Intrinsic sources of high thermal conductivity of CdSiP2 determined by first-principle anharmonic calculations

The high thermal conductivity of CdSiP2 stems from long acoustic phonon lifetime and large optic phonon velocity.

Built-in-polarization effect on relaxation time and mean free path of phonons in AlxGa1−xN/GaN heterostructure

In this paper, we have theoretically investigated the effect of built-in-polarization field on various phonon scattering mechanisms in Al[Formula: see text]Ga[Formula: see text]N/GaN heterostructure. The built-in-polarization field enhances the elastic constant, phonon velocity and Debye frequency of Al[Formula: see text]Ga[Formula: see text]N alloy. As a result, various phonon scattering mechanisms are modified. Important phonon scattering mechanisms such as normal scattering, Umklapp scattering, point defect scattering, dislocation scattering and phonon–electron scattering processes have been considered. The combined relaxation time due to above scattering mechanisms has also been computed as a function of phonon frequency for various Al contents at room temperature. Our result shows that built-in-polarization field suppresses scattering rates leading to enhanced combined relaxation time. Increased relaxation time implies longer phonon mean free path and enhanced optical and thermal transport properties. The result can be used to determine the effect of built-in-polarization field on optical and thermal properties of Al[Formula: see text]Ga[Formula: see text]N/GaN heterostructure and will be useful, particularly, for improvement of thermoelectric performance of Al[Formula: see text]Ga[Formula: see text]N/GaN heterostructure through polarization engineering.

Built-in-polarization field effect on intrinsic and extrinsic thermal conductivities of AlN/GaN/AlN quantum well

In this paper, the effect of built-in-polarization field on lattice thermal conductivity of AlN/GaN/AlN quantum well (QW) has been theoretically investigated. The built-in-polarization field at the hetero-interface of GaN/AlN modifies elastic constant, phonon velocity and Debye temperature of GaN QW. The relaxation time of acoustic phonons (AP) in various scattering processes in GaN with and without built-in-polarization field has been computed at room temperature. The result shows that combined relaxation time of AP is enhanced by built-in-polarization field and implies a longer mean free path. The revised intrinsic and extrinsic thermal conductivities of GaN have been estimated. The theoretical analysis shows that up to a certain temperature the polarization field acts as negative effect and reduces the thermal conductivities. However, after this temperature both thermal conductivities are significantly contributed by polarization field. This gives the idea of temperature dependence of polarization effect which signifies the pyro-electric character of GaN. The intrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 491 Wm -1 K -1 and 409 Wm -1 K -1, respectively i.e., 20% enhancement. However, the extrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 280 Wm -1 K -1 and 245 Wm -1 K -1, respectively i.e., 13% enhancement. The method we have developed may be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self-heating processes and in polarization engineering strategies to optimize the thermoelectric performance of GaN alloys.

Phonon Scattering in Bi2Te3/Sb2Te3 Superlattice

Ultrafast time-resolved measurements were conducted to investigate scattering mechanism of coherent optical and acoustic phonons in Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 superlattice (SL) films. Strong phonon scatterings in the Bi2Te3/Sb2Te3 SLs are attributed to the interfaces of their hetero-structures. Moreover, decreases of acoustic phonon velocity are observed in SLs, coming from phonon folding and softening. Our results show that both the enhanced interface scattering and the reduced phonon velocity contribute to suppressing the heat transport in SLs.

Phonon Scattering in Bi2Te3/Sb2Te3 Superlattice

Thermoelectric materials are characterized with the figure of merit, ZT = S2σT/κ, where T is the temperature, S the Seebeck coefficient, σ the electrical conductivity and κ the thermal conductivity. Many researches have been focused on reducing lattice thermal conductivity through increasing phonon scattering at interfaces. Thin-film superlattices are one of the promising candidates for high ZT thermoelectric materials. Several theoretical models have been used to explain the large ZT in superlattice. One comes from the extra scattering channels at interfaces introduced by the hetero-structure. Another is a result of quantum confinement effect which reduces the phonon group velocity propagating perpendicularly through the superlattice layers through flattening the dispersion curve of acoustic phonons. In this work, ultrafast time-resolved measurements were conducted on Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 superlattice (SL) films to detect coherent acoustic phonons in these materials. Scattering of these phonons is revealed in the Bi2Te3/Sb2Te3 SLs, which comes from the interfaces of the hetero-structure in SL. Also, a decrease of acoustic phonon velocity resulted from folding and flattening of phonons branches is observed. Results show that both interface scattering and the reduced phonon velocity contribute to suppressing the heat transfer process.

SIZE-DEPENDENT MELTING TEMPERATURE AND THERMAL CONDUCTIVITY OF NANOSCALE SEMICONDUCTORS

A model describing size-dependent melting temperature and thermal conductivity of nanosemiconductors is proposed based on Lindermann's melting criterion and Debye model. By the atomic thermal vibration consideration and by introducing intrinsic size effect of phonon velocity and mean free path combined with surface scattering effect, the model predicts that the melting temperature and thermal conductivity of nanosemiconductors decrease as the size reduces. The size effect depends on such material parameters as the vibration entropy, mean free path, the characteristic crystal size and surface roughness. The predictions are in agreement with experimental results of Si nanoparticles, nanowires and thin films.