Transverse Optical Modes Research Articles

Raman activity of the longitudinal optical phonons of the LiNbO3 crystal: Experimental determination and quantum mechanical simulation

AbstractIn this study, the and the two equivalent / polarized Raman spectra of a LiNbO3 single crystal have been recorded and used as a benchmark test for the density functional theory (DFT) calculation of the longitudinal modes and of their Raman activity. The theoretical approach, based on periodic boundary conditions and a linear combination of atomic orbitals (LCAO), provides excellent predictions of phonon wavenumbers and relative bands intensities for both A1 and E Longitudinal Optical (LO) modes and complements a previous paper limited to the study of Transverse Optical (TO) modes. Overall, the present investigation demonstrates that the LCAO approach, as implemented in the CRYSTAL software, gives results of similar accuracy for the TO and the LO phonons features. By means of a band deconvolution scheme applied to the experimental spectra, we present, for the first time, a quantitative comparison between experimental and theoretically predicted polarized Raman band intensities of LiNbO3 LO modes. This analysis highlights the role of the suitable determination of the static and high‐frequency dielectric matrices that are needed for the prediction of the TO/LO frequency split but also the first nonlinear electric susceptibility tensor for an accurate description of the Raman intensity pattern.

First-principles calculations of Raman and infrared spectroscopy for phase identification and strain calibration of hafnia

Using density functional perturbation theory, we computed the phonon frequencies and Raman and IR activities of hafnia polymorphs (P42nmc, Pca21, Pmn21, Pbca OI, brookite, and baddeleyite) for phase identification. We investigated the evolution of Raman and IR activities with respect to epitaxial strain and provide plots of frequency differences as a function of strain for experimental calibration and identification of the strain state of the sample. We found Raman signatures of different hafnia polymorphs: ω(A1g)=300 cm−1 for P42nmc, ω(A1)=343 cm−1 for Pca21, ω(B2)=693 cm−1 for Pmn21, ω(Ag)=513 cm−1 for Pbca (OI), ω(Ag)=384 cm−1 for brookite, and ω(Ag)=496 cm−1 for baddeleyite. We also identified the Raman B1g mode, an anti-phase vibration of dipole moments [ω(B1g)=758 cm−1 for OI and ω(B1g)=784 cm−1 for brookite], as the Raman signature of antipolar Pbca structures. We calculated a large splitting between the longitudinal optical and transverse optical modes [ΔωLO−TO(A1z)=255 cm−1 in Pca21 and ΔωLO−TO(A1)=263 cm−1 in Pmn21] to the same order as those observed in perovskite ferroelectrics and related them to the anomalously large Born effective charges of Hf atoms [Z*(Hf)=5.54].

Formation of epitaxial 3C-SiC layers on Si by rapid vacuum thermal processing

The results of a study of the structure and phase composition of epitaxial layers of silicon carbide (SiC) formed on silicon substrate with orientation (100) under rapid vacuum thermal processing are presented. Planar-view transmission electron microscopy investigation revealed the formation of epitaxial layers of cubic polytype SiC (3C-SiC) on silicon in the process of carbidisation at 1100 °C during 30 s, using a gas mixture of propane (10 %) and argon (90 %) as a carbon source. The formation of a monocrystalline 3C-SiC with polycrystalline inclusions and twins on all possible planes {111} was found. A rather narrow band of 793 cm–1 transverse optical phonon mode SiC on Raman spectra confirms the formation of a cubic polytype SiC. It is noted that the presence of a 180 cm–1 spectral line and a 793 cm–1 half-width band on Raman spectra indicate the presence of deformation defects in SiC.

