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Aluminum scandium nitride alloy (Al1-xScxN) is regarded as a promising material for high-performance acoustic devices used in wireless communication systems. Phonon scattering and heat conduction processes govern the energy dissipation in acoustic resonators, ultimately determining their performance quality. This work reports, for the first time, on phonon scattering processes and thermal conductivity in Al1-xScxN alloys with the Sc content (x) up to 0.26. The thermal conductivity measured presents a descending trend with increasing x. Temperature-dependent measurements show an increase in thermal conductivity as the temperature increases at temperatures below 200K, followed by a plateau at higher temperatures (T> 200K). Application of a virtual crystal phonon conduction model allows us to elucidate the effects of boundary and alloy scattering on the observed thermal conductivity behaviors. We further demonstrate that the alloy scattering is caused mainly by strain-field difference, and less by the atomic mass difference between ScN and AlN, which is in contrast to the well-studied Al1-xGaxN and SixGe1-x alloy systems where atomic mass difference dominates the alloy scattering. This work studies and provides the quantitative knowledge for phonon scattering and the thermal conductivity in Al1-xScxN, paving the way for future investigation of materials and design of acoustic devices.
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Understanding microscopic heat conduction in thin films is important for nano/micro heat transfer and thermal management for advanced electronics. As the thickness of thin films is comparable to or shorter than a phonon wavelength, phonon dispersion relations and transport properties are significantly modulated, which should be taken into account for heat conduction in thin films. Although phonon confinement and depletion effects have been considered, it should be emphasized that surface-localized phonons (surface phonons) arise whose influence on heat conduction may not be negligible due to the high surface-to-volume ratio. However, the role of surface phonons in heat conduction has received little attention thus far. In the present work, we performed anharmonic lattice dynamics calculations to investigate the thickness and temperature dependence of in-plane thermal conductivity of silicon thin films with sub-10-nm thickness in terms of surface phonons. Through systematic analysis of the influences of surface phonons, we found that anharmonic coupling between surface and internal phonons localized in thin films significantly suppresses overall in-plane heat conduction in thin films. We also discovered that specific low-frequency surface phonons significantly contribute to surface--internal phonon scattering and heat conduction suppression. Our findings are beneficial for the thermal management of electronics and phononic devices and may lead to surface phonon engineering for thermal conductivity control.
In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (~1 K) and more recently to low-dimensional materials. Here, we identify graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (~100 K) based on first-principles calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribbons, we predict that phonon Poiseuille flow and Knudsen minimum can be experimentally observed above liquid nitrogen temperature. Further, we reveal the microscopic origin of these intriguing phenomena in terms of the dependence of the effective boundary scattering rate on momentum-conserving phonon-phonon scattering processes and the collective motion of phonons. The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp2 hybrid bonding and weak van der Waals interlayer interactions. As a boundary-sensitive transport regime, phonon hydrodynamics opens up new possibilities for thermal management and energy conversion.
Tin monosulfide (SnS) usually exhibits p-type conduction due to the low formation enthalpy of acceptor-type defects, and as a result n-type SnS thin films have never been obtained. This study realizes n-type conduction in SnS thin films for the first time by using RF-magnetron sputtering with Cl doping and sulfur plasma source during deposition. N-type SnS thin films are obtained at all the substrate temperatures employed in this study (221-341 C), exhibiting carrier concentrations and Hall mobilities of ~2 x 10 18 cm-3 and 0.1-1 cm V-1s-1, respectively. The films prepared without sulfur plasma source, on the other hand, exhibit p-type conduction despite containing a comparable amount of Cl donors. This is likely due to a significant amount of acceptor-type defects originating from sulfur deficiency in p-type films, which appears as a broad optical absorption within the band gap. The demonstration of n-type SnS thin films in this study is a breakthrough for the realization of SnS homojunction solar cells, which are expected to have a higher conversion efficiency than the conventional heterojunction SnS solar cells.
Local conduction at domains and domains walls is investigated in BiFeO3 thin films containing mostly 71o domain walls. Measurements at room temperature reveal conduction through 71o domain walls. Conduction through domains could also be observed at high enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime electrons trapped at defect states are temperature-activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in oxides.
BaSnO_{3}, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining device performance. We report on the experimental measurement of electron saturation velocity in La-doped BaSnO_{3} thin films for a range of doping densities. Predicted saturation velocities based on a simple LO-phonon emission model using an effective LO phonon energy of 120 meV show good agreement with measurements of velocity saturation in La-doped BaSnO_{3} films.. Density-dependent saturation velocity in the range of 1.6x10^{7} cm/s reducing to 2x10^{6} cm/s is predicted for {delta}-doped BaSnO3 channels with carrier densities ranging from 10^{13} cm^{-2} to 2x10^{14} cm^{-2} respectively. These results are expected to aid the informed design of BaSnO3 as the active material for high-charge density electronic transistors.
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