Raman Selection Rules Research Articles

Effect of vanadium doping on structural, dielectric, magnetic, and ferroelectric properties of Ba0.95La0.05TiO3 ceramics

The polycrystalline different Ba0.95La0.05Ti1-xVxO3 (where, x = 0.00 to 0.12) ceramics were synthesized using solid-state reaction method. The toroid- and pellet-shaped samples were sintered at 1200°C temperature for 4 h. According to the outcomes of the X-ray diffraction (XRD) investigation, each sample possesses a tetragonal crystal structure that includes a few impurity peaks. Extra Raman modes at 327 cm−1 (for x = 0.06 to 0.12) may be attributed to localized phonon vibrations in the vicinity of a (substitutional) defect or as a result of the relaxation of Raman selection rules due to defect-induced disorder. The density showed a progressive increase as the concentration of V increases, reaching its maximum at the x = 0.06 sample, after which it drops. The x = 0.06 sample displayed the highest bulk density, with a value of ρB = 4.01 g/cm3. Scanning Electron Micrograph (SEM) showed gradual agglomeration of the grain with V-content due to magnetic dipole interaction between one particle and other neighboring particles and exposed the porous nature of the samples. The TEM image demonstrates that the particles are well crystalline. For all of the compositions, the dielectric constant was larger in lower frequency areas and remained constant in higher frequency region. The sample with x = 0.06 showed a maximum dielectric constant of 1140 at 102 Hz. AC conductivity increased with increasing frequency. The Nyquist plot suggested an appearance of grain boundary (bulk) property of the material. The constant value of the real part of complex initial permeability over long range of frequency indicated the magnetic stability of the materials. For ceramics with x = 0.03, the largest real part of the initial permeability of 17.29 is evident. From the M − H loop, at room temperature, feeble ferromagnetism was observed with vanadium substitution in BLT ceramics. It is found that the highest saturation magnetization for the x = 0.03 sample is 2.61 emu/g. For the composition of x = 0.06, the highest polarization of 2.80 μC/cm2 is observed.

Optical and Raman selection rules for odd-parity clean superconductors

Optical and Raman selection rules for odd-parity clean superconductors

Selective observation of enantiomeric chiral phonons in α -quartz

We report anomalous circularly polarized Raman spectra of phonons in right- and left-handed quartzes. The phonon branches splitting from the E-mode at a finite wave number were found to be chiral with mutually opposite angular momenta. Our analysis reveals the Raman selection rules for chiral phonons. We also find that the conservation of angular momentum should be satisfied between two photons and one chiral phonon in the Raman process. Furthermore, we experimentally proved that the helicity of phonons with a certain frequency and wave vector should be opposite in the other enantiomer. Published by the American Physical Society 2024

Stress Relaxation of Extremely-Thin-Body Ge-on-Insulator p-Type Metal-Oxide-Semiconductor Field-Effect Transistor Along and Observed By Oil-Immersion Raman Spectroscopy

