Phonon Mode Frequencies Research Articles

Wave‐Vector‐Dependent Raman Scattering from Coupled Plasmon–Longitudinal Optical Phonon Modes and Fano Resonance in n‐type Scandium Nitride

Raman scattering from coupled plasmon–longitudinal optical (LO) phonon modes in polar semiconductors is an effective tool to determine electronic properties, such as carrier concentration, mobility, carrier freeze‐out, relaxation times, etc., as well as to understand different types of electron (hole)–phonon interactions. The physics of such coupling mechanism traditionally utilizes the Drude dielectric permittivity that predicts an increase in coupled plasmon–LO phonon mode (LPP+) frequencies with an increase in carrier concentrations. Herein, it is demonstrated that for n‐type epitaxial scandium nitride (ScN) thin films, the frequencies of the coupled plasmon–LO phonon Raman modes exhibit red‐shift with increasing carrier concentrations, which is contrary to the predictions from the Drude theory. Utilizing the generalized Lindhard dielectric function that considers both the frequency and wave‐vector components of free‐electron plasma, it is demonstrated that such a decrease in the frequencies of the coupled plasmon–LO phonon mode in Raman spectra is related to the nonconserved wave vectors due to inelastic scattering from magnesium (Mg) impurities. Modeling of the experimental Raman line shape, intensity, and frequencies illustrates that the wave‐vector dependence of the coupled modes decreases with increasing electron concentrations. An asymmetric broadening of LO Raman modes is observed in films having large electron concentrations (>1020 cm−3) that are explained by Fano resonance.

Strain related relaxation of the GaAs-like Raman mode selection rules in hydrogenated GaAs1−xNx layers

The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAs1−xNx (x = 0.01) layers—with different H doses and similar low-energy irradiation conditions—was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAs1−xNx reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.

A promising thermoelectric response of fully compensated ferrimagnetic spin gapless semiconducting Heusler alloy Zr2MnAl at high temperature: DFT study

In recent times, Heusler compounds have emerged as potential candidates for spintronics and thermoelectric materials owing to their interesting properties such as gapless semiconducting nature with fully compensated ferrimagnetic behavior, 100% spin polarization with high mobility of charge carriers and better thermoelectric figure of merit. Motivated by this fact, we have carried out a systematic theoretical investigation of structural, electronic, magnetic, thermoelectric and lattice dynamical properties of spin gapless Heusler alloy Zr2MnAl within the frame work of density functional theory based first principle calculation. Our ground state total energy calculations confirms the structural stability of antiferromagnetic phase of Zr2MnAl over ferromagnetic phase, which is in agreement with previous study. The positive value for Cauchy’s pressure (C12–C44) predicts ductile nature of the material. The band structure calculation reveals indirect band gap in spin down channel and zero band gap in spin up channel of valence and conduction bands confirming the spin gapless semiconducting nature of the compound. The real frequency of phonon modes throughout the Brillouin zone confirms the structural and dynamical stability of antiferromagnetic phase of Zr2MnAl, which is reported for the first time as per our knowledge. Calculated Seebeck coefficient in spin up and spin down channel reveals that the Zr2MnAl behaves as both n and p type thermoelectric materials with better output efficiency. The calculated thermoelectric figure of merit of 1.4 at 800 K, favors use of an alloy in recovery of waste heat at higher temperature and thermoelectric spin generators. The comprehensive analysis of various chemical potential dependent thermoelectric coefficients for enhancing thermoelectric performance for the material has been carried out. The obtained results propose Zr2MnAl as a potential candidate for high temperature thermoelectric devices. Higher optical gap in the phonon dispersive curve is responsible for decrease in lattice thermal conductivity with temperature, resulting in enhancement in thermoelectric figure of merit.

Femtosecond Pump Probe Spectroscopy Implementation for Semiconductors

Two different pump-probe (PP) setups were developed successfully with different femtosecond pulse lasers. Using a PP setup with an ultra-short pulse laser, the excitation of coherent phonons in GaAs was measured for a calibration and an accuracy test of the developed setup. The frequencies of the coherent phonon modes were in good agreement with reported values [1, 2]. The setups for ZnSe and GaAs were transmission and reflection–type, respectively. When using the ultra-short pulse laser, the signal in the PP experiment was measured by a balanced photo diode.In the other PP experimental setup, built to measure the transient transmittance of bulk ZnSe, the light source and detector differed from the previous PP setup. A strong pulse laser was successfully used for the spectrally resolved pump probe experimental setup. A broadband, high-resolution spectrometer (HR4000CG-UV-NIR) was used as the detector.

