Nonlinear polarization in nitrides revealed with hydrostatic pressure

We use hydrostatic pressure as an instrument to reveal a strong nonlinearity of the electrical polarization in group-III nitride quantum well structures. From the photoluminescence peak energies of the quantum well emission at different applied pressures we obtain the values of the built-in electric field in the wells and the corresponding well-barrier polarization difference. We found that in both the InGaN/GaN and GaN/AlGaN systems the field and the polarization difference increase with pressure much faster than expected from the conventional (linear) model of polarization. This behavior is explained by the dramatic strain dependence of the piezoelectric coefficients of the group-III nitrides, which constitutes the nonlinear piezoelectric effect.