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Phonon Modes (phonon + mode)
Kinds of Phonon Modes Selected AbstractsLattice dynamics of chalcopyrite semiconductors LiAlTe2, LiGaTe2 and LiInTe2PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2009A. V. Kosobutsky Abstract Within density functional perturbation theory using norm-conserving pseudopotentials and a plane-wave basis set calculations of phonon dispersion relations and densities of states of LiAlTe2, LiGaTe2 and LiInTe2 compounds being crystallized into the tetragonal chalcopyrite structure have been performed. Theoretical values of phonon mode frequencies in LiGaTe2 and LiInTe2 are in good agreement with the experimental data available for these crystals obtained by the methods of Raman and infrared spectroscopies. The similarity of the physical and chemical properties of the crystals concerned manifests itself in the similarity of their phonon spectra that are especially close to each other in low- and high-frequency ranges. Phonon modes of the upper phonon band are predominantly caused by the lithium sublattice vibrations and have an upper bound of 350,370 cm,1. In a mid-frequency range a significant downshift of the vibrational frequencies is observed on going from LiAlTe2 to LiGaTe2 and LiInTe2 that is a consequence of the third group cation mass reduction. From calculated electron density maps it follows that Li-containing chalcopyrites are characterized by a less pronounced bond between the first group cation and anion as compared with the Cu- and Ag-based analogs due to the absence of pd-hybridization. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phonon modes of monoclinic BiB3O6PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2005A. Gössling Abstract We present a detailed study of the phonon modes of the monoclinic compound BiB3O6 based on polarized reflectivity measurements on single crystals. The spectra are analysed by means of a generalized Drude,Lorentz model, which allows us to resolve the modes of A and B symmetry. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Annealing crystallization of a-Ge/Al/Si and a-Ge/Si thin filmsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2005F. Fajardo Abstract This work describes the temperature-induced crystallization of amorphous Ge (a-Ge) as a function of the thickness of the a-Ge films (in the 12,2600 nm range), which were deposited both onto c-Si substrates and c-Si substrates covered with aluminium. After deposition, the samples were submitted to cumulative thermal annealing treatments. It is shown that the temperature of crystallization depends on the thickness of the a-Ge films and to the presence (or not) of the Al layer. For an annealing temperature (Ta) of ,700 °C, for example, the Raman spectra of films thinner than ,1000 nm and deposited onto c-Si substrates are completely dominated by the sharp phonon mode of crystalline Si. Films with thicknesses equal to 300, 1000 and 2600 nm, deposited onto Al/c-Si, and treated at Ta = 600 °C, on the other hand, clearly display two additional peaks at 405 and 490 cm,1. They correspond to the Raman modes of Si,Ge and Si,Si modes, suggesting the formation of a SiGe alloy during the thermal anneal of the films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effective mass of InN estimated by Raman scatteringPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010Jung Gon Kim Abstract We have estimated the longitudinal effective mass (m,) of electron in n-type InN films by Raman scattering. The samples were grown by MOVPE (metal organic vapor phase epitaxy) with free carrier concentration of n =6.7×1018 -9.9×1018 cm -3 according to Hall measurement. A weak Raman signal observed at ,430 cm -1 at room temperature was sharpened and shifted to higher frequency toward the A1(TO)-phonon mode at 447 cm -1 with increasing n. This mode was assigned to the lower branch (L - ) of the longitudinal-optic-phonon-plasmon-coupled (LOPC) mode. The line shape was carefully analyzed by a semi-classical line-shape fitting analysis assuming deformation potential and electro-optic coupling mechanisms for the light scattering process. A line-shape fitting analysis was conducted by adjusting three major parameters; electron density, effective mass and plasmon damping rate. The analysis well reproduced values of electron density and mobility deduced by Hall measurement. Electron effective mass of m,*/m0 = 0.05 (±0.01) was also obtained as the best-fit parameter. The result agrees well with previous data obtained by other optical methods. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Vibrational modes in a square cross-section InAs/InP nanowire superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Seiji Mizuno Abstract We study theoretically the phonon modes in a square cross-section nanowire superlattice (NWSL) consisting of InAs and InP. The phonon modes in the NWSL composed of cubic materials are classified into five types, i.e., A1, A2, B1, B2, and E modes. We calculate the dispersion relations of each phonon mode and corresponding displacement fields for this NWSL. The A1, A2, B1, B2, and E modes have features of dilatational, torsional, stretching, shear, and flexural modes, respectively. We examine the effects of both the superlattice modulation along the wire axis and lateral confinement of phonons. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optical properties of Na0.68CoO2 thin filmPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2007M. H. Hsu Abstract The optical reflectance of single-crystalline Na0.68CoO2 (thickness around 3000 Ĺ) thin film has been measured over a wide frequency (40,55000 cm,1) and at temperatures between 20 and 300 K. The room-temperature optical conductivity spectrum displays a metallic character. In addition, one E1u phonon mode at about 518 cm -1 is identified, while the other four phonon peaks are due to the sapphire substrate. As the temperature is lowered, the Drude plasma frequency exhibits little temperature dependence, whereas the scattering rate monotonically decreases with decreasing temperature. Notably, the Drude scattering rate at 20 K is found to approach zero, consistent with the small residual resistivity, indicating high quality of Na0.68CoO2 thin film. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Raman study of strain in GaN/AlN quantum dot multilayered structuresPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005A. Cros Abstract Raman spectroscopy has been used to investigate self-assembled stacks of GaN/AlN quantum dots with increasing number of periods. The E2H phonon modes associated to GaN and AlN are clearly resolved with visible excitation, and their energies allow the simultaneous monitoring of the dot and barrier strain states. The compression of the quantum dots is evidenced by a shift of the E2H phonon mode of circa 29 cm,1 to higher energies with respect to its relaxed value. The strain of the AlN spacer is found to be correlated to that of the dot, with an increase in its tensile component for the samples with fewer periods and a partial relaxation for samples over 50 periods. Additionally, resonant effects of the A1(LO) phonon mode have been investigated by tuning the excitation over a wide range in the ultra-violet region. A discussion of the strain distribution in quantum dots and spacers as a function of the number of periods is performed by means of a theoretical calculation based on the method of inclusions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Ab initio determination of structural and dynamical properties of the InP(110)-S interfacePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004H. M. Tütüncü Abstract We have presented an ab initio pseudopotential study within the local density approximation of the structural properties of the S-treated InP(110) surface for an exchange model. The calculated structural parameters for this model are in good agreement with previous ab initio calculations. With these structural papameters, we have used an ab initio linear-response approach based on the density functional theory to investigate the dynamical properties of this surface. We discuss the similarities and contrast the differences in the phonon dispersion curves of the S-treated InP(110) and the clean InP(110) surfaces. We have found that the rotational phonon mode predicted for the clean surface can be also identified for the S-treated InP(110) surface. In addition to this, two new surface-localized phonon states appear in the acoustic-optical gap range due to the adsorption of S atoms. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Terahertz Time Domain Spectroscopy to Detect Low-Frequency Vibrations of Double-Walled Carbon NanotubesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 27 2010Sunil Kumar Abstract We have measured the frequency-dependent real index of refraction and extinction coefficient (and hence the complex dielectric function) of a free-standing double-walled carbon nanotube film of thickness 200 nm by using terahertz time domain spectroscopy in the frequency range 0.1 to 2.5 THz. The real index of refraction and extinction coefficient have very high values of approximately 52 and 35, respectively, at 0.1 THz, which decrease at higher frequencies. Two low-frequency phonon modes of the carbon nanotubes at 0.45 and 0.75 THz were clearly observed for the first time in the real and imaginary parts of the complex dielectric function along with a broad resonance centred at around 1.45 THz, the latter being similar to that in single-walled carbon nanotubes assigned to electronic excitations. Our experiments bring out a possible application of double-walled carbon nanotube films as a neutral density filter in the THz range. [source] Effects of Phonon Confinement on Anomalous Thermalization, Energy Transfer, and Upconversion in Ln3+ -Doped Gd2O3 NanotubesADVANCED FUNCTIONAL MATERIALS, Issue 4 2010Andreia G. Macedo Abstract There is a growing interest in understanding how size-dependent quantum confinement affects the photoluminescence efficiency, excited-state dynamics, energy-transfer and thermalization phenomena in nanophosphors. For lanthanide (Ln3+)-doped nanocrystals, despite the localized 4f states, confinement effects are induced mostly via electron,phonon interactions. In particular, the anomalous thermalization reported so far for a handful of Ln3+ -doped nanocrystals has been rationalized by the absence of low-frequency phonon modes. This nanoconfinement may further impact on the Ln3+ luminescence dynamics, such as phonon-assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd2O3:Eu3+ and Gd2O3:Yb3+,Er3+ nanotubes, exhibiting up to one order of magnitude larger than previously reported for similar materials, is reported. This anomalous