Search

Hexagonal boron nitride, h-BN, is an ubiquitous layered material with unique properties as a dielectric insulator, strong ultraviolet emitter and natural optical hyperbolic material. In the present days it has attracted attention due to its elec- tronic and photonic applications based on graphene and related two-dimensional materials. The recent availability of good quality single crystals brings the possibility to improve its characterization, both in ambient conditions and at high pressure. We have performed high pressure experiments with h-BN single crystals, including x-ray diffraction (XRD), Raman scattering[1] and infrared spectroscopy[2]. Raman scattering experiments have also been performed on isotopically enriched samples[3]. Single crystal XRD complements previous powder diffraction experiments, where the quality of the sample and the existence of turbostratic defects originated controversial results about the h-BN equation of state. We have characterized under high pressure the Raman active E2g high and low frequency modes, as well as the E1u(TO), E1u- (LO), A2u(TO) and A2u(LO) infrared active modes[3] up to the transition to the wurtzite phase at 10.5 GPa. We interpret the high pressure evolution of the in-plane modes in terms of the intra and interlayer interactions. The softening of the A2u(TO) mode is induced by dynamical buckling of the flat honeycomb layers. The wurtzite phase is metastable and is maintained on decompression down to ambient pressure. The pressure evolution of the frequencies of Raman active modes does not display any systematic departures related to isotopic mass. Only a weak signature of the van der Waals interactions was observed by comparing the pressure de- pendence of the E2g low interlayer shear mode in isotopically pure samples. At low pressures, where van der Waals forces dominate the interlayer interactions, the mode frequencies diverge faster than at high pressures, where Pauli repulsion sets in. Finally, the linewidth of

The pressure dependence of the direct and indirect bandgap transitions of hexagonal boron nitride is investigated using optical reflectance under hydrostatic pressure in an anvil cell with sapphire windows up to 2.5 GPa. Features in the reflectance spectra associated with the absorption at the direct and indirect bandgap transitions are found to downshift with increasing pressure, with pressure coefficients of −26 ± 2 and −36 ± 2 meV GPa–1, respectively. The GW calculations yield a faster decrease of the direct bandgap with pressure compared to the indirect bandgap. Including the strong excitonic effects through the Bethe–Salpeter equation, the direct excitonic transition is found to have a much lower pressure coefficient than the indirect excitonic transition. This suggests a strong variation of the binding energy of the direct exciton with pressure. The experiments corroborate the theoretical predictions and indicate an enhancement of the indirect nature of the bulk hexagonal boron nitride crystal under hydrostatic pressure.

We review the recent progress regarding the physics and applications of boron nitride bulk crystals and its epitaxial layers in various fields. First, we highlight its importance from optoelectronics side, for simple devices operating in the deep ultraviolet, in view of sanitary applications. Emphasis will be directed towards the unusually strong efficiency of the exciton-phonon coupling in this indirect band gap semiconductor. Second, we shift towards nanophotonics, for the management of hyper-magnification and of medical imaging. Here, advantage is taken of the efficient coupling of the electromagnetic field with some of its phonons, those interacting with light at 12 and 6 yin in vacuum. Third, we present the different defects that are currently studied for their propensity to behave as single photon emitters, in the perspective to help them becoming challengers of the NV centres in diamond or of the double vacancy in silicon carbide in the field of modern and developing quantum technologies.

We present a Raman-scattering study of the interlayer and intralayer E2g Raman-active modes of hexagonal boron nitride (h−BN) under hydrostatic pressure for pressures up to the transition to the wurtzite phase (10.5 GPa). Pressure coefficients and Grüneisen parameters are determined for both modes, and are compared to ab initio calculations based on density functional perturbation theory. The pressure coefficient of the low-energy interlayer mode is higher than that of the high-energy intralayer mode owing to the large compressibility of the h−BN crystal along the c direction. Both modes exhibit a sublinear phonon frequency increase with pressure, which is more marked in the case of the low-energy mode. The intensity of the low-energy mode increases with pressure, suggesting an enhancement of the mode polarizability as the honeycomb layers become closer. The Raman spectrum of the metastable wurtzite phase is observed at ambient pressure in the transited sample after decompression.

