Phonon Mode Frequencies Research Articles

Raman scattering study of undoped and As-doped GaN grown with different III/V ratios

We have used Raman scattering to investigate a series of undoped and As-doped GaN epilayers grown on sapphire substrates by plasma-assisted molecular beam epitaxy with different Ga-to-N ratios. We find that the frequency of the E2h phonon mode of GaN is very sensitive to the Ga-to-N ratio used to grow both types of samples, which may be attributed to strain effects arising from the different growth conditions. We observe Raman signatures corresponding to the formation of submicron cubic domains in some of the spectra of the As-doped samples. We discuss the origin of a broad band located at 232 cm−1 that appears in both the undoped and the As-doped epilayers. We speculate that this band, reported in previous Raman studies on As-doped and Bi-doped GaN, may be attributed to the formation of Ga clusters.

Raman scattering by thin film nanomosaic rutile TiO2

Thin TiO2 films with rutile short-range atomic order grown at room temperature have a nanostructure of rectangular domains based on a faulted rutile structure. Raman scattering by nanomosaic rutile in single layer TiO2 and TiO2–Al2O3 nanolaminate films is reported here. In all architectures, the Eg phonon mode frequency is similar to bulk but the A1g mode frequency is significantly blueshifted. These results are different from published Raman scattering spectra of other nanocrystalline rutile TiO2. A model for nanomosaic rutile is presented in which TiO6 octahedra in the faulted regions are severely distorted with a lengthened equatorial Ti–O bond and a shortened apical Ti–O bond. The observed A1g mode blueshift is a consequence of the shortened apical bond.

Raman scattering study of [formula omitted] in the charge disordered and ordered phases

Polarized Raman spectra of sodium vanadium oxide η - Na 1.3 V 2 O 5 single crystals were measured over wide temperature and frequency ranges. In total 27 first-order Raman modes, as well as overtones and combinational lines, are observed. For the charge ordered phase no appearances of new phonon- or spin-related Raman modes are registered on lowering the temperature to 10 K. The phase transition at about 100 K is manifested through an abrupt change of the phonon mode frequencies and dampings.

Size Effect for Ba(ZrxTi1 − x)O3 (x = 0.05) Nano-Particles

This paper focuses on the size effect for barium zirconate titanate (BZT) prepared by a sol-gel method. BZT particles with different crystal sizes were prepared by carefully controlling the annealing temperatures and times of the alkoxide-derived precursor particles. Particle size was calculated from the XRD patterns using Scherrer's equation. The change in the Curie temperature was estimated by XRD pattern measured at different temperatures, and the intrinsic dielectric permittivity for BZT nano-particles were calculated from the phonon mode frequencies using the Lyddane-Sachs-Teller (LST) relation.

Infrared optical properties of MgxZn1−xO thin films (0⩽x⩽1): Long-wavelength optical phonons and dielectric constants

Infrared spectroscopic ellipsometry in the spectral range from ω=360cm−1toω=1500cm−1 and Raman scattering spectroscopy are applied to study the long-wavelength optical phonon modes and dielectric constants of MgxZn1−xO thin films in the composition range 0⩽x⩽1. The samples were grown by pulsed laser deposition on sapphire substrates. X-ray diffraction measurements of the thin film samples reveal the hexagonal wurtzite crystal structure for x⩽0.53 and the cubic rocksalt crystal structure for x⩾0.67. A systematic variation of the phonon mode frequencies with Mg-mole fraction x is found for both hexagonal and cubic MgxZn1−xO thin films. The modified random isodisplacement model matches the observed composition dependence of the phonon mode frequencies for the hexagonal structure thin films [J. Chen and W. Z. Shen, Appl. Phys. Lett. 83, 2154 (2003)], whereas a simple linear approximation scheme is sufficient for the cubic structure part. We observe a discontinuous behavior of the transverse optical phonon modes (decrease), and the static and high-frequency dielectric constants (increase) within the phase transition composition region from the wurtzite structure part to the rocksalt structure part. On the contrary, the longitudinal phonon mode parameters increase almost linearly, and upon phase transition the splitting between the transverse and longitudinal modes increases. We associate this discontinuous behavior with the change of the nearest-neighbor coordination number from fourfold (wurtzite structure) to sixfold (rocksalt structure) in our samples and the associated increase in bond ionicity from ZnO to MgO. Accordingly, we propose that the reduced exciton mass parameter should approximately double upon changing from wurtzite to rocksalt crystal structure.