Background and purpose Strain techniques and channel materials with high carrier mobility are the principal technology boosters for the realization of high-performance logic devices composed of group IV semiconductors such as silicon (Si). In particular, strain engineering plays an important role in improving the performance of metal-oxide semiconductor field-effect transistors (MOSFETs) from the viewpoint of reducing the effective carrier mass. It is important to evaluate the stress relaxation in group IV semiconductor devices for the optimization of the carrier mobility and the device structure. In this paper, we demonstrated the stress evaluation of extremely-thin-body Ge-on-insulator (ETB GOI) p-type MOSFET along the <110> and <100> longitudinal directions by oil-immersion Raman spectroscopy with the high-numerical-aperture (high-NA) lens to realize precise evaluation of strain states in the Ge channels. Experiments Biaxially-strained (001) GOI layer was prepared by applying optimized Ge condensation technique on epitaxially-grown Si/Si0.75Ge0.25/Si-on-insulator substrates with 4-hour slow cooling [1, 2]. After the removal of the thermal oxide, narrow channel patterning was performed by electron-beam (EB) lithography. GOI channel was further thinned down by the cyclic digital etching process (plasma oxidation and wet etching). Surface passivation process was then immediately performed to fabricate GeO x and Al2O3 by plasma oxidation and atomic layer deposition at 300oC, respectively. The source-drain regions were formed with EB evaporated Ni through a lift-off process and Al deposition was performed as the back gate electrode.We used the Raman spectrometer with spectrograph focal length of 2,000 mm. The numbers of grating grooves were 2,400 mm-1 and a visible laser was used as the incident excitation light source whose wavelength was 532 nm. To realize anisotropic stress evaluation assuming the channel region of the ETB GOI MOSFET, we used a high-NA immersion lens for the selective excitation of both LO and TO phonon modes [3]. The high-NA and refractive index n of the oil were approximately 1.4 and 1.5, respectively. Using a high-NA immersion lens, a z-polarized light component can be effectively obtained owing to the aperture angle. Therefore, TO phonons can be excited on the basis of Raman selection rules for (001)-oriented GOI [3, 4]. Results and discussion Figure 1 shows the channel width dependence of the anisotropic biaxial stress of ETB GOI MOSFET along the <100> longitudinal direction obtained by the oil-immersion Raman spectra. Anisotropic biaxial compressive stress at the GOI channel width direction of the GOI channels was observed even when the GOI channel width is wide (GOI channel width ~ 630 nm). The stress along the GOI channel length (the <100> longitudinal direction) is relatively high. These results are clearly different from the stress relaxation in the ETB GOI MOSFET along the <110> longitudinal direction. In previous studies, a clear stress relaxation at the GOI channel width direction was observed as the GOI channel width becomes narrower of the ETB GOI MOSFET along the <110> longitudinal direction (from channel width ~ 200 nm) [5]. The results observed in this study are generally well correlated with the results obtained in strain relaxation measurements of carbon-doped Si nanowires along <100> by X-ray diffraction with synchrotron radiation [6]. The difference in the strain relaxation mechanism depending on the GOI channel direction may affect the electrical properties, and we consider that it is useful to be physical elucidation of the carrier mobility enhancement. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was con-ducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007.

Local symmetry breaking and low-energy continuum in K2ReCl6

Using polarization selective spontaneous Raman scattering, we have investigated the $5d$ transition metal compound ${\mathrm{K}}_{2}{\mathrm{ReCl}}_{6}$ which displays a series of structural phase transitions. We observe a violation of the Raman selection rules in the cubic high temperature phase as well as a low-energy scattering continuum persistent throughout the investigated temperature range from 300 down to 5 K. The continuum couples to one of the phonon modes at temperatures above the lowest structural phase transition at 76 K. We propose a common origin of these observations caused by disorder in the orientation of the ${\mathrm{ReCl}}_{6}$ octahedra which locally breaks the long-range cubic symmetry. Consistent results from the related nonmagnetic compound ${\mathrm{K}}_{2}{\mathrm{SnCl}}_{6}$ support this interpretation.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α-MoO3.

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO3 as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO3 that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO3 is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO3 as an example, although it is generally applicable to all transparent birefringent ALMs.

Raman energy density in the context of acoustoplasmonics

Interactions between elementary excitations are of great interest from a fundamental aspect and for novel applications. While plasmon-exciton have been extensively studied, the interaction mechanisms between acoustic vibrations (phonons) and localized surface plasmons (LSPs) remain quite unexplored. Here, we present a new theoretical framework for the investigation of the interaction between confined acoustic vibrations and LSPs involved in resonant acoustic Raman scattering. We express the Raman scattering process in the framework of Fermi's golden rule and introduce the concept of Raman energy density (RED). Similarly to the Raman-Brillouin electronic density (RBED) introduced for semiconductors, this physical quantity is used as a theoretical tool for the interpretation of resonant Raman scattering mediated by LSPs in metallic nanoparticles. The RED represents the electromagnetic energy density excited in the Raman scattering process and modulated by the acoustic vibrations of the nanoparticle. We show that, similarly to the LDOS and the RBED, the RED can be mapped in the near-field region, and provides a clear picture of the interaction between LSPs and acoustic vibrations giving rise to inelastic scattering measurable in the far-field. We use the newly introduced RED concept to investigate elastic (an)isotropy effects and extract the Raman selection rules of spherical nanoparticles embedded in a dielectric environment.

