Enhanced 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 shows less dependence on the parent AlN or GaN, which is considered to be caused by the Yb-Yb pair interaction in the c-axis, and the difference of electromechanical coupling between the GaN- and AlN-based alloys mainly comes from the enhancement effect of Yb substitution for the piezoelectric response. The largest change in the piezoelectric response relative to the parent nitride is observed in the GaN-based alloy, and is mainly attributed to a small piezoelectric constant of the parent GaN, which makes GaN as the parent nitride difficult to improve its electromechanical coupling coefficient compared with AlN-based alloys. Moreover, our calculations 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 the piezoelectric response. This can serve as a simple guideline to identify alloying components in a search for a massive increase in electromechanical coupling.