We present an infrared reflectivity study of the E1u in-plane phonons of hexagonal BN as a function of temperature in the 40–680 K range. The infrared reflectance spectra of high-quality lamellar single crystals are accurately fitted using Lowndes' factorized form of the dielectric response, where the longitudinal-optic (LO) frequency is an independent adjustable parameter. From this analysis we obtain reliable values for the phonon damping of the IR-active E1u phonons which couple to light and give rise to the phonon-polariton excitations in this hyperbolic material. Anharmonic coupling potentials are estimated from the temperature dependence of the damping parameters. The E1u(LO) mode exhibits a substantially longer lifetime than its transverse-optic counterpart, as it is basically unaffected by isotopic-disorder scattering owing to the low density of phonon states around the E1u(LO) frequency.

The phonon linewidth of isotopically controlled hexagonal boron nitride (h-BN) single crystals has been determined by Raman scattering. The scattering by isotopic mass disorder induces a phonon broadening that is largest for boron 11 fractions around 0.65. Lowest-order perturbation theory does not suffice to explain the dependence of the isotopic broadening on isotopic composition. A multiple-scattering theory based on the coherent potential approximation provides a good quantitative account of the phonon shift and broadening with isotopic composition observed in the experiments. Isotopic-disorder scattering is shown to have a prominent role in limiting the optical-phonon lifetime in h-BN © 2020 American Physical Society

The recent availability of isotopically pure samples of hexagonal boron nitride (h-BN) has allowed the isotopic substitution effects to be studied in this highly interesting layered crystal. Here, we review the application of Raman scattering to investigate phonon anharmonic decay and its particularities in layered crystals, exemplified by h-BN. The modification of the phonon spectrum and anharmonic phonon decay paths in isotopically pure samples is specifically addressed. Detailed information about phonon lifetimes and decay channels is obtained for h-BN from a thorough analysis of temperature-dependent Raman scattering measurements in the light of density functional theory and perturbation theory calculations. The phonon lifetime is substantially increased in isotopically pure crystals, which may have important implications for the development of low-loss h-BN based phonon-polariton devices. On account of the low cation mass, isotopic substitution substantially alters the dominant phonon decay pathways and changes the strength of phonon anharmonic interactions.

Hexagonal boron nitride (h-BN) is attracting a great deal of attention in the emerging field of two-dimensional atomic crystals and van der Waals heterostructures because its extreme optical anisotropy [1] results in optical hyperbolic behavior in the mid-infrared. h-BN is particularly well suited for phonon-polariton applications because of the low optical losses, the strong and broad phonon resonances in the technologically relevant mid-infrared spectral region, and the concurrence of Type I and Type II hyperbolic response in the same material [2]. Considering the central role of the infrared (IR)-active phonons in limiting the phonon-polariton propagation length and the availability today of high-quality h-BN single crystals, we have revisited the polar phonons and dielectric properties of h-BN and we study the anisotropic pressure dependence of the in-plane and out-of-plane IR-active modes up to 10 GPa [3]. Infrared reflectivity spectra at normal incidence on high quality single crystals show strict fulfillment of selection rules and an unusually long E1u(TO) phonon lifetime. Accurate values of the dielectric constants at ambient pressure ¿0¿=6.96, ¿¿¿=4.95, ¿0||=3.37, and ¿¿||= 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonons are revealed in measurements on an inclined facet. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab-initio calculations. While Grüneisen parameters are generally small, the A2u(TO) mode displays an exceptionally large and negative Grüneisen parameter [3]. This unusual behavior results from the softening of the A2u(TO) mode due to a dynamical buckling of the flat honeycomb layers induced by the atomic motions. Such buckling instabilities play a central role in structural phase transitions of lamellar crystals [4]. This work has been supported by the Spanish MINECO/FEDER grant No. MAT2015-71035-R.

We present a detailed analysis of the temperature dependence of the interlayer shear mode that reveals a variation of the four-phonon scattering coupling with isotopic mass. Phonon anharmonic decay and isotopic disorder effects are very weak for the low-energy interlayer shear mode, and the overall temperature dependence is mainly governed by the thermal lattice expansion. This allows us to observe systematic differences in the temperature dependence of the low-energy mode with the isotopic mass that are related to the four-phonon scattering contribution in quartic anharmonicity. The change in the quartic anharmonicity coefficient between the isotopically pure samples is larger than would be purely expected from the mass variation. © 2019 American Physical Society.