Lattice Dynamics and Structural Phase Transitions in Two-Dimensional Ferroelectric Methylhydrazinium Lead Bromide Investigated Using Raman and IR Spectroscopy

Two-dimensional (2D) lead halide perovskites are promising photovoltaic, light-emitting, nonlinear optical, and ferroelectric materials. Their key optoelectronic properties and polar order of organic cations are strongly affected by the dynamic response of the ionic lattice. Herein, we report polarized Raman and IR spectroscopy studies of methylhydrazinium lead bromide (MHy2PbBr4) exhibiting ferroelectric, photoluminescence, and multiple nonlinear optical phenomena. We determined the wavenumbers of longitudinal optical (LO) and transverse optical (TO) modes and their symmetries. These studies reveal that the dielectric response of MHy2PbBr4 is dominated by vibrations of the inorganic sublattice, which exhibits a large LO-–TO splitting (up to 44 cm–1). We also performed temperature-dependent Raman spectroscopy studies to monitor the effect of temperature on lattice dynamics and the effect of structural phase transitions on phonon modes. Raman spectroscopy data reveal that Raman bands are very narrow at 80 K, indicating a well-ordered structure. However, the increase of temperature leads to a very pronounced broadening of bands, indicative of a strong increase of lattice anharmonicity due to increased motional freedom of MHy+ cations and the inorganic network. Although former X-ray diffraction analysis showed that MHy2PbBr4 undergoes an order–disorder phase transition at 351 K, this transition leads to weak wavenumber shifts and broadening of Raman bands, more pronounced for modes related to the inorganic subnetwork. Furthermore, we do not observe any clear anomalies at 371 K, where the second phase transition into another disordered phase occurs. Our results indicate, therefore, that in contrast to many 2D lead halide perovskites, the dynamics of MHy+ cations and the inorganic network in MHy2PbBr4 do not exhibit pronounced slowing down at the order–disorder phase transition temperature but rather decrease gradually over a very large temperature range.

Thermal radiation resonating with longitudinal optical phonon from surface micro-stripe structures on metal-gallium nitride and sapphire

Thermal emissions from metal/dielectric-material stripe structures on the surface of dielectric materials are observed. These emissions are attributed to electric-dipole emissions induced by coherent thermal lattice vibrations in the surface structures of a few micrometers. Thermal emission based on coherent lattice vibrations has been reported via surface phonon polariton (SPhP), where metal ingredients have been excluded, whereas the present emission is independent of SPhP. The structures on undoped GaAs shows the emission close to the longitudinal optical (LO) phonon energy, while the peak energy of the emission from the structures on GaN is located in between the LO and transverse optical (TO) mode energies. The present studies on several vibration modes of α-Al2O3 and the dependence of the emission peak energy on geometrical conditions of surface structures reveal that modification of electric permittivity by the local depolarization field induces additional zero points of the real part of the permittivity, which yields additional local LO-like modes and the shift of the main emission peak energy from the original LO mode. This shift is significant for materials with a large energy difference between the LO and TO modes, such as 270 cm−1 for a mode of α-Al2O3 and 170 cm−1 for GaN. This is contrasted with the case of GaAs with 30 cm−1 difference.

Raman scattering studies of the ambient atmospheric thermal stability of Be in periodic Be/Mo and Be/W multilayer mirrors

The ambient atmospheric thermal stability of beryllium (Be) layers in Be/Mo and Be/W multilayer mirrors was investigated by Raman scattering. The physical characteristic of the transverse optical (TO) mode was considered for structural analysis of the Be layers in the multilayers. With an increase in thermal annealing temperature, two important modifications of this mode were noticed: the TO mode of Be was found to shift to a lower frequency and the peak width became wider. These two facts are related to the deterioration of the crystalline quality of the Be layers upon thermal annealing. The TO mode of the crystalline Be phase completely vanished and high-intensity peaks at the shoulder were detected for the Be/Mo multilayer thermally annealed at 723 K. This evidence is associated with the transformation of polycrystalline into an oxidized amorphous Be phase. In this case, the diffusion of oxygen to the inner period and the destruction of the modulation of the periodic structure of the multilayer was investigated by a secondary ion mass spectrometer. However, the TO mode of Be embedded within W in Be/W multilayers was stable in similar annealing conditions, which revealed the thermal stability.

