Sebastian Volz1
CNRS1
<br/>Phonons are the quasiparticles of lattice vibrations and represent the primary heat carriers in bulk dielectric materials. The thermal conductivity of dielectric membranes is thus typically driven by acoustic phonons that is usually assimilated to a gas of particles, and generally reduces with the membrane thickness due to the increasing frequency of surface scattering events. In the light of the ever-increasing miniaturization of devices with enhanced rates of operation, this reduction in the thermal conductivity causes overheating, low reliability, and reduced lifetime of electronic components. Yet, while the thermal transport via acoustic phonons might be at the limit, the heat dissipation might be enhanced via optical phonons coupled with surface electromagnetic waves.<br/>Over the past decade, substantial research efforts have been devoted to the study of these surface waves, because the surface effects predominate over the volumetric ones in nanostructures with high surface-to-volume ratio. Some types of surface electromagnetic waves may even carry heat and thus improve the thermal performance and stability of nanoscale devices.<br/>One type of such surface waves is the surface phonon-polaritons (SPhPs), which occurs as a hybrid of optical phonons and surface electromagnetic waves. The SPhPs are essentially evanescent waves that propagate along the surface of polar dielectric membranes.<br/>We will first demonstrate the existence and the properties of those SPhPs in thin SiO2 films and especially uncover that the propagation lengths of those waves reach the range of hundreds of micrometers, which is orders of magnitude longer than the typical mean free path of acoustic phonons. Theoretical models predict that such a long propagation length enables SPhPs to conduct more thermal energy than phonons when the membrane thickness is reduced below 100 nm.<br/>We will then corroborate the significant contribution of SPhPs to heat flux in silica membranes, in a restricted temperature range and also, in a wide temperature range, in silicon nitride membranes where SPhP thermal conductivity becomes predominant at 800K. An additional confirmation was finally provided by showing the size dependence of thermal conductivity due to the ballistic contribution of SPhP.