Forbidden Raman Modes Research Articles

Off-axis Raman spectroscopy for nanoscale stress metrology

Raman spectroscopy is an effective tool for stress and compositional metrology in the semiconductor industry. However, its application toward decoupling a complex stress state in semiconductor materials requires the use of liquid immersion lenses that are process line incompatible. In this work, a practical design concept for off-axis Raman spectroscopy is presented. By tilting the incident light away from the normal incident axis, forbidden Raman modes can be accessed allowing determination of the in-plane stress tensor in semiconductor materials. Furthermore, we benchmark off-axis Raman spectroscopy against oil-immersion Raman spectroscopy for stress characterization in 20 nm-wide strained Ge fin field-effect transistor channels. We demonstrate that off-axis Raman allows anisotropic stress metrology without reliance on liquid immersion lenses, highlighting its viability in the process line. The stress state is validated through nanobeam diffraction measurements.

Discovering the forbidden Raman modes at the edges of layered materials.

The edges of layered materials have unique properties that substantially differ from the body regions. In this work, we perform a systematic Raman study of the edges of various layered materials (MoS2, WS2, WSe2, PtS2, and black phosphorus). The Raman spectra of the edges feature newly observed forbidden Raman modes, which are originally undetectable from the body region. By selecting the edge type and the polarization directions of the incident and scattered light, all forbidden Raman modes are distinctly detected. Optical simulations show that the edges of layered materials drastically distort the electromagnetic fields of both the incident and scattered light, so that the light interacts with the edges in a distinct way, which differs from its interactions with the body regions.

Second-Order Raman Scattering in Exfoliated Black Phosphorus.

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp2-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D1, D1', D2 and D2', in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the Ag1 and Ag2 modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D' and D modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these D modes to disorder helps explaining several discrepancies found in the literature.

Resonance Raman effects in transition metal dichalcogenides

Raman spectroscopy is broadly used in the studies of transition metal dichalcogenides to determine the number of layers or other structural parameters. However, unlike the case of graphene, the Raman spectrum varies greatly depending on the excitation energy, and many unusual effects have been reported. The optical absorption spectrum has many features related to exciton states due to the strong coulomb interaction in these materials, and dramatic resonance effects occur when the excitation energy matches one of these exciton states. Several forbidden Raman modes and some unexplained peaks appear near resonance, and Davydov splitting of some Raman modes is observed. Furthermore, the polarization dependence of the some Raman modes also shows excitation energy dependence. In this review, recent progress in resonance Raman studies on transition metal dichalcogenides and some unresolved issues are reviewed. Copyright © 2017 John Wiley & Sons, Ltd.

Selective Raman modes and strong photoluminescence of gallium selenide flakes on sp2 carbon

Two-dimensional materials awakened a strong interest in the scientific and technological communities due to their exceptional properties that can be tuned by the material thickness and chemistry. In order to correlate optical properties with crystallographic structure and morphology, in this work, the authors aim at studying GaSe nanoflakes deposited on highly ordered pyrolytic graphite by means of atomic force microscopy, Raman, and photoluminescence (PL) spectroscopies. The authors found that the basal plane of the flakes can be attributed to the ε-phase expected for bulk samples grown by the Bridgman method. However, a strong difference in the Raman spectra was systematically found at the edge of our GaSe flakes. Forbidden Raman modes located around 250 cm−1 were selectively observed at specific locations. These modes could not be directly attributed to the ε-phase observed in the basal plane or in the bulk. The atomic force microscopy investigations show that high topographical features characterize the regions with the highest photoluminescence and the forbidden Raman modes. This change in crystal orientation at selective locations of the flake could activate the E′(LO) Raman mode at 255 cm−1. The enhancement of the PL emission around 620 nm is also linked to the change in crystal orientation due to an increase in the probability of electronic transitions. These results show the rich physics of GaSe flakes in contrast to what is known from its bulk counterpart with possible applications in photovoltaics and photodetectors for nanoflakes with engineered geometries.

Polarized micro‐Raman spectroscopy and ab initio phonon modes calculations of LuPO4

The vibrational dynamics of lutetium orthophosphate (LuPO4) single crystals was carefully investigated by means of polarized micro‐Raman spectroscopy and ab initio calculations. Eleven of the twelve independent components of the polarizability tensor, expected on the basis of the group theory for LuPO4, were selected in turn and assigned in symmetry. The only B1g(2) Raman mode was not observed, likewise due to either its very small intensity or its nearness in energy with forbidden Raman modes, which spill and could hide it. Both Raman and infrared vibrational modes are evaluated by density‐functional theory calculations using effective core pseudo‐potential. The agreement between calculated and experimental frequencies is very good. On the basis of our ab initio results, and of reduced‐mass ratio considerations, the expected wavenumber of the missing B1g (2) mode falls close to that of Eg(3) mode peaked at about 306 cm−1, and therefore we can definitively conclude that the observation of the missing B1g (2) mode is masked by the spill‐over of this Eg mode. Copyright © 2013 John Wiley & Sons, Ltd.

Evidence for a subtle structural symmetry breaking inEuB6

This work presents a systematic Raman scattering study and first-principles calculations for theEuB6 system. Evidence for the presence of an incipient (∼1 × 10−4 Å) tetragonal symmetry break of its crystalline structure was found. Forbidden Raman modes atωfRm(1)∼1170 cm−1,ωfRm(2)∼1400 cm−1, andωfRm(3)∼1500 cm−1 were observed. Thetetragonal symmetry of ωfRm(2) and ωfRm(3) together with spin-polarized first-principles simulations of the structural and magneticproperties fully support such a break of symmetry. Our data and calculations explain theoccurrence of ferromagnetism in Eu hexaborides, previously reported.

