Identifying Materials for Ultrathin Conduction via Polaritonic Thermal Transport

Microelectronics no longer gain efficiency as they are made smaller. Dennard scaling is dead. With every uptick in clock speed or node size reduction, heat load increases. Balancing the thermal budget necessitates dissipating these continually increasing heat loads. It hasn't happened. As a result, heating now limits performance, shortens lifetimes and requires that up to ≈ 50% of an integrated circuit is turned off, or dark, at any given time. Improving computing requires not just smaller, more capable, devices but more capable methods of moving heat at extremely small scales.<br/><br/>The most obvious solution points to merely utilizing higher thermal conductivity materials. Along these lines, the development of high thermal conductivity synthetic diamond and BAs have been successful. While attractive for larger power electronics (&gt; 1 μm), their utility is limited at the &lt; 100 nm sizes characteristic of Si-based devices. This is a consequence of phonon physics—and thus endemic to materials in general—and therefore not a defect that can be "fixed."<br/><br/>Thermal conductivity increases when phonons move further at higher speeds. As the size of a material decreases, phonons are effectively fenced in. They scatter off the closely spaced boundaries reducing thermal conductivity. Circumventing this so-called size-effect in the thermal conductivity requires a means of either increasing the speed of the heat carriers or in some way making them less sensitive to material boundaries.<br/><br/>Polaritons–quasiparticles created by the hybridization (i.e., mix) of photons and material dipoles—can do both. Their photonic character provides a path to heat transfer closer to that of the speed of light than of sound. Polaritons also localize at material boundaries and therefore skate along interfaces rather than crashing into them. Recognizing these facts, we compare the potential of several dielectrics for moving heat in extremely thin films (t&lt;100 nm) via polartonic transport and compare their utility relative to standard phonon-based conduction. Using established analytical expressions for deriving polaritonic dispersion in conjunction with kinetic theory of polariton transport, a material map is derived linking optical properties with potential for thermal transport in ultrathin films.