Hexagonal boron nitride (h-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state BN10 neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-crystal h-BN. Phonon frequency and lifetime are measured in the 80-600-K temperature range for B10-enriched, B11-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in B10-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes. © 2018 American Physical Society.

The linear optical response of metastable alpha-Ga2O3 is investigated by spectroscopic ellipsometry. We determine the ordinary dielectric function from lattice vibrations up to the vacuum ultraviolet spectral range at room temperature for a sample with a (0001) surface. Three out of four E-u infrared-active phonon modes are unambiguously determined, and their frequencies are in good agreement with density functional theory calculations. The dispersion of the refractive index in the visible and ultraviolet part of the spectrum is determined. High-energy interband transitions are characterized up to 20 eV. By comparison with the optical response of alpha-Al2O3 and with theoretical results, a tentative assignment of interband transitions is proposed.

The giant birefringence of layered h-BN was demonstrated by analyzing the interference patterns in reflectance and transmittance measurements in the mid-infrared to the deep ultraviolet energy range. The refractive index for polarization perpendicular to the c axis is much higher than the refractive index for polarization parallel to the c axis, and it displays a strong increase in the ultraviolet range that is attributed to the huge excitonic effects arising from the unique electronic structure of h-BN. Thus, h-BN is shown to exhibit a giant negative birefringence that ranges from -0.7 in the visible to -2 in the deep ultraviolet close to the band gap. The electronic dielectric constants for polarization perpendicular and parallel to the c axis were determined to be epsilon(perpendicular to)(infinity) = 4.95 and epsilon(parallel to)(infinity) = 2.86, respectively. The anisotropy we find in high-quality h-BN is significantly larger than proposed in previous experimental studies but in excellent agreement with ab initio calculations.

The elastic constants of high-quality, single-crystal hexagonal boron nitride (h-BN) have been measured by means of high resolution Brillouin spectroscopy using a modified reflected light microscope. The sound propagation velocity in the c-axis direction and perpendicular to the c-axis have been obtained from the Brillouin frequency shift, with a proper account taken of the vast difference between the ordinary and extraordinary refractive indices recently reported in the highly anisotropic layered h-BN crystal. The elastic constants c11 and c33 obtained from the Brillouin experiments are somewhat lower than previous determinations based on inelastic x-ray measurements and confirm the overestimation of the h-BN elastic constants by ab initio calculations based on density functional perturbation theory in the local density approximation.

Hexagonal boron nitride is a model lamellar compound where weak, non-local van der Waals interactions ensure the vertical stacking of two-dimensional honeycomb lattices made of strongly bound boron and nitrogen atoms. We study the isotope engineering of lamellar compounds by synthesizing hexagonal boron nitride crystals with nearly pure boron isotopes (10B and 11B) compared to those with the natural distribution of boron (20 at% 10B and 80 at% 11B). On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the van der Waals interactions upon isotope purification. The electron density distribution is more diffuse between adjacent layers in 10BN than in 11BN crystals. Our results open perspectives in understanding and controlling van der Waals bonding in layered materials. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

The electronic structure near the band gap of the corundumlike a phase of Ga2O3 has been investigated by means of optical absorption and spectroscopic ellipsometry measurements in the ultraviolet (UV) range (400-190 nm). The absorption coefficient in the UV region and the imaginary part of the dielectric function exhibit two prominent absorption thresholds with wide but well-defined structures at 5.6 and 6.3 eV which have been ascribed to allowed direct transitions from crystal-field split valence bands to the conduction band. Excitonic effects with large Gaussian broadening are taken into account through the Elliott-Toyozawa model, which yields an exciton binding energy of 110 meV and direct band gaps of 5.61 and 6.44 eV. The large broadening of the absorption onset is related to the slightly indirect character of the material.