STRUCTURAL AND OPTICAL PROPERTIES OF SiC/Si HETEROSTRUCTURES OBTAINED USING RAPID VACCUM-THERMAL CARBIDIZATION OF SILICON

TEM investigation revealed that the rapid vacuum-thermal carbidization of silicon at 1100C leads to the formation of cubic silicon carbide (SiC) layers. The band of the IR transmission spectrum at 798 cm–1 corresponding to the stretching vibration of Si-C and the maximum of Raman spectrum at 793 cm–1 relating to transverse optical phonon mode of SiC confirm the formation of a layer of the cubic SiC politype. The absorption
 band of Si-O-Si (1100 cm–1) was found using the IR-spectroscopy. The dependence of the transmission coefficient on the wavenumber was determined.

Infrared-active phonon modes and static dielectric constants in α-(AlxGa1−x)2O3 (0.18 ≤ x ≤ 0.54) alloys

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(AlxGa1−x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

Magnetic Control of Soft Chiral Phonons in PbTe.

PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Thus, our Letter reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.

Synchrotron based x-ray absorption spectroscopy investigation and temperature dependent ferroelectric properties of Ni doped BaTiO3 nanostructures

In this report, the local structure geometry of BaTi1-xNixO3 (0.0 ≤ x ≤ 0.06) ceramics has been studied through synchrotron based x-ray-absorption spectroscopy, measured at the Ti K, Ba L3 and Ni K-edges at room temperature. The x-ray absorption near edge spectroscopy (XANES) at Ti K-edge confirms the TiO6 octahedron geometry of Ti atom due hybridization between Ti(3d)-O(2p) orbitals and signifies the non-centrosymmetric nature of BaTiO3 samples. However, Ti–O off-center displacement (non-centrosymmetric) is disturbed under the effect of Ni doping as a result of oxygen vacancies formation. More interestingly, XANES studies at Ni K-edge ensure successful substitution of Ni in BaTiO3 as Ni2+ ions. Extended x-ray absorption fine structure (EXAFS) data at Ni K-edge have been fitted to estimate pertinent local structural parameters of the Ni–O, Ni–Ba, Ni–Ti and Ni–Ni shells (viz. bond lengths and disorder parameters) and it also reveal that the structural disorders around the Ti sites in the doped BaTiO3 expand with Ni doping. The mixed-valence states of titanium ion, i.e., Ti4+ and Ti3+ in the doped samples were established with the x-ray photoelectron spectroscopy (XPS). Moreover, XPS divulges the creation of oxygen vacancies due to Ni doping in BaTiO3 matrix. The complementary information about the lattice vibration is analyzed through Raman studies that approve the softening of the transverse optical (TO) mode present at 515 cm−1. The temperature dependent ferroelectric studies affirm that the ferroelectricity vanishes in the doped samples due to the decrement in the off-center displacement between the Ti4+ and O2− ions in the TiO6 octahedral geometry. Differential thermal analysis (DTA) exhibits a ferroelectric tetragonal to para-electric cubic phase transition in the pristine as well as Ni doped BaTiO3 samples.

Anomalous transverse optical phonons in SnTe and PbTe

We present a study of the soft transverse optic phonon mode in SnTe in comparison to the corresponding mode in PbTe using inelastic neutron scattering and ab-initio lattice dynamical calculations. In contrast to previous reports our calculations predict that the soft mode in SnTe features a strongly asymmetric spectral weight distribution qualitatively similar to that found in PbTe. Experimentally, we find that the overall width in energy of the phonon peaks is comparable in our neutron scattering spectra for SnTe and PbTe. We observe the well-known double-peak-like signature of the TO mode in PbTe even down to $T$ = 5 K questioning its proposed origin purely based on phonon-phonon scattering. The proximity to the incipient ferroelectric transition in PbTe likely plays an important role not included in current models.