thermalization induces unexpected energy transfer from Eu3+C2 to S6 crystallographic sites, at 11,K, and 2H11/2,,,4I15/2 Er3+ upconversion emission; it is interpreted on the basis of the discretization of the phonon density of states, easily tuned by varying the annealing temperature (923,1123,K) in the synthesis procedure, and/or the Ln3+ concentration (0.16,6.60%). [source] Pair distribution functions calculated from interatomic potential models using the General Utility Lattice ProgramJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2007Elizabeth R. Cope A new module has been developed for the widely used General Utility Lattice Program (GULP). The phonon-based theory developed by Chung & Thorpe [Phys. Rev. B (1999), 59, 4807,4812] to calculate pair distribution function (PDF) peak widths has been utilized to give a selection of commonly used correlation functions. A numerical library of neutron scattering information is now available within GULP, and is used to produce results that can be compared with neutron scattering experimental data. The influence of different phonon modes on the PDF can be assessed by excluding modes above or below a cut-off frequency. Results are presented for sample crystallographic systems, MgO, SrTiO3 and ,-cristobalite, as well as CaxSr1,xTiO3 at x = 0.5, which makes use of the capability to handle partial occupancies to compare different Ca/Sr ordering arrangements with a disordered model in which every Ca/Sr site has 50% occupancy of both species. [source] Crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 complex perovskites investigated by Raman scatteringJOURNAL OF RAMAN SPECTROSCOPY, Issue 1 2010Anderson Dias Abstract This work investigates the crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 perovskites by Raman spectroscopy. The samples were produced by conventional solid-state processing at 1200 °C. X-ray diffraction showed that single-phase homogeneous materials were produced, which are cubic or pseudo-cubic in symmetry. The existing controversies in the literature for these complex perovskites were investigated by comparing experimental Raman data with group-theory analysis. Ceramics with Ca and W or Mo were found to be cubic, space group Fm3m. For these materials, four Raman-active bands were observed and the fitting parameters showed that the Ba(Ca1/2Mo1/2)O3 ceramic presents bands at lower wavenumbers if compared with the Ba(Ca1/2W1/2)O3 sample. For the Ba(Sr1/2W1/2)O3 material, two hypotheses were investigated for monoclinic or triclinic structures. The experimental results showed 12 Raman-active modes for this ceramic, which is in perfect agreement with the theoretical predictions for a monoclinic (I2/m) structure. Copyright © 2009 John Wiley & Sons, Ltd. [source] Polar phonon mode selection rules in tip-enhanced Raman scatteringJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2009Samuel Berweger Abstract We discuss the use of the symmetry selectivity of phonon Raman scattering to determine nanocrystallographic information of solids using tip-enhanced Raman scattering (TERS). The necessary degrees of freedom arise from the combination of the Raman selection rules reflecting crystal symmetry superimposed by the polarization and k -vector-dependent field enhancement and scattering of the scanning probe tip. The resulting phonon TERS selection rules are discussed, including the use of the crystal Raman tensor and momentum conservation for polar phonon modes. We demonstrate the selection rules for both far-field and tip-enhanced near-field Raman scattering from bulk and nanocrystalline LiNbO3. Copyright © 2009 John Wiley & Sons, Ltd. [source] Size-dependent Raman and infrared studies of PbSe nanoparticlesJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2008F. S. Manciu Abstract Micro-probe Raman and far-infrared absorption spectroscopies were used to prove the existence of optical phonon modes of PbSe nanoparticles prepared by colloidal chemistry and preliminarily characterized by transmission electron microscopy. To the best of our knowledge, this is the first time that evidence of the surface phonon (SP) mode by Raman spectroscopy has been experimentally observed. The wavenumber of the SP mode is consistent with its prediction by a dielectric continuum model. While for different PbSe nanoparticle sizes the observed SP mode does not show any obvious change in its position, there is a clear shift by approximately 4 cm,1 toward higher wavenumber in the appearance of the LO(,) in the Raman spectra from the 3 nm to the 7 nm PbSe nanoparticles. Far-infrared measurements demonstrate the presence of the transverse optical TO(,) and of the coupled phonon modes. Copyright © 2008 John Wiley & Sons, Ltd. [source] Temperature-dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2008M. Ma, czka Abstract The temperature-dependent Raman studies of A2Ti2O7(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2,295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third-order phonon,phonon anharmonic interactions. Moreover, the 312 and 330 cm,1 modes of Er2Ti2O7(Gd2Ti2O7) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd. [source] Anomalous Dynamical Charge Change Behavior of Nanocrystalline 3C-SiC upon CompressionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004Haozhe Liu Using diamond anvil cell (DAC) technique, in situ high-pressure Raman scattering and energy-dispersive