Raman spectra of epitaxial layers of CdSe grown by molecular beam epitaxy have been measured for the cubic (zincblende) and hexagonal (wurtzite) phases. The Raman spectra are examined in the light of density functional calculations for these two highly similar structures. Characteristic Raman frequencies and spectral features associated with the different symmetry are discussed and reliable criteria for phase discrimination based on Raman spectroscopy are proposed. Although LO frequencies are virtually identical in both structures and may be affected by size effects, the observation of a low energy E2 mode at 33cm-1 unambiguously identifies the wurtzite structure and can be used as a specific fingerprint to distinguish between these two phases in CdSe-based nanostructures. The slightly lower LO frequency measured in the zincblende epitaxial layer is ascribed to residual tensile strain. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effect of radiation damage on the defect and alloy structure in InxGa1− xN thin films grown on Si substrates was studied using positron annihilation spectroscopy. Prior to the measurements, the samples were subjected to double He+ implantation at 40 and 100 keV. The results show the presence of cation vacancy-like defects in high concentrations (>1018 cm−3) already in the as-grown samples. The evolution of the annihilation characteristics after the implantation suggests strong alloy disorder rearrangement under irradiation. © 2017 Author(s).

We address the intrinsic optical properties of hexagonal boron nitride in deep ultraviolet. We show that the fine structure of the phonon replicas arises from overtones involving up to six low-energy interlayer shear modes. These lattice vibrations are specific to layered compounds since they correspond to the shear rigid motion between adjacent layers, with a characteristic energy of about 6-7 meV. We obtain a quantitative interpretation of the multiplet observed in each phonon replica under the assumption of a cumulative Gaussian broadening as a function of the overtone index, and with a phenomenological line broadening taken identical for all phonon types. We show from our quantitative interpretation of the full emission spectrum above 5.7 eV that the energy of the involved phonon mode is 6.8±0.5 meV, in excellent agreement with temperature-dependent Raman measurements of the low-energy interlayer shear mode in hexagonal boron nitride. We highlight the unusual properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. © 2017 American Physical Society.

Spectrally and spatially resolved cathodoluminescence (CL) measurements were carried out at 80 K on undoped/Mg-doped GaN core-shell nanorods grown by selective area growth metalorganic vapor phase epitaxy in order to investigate locally the optical activity of the Mg dopants. A study of the luminescence emission distribution over the different regions of the nanorods is presented. We have investigated the CL fingerprints of the Mg incorporation into the non-polar lateral prismatic facets and the semi-polar facets of the pyramidal tips. The amount of Mg incorporation/activation was varied by using several Mg/Ga flow ratios and post-growth annealing treatment. For lower Mg/Ga flow ratios, the annealed nanorods clearly display a donor-acceptor pair band emission peaking at 3.26-3.27 eV and up to 4 LO phonon replicas, which can be considered as a reliable indicator of effective p-type Mg doping in the nanorod shell. For higher Mg/Ga flow ratios, a substantial enhancement of the yellow luminescence emission as well as several emission subbands are observed, which suggests an increase of disorder and the presence of defects as a consequence of the excess Mg doping.

Identifying and mapping the crystalline phases and orientation relationships on the local scale in core-shell ZnO nanowire heterostructures are of primary importance to improve the interface quality, which governs the performances of the nanoscale devices. However, this represents a major difficulty, especially when the expected polytypes exhibit very similar properties as in the case of CdSe. In the present work, we address that issue in ZnO nanowire heterostructures involving a uniform and highly conformal CdSe shell grown by molecular beam epitaxy. It is shown by x-ray diffraction and Raman spectroscopy through the occurrence of the (1010) and (1011) diffraction peaks and of the E mode at 34 cm, respectively, that the CdSe shell is mostly crystallized into the wurtzite phase. By using automated crystal phase and orientation mapping with precession (ASTAR) in a transmission electron microscope and thus by benefiting from highly precise electron diffraction patterns, the CdSe shell is found to crystallize also into the minority zinc blende phase. The wurtzite CdSe shell is epitaxially grown on the top of ZnO nanowires, and some specific orientation relationships are mapped and revealed when grown on their vertical sidewalls. Zinc blende CdSe domains are also formed exclusively in the center of wurtzite CdSe grains located on the vertical sidewalls; both wurtzite and zinc blende CdSe crystalline phases have a strong orientation relationship. These findings reveal that ASTAR is a powerful technique to elucidate the structural properties on the local scale and to gain a deeper insight into their crystalline phases and orientation relationships, which is highly promising for many types of semiconducting nanowire heterostructures.