Phonon modes, dielectric constants, and exciton mass parameters in ternary MgxZn1−xO

ABSTRACTThe long-wavelength optical phonons and dielectric constants of PLD-grown MgxZn1−xO films are studied by combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The ternary alloy MgxZn1−xO exhibits a phase transition with change of coordination number from four-fold coordinated, hexagonal wurtzite structure (ZnO) to six-fold coordinated, cubic rocksalt structure (MgO). It is found that both phonon mode frequencies and dielectric constants change abruptly upon phase transition, which is assigned to the change of coordination number. The change of dielectric constants can be related to the change of electronic properties, for instance, the exciton binding energy. In a simple approach, the exciton binding energy depends on the reduced exciton mass and the static dielectric constant. By comparison with experimental values of the exciton binding energies it is found that the reduced exciton mass must increase by a factor of about two upon face transition from wurtzite to rocksalt structure.

Raman characterization of electronic properties of self-assembled GaN nanorods grown by plasma-assisted molecular-beam epitaxy

We have investigated the Raman scattering of the aligned gallium nitride (GaN) nanorods grown by plasma-assisted molecular-beam epitaxy. It was determined by Raman spectroscopy that the GaN nanorods are relatively strain free. The free carrier concentration, as well as electron mobility of the GaN nanorods, was obtained by the line shape analysis of the coupled A1longitudinal-optical(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017cm−3 and 140cm2Vs, respectively. The local temperature of the nanorod sample was estimated based on the ratio of Stokes to anti-Stokes Raman peak intensity. Since the position of the LO phonon peak was found to be dependent on both the temperature and the LO phonon-plasmon coupling, it is crucial to consider the temperature effect in determining the frequency of the uncoupled LO phonon mode for the line shape analysis. The frequency of the A1(LO) mode of an undoped bulk GaN was used as a reference to determine the frequency of the uncoupled A1(LO) phonon mode of the GaN nanorods.

Dispersion of the high-energy phonon modes inNd1.85Ce0.15CuO4

The dispersion of the high-energy phonon modes in the electron doped high-temperature superconductor Nd$_{1.85}$Ce$_{0.15}$CuO$_4$ has been studied by inelastic neutron scattering. The frequencies of phonon modes with Cu-O bond-stretching character drop abruptly when going from the Brillouin zone center along the [100]-direction; this dispersion is qualitatively similar to observations in the hole-doped cuprates. We also find a softening of the bond-stretching modes along the [110]-direction but which is weaker and exhibits a sinusoidal dispersion. The phonon anomalies are discussed in comparison to hole-doped cuprate superconductors and other metallic perovskites.

Observation of TO1 soft mode in SrTiO3 films by terahertz time domain spectroscopy

We have measured the dielectric properties of pulsed-laser-deposited SrTiO3 (STO) thin films by terahertz (THz) time-domain spectroscopy in the frequency range from 80GHzto1.5THz at temperatures from 20to290K. The measured frequency dispersion of the dielectric constant of the STO thin films show a clear Lorentzian oscillator behavior below 180K and softening of the ferroelectric soft mode of the STO thin films takes place with decreasing temperature. Both eigen frequency and damping of the soft phonon mode show a peak nearly at 110K that can be attributed to the effect on the Ti–O–Ti bending in STO thin film due to the starting of tilting in oxygen octahedra near the cubic to tetragonal phase transition. Dielectric strength saturates near the temperature 60K, however, that is a point of further investigation.

Raman spectroscopy of Si 1− xGe x epilayers

The first systematic investigation of the vibrational properties of epitaxial Si 1− x Ge x alloys in the entire composition range (0 ≤ x ≤ 1) is presented. Reciprocal Space Mapping measurements and a Raman spectroscopy study have been undertaken on alloys grown by Low Energy Plasma Enhanced Chemical Vapour Deposition (LEPECVD) in order to investigate the phonon mode frequency dependence on structural and elastic parameters. It is concluded that the strain relaxation process in the LEPECVD virtual substrates is very effective in the entire composition range and that in these fully relaxed epitaxial SiGe alloys the presence of ordering effects and of local composition fluctuations can be excluded.

A multifractal analysis of optical phonon excitations in quasicrystals

It is shown that the allowed frequencies of optical phonon modes propagating in quasiperiodic AlN–GaN structures display a multifractal behavior. This feature is confirmed through a scaling-exponent analysis of a parametric spectrum of singularities f ( α ) . This is done by using an alternative counting method, avoiding the use of a Legendre transform, which could lead to distortions. The Cantor-like spectrum is also investigated, and scaling properties are shown for Fibonacci, Thue–Morse and double-period quasiperiodic superlattices.

