 12◦ was used in all tests. The feed rate used was 12.5 (1.25) μm rev−1 and the nominal depth of cut was 10 (1)μm. These conditions provide brittle (ductile) mode during machining, with an opaque (mirrorlike) surface. The finished surfaces were observed using scanning electron microscopy (SEM). Fig. 1 shows a

Raman-active phonons in Bi 2Sr 2Ca n−1 Cu nO 2 n+4 d ( n=1,2,3) – phonon assignment and charge redistribution effects

The phonon Raman spectra of Bi 2Sr 2Ca n−1 Cu n O 2 n+4 d ( n=1,2,3), high- T c superconductors have been investigated in a number of single-crystal and polycrystalline samples. From the polarization and doping dependence, and from a comparison with previous reports on other high- T c cuprates, we identify the A 1g and B 1g symmetry modes that are Raman allowed within the ideal body-centered-tetragonal unit cell. On the basis of the assignments proposed here, we discuss the influence of aliovalent substitutions on the phonon modes in Bi 2Sr 2− x La x CuO 6+ d and Bi 2Sr 2Ca 1− y Y y Cu 2O 8+ d . We interpret the observed changes in phonon frequencies and intensities as a combination of internal pressure and charge redistribution effects induced by doping.

Raman spectroscopy as a mapping tool for localized strain in microengineered structures

The behaviour of microengineered devices is routinely modelled using finite element analysis (FEA). Whilst this is becoming increasingly sophisticated, the values of the material data used in FEA packages have considerable associated uncertainty. In particular, there is no general technique to measure localized strain: the FEA is done using an average value taken from, for instance, a displacement measurement followed by calculations on bending stresses. The aim of this paper is to show how Raman spectroscopy can be used to assess localized strain, by first using the technique to calibrate doping profiles in silicon: doping itself being a major contributor to the strain in a device. Localized results are important because many applications require heavily doped silicon structures: in high concentrations the dopant will distort the silicon lattice considerably. In addition, dopants are known to aggregate at certain crystal defects such as dislocations. Thus the distribution of the dopant, and the associated strain, will depend on the original crystal quality and its processing history prior to dopant incorporation. Results have previously been reported on Raman spectroscopy of boron doped silicon [1], using a destructive etch technique to obtain a through-wafer doping profile. To our knowledge no results have been published on localized measurement of doping with the spatial resolution (,1 μm) proposed here, and using a nondestructive technique. There is an additional benefit from the work. Some microengineered devices, in particular those which require resonant motion, may have long term failure mechanisms which are caused by localized strain maxima. The ability to map the distribution of strain within a device, and then correlate this data with points of failure, should lead to more reliable design and manufacturing. The principles of this technique have been reported elsewhere, e.g. [2], as have details of the microline focus spectrometer (MiFS) used in our experiments [3]. Fig. 1 is a schematic of the instrument. Both the band shift and band half-width can be plotted as a function of position to give a direct two-dimensional picture. The established strain induced shift in orientated single crystal silicon and other materials points to the use of shifts in the doped silicon phonon frequency to determine relative strain distributions in the fabricated structures. As a result, strain peaks in the device, including overlays, can be observed. The instrumentation is capable of obtaining spectra with sub-micron resolution and can generate profiles and images representing intensity (species concentration), frequency (stress) and bandwidth (crystallinity). Applied mechanical stresses will result in phonon frequency changes, and structural alterations will change the phonon density of states population and thus the band shape. In the case of silicon these spectral responses to stress have been published [4], although the manifestation of all the theoretically predicted spectral changes has not been observed. A (1 0 0) n-type silicon wafer, of 10 U cm resistivity, was used as the starting material. After cleaning and etching to remove the top 3 μm of the wafer surface, the substrate was loaded into a diffusion furnace alongside a solid source of boron. Diffusion was carried out at 1100 8C for two hours. This is enough to produce a heavily doped p-type surface layer, with a concentration in excess of 3 3 10 ions cmy3 at a depth of 2 μm; confirmation is provided by this process being used to produce an etch stop layer in an ethylenediamine pyrocatechol (EDP)-based process. Substantial changes to the phonon spectrum of silicon in boron-diffused material have been ob-

Raman-active phonons in Bi2Sr2Ca1-xYxCu2O8+d (x=0-1): Effects of hole filling and internal pressure induced by Y doping for Ca, and implications for phonon assignments.