We present a Raman scattering study of optical phonons in hexagonal BN for temperatures ranging from 80 to 600 K. The experiments were performed on high-quality, single-crystalline hexagonal BN platelets. The observed temperature dependence of the frequencies and linewidths of both Raman active E2g optical phonons is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional theory calculations. With increasing temperature, the E2ghigh mode displays strong anharmonic interactions, with a linewidth increase that indicates an important contribution of four-phonon processes and a marked frequency downshift that can be attributed to a substantial effect of the four-phonon scattering processes (quartic anharmonicity). In contrast, the E2glow mode displays a very narrow linewidth and weak anharmonic interactions, with a frequency downshift that is primarily accounted for by the thermal expansion of the interlayer spacing. © 2016 American Physical Society.

We present high-resolution surface Brillouin scattering measurements on cubic GaN layers grown on GaAs substrate. By using a suitable scattering geometry, scattering by surface acoustic waves is recorded for different azimuthal angles, and the surface acoustic wave velocities are determined. A comparison of experimental results with numerical simulations of the azimuthal dependence of the surface wave velocity shows good agreement and allows a consistent set of elastic constants for c-GaN to be determined. © 2016 The Japan Society of Applied Physics.

© 2015 Elsevier B.V. We present a Raman scattering study of LO phonon-coupled modes in Be-doped, p-type In0.53Ga0.47As with hole densities ranging from 2.2×1017 to 2.4×1019 cm-3. Two separate phonon-like coupled modes are observed in the optical-phonon spectral region, corresponding to InAs-like and GaAs-like overdamped modes. With increasing free-hole density, these modes exhibit a redshift and their frequencies approach the respective TO frequencies. Unlike the case of n-type material, no high-frequency L+ coupled mode could be detected. The Raman spectra are analyzed using a dielectric model based on the Lindhard-Mermin susceptibility that takes into account HH and LH intraband transitions as well as HH-LH interband transitions. The model yields good quality fits to the experimental spectra. It is shown that the inter-valence-band processes introduces an additional damping channel that causes the L+ mode to be damped out. The comparison between the Raman spectra and the theoretical line-shape calculations suggests the presence of a residual strain and a reduced sublattice interaction in the most heavily doped samples.

© 2015 American Physical Society. We present a Raman-scattering study of optical phonons in zinc-blende (cubic) GaN for temperatures ranging from 80 to 750 K. The experiments were performed on high-quality, cubic GaN films grown by molecular-beam epitaxy on GaAs (001) substrates. The observed temperature dependence of the optical phonon frequencies and linewidths is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional-theory calculations. The longitudinal-optical (LO) mode relaxation is found to occur via asymmetric decay into acoustic phonons, with an appreciable contribution of higher-order processes. The transverse-optical mode linewidth shows a weak temperature dependence and its frequency downshift is primarily determined by the lattice thermal expansion. The LO phonon lifetime is derived from the observed Raman linewidth and an excellent agreement with previous theoretical predictions is found.

We present Raman-scattering measurements on InGaN epilayers with In content ranging from 17% to 42% which have been implanted with different He+ doses. Raman spectra obtained using near-resonance visible excitation (514.5 nm) and above band-gap UV excitation (325 nm) are measured in order to study the resonant behaviour of the samples and to assess its crystalline quality. The spectra obtained with UV excitation show resonant LO multiphonon scattering up to fifth order, whose relative intensities depend on the indium concentration and implantation dose. An assessment of the degree of lattice disorder introduced by the implantation can be obtained from the analysis of the Raman spectra in terms of relative intensities and line broadenings of the multiphonon peaks. © 2014 IOP Publishing Ltd.

We have performed high-pressure Fourier transform infrared reflectance measurements on a freestanding InN thin film to determine the refractive index of wurtzite InN and its high-pressure rocksalt phase as a function of hydrostatic pressure. From a fit to the experimental refractive-index curves including the effect of the high-energy optical gaps, phonons, free carriers, and the direct (fundamental) band-gap in the case of wurtzite InN, we obtain pressure coefficients for the lowfrequency (electronic) dielectric constant e1 . Negative pressure coefficients of -8.8 × 10-2 GPa-1 and -14.8 × 10-2 GPa-1 are obtained for the wurtzite and rocksalt phases, respectively. The results are discussed in terms of the electronic band structure and the compressibility of both phases.