Time-dependent electron transport through molecular quantum dots in the presence ofexternal irradiation

We present a fully nonequilibrium calculation of the low-temperature transport propertiesof a single molecular quantum dot coupled to the local phonon mode when an ac field isapplied to the gate. The resonant behaviour is shown in the time-averaged differentialconductance as the ac frequency matches the frequency of the local phonon mode, which isa direct consequence of the satellite-phonon-peak structure in the dot electron spectralfunction. Different step structure with and without the external irradiation is found in theI–V curves, and oscillation behaviour is found in the step height as a function of the irradiationintensity.

Optical and transport properties of InSb thin films grown on GaAs by metalorganic chemical vapor deposition

Optical and transport properties of InSb thin films grown on GaAs by metalorganic chemical vapor deposition (MOCVD) have been investigated by far-infrared (FIR) reflectance spectroscopy. The lattice vibration behaviors of a series of MOCVD InSb/GaAs(100) materials grown under different growth conditions were studied. Effects of III–V source ratios on the films crystalline quality were examined. Two additional weak modes in the wavenumber regions of 210–240 cm − 1 were observed and they appeared more prominent at low temperatures. Interference fringe effects modify the FIR reflectance band of the GaAs substrate, which are related to the uniformity of film thickness and crystalline perfection. The dielectric constant, phonon modes and other optical parameters, as well as transport properties including carrier concentration, mobility, effective mass were calculated theoretically and compared with experimental results. The obtained distribution values of the InSb LO phonon mode frequency, line width, relative integrated intensity ratio between the forbidden and defect-related TO phonon and the allowed LO mode are adopted as figures of merit for the quality of the InSb films. The electrical transport properties of carrier concentration, mobility, and effective mass as well as the dielectric constant of these films have been determined by optical method non-destructively.

Family of noble metal nitrides: First principles calculations of the elastic stability

By calculating the elastic stability using the full-potential linearized augmented plane waves (FLAPW) method, three noble metal nitrides ($\mathrm{Ir}{\mathrm{N}}_{2}$, $\mathrm{Ag}{\mathrm{N}}_{2}$, and $\mathrm{Au}{\mathrm{N}}_{2}$) were found to be stable with the fluorite structure. Together with $\mathrm{Pt}{\mathrm{N}}_{2}$, they form a new family of transition metal nitrides, where nitrogen atoms occupy all the tetrahedral interstitials of the metal sublattice, instead of larger octahedral interstitials. Using the topological analysis of charge density (atoms in molecules theory), it was shown that the stability of the new nitrides can be understood based on the simple size effect. The filling of all the small tetrahedral interstitials makes the phases difficult to compress, resulting in large bulk moduli. The frequencies of the Raman-active phonon modes were also given.

Study on strain and piezoelectric polarization of AlN thin films grown on Si

The strain and piezoelectric polarization of AlN thin films grown on Si(111) substrates by metalorganic chemical-vapor deposition were investigated by using x-ray diffraction and Raman measurements. The stress and piezoelectric polarization of the AlN films were analyzed with E2 (high) phonon-mode frequency shifts in Raman spectra. The result indicates that the biaxial compressive stress is about 4.3GPa, and the piezoelectric polarization is about 1.91×10−2C∕m2. Phonons of Si–N bonds were also observed accompanying phonons of AlN crystal in Raman spectra, which indicate the interdiffusion of Si and N atoms during growth.

DISPERSION OF INTERFACE OPTICAL PHONONS AND THEIR COUPLING WITH ELECTRONS IN ASYMMETRICAL WURTZITEGaN/Ga1-xAlxNQUANTUM WELLS

Within the framework of the dielectric continuum model and Loudon's uniaxial crystal model, the properties of frequency dispersion of the interface optical (IO) phonon modes and the coupling functions of electron–IO-phonon interaction in an asymmetrical wurtzite quantum well (QW) are deduced and analyzed via the method of electrostatic potential expansion. Numerical results reveal that in general, there are four branches of IO phonon modes in the systems. The dispersions of the four branches of IO phonon modes are obvious only when the free wavenumber ktin xy plane is small. The degenerating behavior of all the four branches of IO phonon modes in the asymmetric wurtzite QWs has been clearly observed for small kt. When ktis relatively large, with the increase of kt, the frequencies of the IO phonon modes converge to the four definite limiting frequencies in the corresponding wurtzite single planar heterostructure. This feature is obviously different from that in symmetric wurtzite QW, and the mathematical and physical reasons have been analyzed in depth. The calculations of electron–phonon coupling function show that the electrostatic distribution of the IO modes is neither symmetrical nor antisymmetrical, and the high-frequency IO phonon branches and the short-wavelength IO phonon modes play a more important role in the electron–phonon interaction.

