Transverse Optical Modes Research Articles

Raman investigation of GaP–Si interfaces grown by molecular beam epitaxy

Raman spectroscopy was used to investigate the residual strain in thin GaP layers deposited on Si substrates by molecular beam epitaxy. Different growth conditions were used to obtain a clean GaP–Si interface, including migration enhanced epitaxy. The strain induced Raman shifts of the longitudinal and the transverse optical GaP lattice modes were analyzed. The effects of crystalline defects are discussed, supported by high resolution transmission electron microscopy and X-ray scattering studies. Finally, Raman Spectroscopy reveals the presence of disorder (or surface)-activated optical phonons. This result is discussed in the light of surface morphology analyses.

Lattice dynamics and dielectric functions of multiferroic BiFeO3/c-sapphire films determined by infrared reflectance spectra and temperature-dependent Raman scattering

Multiferroic BiFeO3 (BFO) films have been grown on c-sapphire substrates by pulsed laser deposition under different laser energies (EL). The X-ray diffraction and Raman spectra indicate that the films are polycrystalline and exhibit the single rhombohedral (R) phase. The crystal distortion becomes weaker with decreasing the EL, which is described by the ratio of c/a. It was found that different EL values also lead to the variation of the Bi/Fe ratio. Temperature-dependent Raman spectra were carried out to study the phonon mode evolution behaviors. The three A1 transverse optical (TO) phonon modes located at 219, 172, and 142cm−1 shift towards a lower energy side with the temperature due to thermal expansion, thermal disorder and the anharmonic effects of lattice. The E(TO) and three A1(TO) phonon frequencies slightly increase with increasing the EL of the growth condition, which results from the Bi vacancies, the changes of the length and intensity of BiO bonds and the local structure distortion in the FeO6 octahedra. The dielectric functions of the BFO films in the frequency range of 50–8000cm−1 have been extracted by fitting infrared reflectance spectra with the Lorentz multi-oscillator dispersion model. The variation trend of the dielectric functions with different EL can be observed and related to the packing density, surface roughness, and defect states. It was concluded that the EL corresponding to changing the c/a ratio has an obvious influence on the lattice vibrations and intraband transitions of the BFO films.

Multiphonon scattering and photoluminescence of two dimensional ZnS nanosheets grown within Na-4 mica

Two dimensional wurtzite ZnS nanosheets with thickness of 0.6 nm are grown within the interlayer spaces of sodium fluorophlogopite mica (Na-4 mica) using ion-exchange-cum-solution treatment method followed by sulfidation treatment at 873 K. The presence of wurtzite ZnS is confirmed by x-ray diffraction, electron microscopy, and Raman scattering studies. The two dimensional form of ZnS gives rise to a strong quantum confinement with the band gap blue shifted by 1.7 eV. Thickness of the nanosheet is confirmed using atomic force microscopy. Raman scattering studies show higher order transverse optical modes due to increased deformation potential in reduced dimension. In contrast to red shift of optical phonon modes in phonon confinement model, a blue shift observed is ascribed to a compressive stress on ZnS nanosheets grown within Na-4 mica interlayer spaces. An additional band at 315 cm−1 is assigned to surface optical phonon. Unusual broadening in room temperature photoluminescence spectrum may be due to strong coupling of excitons with overtones of longitudinal optical phonon modes.

Structural, dynamical and thermodynamics properties of YBi

We have performed first-principles calculations using the full-potential linear augmented plane-wave, plane wave pseudopotential methods and density linear-response theory to study the structural, electronic, dynamical and thermodynamic properties of the yttrium bismuth YBi. Total energy calculations of the zinc-blende, rocksalt, and cesium chloride phases are investigated. The NaCl structure is found to be the ground state phase of YBi. Bismuth atom induces strong relativistic effects, creating a large spin–orbit splitting of ∼2000meV. Moreover, the phonon band structure is characterized by an appreciable overlap between longitudinal and transverse optical modes. Furthermore we found that the transverse and longitudinal acoustic phonon branches show respectively small variation along the X→W and W→L directions, while the longitudinal optical modes show flatness along a large part of the Brillouin zone. Bismuth atoms show low vibrational amplitudes as compared with yttrium atoms. Moreover we found a strong contribution to heat capacity and entropy from optical (acoustic)-modes of Y (Bi) atoms, with relatively small contribution of Y-acoustic modes and a vanishing contribution of optical modes of Bi.