The phonon Raman spectra of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathit{d}}$ (x=0-1) have been investigated in a number of well-defined single-crystal and polycrystalline samples. From the polarization and Y-doping dependence, and from a comparison with previous reports on Bi-based cuprates, we identify the (6${\mathit{A}}_{1\mathit{g}}$+1${\mathit{B}}_{1\mathit{g}}$) symmetry modes that are Raman allowed within the ideal body-centered-tetragonal unit cell. A large number of extra ``disorder-induced'' phonon bands are observed in the ab-plane polarized spectra. In contrast to most previous reports, we argue that the c-axis polarized phonon band around 629 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is due to the O(2${)}_{\mathrm{Sr}}$ ${\mathit{A}}_{1\mathit{g}}$ vibration, while the exclusively ab-plane polarized band around 463 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is induced by the O(3${)}_{\mathrm{Bi}}$ ${\mathit{A}}_{1\mathit{g}}$ vibration. With increasing Y doping we find that the vibrational modes involving atoms in the ${\mathrm{CuO}}_{2}$ planes rapidly increase in intensity as a result of the reduced metallic screening in the hole-depleted Y-doped samples. We also find that Y substitution gives rise to a substantial hardening of the O(1${)}_{\mathrm{Cu}}$ ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ phonons by \ensuremath{\sim}40 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, whereas the O(2${)}_{\mathrm{Sr}}$ ${\mathit{A}}_{1\mathit{g}}$ phonon is found to soften by \ensuremath{\sim}20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, when x increases from 0 to 1. The phonon frequency changes can be explained by the ``internal pressure'' induced by the decrease in the average Ca/Y ion size and an additional ``charge-transfer'' induced by the change in the Cu and Bi valences with Y doping. \textcopyright{} 1996 The American Physical Society.

The roles of spin wave and phonon in the temperature dependence of magnetization in an itinerant electron ferromagnet

We show that since the phonon frequency changes with magnetization in an itinerant electron ferromagnet, phonons can have an effect on magnetization that is similar to that of spin waves. From a model numerical analysis we find that the size of the phonon effect is generally of the same order of magnitude as that of the spin wave.

Fatigue mechanisms in thin film PZT memory materials

The recent evolution of high-quality thin films of the PZT family has brought closer the use of ferroelectrics in non-volatile memory applications. While the inherent properties of the films are more than acceptable for most memory applications, the reduction in switched charge under repeated ferroelectric switching remains a problem. This so-called fatigue leads to changes of phonon frequencies and Raman cross sections in PbZr0.48Ti0.52O3 (PZT). Simultaneously, a characteristic IR absorption band near 4000 cm-1 shifts towards higher energies. This absorption signal is discussed in terms of polaronic states related to carriers generated by oxygen loss from the samples. It will be shown that the fatigue in PZT can be related to an increase of the local lattice distortion and a formation of defects both caused by oxygen loss from the samples.

Heat transport properties of semiconductors under nonuniform stress

The effects of an inhomogeneous stress field on the heat transport in semiconductors are qualitatively studied. Stress-induced spatial variations in lattice constant results in changes in phonon frequency and hence, in the phonon distribution leading to diffusion of phonons which constitutes heat flux at the macroscopic level. It is also found that this leads to a fractional change in thermal conductivity that is linear with the local stress tensor.

Electron-phonon interactions in oxide superconductors

We present recent results on the lattice dynamics of oxide superconductors as determined by inelastic neutron-scattering experiments. Included are data on the high- T c cuprate superconductors, for example YBa 2Cu 3O 7, as well as Nb-doped SrTiO 3, which is a low- T c system ( T c∼1 K). Particular emphasis is laid on direct indications of electron-phonon coupling such as phonon softening by renormalization or changes in phonon frequencies when passing through T c .