Evaluation of Strained Group IV Semiconductor Devices by Oil-Immersion Raman Spectroscopy

1. Background and purpose Strain techniques are one of the principal technology boosters for the realization of high-performance logic devices composed of group IV semiconductors such as Si, Ge and their alloy. Recently, it is gradually difficult to measure strain states as miniaturization proceeds to the nanometer scale. Raman spectroscopy is one of the most powerful techniques for phonon measurements and can perform strain evaluation with high spatial resolution nondestructively [1]. However, conventional Raman spectroscopy is limited in strain information based on the Raman polarization selection rules [1].In this paper, we introduce oil-immersion Raman spectroscopy with the high-numerical-aperture (high-NA) lens to realize precise evaluation of strain states (for example, anisotropic strain) in group IV semiconductor devices. 2. Oil-immersion Raman spectroscopy In conventional Raman spectroscopy, it is impossible to excite transverse optical (TO) phonon mode due to Raman polarization selection rules under the backscattering geometry from the (001) group IV semiconductor surface. In other words, a uniaxial or isotropic biaxial stress measurements in group IV semiconductors can only be performed by exciting only the longitudinal optical (LO) phonon mode under a conventional (001) backscattering configuration.To realize anisotropic stress evaluation assuming the channel region of the group-IV semiconductor MOSFET, we used a high-NA immersion lens for the selective excitation of both LO and TO phonon modes. The high-NA and refractive index n of the oil were approximately 1.4 and 1.5, respectively. Using a high-NA immersion lens, a z-polarized light component can be effectively obtained owing to the aperture angle. Therefore, TO phonons can be excited on the basis of Raman selection rules for (001)-oriented group IV semiconductors [2, 3]. 3. Results and discussion As an example of oil-immersion Raman measurements results, we evaluated the strain states of extremely-thin-body Ge-on-insulator (ETB GOI) MOSFET channels fabricated by Ge condensation [4, 5] by oil-immersion Raman spectra. We observed that the Raman peak shifted toward a higher wavenumber, indicating that high compressive strain in the GOI channels. In addition, a clear splitting of LO and TO phonon modes was confirmed, implying the anisotropic biaxial strain states induced in the ETB GOI channel.Figure 1 shows the channel width (Wch) dependence of the anisotropic biaxial stress for ETB GOI MOSFET (channel thickness: 10.6 nm) obtained by the oil-immersion Raman spectroscopy. As shown in Fig. 1, the stress relaxation along the GOI channels was observed as the GOI channel width gets narrower. In other words, the uniaxial stress is applied into channels by narrow GOI channel patterning. These behaviors are in good agreement with the previous reports such as strained-SOI [6] and SiGe nano patterns [3][7].In conclusion, oil-immersion Raman spectroscopy is one of the most powerful strain measurements in Group IV semiconductor device structures. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was conducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007.

Raman tensor studies on defective non-van der Waals Bi2O2Se

The Raman tensors of the three modes at ∼55, ∼80, and ∼160 cm−1 for the non-van der Waals layered material Bi2O2Se, which were assigned to Eu, Eg, and A1g, respectively, were experimentally investigated. Two modes at ∼55 and ∼80 cm−1, which were not observable in perfect crystal Bi2O2Se in the backscattering configuration, owing to the Raman selection rule, were activated by defects. These two modes exhibit strong polarization dependence at line defects and the excitation energy; thus, their Raman polarizability tensors exhibit strong dependence on the defect morphology and geometric characteristics of Bi2O2Se. The results of this study confirm that the Raman tensors of nanocrystalline structures can be modulated by defects.

