Large electromechanical response in ZnO and its microscopic origin

The electromechanical coefficient d₃₃ of wurtzite ZnO is determined by direct first-principles density functional calculations which are performed for solids under finite electric fields. Our theoretical d₃₃ value of 12.84 pC∕N turns out to be in good agreement with experiment. This electromechanical response in ZnO (which is the largest among the known tetrahedral semiconductors) is found to originate from the strong coupling between strain and polarization, namely, a notably large β parameter. We further show that the electromechanical response in wurtzite semiconductors bears a previously unknown resemblance to the polarization rotation mechanism in ferroelectric Pb(ZnNb)O₃-PbTiO₃ and Pb(MgNb)O₃-PbTiO₃ single-crystal solid solutions. Our results demonstrate that, different from what is commonly believed, the main effect of electric fields in wurtzite semiconductors is not to elongate the polar chemical bonds, but to rotate those bonds that are non-collinear with the polar axis. This finding also suggests that the electromechanical response in wurtzite materials is governed mainly by the ease of bond bending, which may provide an useful scheme for designing better piezoelectric semiconductors with enhanced performance.