Understanding nanoscale thermal transport is of substantial importance for designing contemporary semiconductor technologies. Heat removal from small sources is well established to be severely impeded compared to diffusive predictions due to the ballistic nature of the dominant heat carriers. Experimental observations are commonly interpreted through a reduction of effective thermal conductivity, even though most measurements only probe a single aggregate thermal metric. Here, we employ thermoreflectance thermal imaging to directly visualise the 2D temperature field produced by localised heat sources on InGaAs with characteristic widths down to 100 nm. Besides displaying effective thermal performance reductions up to 50% at the active junctions in agreement with prior studies, our steady-state thermal images reveal that, remarkably, 1–3 μm adjacent to submicron devices the crosstalk is actually reduced by up to fourfold. Submicrosecond transient imaging additionally shows responses to be faster than conventionally predicted. A possible explanation based on hydrodynamic heat transport, and some open questions, are discussed.

A fully predictive kinetic-collective model using first principles phonon spectra and relaxation times is presented. Thermal conductivity values obtained for Si, Ge, C (diamond), and GaAs in a wide range of sizes and temperatures are in good agreement with experimental data without the use of any fitting parameter. These results open the door to discuss how the precise combination of kinetic and collective contributions to heat transport could provide a useful framework to interpret recent complex experiments displaying non-Fourier behavior.

Several recent experiments have demonstrated the failure of the Fourier heat equation in semiconductors at the nanoscale. We show that a generalized heat transport equation including a hydrodynamic term can explain three of these experiments on a silicon substrate: heater lines experiments both in stationary and nonstationary settings and the nonstationary response in grating experiments. To this end, we solve the hydrodynamic heat equation either numerically or analytically. The non-Fourier response observed in those experimental situations can be easily explained in terms of hydrodynamic concepts, such as friction and vorticity. For instance, the experimentally observed increase of thermal boundary resistance as size decreases can be simply understood as an increase of friction. We conclude that, contrary to common belief, hydrodynamics is a fundamental ingredient of semiconductor heat transport in the nanoscale at room temperature, providing physical insight and a unifying framework on the observed non-Fourier response. The relations between hydrodynamic heat transport with phonon momentum conservation and the Boltzmann transport equation are also provided.

A multiscale hydrodynamic-heat-transport model applicable to arbitrary geometries using finite-element methods is compared with the experimental effective thermal conductivity of silicon thin films and periodic holey membranes for different sizes and temperatures. The range of system length scales and temperatures in which the model predictions agree with experimental data is discussed and quantitatively determined. The model agrees with experimental results when the smallest system size is larger than twice the nonlocal length, an intrinsic property of the material that depends only on temperature. These results open the door to the use of the hydrodynamic equation instead of an effective Fourier model to interpret current heat-transport experimental data.

We carry out a systematic study of the thermal conductivity of four single-layer transition metal dichalcogenides, MX ₂ (M = Mo, W; X = S, Se) from first-principles by solving the Boltzmann transport equation (BTE). We compare three different theoretical frameworks to solve the BTE beyond the relaxation time approximation (RTA), using the same set of interatomic force constants computed within density functional theory (DFT), finding that the RTA severely underpredicts the thermal conductivity of MS ₂ materials. Calculations of the different phonon scattering relaxation times of the main collision mechanisms and their corresponding mean free paths (MFP) allow evaluating the expected hydrodynamic behaviour in the heat transport of such monolayers. These calculations indicate that despite of their low thermal conductivity, the present TMDs can exhibit large hydrodynamic effects, being comparable to those of graphene, especially for WSe ₂ at high temperatures.

This work provides an in-depth study of how the thermal conductivity of stoichiometric [110] Bi₂Te₃ nanowires becomes affected when reducing its diameter from an experimental and theoretical point of view. The thermal conductivity was observed to decrease more than 70% (from 1.78 ± 0.46 W K⁻¹ m⁻¹ to 0.52 ± 0.35 W K⁻¹ m⁻¹) when the diameter of the nanowire was reduced one order of magnitude (from 300 nm to 25 nm). The Kinetic–Collective model was used to understand such a reduction, which can be explained by the impact that surface scattering has in acoustic phonons. The smaller the diameter of the nanowires is, the larger the alteration in the mean free path of the low-frequency phonons is. The model agrees well with the experimental data, and the reduction in the thermal conductivity of the nanowires can be explained in terms of an increment of phonon scattering.