Group III nitrides

Both the theoretical and experimental values of the composition-related band-gap bowing parameter for solid solutions of the III-N compounds feature an appreciable spread. The properties of the binary GaN, AlN, and InN compounds have been studied in detail using both the empirical-pseudopotential method and the methods based on first principles. However, theoretical studies of properties of solid solutions under consideration are typically based on the virtual-crystal approximation or the coherent- potential approximation (CPA); these methods amount conceptually to the replacement of a certain microscopic configuration by an averaged effective medium and, thus, cannot account for experimentally observed compositional dependences of the band gap.

Calculations were done in Empirical Pseudopotentail Method but using the functional pseudopotential form which is continuous in reciprocal space and depends on the local strain that describes the charge redistribution and screening of the pseudopotential due to changes in the local environment. Pseudopotential parameters were fitted to reproduce experimental values of the interband transition energies at the high-symmetry points of the Brillouin zone, effective masses and optical properties of pure binary compounds. In order to take correctly into account the effect of local strains, we included the values of the bandgap deformation potentials in the fitting procedure.

In our work, we studied the alloying effects on electronic properties of disordered AlGaN, InGaN, and InAlN ternary solid solutions using supercells containing 32 atoms with atomic configurations obtained on the basis of density-functional theory. It is shown that, if the internal local strains in the solid solutions and both compositional and positional disorder are taken into consideration, a bowing in the compositional dependences of the band gap Eg(x) is observed, which accounts satisfactorily for the available experimental data. The largest contribution to the band-gap bowing parameter is made by the compositional disorder. The bond-length relaxation reduces the effect of compositional disorder and the volume-deformation effects. The chosen model can satisfactorily account for electronic properties of ternary solid solutions and makes it possible to gain insight into the causes of appearance of bowing in the dependence Eg(x).

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