  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.
