Nonlinear macroscopic polarization in III-V nitride alloys - art. no. 085207

We study the dependence of macroscopic polarization on composition and stra in in wurtzite III-V nitride ternary alloys using ab initio density-functio nal techniques. The spontaneous polarization is characterized by a large bo wing, strongly dependent on the alloy microscopic structure. The bowing is due to the different response of the bulk binaries to hydrostatic pressure and to internal strain effects (bond alternation). Disorder effects are ins tead minor. Deviations from parabolicity (simple bowing) are of order 10% i n the most extreme case of AlInN alloys, much less at all other composition s. Piezoelectric polarization is also strongly nonlinear. At variance with the spontaneous component, this behavior is independent of microscopic allo y structure or disorder effects, and due entirely to the nonlinear strain d ependence of the bulk piezoelectric response. It is thus possible to predic t the piezoelectric polarization for any alloy composition using the piezoe lectricity of the parent binaries.