New effects of the electron–phonon interaction in dielectrics: (50th anniversary of the Institute of Spectroscopy, Russian Academy of Sciences)

We have studied the electron–phonon interaction (EPI) in multiferroic PrFe3(BO3)4 applying reflection methods in far-infrared (terahertz) spectral range and theoretical simulations. A specific feature of PrFe3(BO3)4 is that the 4f-electron excitation of the Pr3+ ion falls into a range between the longitudinal (LO) and transverse (TO) phonon mode frequencies. Simultaneously, inversion of the electronic oscillator occurs: its LO frequency becomes smaller than the TO frequency. With lowering temperature, a coupled electron–phonon mode emerges, and a new effect is observed: the reststrahlen band corresponding to a nondegenerate phonon mode splits. Yet another effect caused by the EPI is the appearance of a gap in the spectrum of electronic excitations of a stoichiometric rare-earth easy-axis antiferromagnet placed into an arbitrarily weak external magnetic field directed along the easy magnetization axis.

Spin-charge-lattice coupling in YBaCuFeO5: Optical properties and first-principles calculations

We combined spectroscopic ellipsometry, Raman scattering spectroscopy, and first-principles calculations to explore the optical properties of YBaCuFeO5 single crystals. Measuring the optical absorption spectrum of YBaCuFeO5 at room temperature revealed a direct optical band gap at approximately 1.41 eV and five bands near 1.69, 2.47, 3.16, 4.26, and 5.54 eV. Based on first-principles calculations, the observed optical excitations were appropriately assigned. Analysis of the temperature dependence of the band gap indicated anomalies in antiferromagnetic phase transition at 455 and 175 K. Additionally, a hardening in the frequency of the Eg phonon mode was observed at 175 K. The value of the spin–phonon coupling constant was 15.7 mRy/Å2. These results suggest a complex nature of spin–charge–lattice interactions in YBaCuFeO5.

Time-resolved, nonequilibrium carrier and coherent acoustic phonon dynamics in (Cd, Mg)Te single crystals for radiation detectors

We present time-resolved nonequilibrium carrier and coherent acoustic phonon dynamics in (Cd, Mg)Te single crystals intended for detection of high-energy x-ray photons. Our (Cd, Mg)Te crystals were grown using a vertical Bridgman method and during the process were doped either with In (a shallow dopant) or Ge (a deep impurity). Next they were cut into platelets and, subsequently, tested as as-grown specimens, or, before testing, long-term annealed in a temperature range of 750 °C–800 °C to improve their crystalline properties. All samples exhibited high resistivities of 109–1010 Ω cm, as required for x-ray detection applications. We have performed femtosecond optical spectroscopy, pump-probe measurements on all samples in both one-color (high-intensity 800 nm pump and low-intensity 800 nm probe) and two-color (high-intensity 400 nm pump and low-intensity 800 nm probe) configurations, and analyzed the results using our carrier dynamics rate-equation model. The model allowed us to perform a comprehensive, time-resolved analysis of relaxation dynamics of photoexcited carriers and investigate the role of carrier traps present in all tested samples. Two-color measurements also allowed us to observe and analyze the propagation of acoustic coherent phonons excited by the high-energy pump pulse. We observed that the dependence of phonon mode frequency, after including a correction of the experimentally measured crystal index of refraction dependence on the probe wavelength, was dispersionless and led to the value for the sound velocity of 3367 m s−1, which is in excellent agreement with the literature data. The intrinsic lifetime of coherent phonons in (Cd, Mg)Te was estimated to be of the order of 10 ns. Finally, we conclude that among our tested samples, the In-doped, annealed (Cd, Mg)Te crystal exhibits the best characteristics for radiation detection applications.

Temperature-dependent ultraviolet Raman scattering and anomalous Raman phenomenon of AlGaN/GaN heterostructure.

