Future of silicon-based microelectronics relies on solving the heat dissipation problem. A solution may lie in a nanoscale phenomenon known as ballistic heat conduction, which implies heat conduction without heating the conductor. But, attempts to demonstrate this phenomenon experimentally are controversial and scarce whereas its mechanism in confined nanostructures is yet to be fully understood. Here, we experimentally demonstrate quasiballistic heat conduction in silicon nanowires (NWs). We show that the ballisticity is strongest in short NWs at low temperatures but weakens as the NW length or temperature is increased. Yet, even at room temperature, quasi-ballistic heat conduction remains visible in short NWs. To better understand this phenomenon, we probe directionality and lengths of phonon flights. Our experiments and simulations show that the quasi-ballistic phonon transport in NWs is the Lévy walk with short flights between the NW boundaries and long ballistic leaps along the NW.

Improving heat dissipation in increasingly miniature microelectronic devices is a serious challenge, as the thermal conduction in nanostructures is markedly reduced by increasingly frequent scattering of phonons on the surface. However, the surface could become an additional heat dissipation channel if phonons couple with photons forming hybrid surface quasiparticles called surface phonon-polaritons (SPhPs). Here, we experimentally demonstrate the formation of SPhPs on the surface of SiN nanomembranes and subsequent enhancement of heat conduction. Our measurements show that the in-plane thermal conductivity of membranes thinner than 50 nm doubles up as the temperature rises from 300 to 800 kelvin, while thicker membranes show a monotonic decrease. Our theoretical analysis shows that these thickness and temperature dependencies are fingerprints of SPhP contribution to heat conduction. The demonstrated thermal transport by SPhPs can be useful as a previously unidentified channel of heat dissipation in a variety of fields including microelectronics and silicon photonics.

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We detect thermally excited surfaces waves on a submicron SiO 2 layer, including Zenneck and guided modes in addition to Surface Phonon Polaritons. The measurements show the existence of these hybrid thermalelectromagnetic waves from near-(2.7 µm) to far-(11.2 µm) infrared. Their propagation distances reach values on the order of the millimeter, several orders of magnitude larger than on semi-infinite systems. These two features; spectral broadness and long range propagation, make these waves good candidates for near-field applications both in optics and thermics due to their dual nature.PACS numbers: 44.40.+a,71.36.+c,78.20.-e Keywords: thermal emission, nearfield thermal radiation, Zenneck modes, surface phonon polaritons.Thermal radiation through surface wave diffraction is usually only considered as the result of Surface Phonon Polaritons (SPhPs). SPhPs are hybrid evanescent electromagnetic surface waves generated by the phononphoton coupling, at the interface of polar and dielectric materials (such as SiO 2 and air) 1-4 . The influence of SPhPs on the thermal performance of nanostructured materials has been studied intensively over the last decade, providing an alternative channel of heat conduction when the objects are scaled down 5,6 . Due to this behaviour, they are essential for the improvement of the thermal stability in micro and nanoelectronics 7-9 , microscopy 10 , near-field thermophotovoltaics 11 and for thermal radiation 12-14 . In addition, SPhPs provide coherent thermal radiation in mid-infrared 13,14 . This feature is now widely used to control thermal radiation but in a frequency range that is limited to the midinfrared because it implies the coupling to transverse optical phonons 1,15 . But this narrow spectrum (typically 8.6 − 9.3 µm at a SiO 2 -air interface) in addition to propagation lengths in the range of the wavelength decrease the field of use of SPhPs for many applications such as thermal transport at nanoscale, infrared nanophotonics and coherent thermal emission.In this letter we demonstrate through experiment that coherent thermal emission, resulting from surface waves, can be extended spectrally. We also prove experimentaly that these surface waves have a long propagation range, when considering isolated submicron layers. Indeed, if the film is thinner than the penetration depth of a) Corresponding author:thomas.antoni@centralesupelec.fr the wave inside the material, the electromagnetic mode can be coupled on both its interfaces allowing for the long-range propagation of two other types of electromagnetic surface waves; Zenneck and subwavelength Transverse Magnetic (TM) guided modes 16 . The propagation length is increased as a consequence of the dramatic decrease in the overlap of the mode with the material, hence its absorption. For example, it is almost two orders of magnitude larger than the wavelength for a 1 µm thick suspended SiO 2 membrane 5 . To prove those predictions, we fabricated a submicron glass layer and characterized its thermal emission by means of Fourier Tran...

We measure the thermal conductivity of silicon phononic crystals with asymmetric holes at room and liquid helium temperatures and study the effect of thermal rectification, phonon boundary scattering, neck transmission, and hole positioning. Also, we compare the influence of asymmetric holes on thermal conductivity reduction with the one of conventional circular holes. This reduction is almost 40% larger in the case of pacman shaped holes as compared with circular ones for the same parameters of phononic crystals. Our experimental results can be used to significantly improve the efficiency of thermoelectric devices by using pacman-shaped holes in phononic crystals.

Modern thermoelectric devices incline toward inexpensive, environmentally friendly, and CMOS-compatible materials, such as silicon. To improve the thermoelectric performance of silicon, researchers try to decrease its thermal conductivity using various nanostructuring methods. However, most of these methods have limited efficiency because they are costly and damaging for the internal structure of silicon. Here, we propose a cost-effective, large-area, and maskless nanofabrication method that creates external nanocones on the silicon surface while preserving its interior. Our experiments show that these nanocones reduce the thermal conductivity of thin silicon membranes by more than 40%. Using a modified Callaway–Holland model, we study how the thermal conductivity is affected by various phonon scattering processes in the 4–295 K temperature range. We conclude that the nanocones generate additional surface scattering, which causes the thermal conductivity reduction. The proposed nanocones and their simple fabrication method are promising for the planar thermoelectric devices based on silicon.

We experimentally demonstrate length and temperature limits of ballistic thermal conduction in silicon nanowires.

Based on the ability of plane structures to simultaneously optimize the propagation, confinement, and energy of surface plasmon-polaritons or surface phonon-polaritons, we develop the polaritonic figure of merit Z = βRΛ/δ, where βR, Λ and δ are the longitudinal wave vector, propagation length, and penetration depth, respectively. Explicit and analytical expressions of Z are derived for a single interface and a suspended thin film, as functions of the material permittivities and the film thickness. Higher Z are obtained for thinner films and smaller energy losses. The application of the obtained results for a SiC-air interface and a SiC thin film suspended in air shows that both structures are able to maximize the presence of polaritons at a frequency near to, but different than that at which the real part of the SiC permittivity exhibits a dip. Furthermore, using the temperature change of this dip, we show that the propagation length, confinement and energy of polaritons increases with its deepness, which provides an effective way to enhance the overall Z of polaritonic structures.