Micro‐Raman analysis and finite‐element modeling of 3 C‐SiC microstructures

Micro‐ and nano‐electromechanical systems (MEMS and NEMS) fabricated in 3 C‐SiC are receiving particular attention thanks to the material physical properties: its wide band gap (2.3 eV), its ability to operate at high temperatures, its mechanical strength and its inertness to the exposure in corrosive environments. However, high residual stress (which is normally generated during the hetero‐epitaxial growth process) makes the use of 3 C‐SiC in Si‐based MEMS fabrication techniques very limited leading to a failure of micro‐machined/sensor structures.In this paper, micro‐Raman characterizations and finite‐element modeling (FEM) of microstructures realized on poly and single‐crystal (100) 3 C‐SiC/Si films are performed. Transverse optical (TO) Raman mode analysis reveals the stress relaxation on the free standing structure (796.5 cm−1) respect to the stressed unreleased region (795.7 cm−1). Also, microstructures as cantilever, bridge and planar rotating probe show an intense stress field located around the undercut region. Here, the TO Raman mode undergoes an intense shift, up to 2 cm−1, ascribed to the modification of the Raman stress tensor. Indeed, the generalized axial regime, described by diagonal components of the Raman stress tensor, cannot be applied in this region. Raman maps analysis and FEM simulations show the ‘activation’ of the shear stress, i.e. non‐diagonal components of the stress tensor. The stress‐Raman modes shift correlation, in the case of fully non‐diagonal stress tensors, has been investigated. The aim of future works will be to minimize the stress field generation and the defects density within the epitaxial layer. Copyright © 2012 John Wiley & Sons, Ltd.

Far infrared and microstructural studies of mechanically activated nickel manganite

Nickel manganite powder synthesized by calcination of a stoichiometric mixture of manganese (MnO Aldrich 99.9%) and nickel oxide (NiO Merck, 99.5%) containing 0.5wt% CoO and Fe2O3, was additionally mechanically activated in a high energy planetary ball mill for 5–60min. The resulting powders were uniaxially pressed into disc shape pellets and then sintered for 60min at 900°C, 1050°C and 1200°C. Morphological changes of the obtained nickel manganite ceramics induced by mechanical activation were monitored using scanning electron microscopy, while changes in structural characteristics were followed using X-ray powder diffraction. Room temperature far infrared reflectivity spectra for all sintered samples were recorded in the frequency range between 50cm−1and 1200cm−1. The observed spectra for all samples showed the presence of the same oscillators, but their intensities depended on the sintering temperature and the time of mechanical acivation. Transversal and longitudinal optical modes were calculated for six ionic oscillators (four strong, and two shoulders) belonging to the nickel manganite partially inverse spinel structure.

The vibrations of vitreous silica observed in hyper‐Raman scattering

Hyper‐Raman scattering (HRS) is performed in v‐SiO2, and the results are compared to RS and infrared absorption data. Mode analyses are tested based on two models, either the local C2v symmetry of the Si–O–Si bonds or the Td symmetry of the SiO4 tetrahedra. The HRS boson‐peak (BP) signal is dominated by the quasi‐local librations of tetrahedra. However, we find that it also contains a weak dipolar contribution with a different spectral signature, suggesting that the BP modes are not fully hybridized. The transverse optic (TO) modes can be assigned to molecular‐like excitations according to either model, with the exception of the lowest frequency TO4 which involves highly cooperative Si–O–Si motions. On the opposite, the strong scattering from all longitudinal optic (LO) modes clearly follows the average isotropic medium ∞∞m selection rules. This long range cooperativity presumably results from the associated internal electric field. Finally, the strong LO HRS signals enabled observing a new longitudinal component at 1078 cm−1. Through the example of v‐SiO2, the article also highlights new perspectives provided by HRS for the description of the vibrations in disordered materials in general. Copyright © 2012 John Wiley & Sons, Ltd.

