Electromechanical coupling in Yb-substituted III-V nitride alloys
Group-III nitride alloys are currently used in various microwave communication applications because of the giant enhancement in electromechanical coupling after alloying with rocksalt nitrides such as YbN or ScN. Herein, the Yb-substitution induced enhancement for electromechanical coupling in wurtzite III-V nitrides is studied via theoretical calculations and experiments. The substitution induced mechanical softening and local strain can enhance electromechanical coupling. The mechanical softening induced by Yb substitution shows less dependence on the parent AlN or GaN, which is caused by the Yb-Yb pair interaction in the c-axis direction, and the difference of electromechanical coupling between the GaN- and AlN-based alloys mainly comes from their enhancement effect of Yb substitution for piezoelectric response. The largest change in piezoelectric response relative to the parent nitride is observed in GaN-based alloy, which is mainly considered as a consequence of small piezoelectric constant of the parent GaN. Our calculations also reveal that the substitutional element with a closer ionic size to the host cation is easier to substitute into the host nitride, and produces a larger internal strain to partly contribute to the enhancement in piezoelectric response. This can serve as a simple guideline to identify alloying components in a search for a massive increase in electromechanical coupling.