First-principles study on piezoelectric constants in strained BN, AlN, and GaN

Piezoelectric constants at the states suffering volume-conserving strains have been calculated by using geometric quantum Berry phase approach for bulk BN, AlN, and GaN with wurtzite structure. Two step calculations have been carried out: for given strains the first-principles molecular dynamics method has been applied to relax the electronic ground state and the ionic position in the unit cell, after then the piezoelectric constants have been calculated by applying the other probing strains. Very large changes have been found in piezoelectric constants at the states distorted differently. This nonlinear piezoelectricity is shown to come from the structural parameters in the distorted unit cell for each strained state.