Light depolarization induced by metallic tips in apertureless near-field optical microscopyand tip-enhanced Raman spectroscopy

We have investigated the depolarization effects of light scattered by sharp tips used forapertureless near-field optical microscopy. Dielectric and metal coated tips have beeninvestigated and depolarization factors between 5 and 30% have been measured, changingas a function of the incident light polarization and of the tip shape. The experimentalresults are in good agreement with theoretical calculations performed by the finite elementmethod, giving a near-field depolarization factor close to 10%. The effect of depolarizationhas been investigated in polarized tip-enhanced Raman spectroscopy (TERS)experiments; the depolarization gives rise to forbidden Raman modes in Si crystals.

The influence of structural disorder on the phonon modes in zinc oxide

Abstract Undoped zinc oxide thin films and nanostructured layers were grown by pulsed laser deposition on different substrates. They were characterized by scanning electron microscopy and Raman backscattering spectroscopy. Larger substrate mismatch leads to higher structural disorder in the thin films. Simultaneously, the intensity of the phonon mode at 580 cm −1 increases. However, for the nanostructured layers it remains constant. These observations are discussed in terms of the disorder activation of forbidden Raman modes.

Evidence of magnetic polaronic states inLa0.70Sr0.30Mn1−xFexO3manganites

We have performed x-ray, electrical resistivity, and Raman scattering measurements in ${\mathrm{La}}_{0.70}{\mathrm{Sr}}_{0.30}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ manganites, with x varying from 0 to 0.40. The appearance of new peaks in the Raman spectra as the Fe content increases is attributed to either otherwise forbidden Raman modes that become active due to symmetry breaking effect or to the phonon density of states features, which arise due to a disorder-induced phenomenon. The softening of some phonon modes suggests the evidence of magnetic polaronic states in the insulating paramagnetic phase of these manganites. Our observations are discussed in connection with the x-ray diffraction and electrical resistivity data.

Weak ferromagnetism above TN in Gd 2CuO 4

In this work we report magnetization and Raman measurements performed in single crystals of Gd 2CuO 4. The data show the presence of two forbidden Raman modes (fRm), B 1g ∗ and B 2g ∗ , and weak ferromagnetism (WF) in the CuO 2 planes up to temperatures well above T N Cu ∼280 K . We have studied the T-dependence of the intensity of these fRm and the WF component between 2 and 800 K. The WF component and the relative intensity of the two fRm decrease with increasing T and they vanish at T∼650 and 520 K, respectively. Recent neutron diffraction experiments observed a tetragonal to orthorhombic phase transition at T≅650 K . These results are interpreted in terms of structural distortions within the CuO 2 planes that lead to the existence of a distribution of coexisting tetragonal and orthorhombic crystallographic domains in this system.

Weak ferromagnetism and Raman scattering in Eu 2− xPr xCuO 4

Magnetization and Raman experiments performed in single crystals of Eu 2− x Pr x CuO 4 (0≲ x≲1) are interpreted in terms of local distortions within the CuO 2 planes. We find that samples grown in Pt crucibles contain Pt impurities, that are partially responsible for these distortions. We observed: (i) the appearance of two forbidden Raman modes ( f RM) and (ii) a weak-ferromagnetic (WF) component when the crystals are field cooled in the ab-plane. The WF component, the onset temperature where 3D long-range WF is first observed, T WF, the coercive field, H C, and the intensity of the f RM decrease as the Pt content decreases. For crystals with a similar Pt content, the above quantities are larger for samples with smaller lattice unit cell volume, V. The crystal with the highest Pt content and smallest V ( x=0) shows, for T→0, an average canted Cu moment of 5.5(3)×10 −3 μ B/Cu. The intensity of the f RM decrease with T, suggesting that the number of distortions decrease as V expands. This is consistent with recent neutron diffraction results in Eu 2CuO 4, where a tetragonal to orthorhombic phase transition was observed at T≲200 K. We concluded that the Pt impurities and/or the reduction of V beyond a critical value, V c, are the origin of these orthorhombic distortions which in turn are responsible for the f RM and WF observed in these compounds.

Distortion‐induced intensity of Raman modes of molecular units in different crystal orientations

AbstractUsing the principles of the theory of distortion‐induced activity of forbidden Raman modes of molecular units in differnt crystal orientations and the theory of distortion‐induced Raman intensities of different modes of molecular units adsorbed onto metal surfaces, the distortion‐induced intensities of different Raman modes of molecular units in differnt crystal orientations is discussed. An example of the intensities of ν1s, ν2s, ν3s and ν4s (where s represents the SO ion) of (NH4)2SO4 [D] single crystal in Ag, B1g, B2g B3g orientations is discussed to demonstrate the usefulness of the results of these theories.

The theory of distortion-induced activity of forbidden Raman modes of molecular units in different crystal orientations

Using the principles of the theory of distortion-induced activity of forbidden modes of molecular units in crystals, a quantitative theory of distortion-induced activity of forbidden Raman modes of molecular units in different crystal orientations is discussed. An example of the observation of nu 1s and nu 1n modes (Ag symmetry) of (NH4)2SO4 in B2g and B3g polarisations has been discussed to demonstrate the usefulness of the results of the theory.