Understanding the relative importance of interface scattering and phonon-phonon interactions on thermal transport in superlattices (SLs) is essential for the simulation of practical devices, such as quantum cascade lasers (QCLs). While several studies have looked at the dependence of the thermal conductivity of SLs on period thickness, few have systematically examined the effect of varying material thickness ratio. Here, we study through-plane thermal conduction in lattice-matched In0.53Ga0.47As/In0.52Al0.48As SLs grown by metalorganic chemical vapor deposition as a function of SL period thickness (4.2 to 8.4 nm) and layer thickness ratio (1:3 to 3:1). Conductivities are measured using time-domain thermoreflectance and vary between 1.21 and 2.31 W m−1 K−1. By studying the trends of the thermal conductivities for large SL periods, we estimate the bulk conductivities of In0.53Ga0.47As and In0.52Al0.48As to be approximately 5 W m−1 K−1 and 1 W m−1 K−1, respectively, the latter being an order of magnitude lower than theoretical estimates. Furthermore, we find that the Kapitza resistance between alloy layers has an upper bound of ≈0.1 m2 K GW−1, and is negligible compared to the intrinsic alloy resistances, even for 2 nm thick layers. A phonon Boltzmann transport model yields good agreement with the data when the alloy interfaces are modeled using a specular boundary condition, pointing towards the high-quality of interfaces. We discuss the potential impact of these results on the design and operation of high-power QCLs comprised of In1−xGaxAs/In1−yAlyAs SL cores.

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.

While atomic vibrations dominate thermal conduction in the amorphous and face-centered cubic phases of Ge2Sb2Te5, electrons dominate in the hexagonal closed-packed (hcp) phase. Here we separate the electron and phonon contributions to the interface and volume thermal resistances for the three phases using time-domain thermoreflectance and electrical contact resistance measurements. Even when electrons dominate film-normal volume conduction (i.e., 70% for the hcp phase), their contribution to interface heat conduction is overwhelmed by phonons for high-quality interfaces with metallic TiN.

Thin diamond films of thickness near 1 μm can have highly nonuniform thermal conductivities owing to spatially varying disorder associated with nucleation and grain coalescence. Here, we examine the nonuniformity for nanocrystalline chemical vapor deposited diamond films of thickness 0.5, 1.0, and 5.6 μm using picosecond thermoreflectance from both the top and bottom diamond surfaces, enabled by etching a window in the silicon substrate. The extracted local thermal conductivities vary from less than 100 W m−1 K−1 to more than 1300 W m−1 K−1 and suggest that the most defective material is confined to within 1 μm of the growth surface.

Thermal conduction in periodic multilayer composites can be strongly influenced by nonequilibrium electron-phonon scattering for periods shorter than the relevant free paths. Here we argue that two additional mechanisms-quasiballistic phonon transport normal to the metal film and inelastic electron-interface scattering-can also impact conduction in metal/dielectric multilayers with a period below 10 nm. Measurements use the 3ω method with six different bridge widths down to 50 nm to extract the in- and cross-plane effective conductivities of Mo/Si (2.8 nm/4.1 nm) multilayers, yielding 15.4 and 1.2 W/mK, respectively. The cross-plane thermal resistance is lower than can be predicted considering volume and interface scattering but is consistent with a new model built around a film-normal length scale for phonon-electron energy conversion in the metal. We introduce a criterion for the transition from electron to phonon dominated heat conduction in metal films bounded by dielectrics.

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates including 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 the thermal resistances at the GaN-diamond interfaces for two generations (1st and 2nd) of GaN-on-diamond substrates using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance (TTR) techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the 2nd 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 thickness by 75%, 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.

Extreme ultraviolet (EUV) lithography requires nanostructured optical components, whose reliability can be influenced by radiation absorption and thermal conduction. Thermal conduction analysis is complicated by sub-continuum electron and phonon transport and the lack of thermal property data. This paper measures and interprets thermal property data, and their evolution due to heating exposure, for Mo/Si EUV mirrors with 6.9 nm period and Mo/Si thickness ratios of 0.4/0.6 and 0.6/0.4. We use time-domain thermoreflectance and the 3ω method to estimate the thermal resistance between the Ru capping layer and the Mo/Si multilayers (RRu-Mo/Si = 1.5 m2 K GW−1), as well as the out-of-plane thermal conductivity (kMo/Si 1.1 W m−1 K−1) and thermal anisotropy (η = 13). This work also reports the impact of annealing on thermal conduction in a co-deposited MoSi2 layer, increasing the thermal conductivity from 1.7 W m−1 K−1 in the amorphous phase to 2.8 W m−1 K−1 in the crystalline phase.