Infrared dielectric functions and Brillouin zone center phonons of α−Ga2O3 compared to α - Al2O3

We determine the anisotropic dielectric functions of rhombohedral $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high-frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, the binary parent compound for the emerging alloy of $\ensuremath{\alpha}$-(${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{)}_{2}{\mathrm{O}}_{3}$, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations, we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single-mode or more complex mode behaviors.

Two-dimensional stanene: Electron-phonon interaction

The electron-phonon interaction is studied using density functional and many-body perturbation theories. It has been found that the temperature dependence of the electron linewidth and the electron scattering rate along the high symmetry line of the Brillouin Zone (BZ) and the contribution of acoustic and optical phonon modes to the electron scattering rate were determined and reveal an overall dominance of out-of-plane transversal optical modes (ZO).

Investigation of Be doped GaSb with varying hole concentrations using single magnetic field Hall measurement and Raman spectroscopy

Raman scattering results on Beryllium doped Gallium antimonide (GaSb) epi-layers with varying hole concentrations (3 × 1017 cm−3 to 5 × 1019 cm−3) are presented in this study. The hole concentrations are determined from single magnetic field Hall measurements and corrected taking into consideration two band conduction. The Raman spectra show four distinct features: Longitudinal optical (LO) phonon mode from the surface accumulation/depletion region, disorder activated forbidden transverse optical (TO) phonon mode, LO phonon coupled Plasmon (LOPC) mode and the uncoupled LO phonon mode from the bulk region. A detailed Raman line-shape analysis using deformation potential and electro-optic contribution has been carried out to reproduce the experimental results. The effect of plasmon damping on the LO phonon coupled Plasmon (LOPC) mode is discussed. Free hole concentrations are derived from the Raman line-shape analysis and compared with that from the Hall measurements.

Tuning the onset pressure of pressure-induced phase transition in indium phosphide by extrinsic doping

In-situ angle-dispersive X-ray diffraction (ADXRD) and Raman scattering were carried out to delineate the phase transitions under pressure in p-type zinc-doped indium phosphide (InP:Zn) at ambient temperature. The ADXRD measurements with pressure up to 17.3(2) GPa reveal that InP:Zn transforms from zinc-blende (ZB) to rock-salt (RS) structure in a pressure range of 8.1(2) - 8.6(1) GPa. The Raman scattering measurements indicate that the phase transition proceeds continuously over a wide range of pressure and reaches completion at the highest pressure applied (14.5(2) GPa) with significant metallization, as suggested by the disappearance of the transverse optical (TO) and longitudinal optical (LO) modes. In contrast, originally absent at ambient pressure, additional Raman shift of Zn local vibrational modes (LVMs), appeared at 276 cm-1 and 420.7(1) cm-1 within the pressure range of 2.2(1) up to 10.1(2) GPa, which can be attributed to Zn3P2 and VPZnInVP LO modes, respectively. The zero-pressure isothermal bulk moduli and its first order pressure derivatives for the zinc-blende InP:Zn are 79.44(2) GPa and 3.92(1), respectively. A total enthalpy change of about 45.45 KJ•mole-1 was derived for the ZB to RS structure transformation occurring at the pressure of 8.6(1) GPa. The present results suggest that diffusion of Zn in InP:Zn may have been governed by the modified substitution and interstitial (S–I) mechanism. Moreover, comparisons on the pressure-dependent behaviors in InP:Zn, InP:S, and InP:Fe extrinsically doped systems revealed that the exact type of the defects introduced into the lattice upon doping may have played crucial roles in the pressure-induced phase transitions.

