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  1. Thermal rectification is an asymmetric heat transfer process where directionally dependent transport occurs along a given axis. In this work, geometric parameters that govern thermal rectification in solids composed of various semiconducting materials were investigated utilizing metalattice data for seven materials with pore sizes ranging between 2 and 30 nm. Using numerical simulation, thermal rectification was calculated at different thermal biases in single material systems, including silicon, cubic boron nitride, and diamond, among others. The largest thermal rectification for each material was exhibited in bilayer sample stacks that were thermally matched (i.e., the thermal resistance of each layer in the stack is equal in either forward or reverse direction). Of the materials tested, diamond provided the highest thermal rectification for all cases, with its best case achieving a thermal rectification of 57.2%. This novel thermal functionality will find application in advanced applications for temperature regulation, including resonator systems where thermal effects may significantly alter and/or degrade performance. 
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  2. This article provides a broadband dielectric characterization of different silicate substrates up to 115 GHz, to fill the gap in the properties of different kinds of glasses in a broad part of the mm-wave spectrum. Both the internal structure (crystalline or amorphous) and the chemistry of the substrates influence the permittivity and loss tangent of the material. Quartz and sapphire are crystalline materials that exhibit a low loss in the mm-wave frequency range. Amorphous silicates generally have higher loss values than crystalline materials, and within the glasses, the level of impurities added also affects the dielectric loss. Several characterization techniques have been employed to cover a broad frequency band. The limitations of the different characterization techniques are also included. Once the dielectric properties of substrates are characterized, a metasurface has been designed and fabricated at 100 GHz to increase the reflection in window glass and provide coverage on areas that would otherwise be shadowed. The measurement results are in good agreement with the simulations. 
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  3. AlN thin films are enabling significant progress in modern optoelectronics, power electronics, and microelectromechanical systems. The various AlN growth methods and conditions lead to different film microstructures. In this report, phonon scattering mechanisms that impact the cross-plane (κz; along the c-axis) and in-plane (κr; parallel to the c-plane) thermal conductivities of AlN thin films prepared by various synthesis techniques are investigated. In contrast to bulk single crystal AlN with an isotropic thermal conductivity of ∼330 W/m K, a strong anisotropy in the thermal conductivity is observed in the thin films. The κz shows a strong film thickness dependence due to phonon-boundary scattering. Electron microscopy reveals the presence of grain boundaries and dislocations that limit the κr. For instance, oriented films prepared by reactive sputtering possess lateral crystalline grain sizes ranging from 20 to 40 nm that significantly lower the κr to ∼30 W/m K. Simulation results suggest that the self-heating in AlN film bulk acoustic resonators can significantly impact the power handling capability of RF filters. A device employing an oriented film as the active piezoelectric layer shows an ∼2.5× higher device peak temperature as compared to a device based on an epitaxial film.

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    Abstract Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generation high-power and high-frequency electronic devices. Recent efforts have focused on realizing vertical power device structures such as in situ oxide, GaN interlayer based vertical trench metal–oxide–semiconductor field-effect transistors (OG-FETs). Unfortunately, the higher-power density of GaN electronics inevitably leads to considerable device self-heating which impacts device performance and reliability. Halide vapor-phase epitaxy (HVPE) is currently the most common approach for manufacturing commercial GaN substrates used to build vertical GaN transistors. Vertical device structures consist of GaN layers of diverse doping levels. Hence, it is of crucial importance to measure and understand how the dopant type (Si, Fe, and Mg), doping level, and crystal quality alter the thermal conductivity of HVPE-grown bulk GaN. In this work, a steady-state thermoreflectance (SSTR) technique was used to measure the thermal conductivity of HVPE-grown GaN substrates employing different doping schemes and levels. Structural and electrical characterization methods including X-ray diffraction (XRD), secondary-ion mass spectrometry (SIMS), Raman spectroscopy, and Hall-effect measurements were used to determine and compare the GaN crystal quality, dislocation density, doping level, and carrier concentration. Using this comprehensive suite of characterization methods, the interrelation among structural/electrical parameters and the thermal conductivity of bulk GaN substrates was investigated. While doping is evidenced to reduce the GaN thermal conductivity, the highest thermal conductivity (201 W/mK) is observed in a heavily Si-doped (1–5.00 × 1018 cm−3) substrate with the highest crystalline quality. This suggests that phonon-dislocation scattering dominates over phonon-impurity scattering in the tested HVPE-grown bulk GaN substrates. The results provide useful information for designing thermal management solutions for vertical GaN power electronic devices. 
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