Thermal Conductivity of Aluminum Scandium Nitride for 5G Mobile Applications and Beyond.

Radio frequency (RF) microelectromechanical systems (MEMS) based on Al1-xScxN are replacing AlN-based devices because of their higher achievable bandwidths, suitable for the fifth-generation (5G) mobile network. However, overheating of Al1-xScxN film bulk acoustic resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone's ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. In this work, the ramifications of tailoring of the piezoelectric response and microstructure of Al1-xScxN films on the thermal transport have been studied. The thermal conductivity of Al1-xScxN films (3-8 W m-1 K-1) grown by reactive sputter deposition was found to be orders of magnitude lower than that for c-axis-textured AlN films due to alloying effects. The film thickness dependence of the thermal conductivity suggests that higher frequency FBAR structures may suffer from limited power handling due to exacerbated overheating concerns. The reduction of the abnormally oriented grain (AOG) density was found to have a modest effect on the measured thermal conductivity. However, the use of low AOG density films resulted in lower insertion loss and thus less power dissipated within the resonator, which will lead to an overall enhancement of the device thermal performance.

Dynamical Magnetic Field Accompanying the Motion of Ferroelectric Domain Walls.

The recently proposed dynamical multiferroic effect describes the generation of magnetization from temporally varying electric polarization. Here, we show that the effect can lead to a magnetic field at moving ferroelectric domain walls, where the rearrangement of ions corresponds to a rotation of ferroelectric polarization in time. We develop an expression for the dynamical magnetic field, and calculate the relevant parameters for the example of 90° and 180° domain walls, as well as for polar skyrmions, in BaTiO_{3}, using a combination of density functional theory and phenomenological modeling. We find that the magnetic field reaches the order of several µT at the center of the wall, and we propose two experiments to measure the effect with nitrogen-vacancy center magnetometry.