We perform high-pressure Raman-scattering measurements on different In xGa1-xN/Si(111) epilayers (0.19 < x < 0.45). We find that the experimental pressure coefficient of the A 1(LO) mode measured in these samples is larger than that expected from the linear interpolation between the corresponding values of GaN and InN. Similar measurements in InGaN epilayers grown on GaN/sapphire templates yield much lower values, below the linearly interpolated pressure coefficients. We conclude that the phonon pressure coefficients measured in InGaN are mainly determined by the different compressibility of the substrate and epilayer material. Neglecting substrate effects may yield highly inaccurate phonon pressure coefficients and mode Grüneisen parameters.

CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl2 heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl2 heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl2 heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl2 heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells.

Charge homogeneity in heavily Si-doped ammonothermal GaN has been studied by confocal micro-Raman measurements. The polarized Raman spectra indicate a high crystalline quality of the sample. On the N-polarity growth sector we observe both branches (L+, L-) of the phonon-plasmon coupled modes. The free-electron density, which is derived from a line-shape fit to the L+ mode, displays an in-depth gradient with a decreasing density when the top surface is approached. The Raman spectra of the Ga-polarity growth sector do not show any trace of phonon-plasmon coupled modes, suggesting a lower Si incorporation and/or the presence of compensating deep levels. © 2014 The Japan Society of Applied Physics.

Recent investigations have revealed the great potential of Raman spectroscopy for the characterization of clinker minerals and commercial Portland cements. The usefulness of this technique for the identification of anhydrous, hydrated, and carbonated phases in cement-based materials has been demonstrated. In the present work, the application of micro-Raman spectroscopy for the characterization of the main clinker phases of calcium aluminate cements and calcium sulfoaluminate cement is explored. The main stable hydrated phases as well as several important carbonated phases are investigated. Raman measurements on the following phases are reported: (i) pure, unhydrated phases: CA, C12A7, CA2, C2AS, cubic-C3A, C4AF, and C4A3inline image; (ii) hydrated phases: ettringite, monosulfoaluminate, and hydrogarnet (C3AH6); (iii) carboaluminate phases: hemicarboaluminate and monocarboaluminate. The present results, which are discussed in terms of the internal vibrational modes of the aluminate, carbonate, and sulfate molecular groups as well as stretching O–H vibrations, show the ability of Raman spectroscopy to identify the main hydrated and unhydrated phases in the aluminate and sulfoaluminate cements. The Raman spectra obtained in this work provide an extended database to the existing data published in the literature., Peer Reviewed, Postprint (published version)

We have performed Raman-scattering measurements under high hydrostatic pressure on CdO thin films grown by metal-organic vapor phase epitaxy on sapphire substrates. The pressure dependence of the second-order Raman bands is discussed in terms of ab initio lattice-dynamical calculations, which allow us to obtain mode Grüneisen parameters for the zone-center TO and LO modes of CdO. Our experiments and calculations suggest that at low pressures (<4¿GPa) the Raman spectra are dominated by second-order modes, while at higher pressures (>4¿GPa) the spectra mainly display contributions from disorder-activated first-order modes.

Recent investigations have revealed the great potential of Raman spectroscopy for the characterization of clinker minerals and commercial Portland cements. The usefulness of this technique for the identification of anhydrous, hydrated, and carbonated phases in cement-based materials has been demonstrated. In the present work, the application of micro-Raman spectroscopy for the characterization of the main clinker phases of calcium aluminate cements and calcium sulfoaluminate cement is explored. The main stable hydrated phases as well as several important carbonated phases are investigated. Raman measurements on the following phases are reported: (i) pure, unhydrated phases: CA, C12A7, CA2, C2AS, cubic-C 3A, C4AF, and C4A3S̄; (ii) hydrated phases: ettringite, monosulfoaluminate, and hydrogarnet (C 3AH6); (iii) carboaluminate phases: hemicarboaluminate and monocarboaluminate. The present results, which are discussed in terms of the internal vibrational modes of the aluminate, carbonate, and sulfate molecular groups as well as stretching O-H vibrations, show the ability of Raman spectroscopy to identify the main hydrated and unhydrated phases in the aluminate and sulfoaluminate cements. The Raman spectra obtained in this work provide an extended database to the existing data published in the literature. © 2013 The American Ceramic Society.