Nonlinearities and piezoelectric fields in AlN/GaN wurtzite heterostructures

A one-dimensional model for examining the combined influence of lattice mismatch, spontaneous polarization, piezoelectricity, strain, and nonlinear permittivity effects is described. Nonlinear permittivity effects in two-layer AlN/GaN structures used in, e.g., heterojunction field effect transistors are shown to have a significant influence (approximately 20%) in the absence of interface charge screening due to strong electric fields in the thin-film AlN layer. Considerable reductions occur, however, in the electric field when interface charge screening is effectively reducing the importance of nonlinear permittivity effects. Determination of dynamic strain characteristics is performed for the case of a strong dc electric field in the AlN layer (due to short-circuit dc voltage conditions over the full structure) superimposed by a small ac voltage and it is shown that resonance frequencies exist being weakly affected by permittivity nonlinearities. As a corollary, it is found that a nonlinear permittivity analysis indicates that the material sound speed becomes electric field dependent due to piezoelectric coupling. (c) 2006 American Institute of Physics.