It has been shown that, in single crystals and films of a strongly correlated material, namely, vanadium dioxide, upon a thermally stimulated phase transition from the low-temperature monoclinic phase to the high-temperature tetragonal phase, the narrow-line Raman spectrum of the insulating (monoclinic) phase transforms into the broad-band Raman spectrum, which contains two peaks at 500 and 5000 cm−1 with widths of 400 and 3500 cm−1, respectively. It has been found that, as the temperature of the monoclinic phase approaches the structural phase transition temperature (340 K), the line profile of soft-mode phonons at a frequency of 149 cm−1 with A g symmetry and the line profile of phonons at a frequency of 201 cm−1 with A g symmetry acquire an asymmetric shape with a Fano antiresonance that is characteristic of the interaction of a single phonon vibration with a continuum of strongly correlated electrons. It has been demonstrated that the thermal transformation of peaks in the Raman spectra of the VO2 metallic phase is in quantitative agreement with the theory of Raman scattering in strongly correlated materials.

Raman studies of Mg-doped InN films with a Mg content from NMg=3.3×1019to5.5×1021cm−3 are reported. Raman and secondary ion mass spectroscopy data on the Mg content have been found to correlate well. Lattice dynamics of hexagonal InN with substitutional impurities and vacancies has been investigated in the framework of the cluster approach. Energy positions of local vibrational modes in InN have been calculated and compared with experimental findings. It is concluded that Raman spectroscopy is a good tool for quantitative characterization of Mg-doped InN.

A strong resonant behavior of the Raman scattering from LO-phonons in n-InGaN alloys at excitation near the interband absorption threshold was observed. An approach has been developed to describe the resonant Raman cross sectional profile in the presence of a Burstein-Moss shift of the interband optical transitions. It has been shown that a simultaneous study of absorption, photoluminescence, and Raman spectra provides reliable information about the band gap and can be efficient for the alloy characterization. Our data show that the band gap composition dependence of InGaN is characterized by the strongly nonlinear behavior with the large bowing parameter of 2.5-2.6 eV.

We present results of a detailed study of X-ray, photoluminescence, and Raman measurements of hexagonal InN, In-rich InxGa1-xN (0.36 < x < 1) alloys, and InN samples annealed in oxygen. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

It is shown that such parameters of GaN/AlxGa1-xN superlattice as the period, build-in strain, composition of the alloy, and individual layer thicknesses can be extracted from the energy positions, intensities, and line shapes of various optical and acoustical modes detected in Raman scattering.

We suggest on approach including lattice dynamics calculations and Raman measurements of individual phonon modes in hexagonal GaN/AlN and GaN/AlGaN superlattices, which can prove to be a valuable tool in quantitative characterization of these nanostructures. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

It is shown that such parameters of GaN/AlxGa1-xN superlattice as the period, built-in strain, composition of the alloy, and individual layer thicknesses can be extracted from the energy positions, intensities, and line shapes of various optical and acoustic modes detected in Raman scattering.

Studies of first- and second-order Raman scattering in hexagonal ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys are reported. The dependences of frequencies of all Raman-allowed optical phonons versus Al content are traced in detail in the entire composition range. The one-mode behavior of LO phonons and the two-mode behavior of the other phonons is established. It is shown that the composition dependences of ${A}_{1}(\mathrm{TO}),$ ${A}_{1}(\mathrm{LO}),$ ${E}_{1}(\mathrm{LO}),$ and ${E}_{2}(\mathrm{low})$ phonon energies are convenient tools for the quantitative characterization of the Al content in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys. The energy position of the ${B}_{1}(\mathrm{high})$ silent mode is proposed. A narrow gap separating the dispersion regions of transverse and longitudinal optical phonons is revealed in the phonon density-of-state function. The composition dependence of the phonon line broadening is investigated experimentally and theoretically. It is shown that the broadening is due to elastic phonon scattering by the composition fluctuations. A theoretical approach is used where the statistical and dynamical aspects of the phonon scattering are treated separately. The type, size, and number of the fluctuations responsible for the phonon line broadening are estimated. The theory is qualitatively consistent with the observed composition dependences.

The effects of the layer thickness and alloy composition on the Raman spectra of hexagonal GaN/Al x Ga 1-x N multilayer structures have been investigated. Our findings show that the A 1 (TO) optic mode can be regarded as propagating through the structure, whereas other optic modes are confined to the GaN or Al x Ga 1-x N layer. We report the first observation of folded acoustic modes in short-period GaN/Al x Ga 1-x N superlattices. The dispersion of the LA phonon branch was determined by using different Raman scattering configurations. The sound velocity for GaN/Al x Ga 1-x N SL with Al content x = 0.28 was found to be 8410 m/s.

