ABSTRACT
This chapter reviews the different models that have been proposed over the years and how they compare to experimental measurements. It outlines both linear and nonlinear models and discusses the experimental determination of values of spontaneous polarization in polymorphic crystals. The direct piezoelectric effect is the generation of an electric dipole moment in certain crystals if a stress is applied and the electric dipole moment is proportional to the applied stress. Spontaneous polarization develops from the atomic layout in the bulk materials. The chapter proposes the various nonlinear models of piezoelectricity in III-N semiconductors. Such nonlinearities in the polarization were first reported for III-N by K. Shimada et al., expressing theoretically and calculating the dependence of the polarization on deviatoric strain, but without expressly providing a set of nonlinear coefficients. Piezoelectricity in III-N semiconductors is an effect of very strong magnitude, with fields measured in the MV/cm a commonly reported feature.
