Properties of strained wurtzite GaN and AlN: Ab initio studies

The structural, dielectric, lattice-dynamical, and electronic properties of biaxially and uniaxially strained group-III nitrides are studied ab initio using a pseudopotential-plane-wave method. A linear-response approach to the density-functional theory is used to calculate the dielectric constants, the dynamical effective charges, and the phonon frequencies. For a given strain the atomic coordinates are determined from the equilibrium condition. The elastic properties of GaN and AlN are characterized in terms of ratios of the elastic stiffness constants, which allow for a critical comparison with literature data; unreliable ones are pointed out. Electronic as well as phonon deformation potentials and the respective strain and stress coefficients are determined. We show that the quasicubic approximation does not hold for the electronic interband deformation potentials of GaN but for those of AlN. Seeming discrepancies between experimental and theoretical results can be widely resolved using suitable parameters and correct stress-strain relations. We find that the stress obtained from biaxial-strain-induced shifts of the high-frequency E2 phonon or excitonic transitions should be higher than determined by other authors.