Elah Bozorg-Grayeli

Low Thermal Resistances at GaN–SiC Interfaces for HEMT Technology

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.

Improved Thermal Interfaces of GaN–Diamond Composite Substrates for HEMT Applications

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.

Phonon and electron transport through Ge2Sb2Te5 films and interfaces bounded by metals

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.

Thermal conduction inhomogeneity of nanocrystalline diamond films by dual-side thermoreflectance

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.

Phonon Dominated Heat Conduction Normal to Mo/Si Multilayers with Period below 10 nm

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.

Thermal conduction in lattice–matched superlattices of InGaAs/InAlAs

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 low...

Temperature-Dependent Thermal Properties of Phase-Change Memory Electrode Materials

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 m2K/GW, increasing to 11.9 m2K/GW postanneal. The W-WNx stacks demonstrate an as-deposited TBR of 3.9 m2K/GW, decreasing to 3.6 m2 K/GW postanneal. These resistances are equivalent to electrode films with thickness on the order of tens of nanometers.

High temperature thermal properties of thin tantalum nitride films

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.

Thermal characterization of GaN-on-diamond substrates for HEMT applications

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.

Thermal conduction properties of Mo/Si multilayers for extreme ultraviolet optics

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.