Optical properties of corundum-structured In2O3

The optical properties of a single-phase corundum-structured In2O3 epilayer grown by a mist chemical vapor deposition method have been studied. Raman scattering measurements on a c face and on a lateral face reveal all of the seven Raman-active modes of the corundum structure, with good adherence to the Raman selection rules. Three out of the four infrared-active modes are observed in the spectroscopic ellipsometry measurements. The phonon frequencies obtained from Raman and ellipsometry measurements are in excellent agreement with density functional perturbation theory calculations. No trace of bixbyite phase was detected in the spectra. In the UV region, the imaginary part of the dielectric function shows two distinct onsets of strong absorption associated with direct band-to-band transitions at 3.38 and 3.86 eV.

Anharmonic Lattice Dynamics in Sodium Ion Conductors.

We employ terahertz-range temperature-dependent Raman spectroscopy and first-principles lattice dynamical calculations to show that the undoped sodium ion conductors Na3PS4 and isostructural Na3PSe4 both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice–Na+ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na3PSe4 shows an almost purely displacive character with the soft modes disappearing in the cubic phase as the change in symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na3PS4 instead shows an order–disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining Raman active in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice–mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity of these Na+ ion conductors.

Magnons and magnetic fluctuations in atomically thin MnBi2Te4

Electron band topology is combined with intrinsic magnetic orders in MnBi2Te4, leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin fluctuations in atomically thin MnBi2Te4 flakes using Raman spectroscopy. In a two-septuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.

Probing nonadiabatic dynamics with attosecond pulse trains and soft x-ray Raman spectroscopy.

Linear off-resonant x-ray Raman techniques are capable of detecting the ultrafast electronic coherences generated when a photoexcited wave packet passes through a conical intersection. A hybrid femtosecond or attosecond probe pulse is employed to excite the system and stimulate the emission of the signal photon, where both fields are components of a hybrid pulse scheme. In this paper, we investigate how attosecond pulse trains, as provided by high-harmonic generation processes, perform as probe pulses in the framework of this spectroscopic technique, instead of single Gaussian pulses. We explore different combination schemes for the probe pulse as well as the impact of parameters of the pulse trains on the signals. Furthermore, we show how Raman selection rules and symmetry consideration affect the spectroscopic signal, and we discuss the importance of vibrational contributions to the overall signal. We use two different model systems, representing molecules of different symmetries, and quantum dynamics simulations to study the difference in the spectra. The results suggest that such pulse trains are well suited to capture the key features associated with the electronic coherence.

Zenith-angle resolved polarized Raman spectroscopy of graphene

Angle resolved polarized Raman (ARPR) spectroscopy has been widely used to study basic properties of two-dimensional materials (2DMs), such as underlying symmetry, mode assignment, crystallographic orientation and optical anisotropy. The substrate effect has never been uncovered in ARPR spectroscopy. In this work, we investigated zenith-angle resolved polarized Raman (ZRPR) spectra of the G mode of graphene (1LG) deposited on different substrates as well as graphite under in-plane and out-of-plane configurations. In contrast to the independent behavior in normal incidence geometry, the G mode intensity exhibits obvious zenith-angle dependence. In particular, this polarization behavior is sensitive to the underlying substrates underneath 1LG. The ZRPR intensity of the G mode can be well understood by a model considering both Raman selection rule and zenith-angle resolved interference effect. This work enriches the understanding of polarized Raman spectroscopy of 2DMs and the approach can be applicable to other 2DMs, especially in-plane anisotropic 2DMs.

Electrical control of orbital and vibrational interlayer coupling in bi- and trilayer 2H−MoS2

Manipulating electronic interlayer coupling in layered van der Waals (vdW) materials is essential for designing opto-electronic devices. Here, we control vibrational and electronic interlayer coupling in bi- and trilayer 2H-MoS$_2$ using large external electric fields in a micro-capacitor device. The electric field lifts Raman selection rules and activates phonon modes in excellent agreement with ab-initio calculations. Through polarization resolved photoluminescence spectroscopy in the same device, we observe a strongly tunable valley dichroism with maximum circular polarization degree of $\sim 60\%$ in bilayer and $\sim 35\%$ in trilayer MoS$_2$ that are fully consistent with a rate equation model which includes input from electronic band structure calculations. We identify the highly delocalized electron wave function between the layers close to the high symmetry $Q$ points as the origin of the tunable circular dichroism. Our results demonstrate the possibility of electric field tunable interlayer coupling for controlling emergent spin-valley physics and hybridization driven effects in vdW materials and their heterostructures.

