Phonon focusing and temperature dependences of the thermal conductivity of silicon nanowires

The influence of phonon focusing on the anisotropy and temperature dependences of the thermal conductivity of silicon nanowires (NWs) has been studied using the three-mode Callaway theory. The calculated temperature dependences of the thermal conductivity of silicon NWs with diameters above 50 nm agree well with experimental data in the 20–300 K range. The temperatures of transitions from the boundary-scattering to volume-relaxation mechanisms are determined. Variation of the thermal conductivity anisotropy depending on temperature is analyzed. The free paths of phonons with various polarizations in the boundary scattering regime in silicon NWs significantly differ and depend to a considerable degree on the phonon focusing. The free paths reach maxima in the directions of phonon focusing and exceed values for other oscillatory modes. However, in the isotropic medium model, the phonon free paths for various polarizations coincide and are fully determined by the geometric parameters of NWs.     

Effect of dispersion and damping of thermal phonon states on the longitudinal ultrasonic absorption in germanium crystals

A method has been proposed for approximating a phonon spectrum of cubic crystals, which has been obtained from data on inelastic neutron scattering for symmetric directions, over the entire Brillouin zone in the form appropriate for studying relaxation characteristics of phonon systems. The effect of dispersion and damping of thermal phonon states on the longitudinal ultrasonic absorption in anharmonic processes of scattering with the participation of three longitudinal phonons has been investigated for germanium crystals. It has been shown that the inclusion of the dispersion leads to a decrease in the anisotropy of ultrasonic absorption in the LLL relaxation mechanism and makes it possible to fit the results obtained from calculations of the ultrasonic absorption coefficients to the experimental data in the low-temperature range. The temperature dependence and anisotropy of the relaxation rate of longitudinal thermal phonons in germanium crystals have been determined from experimental data on ultrasonic absorption. The performed analysis has refined values of the relaxation parameters obtained from the interpretation of the data on thermal conductivity of germanium crystals with different isotopic compositions in the isotropic-medium model.     

The effect of normal phonon-phonon scattering processes on the maximum thermal conductivity of isotopically pure silicon crystals

The effect of normal phonon-phonon scattering processes on the thermal conductivity of silicon crystals with various degrees of isotope disorder is considered. The redistribution of phonon momentum in normal scattering processes is taken into account within each oscillation branch (the Callaway generalized model), as well as between different oscillation branches of the phonon spectrum (the Herring mechanism). The values of the parameters are obtained that determine the phonon momentum relaxation in anharmonic scattering processes. The contributions of the drift motion of longitudinal and transverse phonons to the thermal conductivity are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons in the Herring relaxation model represents an efficient mechanism that limits the maximum thermal conductivity in isotopically pure silicon crystals. The dependence of the maximum thermal conductivity on the degree of isotope disorder is calculated. The maximum thermal conductivity of isotopically pure silicon crystals is estimated for two variants of phonon momentum relaxation in normal phonon-phonon scattering processes.     

Anisotropy of the mean free paths of phonons in single-crystal films of germanium,silicon, and diamond at low temperatures

The physical aspects of the influence of the elastic energy anisotropy of crystals on the anisotropy of the mean free paths of phonons in single-crystal films of germanium, silicon, and diamond in the diffuse scattering of phonons at the boundaries of the samples have been considered. It has been shown that, for sufficiently wide films of germanium, silicon, and diamond with the {100} and {111} orientations and the lengths of less than or equal to their width, the phonon mean free paths are isotropic (independent of the direction of the temperature gradient in the plane of the film). The anisotropy of the phonon mean free paths depends primarily on the orientation of the film plane and is determined by the focusing and defocusing of phonon modes. For single-crystal films of germanium, silicon, and diamond with the {100} and {111} orientations and lengths much larger than their width, the phonon mean free paths are anisotropic.     

Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb₃Ga₅O₁₂ (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at T _min = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb³⁺ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb³⁺ ion.     

Normal phonon-phonon scattering processes and the thermal conductivity of germanium crystals with isotope disorder

The influence of the normal phonon-phonon scattering processes on the thermal conductivity was theoretically studied for germanium crystals with various degrees of the isotope disorder. The theory takes into account redistribution of the phonon momentum in the normal scattering processes both inside each oscillation branch (Simons mechanism) and between various phonon oscillation branches (Herring mechanism). Contributions to the thermal conductivity due to the drift mobility of the longitudinal and transverse phonons are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons according to the Herring relaxation mechanism leads to a significant suppression of the drift motions (and to the corresponding drop in contribution to the thermal conductivity) of the longitudinal phonons in isotopically pure germanium crystals. The results of the thermal conductivity calculations involving the Herring relaxation mechanism agree well with the experimental data available for germanium crystals with various degrees of the isotope disorder.     