X-ray diffraction (EDXRD) experiments were used at room temperature to study 3C-SiC with an average grain size of 30 nm. In contrast to its bulk counterpart, a decrease of the Born's transverse effective charge of these nanocrystals was observed with increasing pressure from measurements of the longitudinal and transverse optical phonon modes (longitudinal optical,transverse optical) splitting. This is therefore indicative of a diminishing ionicity of nanocrystalline 3C-SiC on compression. [source] Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics SimulationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2001Patrick K. Schelling We present results of molecular-dynamics simulations of the thermal conductivity, ,, of ZrO2 and Y2O3 -stabilized ZrO2 (YSZ). For both pure ZrO2 and YSZ with low concentrations of Y2O3, we find that the high-temperature , is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y2O3 concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to ,. [source] Steady-state picosecond stimulated Raman scattering in two host-crystals for Ln3+ and Ln2+ lasantsLASER PHYSICS LETTERS, Issue 8 2006A. A. Kaminskii Abstract Efficient room-temperature high-order Stokes and anti-Stokes steady-state generation in two wide-used hostcrystals LuVO4 and CaF2 for lanthanide lasants has been discovered. All recorded ,(3)-lasing components are identified to the SRS-promoting phonon modes of these crystals. We classi- fied lutetium vanadate and fluorite as media for promising for up- and down-Raman laser-frequency converters, as well as for self-SRS lasers. (© 2006 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Vibrational properties of GaP and GaP1,xNx under hydrostatic pressures up to 30 GPaPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007M. P. Jackson Abstract We report on Raman scattering of the vibrational modes of GaP and the dilute nitride ternary GaP0.979N0.021 under hydrostatic pressures of up to 30 GPa. We measured the pressure induced shift of the TO and LO modes of GaP up to the phase transition (I,II) and determined mode Grüneisen parameters of 1.12 and 0.98. The phase transition was observed near to 24.5 GPa. We have also studied the dependence of the vibrational spectrum of the GaP0.979N0.021 up to 20 GPa. The zincblende optical phonons of the ternary alloy show great similarities to those of binary GaP, with no significant changes to the Grüneisen parameters or the phase transition. The pressure-induced shift of local nitrogen mode frequency in GaP0.979N0.021 was studied up to 10 GPa and found to be considerably larger than that of the phonon modes in GaN, indicating that the local mode does not show a GaN-like behaviour under pressure. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Surface morphology effects on the optical phonon modes in InAsxSb1,x epilayers on GaAs(001)PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2006P. Jayavel Abstract We have studied Raman scattering from longitudinal- and transverse-optical (LO and TO) phonon modes in InAsxSb1,x epilayers grown on GaAs(001) substrates with various arsenic compositions (x). InSb-like and InAs-like LO and TO phonon modes of the epilayers are observed. We find that the peak intensity ratio of the InAs-like LO to TO-mode decreases for x < 0.6 while it increases for x > 0.6. This intensity ratio change is attributed to two- and three-dimensional (2D and 3D) growth mode of the epilayers using atomic force microscopy. Further, the intensity ratio depends on the root mean square surface roughness of the alloy. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phonon modes of monoclinic BiB3O6PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2005A. Gössling Abstract We present a detailed study of the phonon modes of the monoclinic compound BiB3O6 based on polarized reflectivity measurements on single crystals. The spectra are analysed by means of a generalized Drude,Lorentz model, which allows us to resolve the modes of A and B symmetry. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Pressure evolution of the phonon modes and force constants of Tb3Al5O12 and Lu3Al5O12PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003K. Papagelis Abstract The pressure evolution of the Raman modes of Tb3Al5O12 and Lu3Al5O12 has been measured at room temperature and analyzed theoretically with the rigid ion model. Satisfactory agreement between experiment and theory has been found. The extracted pressure variation of the bond bending and bond stretching force constants shows that the compressibility of the various polyhedra types increases with increasing cation coordination, indicating that the dodecahedra play an important role in the compressibility of the garnet structure. Finally, an estimation of the pressure coefficients for the transverse optical infrared active mode frequencies is given. [source] Vibrational modes in a square cross-section InAs/InP nanowire superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Seiji Mizuno Abstract We study theoretically the phonon modes in a square cross-section nanowire superlattice (NWSL) consisting of InAs and InP. The phonon modes in the NWSL composed of cubic materials are classified into five types, i.e., A1, A2, B1, B2, and E modes. We calculate the dispersion relations of each phonon mode and corresponding displacement fields for this NWSL. The A1, A2, B1, B2, and E modes have features of dilatational, torsional, stretching, shear, and flexural modes, respectively. We examine the effects of both the superlattice modulation along the wire axis and lateral confinement of phonons. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Luminescence and vibrational properties of erbium-implanted nanoporous GaNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008C. B. Soh Abstract Implantation of erbium (Er) into GaN is useful in creating selected areas to emit at the green, yellow and infrared wavelengths. Enhanced erbium activation is obtained when erbium is implanted into porous GaN formed by electrochemical etching than into as-grown GaN. This is due to the increase in surface areas for light extraction and the availability of more free surfaces to accommodate strain when it is annealed. Furnace annealing at 1100 °C for 30 mins in nitrogen gives rise to higher band-edge photoluminescence intensity. Apart from the host GaN phonon modes, we have also observed disorder-induced lattice vibrations at 170, 200 and 350-365 cm,1 from Er-implanted porous GaN. The E2 (high) mode of GaN also shifts towards higher energy at higher annealing temperatures, indicative of more erbium occupying the VGa site (ionic radii of Er > Ga) and hence increasing the compressive stress in the GaN crystal lattice. The prominent defect-induced local vibrational modes in Er-doped nanoporous GaN are also observed in ultraviolet resonant Raman scattering. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] AlGaN epitaxial layers grown by HVPE on sapphire substratesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006V. Soukhoveev Abstract Growth of AlxGa1,xN (x , 0.0-1.0) alloy layers by hydride vapor pahse epitaxy (HVPE) is reported. Crack-free undoped AlGaN layers from 0.1 to 2 ,m thick were grown on 2, SiC and 2,and 4, sapphire substrates. For AlxGa1,xN (x , 0.7-0.8) layers grown on sapphire, high-resolution symmetric/asymmetric X-ray diffraction (XRD) measurements resulted in ,-scan rocking curve widths ranging from 250 to 650 arcsec and from 1400 to 1900 arcsec, for the (00.2) and (10.2) reflections, respectively. Minimum XRD-estimated screw and edge dislocation densities in AlGaN layers grown on sapphire were <6 x 108 cm,2 and <2 x 109 cm,2, respectively. Raman studies revealed that the composition dependences of the phonon modes of HVPE-grown AlxGa1,xN layers are in a good agreement with the one, which have been observed for MOCVD-grown AlxGa1,xN alloys earlier. Grown layers had n-type electrical conductivity for the composition range up to x = 0.4. The layer became highly resistive for higher AlN contents. First results on 40 µm thick crack free AlGaN growth on SiC substrates are reported. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Interface and confined polar optical phonons in spherical ZnO quantum dots with wurtzite crystal structurePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004Vladimir A. Fonoberov Abstract We derive analytically the interface and confined polar optical-phonon modes for spherical quantum dots with wurtzite crystal structure. While the frequency of confined optical phonons in zincblende nanocrystals is equal to that of the bulk crystal phonons, the confined polar optical phonons in wurtzite nanocrystals are shown to have a discrete spectrum of frequencies different from those in bulk crystal. The calculated frequencies of confined polar optical phonons in wurtzite ZnO quantum dots are found to be in excellent agreement with experimental resonant Raman scattering data. The derived analytical expression for phonon modes can facilitate interpretation of experimental data obtained for ZnO quantum dots. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Well-width dependence of coupled Bloch-phonon oscillations in biased InGaAs/InAlAs superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004Michael Först Abstract The coupling between Bloch oscillations and longitudinal optical (LO) phonons is investigated in ternary In0.53Ga0.47As/In0.52Al0.48As superlattices of different well widths. In femtosecond time-resolved studies, a strong increase of the coherent LO phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with the different optical phonon modes of the ternary semiconductor materials. In a narrow-well superlattice where electronic minibands are energetically shifted close to the confining barrier potential, the phonon amplitudes are asymmetrically enhanced on the high-frequency edge of the resonance. Here, at high electric fields, field induced tunneling into above-barrier continuum states leads to a rapid dephasing of Bloch oscillations. The associated polarization change provides an additional excitation process for coherent LO phonons. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Decoherence of quantum tunneling by coherent phonon modes in symmetric double-well systemPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004Akira Igarashi Abstract We investigate effect of the decoherence in quantum tunneling in one-dimensional symmetric double-well potential with coherent or stochastic perturbation. A coupling with some vibrational modes can cause the decoherence in the wavepacket dynamics. Furthermore, we discuss the controllability of the quantum tunneling by external field based on optimal control theory. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||