Temperature-dependent ultraviolet (UV) Raman scattering from AlGaN/GaN heterostructure is investigated. Compared to the visible Raman spectrum, four new peaks at 600, 700, 780, and 840 cm-1 are observed in the UV Raman spectrum. The peak at 780 cm-1 is from the AlGaN A1(LO) mode. According to the calculated dispersion relations of the interface phonon modes in the AlGaN/GaN heterostructure, the peaks at 600 and 840 cm-1 correspond to interface phonon modes. Meanwhile, the peak at 700 cm-1 is attributed to the disorder-active mode near the 2DEG interface. Due to the near-resonant enhancement effect, the intensities of the GaN A1(LO) mode, interface phonon modes, disorder active mode and the AlGaN A1(LO) mode exhibit different temperature dependence. Furthermore, the frequencies of the interface phonon modes and the disorder active mode show anomalous temperature dependence, which can be attributed to the strong built-in electric field near the 2DEG interface.

Investigation of Structural and Elastic Stability, Electronic, Magnetic, Thermoelectric, Lattice-Dynamical and Thermodynamical Properties of Spin Gapless Semiconducting Heusler Alloy Zr2MnIn Using DFT Approach

In recent times, spin gapless semiconductors (SGS) have attracted much attention as a promising candidate for spintronics and thermoelectric applications due to their high carrier concentration and good thermoelectric figure of merit. In this paper, we have carried out a systematic theoretical investigation of the structural, elastic, thermal, electronic, magnetic, thermoelectric, lattice dynamical and thermodynamical properties of Zr2MnIn using density functional theory (DFT) based first principle calculations. The band structure calculation shows indirect band gap in a spin down channel and zero band gap in a spin up channel of valence and conduction bands confirming the spin gapless semiconducting nature of Zr2MnIn. The structural and dynamical stability of the antiferromagnetic phase of Zr2MnIn has also been investigated. Magnetization in Zr2MnIn originates due to the d state electrons of Zr atoms, which follows the Slater Pauling rule: Mt = Zt − 18. Phonon dispersion curves exhibit real frequency of phonon modes throughout the Brillouin zone, which confirms the dynamical stability of the antiferromagnetic phase of Zr2MnIn. Thermodynamical properties including specific heat and Debye temperature have been calculated using phonon density of states. A higher value of the thermoelectric figure of merit 1.25, predicts that this alloy as good thermoelectric properties with better output efficiency.

Phonon dispersion using the ratio of zero-time correlations among conjugate variables: Computing full phonon dispersion surface of graphene

We present a robust family of methods (ZTR) to compute the phonon dispersion curves based on the ratio of zero-time correlations of conjugate variables in the reciprocal space. This general technique extracts the normal mode frequency corresponding to wave vector q and polarization p, from the ratio of the correlation of the nth derivative of displacement to the n-1th derivative in reciprocal space at zero time. A particular version of this method using the ratio of velocity to displacement (n=1) is previously known but seldom employed in atomistic simulations. For n=2, the method involves velocities and accelerations — dynamical variables that are more well-defined than equilibrium displacements in atomistic simulations. We test the ZTR methods and demonstrate that both ZTR methods (n=1,2) can accurately resolve the phonon mode frequencies while offering a significant improvement to the computational speed. We also illustrate the ability of the ZTR methods to handle anharmonicity and phonon softening at high temperatures. Finally, we demonstrate the power of the ZTR approach by computing the full phonon dispersion surface for graphene across the entire Brillouin zone with 3600 wave vectors and six polarizations at finite temperatures — a challenging task for the traditional methods.

Promoted high temperature carrier mobility and thermoelectric performance of InTe enabled by altering scattering mechanism

Sb nano-precipitates, which serve to alter the vibrational frequency of localized phonon modes that facilitate inter-valley scattering, emerge in pristine InTe after addition of Sb. As a result, exponentially declined carrier mobility is promoted by nearly 50% at 723 K in InTe–Sb0.01, leading to enhanced thermoelectric performance.

