Polar AlN/GaN interfaces: Structures and energetics

Abstract

The structures and energies of {0001} interfaces between GaN and AlN are studied by both ab initio methods and molecular dynamics using the Tersoff empirical inter-atomic potential. Based on experimental observations, structural configurations depending on polarity and atomic stacking are considered. It is evidenced by both ab initio and empirical calculations that III-polar interfaces are energetically favourable compared to the N-polar. In addition, the ab initio analysis shows that the wurtzite interfacial stacking is energetically preferable compared to zinc blende. A linear dependence between the bandgap energy and the strain in AlN/GaN heterostructures is found. It is shown that the bandgap increases with increasing c/a ratio while an inverse proportionality relationship is observed in the case of lattice parameter a. However, biaxial strain is found to flatten this variation considerably. Empirical potential calculations yield the interfacial energies, taking into account the relaxation of the lattice mismatch due to arrays of misfit dislocations and in combination with ab initio methods estimate that the energetically favourable III polarity interface exhibits at least 18% larger critical thickness than the N polar.