Relaxation times and mean free paths of phonons in the boundary scattering regime for silicon single crystals

The phonon focusing in cubic dielectric crystals and its influence on the heat transfer in the boundary phonon scattering regime at low temperatures have been investigated. The mean free paths of phonons of different polarizations in samples of infinite and finite lengths with circular and square cross sections have been calculated in the anisotropic continuum model. For samples of infinite length with circular and square cross sections in the case of the equality of the cross-sectional areas, the angular dependences of the mean free paths normalized by the Casimir length almost completely coincide. It has been shown that the anisotropy of the mean free paths decreases significantly upon changing over from infinite samples to samples of finite length. For silicon crystals, the anisotropy of the phonon mean free paths has been analyzed for each of the branches of the phonon spectrum. It has been found that the mean free paths for phonons of each vibrational mode reach maximum values in the directions of focusing, and, in these directions, they exceed the mean free paths for phonons of the other vibrational modes.     

The mechanism of Landau-Rumer relaxation of thermal and high-frequency phonons in cubic crystals of germanium,silicon, and diamond

Transverse phonon relaxation according to the Landau-Rumer mechanism is considered for an isotropic medium and crystals of germanium, silicon, and diamond possessing a cubic symmetry. The energy of elastic deformation caused by the anharmonicity of vibrations of the cubic crystal lattice is expressed via the second-and third-order moduli of elasticity. Using the known values of these elastic moduli, parameters determining the frequencies of the transverse phonon relaxation in the Landau-Rumer mechanism are evaluated for the germanium, silicon, and diamond crystals. It is shown that the dependence of the relaxation frequency on the wavevector of thermal and high-frequency phonons sharply differs from the classical Landau-Rumer relationship both in the isotropic medium and in the cubic crystals. It is established that the observed peculiarities in the relaxation frequency are related to the angular dependence of the probability of anharmonic scattering and the anisotropy of elastic properties of the germanium, silicon, and diamond crystals. A new method is proposed for the experimental determination of the relaxation frequency of high-frequency phonons as a function of the wavevector using the temperature dependence of the coefficient of absorption of high-frequency ultrasound.     

Thermal conductivity of polycrystalline zinc selenide

The thermal conductivity of optically transparent zinc selenide polycrystals fabricated by vapor deposition was experimentally studied in the temperature range 80–400 K in the as-deposited state and after deformation along the crystal growth direction followed by recrystallization. In the low-temperature range, textured ZnSe samples exhibit anisotropy of the thermal conductivity, which also persisted after their deformation and recrystallization. The anisotropy of the thermal conductivity is caused by phonon scattering by dislocations oriented along the crystal growth direction. The thermal conductivity of ZnSe at T>270 K is shown to be limited by the scattering of acoustic phonons by optical phonons.     

Peripheral phonons and phonon conductivity of the doped semiconductor

The percentage contribution of the peripheral phonons towards the total phonon conductivity of P-doped Ge has been studied with the help of the Ziman expression of the electron-phonon scattering relaxation rate and the Callaway integral of the lattice thermal conductivity. It is found that the percentage contribution of the peripheral phonons increases with the increase of temperature. The entire study is limited to the temperature range 1–5 K.     

Anharmonic processes of scattering and relaxation of slow quasi-transverse phonons in cubic crystals

Relaxation of slow quasi-transverse phonons in anharmonic processes of scattering in cubic crystals with positive (Ge, Si, diamond) and negative (KCl, NaCl) anisotropies of the second-order elastic moduli has been considered. The dependences of the relaxation rates on the direction of the wave vector of phonons in scattering processes with the participation of three quasi-transverse phonons (the TTT relaxation mechanisms) are analyzed within the anisotropic continuum model. It is shown that the TTT relaxation mechanisms in crystals are associated with their cubic anisotropy, which is responsible for the interaction between noncollinear phonons. The dominant contribution to the phonon relaxation comes from large-angle scattering. For crystals with significant anisotropy of the elastic energy (Ge, Si, KCl, NaCl), the total contribution of the TTT relaxation mechanisms to the total relaxation rate exceeds the contribution of the Landau-Rumer mechanism either by several factors or by one to two orders of magnitude depending on the direction. The dominant role of the TTT relaxation mechanisms as compared to the Landau-Rumer mechanism is governed, to a considerable extent, by the second-order elastic moduli. The total relaxation rates of slow quasi-transverse phonons are determined. It is demonstrated that, when the anharmonic processes of scattering play the dominant role, the inclusion of one of the relaxation mechanisms (the Landau-Rumer mechanism or the mechanisms of relaxation of the slow quasi-transverse mode by two slow or two fast modes) is insufficient for describing the anisotropy of the total relaxation rates in cubic crystals.     

Peripheral phonons and phonon conductivity of a metal at low temperatures

The role of the peripheral and non-peripheral phonons in the estimation of the lattice thermal conductivity of a metal has been studied at low temperatures by calculating their separate contributions towards the total lattice thermal conductivity. The study is made in the temperature range 0.4–2.5 K with the help of the Ziman expression for the scattering of phonons by the charge carriers and the Callaway expression of the phonon conductivity, and Sb is taken as an example. The separate percentage contributions due to peripheral and non-peripheral phonons have also been studied and it is found that the percentage contribution due to peripheral phonons increases with increasing temperature while the percentage contribution due to non-peripheral phonons decreases with increasing temperature. The percentage contributions of the lattice thermal resistivities due to electrons and holes towards the total lattice thermal resistivity of Sb have also been reported in the present note.     