This paper uses a 2D finite volume numerical model to predict the steady state and transient temperature distributions in a High-Electron-Mobility Transistor (HEMT). The numerical predictions are confirmed through comparison with analytical solutions of the one and two dimensional steady and transient heat equations. We analyze the thermal performance of several HEMT geometries with varying substrate materials. Devices with wider gates lying directly on highly conductive substrates (e.g., diamond) have significantly less thermal resistance, by as much as 90 percent. Finally, we investigate the temperature response to a frequency modulated heating event. This result indirectly applies to thermal measurements using 3ω electrical thermometry.

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 for the GaN-SiC TIR and 157-182 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.

Due to their high thermal conductivity, diamond substrates are seen as a way to minimize the thermal resistance present in High Electron Mobility Transistor (HEMT) structures based on GaN. Single-crystal AlN transition layers facilitate the growth of high quality GaN on diamond, but such layers may increase the total thermal resistance of the composite substrate. This manuscript measures the thermal conductivity and interface resistance of a 1.4 μm diamond film on a polycrystalline AlN substrate using picosecond time-domain thermoreflectance (TDTR) and nanosecond thermoreflectance. Varying beam widths are used to extract the thermal conductivity anisotropy of the diamond film.

Summary form only given. GaN HEMT technology is transforming applications in communications, radar, and electronic warfare by offering more than 5x higher RF transmit power over the existing GaAs-based technologies. The high breakdown voltage and current handling capability of GaN HEMTs enables, for the same device size, a 10x increase in RF power using GaN-based devices in place of conventional GaAs-based devices. However the ultimate power and performance of GaN technology cannot be exploited in real applications due to thermal limitations on performance and reliability. The high power density in GaN HEMTs translates to mega-Watts/cm 2 heat dissipation at the device gate region. Increasing the heat conductance near the GaN device junction is critical to reduce device junction temperature for reliable operation and performance. NGAS will report on revolutionary methods being developed to directly integrate high quality, high thermal conductivity diamond materials with more than 4x greater thermal conductivity over existing state-of-the-art GaN HEMT technology.

Heat transfer across nanoscale metal/dielectric multilayers involves multiple thermal conduction mechanisms. Electron or phonon interface scattering can augment the thermal conductivity anisotropy in multilayer composites. Weak electron-phonon coupling and quasi-ballistic phonon transport normal to the metal film further increase the anisotropy for metal-dielectric multilayers with period shorter than the relevant free paths. This paper models these physical mechanisms using an approximate thermal resistor network with support from the Boltzmann transport equation. We measure the in- and cross-plane thermal conductivity of a Mo/Si (2.8 nm/4.1 nm) multilayer as 15.4 and 1.2 W/mK, respectively, which agree with the proposed theoretical model. This work introduces a criterion for the transition from electron to phonon dominated heat conduction in metal films bounded by dielectrics.

Phase change memory uses brief pulses of electrical current to induce phase transitions in chalcogenide material regions with dimensions near or even far below 50 nm. The strongly differing electrical conductivities of the crystalline and amorphous phases allow data storage at densities in excess of terabits per square inch. Nanoscale conduction heat transfer governs the figures of merit in these devices, which include the energy and time required for switching, and has received much attention through both measurements and simulations over the last two decades. This chapter reviews the recent progress on thermal conduction phenomena relevant for phase change memory, including a summary of the physical mechanisms involved as well as the most useful simulation and measurement techniques. Experimental work has focused on separating the intrinsic and boundary resistances of thin film phase change materials, as well as the individual contributions of electrons and phonons to the effective conductivity of the hexagonal crystalline phase. Simulations have focused on improving device geometries and switching characteristics, and continue to need improvements in the areas of crystallization modeling and the impact of phase distribution on electrical and thermal transport. Future research requires a more detailed understanding of electron-phonon coupling and its impact on electrical and thermal conduction in the crystalline phase, as well as greater insight into thermoelectric transport and its impact on device behavior. This progress will be critical for the development of innovative memory strategies including multibit storage.