Ultrafast carrier and coherent phonon dynamics in semi-insulated and n-type 4H-SiC

We investigated ultrafast carrier and phonon dynamics in semi-insulated (SI) and n-type 4H-SiC using time-resolved reflectivity measurements. In the n-type 4H-SiC, carriers are excited by an inter-conduction band transition, and thermalized by electron-electron scattering within 20 fs. We observed coherent phonons of folded phonon modes, which become Raman active as a result of the zone folding, and those of A1-symmetry longitudinal optical (A1-LO) phonon mode. In the n-type SiC, the A1-LO coherent phonon forms a coupled mode with a plasmon, resulting in an asymmetrically broadened Fourier transform spectrum. The polarization dependence and the sine-type initial phase indicate that the impulsive stimulated Raman scattering is the mechanism generating coherent phonons of E2-symmetry transverse optical and A1-LO modes in both the SI and n-type 4H-SiC.

Surface-enhanced Raman scattering of SnO2bulk material and colloidal solutions

Surface-enhanced Raman scattering (SERS) effects on tin dioxide in the form of bulk material, nanostructured thin films, and colloidal solutions were investigated. Raman spectra are characterized by the three Raman scattering peaks at 478, 633, and 776 cm${}^{\ensuremath{-}1}$, assigned to the ${E}_{g}$, ${A}_{1g}$, and ${B}_{2g}$ modes, typical of rutile SnO${}_{2}$. In the presence of the silver nanoparticles, in addition to the enhancement intensity of some of the fundamental tin dioxide rutile Raman features, the appearance of a new Raman scattering peak at about 600 cm${}^{\ensuremath{-}1}$ is observed. These spectral features are observed, in the presence of silver nanoparticles, also in other SnO${}_{2}$-based systems such as pulsed laser deposited thin films, with different stoichiometry, and in water colloidal solutions. The observed SERS effects are explained in terms of electric field gradient mechanism that are generated near a metal surface. In particular, the appearance of a peak near 600 cm${}^{\ensuremath{-}1}$ is attributed to the Raman activation of the infrared ${E}_{u}$ transverse optical mode.

Micro-Raman Analysis of a Micromachined 3C-SiC Cantilever

In this work, Raman microscopy is used to study the stress distribution on 3C-SiC cantilevers. Also we compare the strain distribution observed on the microstructure, using the shift of the transverse optical (TO) mode in micro-Raman maps, with the values predicted using a recent analytic theory [1]. Along the width of the cantilever is observed a reduction of stress ascribed to the etching processes that removes a thin layer of the interface between the 3C-SiC film and the substrate close to the edge of the microstructure. It is possible to show that this variation can be ascribed to a non-linearity of the strain field along the 3C-SiC film thickness. Also, helped by Finite Element Modelling (FEM), we determined the stress tensor along the cantilever. This result shows that, for a complete stress description of the cantilevers, it is necessary taking into account the role of diagonal and off-diagonal stress tensorial components.

New structural and vibrational opportunities combining Hyper‐Rayleigh/hyper‐Raman and Raman scattering in isotropic materials

Spontaneous hyper‐Raman and Raman scattering together with hyper‐Rayleigh scattering techniques were combined to study and characterize isotropic materials at the molecular range scale. We also demonstrated new transverse optical–longitudinal optical features in hyper‐Raman in the 90° scattering geometry where it is possible to selectively enlighten or cancel longitudinal optical modes. Noteworthy, it is possible to collect both transverse optical and longitudinal optical modes when collecting the scattered light perpendicularly to the plane of incidence. We detail and illustrate in the field of ionic liquids advances in nonlinear vibrational studies that give unique information about the nature and multipolar symmetry of the scatters. The vibrational multipolar approach, which is a generalized procedure to analyze and interpret all these techniques in isotropic materials, is introduced and applied to silica glass. For silica, we proposed a statistical elementary structural unit with nanometric dimension that fits both the octupolar hyper‐Rayleigh response and the multipolar spectral activity. It is derived from the α‐Cristobalite structure, with a local octupolar S4 symmetry, which consists of a central [SiO4] tetrahedron embedded in a super Si4/4 tetrahedron with bent Si–O–Si bonds. In that respect, the silica glass is thought to be a random medium with inhomogeneous density at the nanometric short‐range scale. Copyright © 2012 John Wiley & Sons, Ltd.