Polar interface optical phonon modes and Fröhlich electron–phonon interaction Hamiltonians in wurtzite quantum well wires

Within the framework of the dielectric continuum approximation and Loudon's uniaxial crystal model, the interface optical (IO) phonon modes and the corresponding Fröhlich electron–phonon interaction Hamiltonian in a wurtzite AlN/GaN/AlN quantum well wire (QWW) are derived and studied. Numerical calculations are mainly focused on the frequency dispersion of the IO phonons and electron–phonon interaction coupling function. Results reveal that, in general, there are four branches of IO phonon modes in the systems. The dispersions of the four branches of IO phonon modes are obvious only when the axial direction wave number kz or the azimuthal quantum number m is small. The degenerating behaviour of the IO phonon modes in wurtzite QWW has also been observed for small kz or m. When kz or m are relatively large, with the increasing of them, the frequencies of these IO phonon modes converge to the two definite limiting frequencies in wurtzite single planar heterostructure, and this feature has been explained reasonably from the mathematical and physical viewpoints. The calculations of the electron–phonon coupling function show that, though some branches of IO phonon modes exchange their localized positions with each other at a large m, there always exist two branches of IO phonon modes localized on each interface. The high-frequency IO phonon modes compared with the low-frequency ones play a more important role in the electron–phonon interaction. Detailed comparison of the dispersion behaviours of the IO phonons and electron–IO phonon couplings properties in wurtzite QWWs with those in zinc-blende QWWs has also been made.

Interface and surface optical phonons in a free-standing [formula omitted] cylindrical quantum-well wire

Under dielectric continuum approximation, interface optical (IO) and surface optical (SO) phonon modes as well as the corresponding Fröhlich electron–phonon interaction Hamiltonian in a free-standing cylindrical quantum-well wire (QWW) were derived and studied. Numerical calculations on GaAs / Al x Ga 1 - x As cylindrical QWWs have been performed. Results reveal that, in the case of taking the “two-mode” behavior of the Al x Ga 1 - x As material into account, there have four branches of IO phonon modes and two branches of SO phonon mode, and the dispersion frequencies of IO or SO phonon modes sensitively depend on the Al mole fraction x in Al x Ga 1 - x As material and the wave-vector in z direction k z . With the increasing of k z and quantum number m, the frequency of each IO mode approaches one of the two frequency values of the single GaAs / Al x Ga 1 - x As heterostructure, and the electrostatic potential distribution of the phonon mode tends to be more and more localized at a certain interface or surface, meanwhile, the coupling between the electron-IO or -SO phonon becomes weaker.

Lattice dynamics ofNaAlH4from high-temperature single-crystal Raman scattering andab initiocalculations: Evidence of highly stableAlH4−anions

Polarized Raman scattering on single crystals of has been used to determine the symmetry properties and frequencies of the Raman-active vibrational modes over the temperature range from 300 to 425 K, i.e., up to the melting point . Significant softening (by up to 6%) is observed in the modes involving rigid translations of cations and translations and librations of . Surprisingly, the data indicate mode softening of less than 1.5% for the Al- stretching and Al- bending modes of the anion. These results show that the anion remains a stable structural entity even near the melting point. First-principles linear response calculations of phonon mode frequencies are in reasonably good agreement with the Raman results. The phonon mode Gr\"uneisen parameters, calculated using the quasiharmonic approximation, are found to be significantly higher for the translational and librational modes than for the Al- bending and stretching modes, but cannot account quantitatively for the dramatic softening observed near in the former two types of modes, suggesting an essentially anharmonic mechanism. The effect of zero-point vibrations on the calculated lattice parameters is found to be large (expansion by 1.2 and 1.5 % in the and parameters, respectively), as expected for a compound with many light elements. We discuss the implications of the observed mode softening for the kinetics of hydrogen release and hypothesize that breaking up the anions is the rate limiting step. The enhanced kinetics of absorption and desorption in Ti-doped powders is attributed to the effectiveness of Ti in promoting the breakup of the anions.

Temperature-dependent Raman scattering studies in ZnSe nanoparticles

A study of the temperature-dependent Raman scattering of nanocrystalline ZnSe is presented and experimental results show weakly higher degree of anharmonicity in bulk compared to that in nanoparticles. Two different decay times τ 0 and τ 1 contributed to the phonon lifetime, temperature-independent τ 0 dominates for nanoparticles, while temperature-dependent τ 0 is important for bulk. Moreover, the asymmetry of the spectra is enhanced with the decreasing temperature, and surface phonon modes are considered as the main contribution to the asymmetry of the spectra. The frequency of the surface phonon mode softens and the ratio of the integrated intensities I LO/ I SP increases with the increasing temperature.