Theoretical investigation of lattice dynamics, dielectric properties, infrared reflectivity and Raman intensity spectra of Nowotny chimney-ladder semiconducting silicide Ru2Si3

Abstract Semiconducting silicides are highly promising materials for applications in thermoelectric, photovoltaic and optoelectronic devices. Here, using ab-initio density functional theory, the lattice dynamics, dielectric properties, Infrared (IR) refletivity and Raman intensity spectra of Nowotny chimney-ladder semiconducting silicide Ru2Si3 are investigated. The zone-center phonon mode frequencies are found to be in the range 80–510 cm−1. The Born effective charge tensor, oscillator strength and Infrared activity is found to largest for the mode with frequency 422 cm−1 which is primarily contributed by displacements of Si atoms. The largest Raman activity is obtained for mode with frequency 479 cm−1 which is also dominated by vibrations of Si atoms. Modified Becke-Johnson (MBJ) exchange potential is used to compute the band gap of Ru2Si3 which improves the band gap by ∼8% as compared to that obtained using LDA. The computed Infrared reflectivity and Raman intensity spectra are expected to provide benchmark first-principles theoertical results for comparison with the experiments.

\u201cGreen\u201d Aqueous Synthesis and Advanced Spectral Characterization of Size-Selected Cu2ZnSnS4 Nanocrystal Inks

Structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 (CZTS) nanocrystal inks produced by a “green” method directly in aqueous solutions were characterized. A size-selective precipitation procedure using 2-propanol as a non-solvent allows separating a series of fractions of CZTS nanocrystals with an average size (bandgap) varying from 3 nm (1.72 eV) to 2 nm (2.04 eV). The size-selected CZTS nanocrystals revealed also phonon confinement, with the main phonon mode frequency varying by about 4 cm−1 between 2 nm and 3 nm NCs.

Ab initio study of the lattice dynamical and thermodynamic properties of SbXI (X= S, Se, Te) compounds

The Born effective charges, high-frequency dielectrics constants, vibrational and thermodynamics properties of paraelectric SbSeI, SbTeI, SbSI and ferroelectric SbSI phase have been calculated by ab-initio method within the local density approximation. The high-frequency dielectric and Born effective charge tensor components are analyzed within density functional perturbation theory. We find remarkably enhanced/reduced Born effective charges and anomalously high-frequency dielectric constants for SbXI compounds. The phonon dispersion spectra and density of phonon states of these compounds were acquired using the direct method based on calculated Hellmann-Feynman forces. A comparison with the phonon dispersion curves of SbXI compounds displays minor differences that can attributed to the atomic-mass difference among S, Se and Te. The Raman and infrared active phonon mode frequencies of compounds are predicted at the Γ point. The thermodynamic properties of these compounds such as the internal energy, Helmholtz free energy, entropy, heat capacity, and Debye temperature are also investigated and discussed in the framework of the harmonic approximation. The obtained results were compared with available experimental and other theoretical data.

Phonon modes, dielectric properties, infrared reflectivity, and Raman intensity spectra of semiconducting silicide BaSi2: First principles study

In this study, we investigated the phonon modes, dielectric properties, infrared (IR) reflectivity, and Raman intensity spectrum of semiconducting silicide BaSi2 within the framework of the first principles density functional theory. The zone-center phonon mode frequencies formed two major bands in the ranges of 50–150 cm−1 and 270–500 cm−1. The modes in the frequency band 100–150 cm−1 had high Born effective dynamical charges and oscillator strengths, and they contributed significantly to the mode effective charge, static dielectric permittivity, IR activities, and reflectivity spectrum. These modes were attributable to displacements of isolated Si clusters relative to one another without internal distortions. The computed Raman activities and intensities were significant for modes in the frequency band of 270–500 cm−1. These modes were primarily associated with silicon atom vibrations in the Si clusters in BaSi2 and the internal modes of the isolated Si clusters with Td symmetry. The band gap was also computed for BaSi2 using the modified Becke–Johnson exchange potential and it was in good agreement with the previously reported experimental values. The computed Raman intensity and IR reflectivity spectrum can be used as benchmark first principles theoretical results to compare with experimental results.

Experimental and ab-initio study of the structural, electronic and vibrational properties of ZnTe

Polycrystalline ZnTe thin films have been deposited by industrially viable electron beam evaporation technique. The effect of substrate temperature on its structural, vibrational and optoelectronic properties has been investigated. The results are complemented with the ab initio calculations performed using density functional theory for bulk ZnTe. X-ray diffraction studies reveal that the films crystallize in zinc blende structure and the highest crystallinity has been observed for the films deposited at 373 K. The surface morphology and roughness of the films have been studied through scanning electron microscopy and atomic force microscopy. Raman spectroscopy has been used to study the vibrational properties of the films and the frequency of the optical phonon modes are in close agreement with that of the ab initio calculations. Optical absorption and photoluminescence spectroscopy has been used to investigate the optoelectronic properties of ZnTe thin films. The individual orbital contributions to the valence and conduction band edges have been analysed through the density functional theory.