Ultralow Beam Divergence and Increased Lateral Brightness in Optically Pumped Midinfrared Laser

An optically pumped edge-emitting semiconductor laser emitting near 4.1 μm was designed with weak transverse mode confinement resulting in an exceptionally large transverse optical mode size. Consequently, a laser device is reported that exhibits a fast-axis divergence angle of ~4.2° full-width at half-maximum (FWHM). More notably, a large reduction of the lateral axis divergence is also observed as a result of the increased lateral coherency. The reduced confinement factor and differential gain serves to suppress lateral filamentation. Despite the higher threshold and lower power, a lateral divergence angle of ~3.2° FWHM and increased lateral brightness is achieved.

Ultrafast large-amplitude relocation of electronic charge in ionic crystals

The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100fs time resolution. Photoexcitation of the prototype material KH(2)PO(4) induces coherent low-frequency motions of the PO(4) tetrahedra in the electronically excited state of the crystal while the average atomic positions remain unchanged. Time-dependent maps of electron density derived from the diffraction data demonstrate an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions. Coherent longitudinal optical and tranverse optical phonon motions that dephase on a time scale of several picoseconds, drive the charge relocation, similar to a soft (transverse optical) mode driven phase transition between the ferro- and paraelectric phase of KH(2)PO(4).

Exciton-phonon coupling in individual ZnTe nanorods studied by resonant Raman spectroscopy

The exciton-phonon coupling in high-quality cubic phase zinc telluride (ZnTe) nanorods (NRs) is investigated by resonant micro-Raman spectroscopy near the direct bandgap of ZnTe. The scattering cross section of longitudinal optical (LO) phonon is enhanced significantly in the resonant process, where the enhancement factor of LO modes is much higher than that of the transverse optical (TO) modes, indicating a dominant Fr\"ohlich electron-phonon interaction mechanism. Up to fifth-order LO phonons are observed by resonant Raman scattering at room temperature. The Huang-Rhys factor of individual NRs---and thus the exciton-LO coupling strengths---is evaluated, showing increasing with the NR diameter. Surface optical (SO) phonon and its high-order overtones are observed between $n$LO and ($n$ \ensuremath{-} 1)LO + TO for the first time, whose positions are consistent with a dielectric continuum model. Strong acoustic phonon-exciton coupling induces a high-frequency shoulder above each $n$LO peaks with two maxima located around 14 cm${}^{\ensuremath{-}1}$ and 32 cm${}^{\ensuremath{-}1}$, which are assigned to transverse acoustic and longitudinal acoustic phonons, respectively. The resonant multiphonon scattering process involving acoustic and LO phonons is discussed based on an exciton-intermediated cascade model, where a scattering sequence of acoustic phonon followed by LO phonons is favorable. These results advance the understanding of electron-phonon coupling and exciton scattering in quasi-one-dimensional systems, especially in the scarcely documented ZnTe compound, facilitating the development and optimization of NR-based optoelectronic devices.

Strain Field Analysis of 3C-SiC Free-Standing Microstructures by Micro-Raman and Theoretical Modelling

In this article we compare the strain distribution observed in 3C-SiC/Si (100) cantilevers, using the shift of the transverse optical (TO) mode in micro-Raman maps, with the values predicted using a recent analytic theory []. By taking advantage of an under etching of the microstructures during the fabrication processes, that removes a thin layer of highly defective SiC close to the film/substrate interface near the edges of the microstructures, we show that the variation of the experimental measured strain can be ascribed to a non-linearity of the strain field along the 3C-SiC film thickness.