Abstract : We report the first experimental observation of folded acoustical modes in strained hexagonal GaN/A1(x)Ga(1)(-x)N superlattices by Raman scattering. The dispersion of LA phonon branch was determined by using different scattering configurations. It was found that the zone center gap of folded phonon branch is beyond the instrumental resolution (approximately equal 0.5 cm (-1)) The sound velocity for GaN/A1(x)Ga(1)(-x)N SL with Al content x= 0.28 was found to be 8410 m/s.

We present experimental Raman data on dynamic properties of AlxGa1-xN alloys in the entire compositional range (0 <x <1). Special attention is given to the behavior of optical phonon modes in Al-rich alloys, This investigation is supported by a thorough polarization analysis. The one-mode type behavior of A(1)(LO) and E-1(LO) phonons and two-mode type behavior of E-2(high) and E-1(TO) phonons has been confirmed, and the behavior of all these phonons for Al-rich compositions has been traced in detail, For the first time the two-mode behavior of A(1)(TO) and E-2(low) phonons has been revealed by Raman spectroscopy. The behavior of phonons in hexagonal AlxGa1-xN is compared with recent theoretical predictions. The information useful for the compositional characterization of AlxGa1-xN alloys is provided.

Results of a comprehensive study of the behavior of the A(1)(TO) phonon mode in hexagonal AlxGa1-xN alloys in the entire compositional range are described. It has been found that the Raman spectrum of AlxGa1-xN, with a Ga content (1-x)<0.3, exhibits a large broadening with a complex structure. We attribute this structure to a manifestation of the phonon density of states in the region of vibrations of the optical A(1)(TO) branch, and to the appearance of a gap mode in AlN. Both effects are due to the substitution of heavier ea atoms in the cation sublattice of,AlN. A theoretical approach is suggested which describes changes in the vibrational spectrum at a sufficiently strong perturbation resulting from isoelectron substitution. In the framework of the developed model, the dependence of the intensity and band shape of the gap mode on Ga content are calculated and compared with the experimental Raman data. The experimental and theoretical dependences are shown to be in good agreement in the region (1-x)<0.12-0.15. The obtained results indicate that the formation of an A(1)(TO) gap mode in the regions of low and intermediate Ga contents is caused by statistical Ga clusters in the cation sublattice of the solid solution. In a limited range of Ga contents, the behavior of the A(1)(TO) phonon mode in AlxGa1-xN can be considered as a two-mode behavior.

lRaman spectroscopic studies of multilayered GaN/AlxGa1-xN structures grown by the MOCVD and MBE techniques have been carried out for the first time. Dependence of frequencies of optical phonons on Al content has been investigated. Our observations have shown that E-2.(low) and LO phonons of A(1) and E-1 symmetry are confined to the GaN or AlxGa1-xN layer. However, it has been established that positions of A(1)(TO), E-1(TO) and E-2(high) phonon modes correspond to the Al content averaged over the whole structure. In addition, angular dispersion of transverse polar phonons was revealed in the layered structures, These findings show that the A(1)(TO), E-1(TO) and E-2(high) phonon modes can be regarded as propagating through the multilayered structure, The effect of the layer thickness on the Raman spectra have been investigated.

The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and (11̄02) sapphire substrates. All six Raman-active optical phonons were observed and assigned: E2(low) at 87 cm−1, E2(high) at 488 cm−1, A1(TO) at 447 cm−1, E1(TO) at 476 cm−1, A1(LO) at 586 cm−1, and E1(LO) at 593 cm−1. The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be ε⊥0/ε∥0=0.91. The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.

The results of Raman spectroscopic studies of the disorder effects in hexagonal AlxGa1−xN epitaxial layers grown by MBE and HVPE on different substrates for a large range of Al concentrations are presented. The width of the nonpolar phonon line with E2 symmetry results from the inhomogeneous broadening due to spatial fluctuations in the Al content. The abnormally small broadening of the A1(TO) polar phonon mode for x or (1−x)≪1 and the large broadening for x≅0.5–0.7 are attributed to the specific frequency dependence of the density of states for the branch with the directional dispersion in pure crystals. Thus the Raman spectrum is found to be highly sensitive to the composition of AlxGa1−xN epitaxial layers and its inhomogeneity. It is shown that in the estimation of the crystal composition, on the basis of Raman data, the influence of the homogeneous strain effects could be excluded via measuring a linear combination of two Raman line frequencies.

We present the results of a detailed study of the first- and second-order Raman scattering and IR reflection from hexagonal InN layers grown on (0001) and (1 (1) over bar 02) sapphire substrates. All six Raman active optical phonons were measured and assigned: E-2(low) at 87 cm(-1), E-2(high) at 488 cm(-1), A(1)(TO) at 447 cm(-1), E-1(TO) at 476 cm(-1), A(1)(LO) at 586 cm(-1), and E-1(LO) at 593 cm(-1). The static dielectric constants of InN for the ordinary and extraordinary directions were estimated to be epsilon(perpendicular to 0) = 13.1 and epsilon(\\0) = 14.4, respectively. The phonon dispersion curves and phonon density-of-state function for hexagonal InN were calculated by scanning throughout the BZ.