Phonon conductivity of deformed lithium flouride crystals

The Callaway model has been applied to investigate the effect of (1) vibrating dislocation scattering, (2) static dislocation scattering, and (3) point-defect scattering of phonons on the thermal conductivity of deformed LiF crystals. It is found that the point-defect scattering of phonons does not produce any appreciable thermal resistance but that the other two scattering processes have a very strong influence on the thermal conductivity. The correction term due to three-phonon normal processes is also found to be significant.     

Low-temperature anisotropy of the thermal conductivity of single-crystal nanofilms and nanowires

The effect of phonon focusing on the phonon transport in single-crystal nanofilms and nanowires is studied in the boundary scattering regime. The dependences of the thermal conductivity and the free path of phonons on the geometric parameters of nanostructures with various elastic energy anisotropies are analyzed for diffuse phonon scattering by boundaries. It is shown that the anisotropies of thermal conductivity for nanostructures made of cubic crystals with positive (LiF, GaAs, Ge, Si, diamond, YAG) and negative (CaF₂, NaCl, YIG) anisotropies of the second-order elastic moduli are qualitatively different for both nanofilms and nanowires. The single-crystal film plane orientations and the heat flow directions that ensure the maximum or minimum thermal conductivity in a film plane are determined for the crystals of both types. The thermal conductivity of nanowires with a square cross section mainly depends on a heat flow direction, and the thermal conductivity of sufficiently wide nanofilms is substantially determined by a film plane orientation.     

表面低配位原子对声子的散射机制

由于纳米结构具有极高的表体比,声子-表面散射机制对声子的热输运性质起到关键作用.提出了表面低配位原子对声子的散射机制,并且结合量子微扰理论与键序理论推导出该机制的散射率.由于散射率正比于材料的表体比,这种散射机制对声子输运的重要性随着纳米结构尺寸的减小而增大.散射率正比于声子频率的4次方,所以这种散射机制对高频声子的作用远远强于对低频声子的作用.基于声子玻尔兹曼输运方程,计算了硅纳米薄膜和硅纳米线的热导率,发现本文模型比传统的声子-边界散射模型更接近实验值.此发现不仅有助于理解声子-表面散射的物理机制,也有助于应用声子表面工程调控纳米结构的热输运性质.     

Thermal conductivity reduction and thermoelectric figure of merit increase by embedding nanoparticles in crystalline semiconductors

Atomic substitution in alloys can efficiently scatter phonons, thereby reducing the thermal conductivity in crystalline solids to the "alloy limit." Using In0.53Ga0.47As containing ErAs nanoparticles, we demonstrate thermal conductivity reduction by almost a factor of 2 below the alloy limit and a corresponding increase in the thermoelectric figure of merit by a factor of 2. A theoretical model suggests that while point defects in alloys efficiently scatter short-wavelength phonons, the ErAs nanoparticles provide an additional scattering mechanism for the mid-to-long-wavelength phonons.     

Phonon-Resonanzstreuung und Wärmeleitung in einer Silizium-Korngrenze

The thermal conductivity of a grain-boundary in silicon bicrystals has been measured in the temperature range from 12°K to 150°K using a newly developed method. The analysis of the experimental results shows that the thermal conductivity is governed by two scattering processes: the acoustical mismatch of both crystal halves and the resonant scattering of the phonons by impurity atoms of the grain boundary. A new method is thereby presented for the direct observation of phonon resonant scattering.     

Thermal conductivity of alloys of the systems Sn-Pb and Sn-In in the solid and liquid states

Results are presented of the measurement of the thermal conductivity of alloys of the eutectic systems Sn-Pb and Sn-In in the solid and liquid phases from 100 to 600 °C by a steady-state method. The thermal conductivity of the investigated alloys depends linearly on the temperature. The contribution of the phonon and electron components to the thermal conductivity are estimated. It is concluded from the results that the presence of foreign atoms in the investigated alloys has no significant effect on the thermal conductivity, which is determined principally by the scattering of the electrons by the phonons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 25–28, December, 1970.     

The effect of the electron-phonon coupling on the thermal conductivity of silicon nanowires

The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire.     

Temperature and concentration dependences of thermal conductivity of solid solutions of gadolinium and dysprosium sulfides

Thermal conductivity of a number of solid solutions of gadolinium and dysprosium sulfides has been studied experimentally within the temperature range 80-400 K. The work offers the data on thermal conductivity coefficient and lattice thermal conductivity of the studied samples. It was found that replacement of gadolinium ions by dysprosium ions leads to significant decrease of the samples?? thermal conductivity and changes its temperature dependence character due to the resonance scattering of phonons by paramagnetic ions of dysprosium. Influence of this mechanism of phonon scattering conditions the area of anomalous change observed on the concentration dependence of thermal conductivity coefficient.