Thermal stability of InGaAs/InGaAsP quantum wells

We report a cross investigation of the effect of interdiffusion on the photoluminescence and Raman spectra of a single quantum well of InGaAs (80 Å wide) sandwiched between two large InGaAsP barriers. First, we investigate the blue shift of the recombination line (2 K) after annealing at 650 and 750 °C from 15 min to 2 h. We assume one single diffusivity coefficient for all atomic species (i.e., conservation of the lattice matching after annealing) and deduce the amount of intermixing through a model calculation. We find average diffusivity coefficients D=9.5×10−3 Å2 s−1 and D=2×10−1 Å2 s−1 at 650 and 750 °C, respectively. This agrees well with previous measurements reported for the parent system InGaAs/InP and supports an activation energy EA=2.54 eV. Next we investigate, on the same series of samples, the change in phonon frequency associated with the GaAs longitudinal-optical-like mode in the active InGaAs layer. To connect quantitatively the change in wave number with the change in arsenic composition, versus annealing sequence, one must use a carefully checked calibration curve. We show that the linear relationship Δy/Δω=4×10−2(cm) provides a satisfactory agreement with the results of our photoluminescence investigation. This gives for the amount of arsenic leaving the well Δy≊−5% per h1/2 at 650 °C and −20% per h1/2 at 750 °C, respectively.

Phonons and electron-phonon interaction in high T c superconductors

The phenomenon of high T c superconductivity and its present status is briefly reviewed. Electron-phonon interaction seems to play a role in the superconducting pair formation, although it does not suffice to account for the highest T c reached (≅ 130 K). A review of the optically active (Raman and ir) phonons is presented using YBa 2Cu 3O 7 ( T c = 92 K) as a paradigm, with particular emphasis on electron-phonon coupling phenomena. Observed changes in phonon frequencies, widths and spectral strengths near T c are discussed. Some consideration is given to the coupling of the phonons with crystal field levels in NdBa 2Cu 3O 7. Resonant Raman profiles are also discussed and compared with theoretical predictions.

Temperature dependence of the phonon frequencies, linewidths, and Raman-continuum scattering of single-domain Y0.56Pr0.44Ba2Cu3O7.

We report detailed Raman-scattering measurements from a single-domain crystal of composition ${\mathrm{Y}}_{0.56}$${\mathrm{Pr}}_{0.44}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (${\mathit{T}}_{\mathit{c}}$=40 K). Through a polarization analysis, 13 of the 15 Raman-active phonons are unambiguously identified. The frequencies and linewidths of the five ${\mathit{A}}_{\mathit{g}}$ phonons are studied as a function of temperature. Unlike ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, there are no pronounced superconductivity-induced changes in phonon frequency and linewidth. At all temperatures, the ${\mathit{B}}_{1\mathit{g}}$-like phonon at 320 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is intrinsically much broader than in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The linewidths of the 440- and 515-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonons decrease sharply with cooling, to an extent that cannot be explained by the simplest model of anharmonic decay. We identify Raman-scattering continua for photons polarized both within and perpendicular to the ${\mathrm{CuO}}_{2}$ planes. Like ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, the ${\mathrm{CuO}}_{2}$-plane continuum interferes strongly with the phonon scattering at 130 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Unlike ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, the ${\mathrm{CuO}}_{2}$-plane continuum of ${\mathrm{Y}}_{0.56}$${\mathrm{Pr}}_{0.44}$${\mathrm{Ba}}_{2}$${\mathrm{Co}}^{3}$${\mathrm{O}}_{7}$ has a low-frequency tail that disappears upon cooling.

Temperature effects in phonon dispersion of SmB 6 intermediate valence semiconductor

Temperature dependence of previously discovered anomalies in SmB 6 acoustic and low-lying phonon branches was investigated on three-axis and time-of-flight spectrometers. The largest changes in phonon frequencies with respect to the room temperature data were found for longitudinal branches while all transverse phonons hardened slightly. The unusual nature of the previously observed dispersionless mode in the energy gap between acoustic and optic phonons was confirmed. Some new peculiarities of the SmB 6 excitation spectra were obtained at low temperature.

Raman analysis of Si/Ge strained-layer superlattices under hydrostatic pressure

Raman scattering was used to study optical phonons in a Si12Ge4 strained-layer superlattice on c-Si(001) that was subjected to hydrostatic pressure at room temperature. The change of phonon frequency with pressure, dω/dP, for the principal quasi-confined LO mode in the Ge layers, is found to be significantly smaller than that for bulk crystalline Ge. This difference is shown to be due to the tuning of biaxial strain in the Ge layers and the pressure response of the confined mode as hydrostatic pressure is varied. Both strain and confinement make comparable contributions to dω/dP for the Ge layers in the superlattice examined here.