Characterization of crystallographic orientation and lattice disorder in hydroxyapatite thin films by Raman scattering

This report presents the way of evaluating the crystallographic orientation and crystallinity of hydroxyapatite (HAp) thin films by Raman scattering spectroscopy. According to the Raman selection rule, the symmetric stretching mode (ν1) of P−O bonds in an isolated PO43− units is Raman active, whereas the asymmetric stretching mode (ν3) is not. Nevertheless, a weak ν3 signal is usually observed in the Raman spectra of HAp powder crystals. We will show that the ν3 signal is entirely absent for uniaxially oriented HAp films, whereas it appears in the case of randomly-oriented ones. This principle is feasible for discriminating the uniaxial orientation from a random one in HAp crystals. Another simple approach for evaluating crystallinity is exploiting the line width of the ν1 PO43− Raman signal. If the full-width at half maximum (FWHM) of the ν1 signal is smaller than 7.4 cm−1, the HAp crystal is uniaxially oriented in one direction, whereas if the FWHM value is larger than 7.4 cm−1, the film is composed of randomly-oriented crystallites.

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.

Predicting Raman line shapes from amorphous silicon clusters for estimating short‐range order

AbstractA theoretical model obtained based on direct matrix method and bond polarization model (DMM‐BPM) has been used to predict Raman scattering line shape for amorphous silicon (a‐Si). These line shapes can describe the observed nonunique peak position from these a‐Si crystallites, unlike crystalline Si, that varies between 470 and 485 cm−1. The model has been validated using three different sets of Raman scattering data by fitting it with the proposed model. The Raman spectra are obtained from a‐Si samples prepared by three different methods by three different research groups reported earlier. Clear consonance with the experimentally observed and theoretical Raman line shape was obtained. The model has also been used to estimate the distance up to which the crystallinity remains intact in a‐Si. The used model has been compared with the existing phonon confinement model, and a discrepancy (in the latter) therein has been discussed. Rationale behind adopting this model has been explained using the constraints related to the Raman selection rule, their relaxation, or breakdown in appropriate materials' regimes. A handy empirical relation between Raman peak position and amorphous cluster size has been proposed.

Synthesis of 2D ternary layered manganese phosphorous trichalcogenides towards ultraviolet photodetection

Manganese phosphorous selenium (MnPSe3), as a representative of layered metal phosphorus trichalcogenides (MPTs), has gained significant attention due to its direct bandgap, high carrier mobility, large absorption coefficient, which indicate great potential in photoelectric application. Herein, high-quality two-dimensional (2D) MnPSe3 flakes were mechanically exfoliated from the corresponding bulk crystals synthesized by chemical vapor transport (CVT) methods. The systematic investigation was applied to the lattice vibrations of MnPSe3via angle-resolved polarized Raman spectroscopy (ARPRS), and the Raman vibration modes were determined based on Raman selection rules and crystal symmetry. Impressively, the photodetectors based on 2D MnPSe3 flakes exhibit excellent photoresponse to the ultraviolet light with a responsivity up to 22.7 A W−1 and a detectivity of 2.4 × 1011 Jones. The high performance in the ultraviolet range signifies that 2D MnPSe3 is expected to be a powerful candidate for future ultraviolet photodetection.

Effect of isotope disorder on the Raman spectra of cubic boron arsenide

Boron arsenide (c-BAs) is at the forefront of research on ultrahigh thermal conductivity materials. We present a Raman scattering study of isotopically tailored cubic boron arsenide single crystals for 11 isotopic compositions spanning the range from nearly pure c-$^{10}$BAs to nearly pure c-$^{11}$BAs. Our results provide insights on the effects of strong mass disorder on optical phonons and the appearance of two-mode behavior in the Raman spectra of mixed crystals. Strong isotope disorder also relaxes the one-phonon Raman selection rules, resulting in disorder-activated Raman scattering by acoustic phonons.