Tantalum Nitride (TaN) films carry high heat fluxes in a variety of applications including diffusion barriers in magnetoresistive random access memory and buffer/absorbers in extreme ultraviolet masks. The thicknesses of these films are usually of the same order as the thermal energy carrier mean free path, which complicates the study of heat conduction. This paper presents thermal (cross-plane) and electrical (in-plane) conductivity measurements on TaN films with thicknesses of 50, 75, and 100 nm. Picosecond thermoreflectance is used to extract the thermal boundary resistance between TaN and Al and the intrinsic thermal conductivity of TaN for temperatures of 300–700 K. The data and the relative importance of boundary resistances, electron-boundary scattering, and electron-defect scattering are interpreted using the electrical and thermal transport data. These data facilitate comparison of the phonon and electron contributions to thermal conduction in TaN.

The programming current required to switch a phase-change memory cell depends upon the thermal resistances in the device. In many designs, significant heat loss occurs through the electrode. This letter investigates the thermal properties of a multilayer electrode stack. This material offers greater thermal resistance than single-material electrodes due to the presence of multiple thermal boundary resistances (TBRs), reducing heat loss from the device and potentially lowering the programming current. Picosecond time-domain thermoreflectance interrogates the temperature-dependent thermal conductivity of three as-deposited and postannealed electrode materials: carbon, titanium nitride, and tungsten nitride. These data are used to extract the temperature-dependent, as-deposited, and postannealed TBR in two multilayer electrode stacks: carbon-titanium nitride and tungsten-tungsten nitride. The C-TiN stacks demonstrate an as-deposited TBR of 4.9 m(2)K/GW, increasing to 11.9 m(2)K/GW postanneal. The W-WN(x) stacks demonstrate an as-deposited TBR of 3.9 m(2)K/GW, decreasing to 3.6 m(2)K/GW postanneal. These resistances are equivalent to electrode films with thickness on the order of tens of nanometers.

Phase change memory (PCM) uses rapid heating and cooling to induce switching in sub-micron memory cells. The rapid rates of heating and nanoscale dimensions require accurate modeling of thermal transport phenomena in the constituent materials. This motivates improved understanding of the thermal properties of Ge2Sb2Te5 (GST) thin films and PCM electrode materials. We report measurements of thermal conductivity and interface resistance of GST and electrode materials by applying nanosecond pump-probe thermoreflectance to multilayer structures of GST-C, GST-TiN, and GST-Ti. We measure the total thermal resistance of the stack from the transient thermal response, separating the intrinsic and boundary resistance terms using a 1-D resistor model of the stack. The intrinsic conductivities for GST are 0.20 W/(m K) for GST-C, 0.33 W/(m K) for GST-TiN, 0.27 W/(m K) for low temperature deposited GST-Ti, and 0.69 for high temperature deposited GST-Ti. The thermal boundary resistances are 27.5 m(2)K/GW for GST-C, 5.2 m(2)K/GW for GST-TiN, 49.8 m(2)K/GW for low temperature GST-Ti, and 11.4 m(2)K/GW for high temperature GST-Ti.

We report the thickness and phase dependent thermal properties of Ge2Sb2Te5 (GST) films using electrical Joule heating thermometry (3ω method) and validate the data with optical thermometry. The intrinsic thermal conductivity of GST increases from 0.26 W/mK for the amorphous phase to 0.57 W/mK for the crystalline phase. The thermal boundary resistance between GST and Si3N4 films decreases from 134×10−9 m2KW−1 to 25×10−9 m2KW−1 after the phase change. The effective thermal resistance of GST shows a nonlinear dependence on the film thickness that infers the presence of microstructural defects. We separate the contributions of defects from intrinsic thermal transport and summarize possible causes for their thickness dependent behavior. An electrothermal simulation of PCM cell estimates the potential impact of the measured properties. This work improves the quality of PCM simulations by offering detailed knowledge of the distribution of thermal resistances.