Nonlinear behavior of spontaneous and piezoelectric polarization in III-V nitride alloys

We investigate spontaneous and piezoelectric polarization in wurtzite III-V nitride ternary alloys as a function of composition and microscopic structure, using ab initio density-functional techniques and Berry phase method. We find a strong nonlinear dependence of spontaneous polarization on composition in chemically disordered alloys. Nonlinearity is quite pronounced in those alloys (e.g. AlInN and InGaN) made of largely lattice-mismatched binaries. Spontaneous polarization bowing strongly depends on the microscopic structure of the alloy. Chemical ordering in the form of super-lattice structure may increase the bowing up to a factor of five. Piezoelectric polarization is also nonlinear, in random alloys the nonlinearity is entirely due to a nonlinear strain dependence on piezoelectric polarization in pure binary compounds, therefore piezoelectric coefficients follow Vegard's law. In chemically ordered InGaN and AlInN alloys piezoelectric coefficients deviate from Vegard's law, and this effect reduces the strength of the piezoelectric polarization up to 38% of its value in AlInN alloy.