Vibrational and elastic properties of silicate spinels A2SiO4 (A = Mg, Fe, Ni, and Co)

A six-parameter bond-bending force constant model is used to calculate the zone-center (Γ = 0) Raman and infrared phonon mode frequencies, elastic constants and related properties, the Debye temperatures, and sound velocities along high-symmetry directions for A2SiO4 (A = Mg, Fe, Ni, and Co) spinels. The main outcomes of the calculations are that the interactions between Si and O atoms (first-neighbor interaction) are stronger than those between A and Oatoms (A = Mg, Fe, Ni, and Co) (second-neighbor interaction). The elastic constants C11, C12, and C44 decrease in the order Mg > Fe > Ni > Co. The calculated bulk modulus, Poisson's ratio, and anisotropy decrease in the sequence Fe2SiO4 → Ni2SiO4 → Co2SiO4 → Mg2SiO4. On comparison, we find overall good agreement with the available experimental and previously calculated data.

Editors' Choice—Optical Emission from Germanium Nanocrystals

We analyze the intense photoluminescence (PL) observed at energies from 600 to 1100 meV for a large number of Si1-xGex epitaxial layers grown by molecular beam epitaxy. In the present work we show that this previously unexplained broad PL peak can be assigned to Ge nanocrystals (NCs) self-assembled within the SiGe layers. These NCs are assumed to be lattice matched to the SiGe in the vertical, growth direction. A consequence of this assignment is that as the Ge-fraction in the SiGe layer increases the vertical strain in the NCs changes from compressive to tensile at x ∼ 0.36, lowering the NC bandgap (BG) below that of bulk Ge. We examine the PL results for more than 60 samples exhibiting this broad PL peak by examining how it follows the strained Ge BG for x from 0.05 to 0.53. The PL is resolvable as two narrower peaks separated by the momentum conserving phonon energy for Ge. Strain and confinement shifted NC bound exciton energies calculated numerically agree well with the measured ones. When Raman scattering results were examined for some of the same samples, the phonon mode frequencies obtained provided valuable corroborative evidence for the presence of the Ge NCs.

Cleaved-edge photoluminescence spectroscopy of multilayer heterostructures

The capabilities of a technique based on combined photoluminescence and Raman-spectroscopy measurements upon lateral scanning across the cleaved edge of heterostructures for monitoring the strain profile and the thickness of epitaxial layers are demonstrated. The characteristics of a laser heterostructure with InGaAs/GaAsP quantum wells are investigated by this technique. It is shown that photoluminescence from different layers of the structure can be recorded separately. It is established that both the frequency of the InP-like phonon mode and the photoluminescence energy can be used to determine the composition of the In x Ga1–xP alloy. The two methods yield close results.

Flexocoupling-induced soft acoustic modes and the spatially modulated phases in ferroelectrics

Using the Landau Ginzburg Devonshire theory and scalar approximation, we derived analytical expressions for the singular points (zeros, complex ranges) of the acoustic phonon mode (A mode) frequency in dependence on the wave vector k and examined the conditions of the soft A modes appearance in a ferroelectric depending on the magnitude of the flexoelectric coefficient f and temperature T. We predict that if the magnitude of the flexocoefficient f is equal to the temperature-dependent critical value fcr(T) at the temperature T=T_IC, the A mode frequency tends to zero at k=kr_0 and the spontaneous polarization becomes spatially modulated in a temperature range T<T_IC.The comparison of calculated physical properties with measured ones are performed for some ferroelectrics with SMP phases.In particular, temperature dependence of the calculated direct and inverse static dielectric susceptibility is in an agreement with experimental data in Sn2P2(SexS1-x)6 that gives us additional background to predict flexo-coupling induced soft acoustic amplitudon-type mode in the SMP phase.The available experimental data on neutron scattering in organic incommensurate ferroelectric (CH3)3NCH2COO*CaCl2*2H2O are in a semi-quantitative agreement with our theoretical results. To quantify the theory, it is necessary to measure the frequency dependence of the A mode in a uniaxial ferroelectric with a spatially modulated phase in the temperature interval near its occurrence.