Origin of enhanced piezoelectric constants of MgNbAlN studied by first-principles calculations

We theoretically investigate the piezoelectric properties of Mg–Nb codoped wurtzite AlN by first-principles calculations and find the following two points. First, doping induces a remarkable softening of elastic constants, especially the diagonal component along the c -axis. This results in the increased piezoelectric strain constant d 33 , which is about 4.3 times larger than that of wurtzite AlN. Second, the c -axis lattice length increases up to the dopant concentration of around 60 at. %, and then decreases. These features are in excellent agreement with the experimental results previously reported.