The peak power density of GaN high-electron-obility transistor technology is limited by a hierarchy of thermal resistances from the junction to the ambient. Here, we explore the ultimate or fundamental cooling limits for junction-to fluid cooling, which are enabled by advanced thermal management technologies, including GaN-diamond composites and nanoengineered heat sinks. Through continued attention to near-junction resistances and extreme flux convection heat sinks, heat fluxes beyond 300 kW/cm(2) from individual 2-mu m gates and 10 kW/cm(2) from the transistor footprint will be feasible. The cooling technologies under discussion here are also applicable to thermal management of 2.5-D and 3-D logic circuits at lower heat fluxes.

Here we study single-crystalline silicon nanobeams having 470 nm width and 80 nm thickness cross section, where we produce tortuous thermal paths (i.e. labyrinths) by introducing slits to control the impact of the unobstructed "line-of-sight" (LOS) between the heat source and heat sink. The labyrinths range from straight nanobeams with a complete LOS along the entire length to nanobeams in which the LOS ranges from partially to entirely blocked by introducing slits, s =3D 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from similar to 47 W m(-1) K-1 for straight beam to similar to 31 W m(-1) K-1 for slit width of 395 nm. A model prediction through a combination of the Boltzmann transport equation and ab initio calculations shows an excellent agreement with the experimental data to within similar to 8%. The model prediction for the most tortuous path (s =3D 395 nm) is reduced by similar to 14% compared to a straight beam of equivalent cross section. This study suggests that LOS is an important metric for characterizing and interpreting phonon propagation in nanostructures.

The temperature rise in AlGaN/GaN high-electron-mobility transistors depends strongly on the GaN-substrate thermal interface resistance (TIR). We apply picosecond time-domain thermoreflectance measurements to GaN-SiC composite substrates with varying GaN thickness to extract both the TIR and the intrinsic GaN thermal conductivity at room temperature. Two complementary data extraction methodologies yield 4-5 m(2) . K/GW for the GaN-SiC TIR and 157-182 W/m . K for the GaN conductivity. The GaN-SiC interface resistance values reported here, as well as the TIR experimental uncertainties documented in this letter, are substantially lower than those reported previously for this material combination.

The heat flow at the interfaces of complex nanostructures is three-dimensional in part due to the nonplanarity of interfaces. One example common in nano systems is the situation when a significant fraction of the interfacial area is composed of sidewalls that are perpendicular to the principal plane, for example, in metallization structures for complementary metal-oxide semiconductor transistors. It is often observed that such sidewall interfaces contain significantly higher levels of microstructural disorder, which impedes energy carrier transport and leads to effective increases in interfacial resistance. The impact of these sidewall interfaces needs to be explored in greater depth for practical device engineering, and a related problem is that appropriate characterization techniques are not available. Here, we develop a novel electrothermal method and an intricate microfabricated structure to extract the thermal resistance of a sidewall interface between aluminum and silicon dioxide using suspended nanograting structures. The thermal resistance of the sidewall interface is measured to be similar to 16 +/- 5 m(2) K GW(-1), which is twice as large as the equivalent horizontal planar interface comprising the same materials in the experimental sample. The rough sidewall interfaces are observed using transmission electron micrographs, which may be more extensive than at interfaces in the substrate plan in the same nanostructure. A model based on a two-dimensional sinusoidal surface estimates the impact of the roughness on thermal resistance to be similar to 2 m(2) K GW(-1). The large disparity between the model predictions and the experiments is attributed to the incomplete contact at the Al-SiO2 sidewall interfaces, inferred by observation of underetching of the silicon substrate below the sidewall opening. This study suggests that sidewall interfaces must be considered separately from planar interfaces in thermal analysis for nanostructured systems.

Time-domain thermoreflectance (TDTR) is a powerful method for measuring thermal properties, such as thermal conductivity and thermal boundary resistance, of a broad variety of thin-film materials and interfaces. Dual-side TDTR, in which measurements are performed on the top and bottom sides of a suspended region of a thin film of interest, has recently emerged as an effective way to investigate the thermal properties of a film that is thermally anisotropic and/or inhomogeneous. Despite its experimental versatility, dual-side TDTR has yet to be fully interrogated. In this work, we examine the thermal conductivity and boundary resistance of anisotropic and/or inhomogeneous suspended thin films, extracted by dual-side TDTR on these films via numerical simulation. We start from a simple case of an anisotropic or inhomogeneous suspended membrane and then consider the combined case where the suspended membrane is both anisotropic and inhomogeneous. Taken together with analysis of measurement sensitivity, we aim to provide a general guideline for data extraction methodologies for dual-side TDTR on anisotropic and/or inhomogeneous suspended thin films.

A novel, mid-IR scanned-wavelength laser absorption diagnostic was developed for time-resolved, interference-free, absorption measurement of methane concentration. A differential absorption (peak minus valley) scheme was used that takes advantage of the structural differences of the absorption spectrum of methane and other hydrocarbons. A peak and valley wavelength pair was selected to maximize the differential cross-section (sigma(peak) (minus) (valley)) of methane for the maximum signal-to-noise ratio, and to minimize that of the interfering absorbers. Methane cross-sections at the peak and valley wavelengths were measured over a range of temperatures, 1000 to 2000 K, and pressures 1.3 to 5.4 atm. The cross-sections of the interfering absorbers were assumed constant over the small wavelength interval between the methane peak and valley features. Using this diagnostic, methane concentration time histories during n-heptane pyrolysis were measured behind reflected shock waves in a shock tube. The differential absorption scheme efficiently rejected the absorption interference and successfully recovered the vapor-phase methane concentration. These measurements allowed the comparison with methane concentration time-history simulations derived from a current n-heptane reaction mechanism (Sirjean et al 2009 A high-temperature chemical kinetic model of n-alkane oxidation JetSurF version 1.0).

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates, including the high-thermal-conductivity diamond, are promising, but high thermal resistances at the interfaces between the GaN and diamond can offset the benefit of a diamond substrate. We report on measurements of thermal resistances at GaN-diamond interfaces for two generations (first and second) of GaN-on-diamond substrates, using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the second generation samples, electrical heating and thermometry in nanopatterned metal bridges confirms the TDTR results. This paper demonstrates that the latter generation samples, which reduce the AlGaN/AlN transition layer thickness, result in a strongly reduced thermal resistance between the GaN and diamond. Further optimization of the GaN-diamond interfaces should provide an opportunity for improved cooling of HEMT devices.