An infrared study of antiferromagnetic medium and high entropy rocksalt structure oxides

The infrared reflectance of a set of antiferromagnetic entropy-stabilized rocksalt structure oxides is measured in order to study the effect of a random array of cations, spin–phonon coupling, and anharmonicity on phonon mode behavior. Samples were prepared by conventional solid state reaction from quaternary or quinary equimolar mixtures of CoO, ZnO, CuO, NiO, and MgO. They were characterized by magnetometry, x-ray diffraction, and reflectance spectroscopy. It is found that the Néel temperature (TN) depends on the concentration of magnetic ions. The far-infrared reflectance spectra consist of one strong reststrahlen band and a weaker low frequency band, which is unexpected in a rocksalt structure. The resonance frequency of the main transverse optical (ωTO) mode varies between 355<ωTO<386 cm−1 and is controlled by both the average reduced mass of a cation–anion pair and the average effective charge. Unlike the parent compounds CoO and NiO, no splitting of ωTO due to spin–phonon coupling appeared in the reflectance spectra below TN. This is possibly due to the high scattering rate of the associated transverse optical mode, which is connected to static disorder.

Polarized Raman spectroscopy of GaP nanowires under 5% elastic strain

Optical porperties of highly-strained gallium phosphide nanowires were investigated via polarized Raman spectroscopy. 5% elastic strain was created in individual nanowire lying on nickel substrate by the means of atomic force microscopy. Micro-Raman mapping along the nanowire cross section in parallel and perpendicular polarization was carried out. Strain-induced effects on transverse optical mode position and shape were analyzed. The pronounced splitting of the mode due to high level of strain was observed. It was found that in parallel polarization the mode shape is sensitive to the position of the pumping spot which can be attributed to enhanced light-nanowire coupling effects.

Super-Planckian thermal radiation between 2D phononic hBN monolayers

Near-field radiation heat transfer (NFRHT) in two-dimensional (2D) hexagonal boron nitride (hBN) monolayer and its composite symmetric-based structure was investigated. Contrary to three-dimensional (3D) materials, the splitting of modes (in 2D materials) into the longitudinal optical (LO) and transverse optical (TO) phononic modes disappears at the Γ-point even in some polar materials. The conductivity of atomic thin hBN monolayer depends on the LO phonon frequency under the long wavelength limit. Exploiting such concept, the electromagnetic local density of states (EM-LDOS) accounting for the near-field radiation spectrum was evaluated in free-space close to an interface with the hBN monolayer as well as composite structures. An intense narrow peak of LDOS was found in the case of hBN monolayer caused by the surface phonon polariton (SPhP) resonance. This peak was modified in the case of hBN-graphene composite due to hybridization, which strongly depends on two factors – the distance of observer (in vacuum) from interface and graphene Fermi energy. It was found that the flux density between monolayer hBN gets 2.8-fold enhanced as compared to the monolayer graphene configuration under 0.11 eV Fermi energy applied at a vacuum gap of 15 nm. The possibility of tuning Fermi energy has great potentials in designing thermally controlled devices.

Ion-beam synthesis of InSb nanocrystals at the Si/SiO2 interface

The formation of InSb nanocrystals at the bonding Si/SiO2 interface of silicon-on-insulator structure was obtained as a result of the In and Sb atom diffusion from the ion-implanted SiO2 and Si regions, respectively, toward the interface. After the annealing at 1000 °C, the Raman scattering peaks corresponding to the transverse and longitudinal optical phonon mode in the monocrystalline InSb matrix were obtained. As the annealing temperature grew to 1100 °C, the transverse optical phonon mode vanished and the longitudinal optical phonon mode dominated in the spectrum. This effect is explained by matching InSb and Si lattice constants under ion-beam synthesis conditions.

TO-phonon anisotropies in a highly doped InP (001) grating structure

For zinc blende semiconductors, such as InP, the Raman selection rules for a backscattering configuration from the (001) surface forbid the transversal optical (TO) phonon mode, whereas the longitudinal optical mode is allowed. However, when InP is highly doped with Si atoms, InP-Si clusters with the reduced C3v symmetry allow TO modes in the Raman spectrum with the backscattering configuration. Here, we demonstrate that the amplitude of the TO modes can be modulated spatially by using a highly doped InP grating. By exciting the sample with a laser linearly polarized parallel and perpendicular to the grating grooves, we observe a change in amplitude of the phonon optical response for the TO mode.