Confinement effects on the vibrational properties of III-V and II-VI nanoclusters

We present a first-principles study of the confinement effects on the vibrational properties of thousand atoms (radii up to 16.2 \AA{}) colloidal III-V and II-VI nanoclusters. We describe how the molecular-type vibrations, such as surface-optical, surface-acoustic, and coherent acoustic modes, coexist and interact with bulk-type vibrations, such as longitudinal and transverse acoustic and optical modes. We link vibrational properties to structural changes induced by the surface and highlight the qualitative difference between III-Vs and II-VIs. We describe the size dependence of the vibrations and find good agreement for Raman shifts and for the frequency of coherent acoustic modes with experiments.

Stress fields analysis in 3C–SiC free-standing microstructures by micro-Raman spectroscopy

The study of the residual stress in the 3C–SiC/Si hetero-epitaxy is a crucial step for the development of SiC based sensors and microsystems. In this work, Raman microscopy is used to study the stress distribution on 3C–SiC cantilevers. The shift of transverse optical (TO) mode is linked to the stress fields present in the structures, whereas the full width at half maximum (FWHM) is an index of the lattice order. Micro-Raman map allows an accurate study of stress fields acting on the structure. Along the width of the cantilever, a trend of the TO Raman shift, moving towards higher frequency from the centre to the edges of the microstructure, has been observed. This is ascribed to etching processes removing some SiC portion at cantilever edges. Also, close to the anchorage zone, an undercut region is generated due to Silicon over-etch. The undercut region shows a worst lattice order (increased TO-FWHM) with respect to the unreleased and free standing regions. Close to the undercut, the stress field cannot be considered biaxial. Helped by Finite Element Modelling, we determined the stress tensor along the cantilever. The results show that, for a complete stress description of the cantilevers, it is necessary to take into account the role of diagonal and off-diagonal stress tensorial components.

Raman Spectroscopy of Sr x Pb1-x TiO3 Thin Films

Raman spectra of Sr x Pb1-x TiO3 sol-gel films of different compositions, were measured and analysed at room temperature. The lowest transverse optic (TO) mode of E symmetry (ferroelectric soft mode) was identified in all our samples and its frequency was found to depend on composition. For the x = 0.26 and 0.47 samples, their temperature dependent ferroelectric soft mode responses were determined up to the paraelectric phases.

Tuning the Kapitza resistance in pillared-graphene nanostructures

The pillared-graphene architecture is a conceivable way of conjoining graphene nanoribbons and carbon nanotubes (CNTs) in nanoelectronics. Especially promising is its capability to dissipate thermal energy in thermal management applications. However, the thermal boundary resistance (Kapitza resistance) at the graphene nanoribbon-CNT interface is a phonon barricade and a bottleneck for efficacious heat extraction. Parallel to strain studies on thermal conductance, this work is a first report on the effects of mechanical strain on the interfacial phonon dynamics in the pillared-graphene nanostructure (PGN). Molecular dynamics simulations are employed to derive the changes in phononics as axial, torsional, and compound strains of various degrees are applied on the PGN. The pillar lattice structure behaves dissimilarly to the different types of strains. In-plane transverse optical mode softening as induced by torsional strain is more effective than LO softening (triggered by tension) in minimizing the thermal boundary resistance. Essentially, it is shown that there is a strong relationship between strained PGN pillar lattice structure, interfacial phononics, and thermal boundary resistance.

Structural, optical and electrical properties of cubic AlN films deposited by laser molecular beam epitaxy

Cubic AlN films were successfully deposited on TiN buffered Si (100) substrates by a laser molecular beam epitaxy (LMBE) technique, and their crystal structure and optical and electrical properties were studied. The results indicate that cubic AlN films show the NaCl-type structure with a (200) preferred orientation, and the lattice parameter is determined to be 0.4027 nm. The Fourier transform infrared (FTIR) pattern of the cubic AlN film displays sharp absorption peaks at 668 cm−1 and 951 cm−1, corresponding to the transverse and longitudinal optical vibration modes. Ellipsometric measurements evidence a refractive index of 1.66–1.71 and an extinction coefficient of about zero for the cubic AlN film in the visible range. Capacitance–voltage (C–V) traces of the metal–insulator–semiconductor (MIS) device exhibit that the cubic AlN film has a dielectric constant of 8.1, and hysteresis in the C–V traces indicates a significant number of charge traps in the film.