  We report first principles density functional calculations of the spontaneous
polarizations, piezoelectric constants and elastic constants for the II-VI
wurtzite structure ZnO, MgO and CdO. Using our self interaction corrected
implementation of density functional theory we obtain polarization values of we
obtain polarization values of -0.05, -0.17 and -0.10 C/m$^2$, and piezoelectric
constants, $e_{33}$ ($e_{31}$) of 1.34 (-0.57), 2.26 (-0.38) and 1.67 (-0.48)
C/m$^2$ for structurally relaxed ZnO, MgO and CdO respectively. These
properties are consistently larger in magnitude than the corresponding GaN, AlN
and InN analogues. Therefore we predict that larger internal fields will be
attainable in ZnO-based polarization field effect transistors than in
equivalent GaN-based devices.
