Influence of strain and internal electric fields on band gaps in short period nitride based superlattices

Abstract Short period superlattices of the form m GaN/ n AlN, where m , n denote integer numbers of monolayers, and with growth direction along the wurtzite c -axis are studied by ab initio calculations. The dependence of the band gaps on composition is compared with results obtained previously for m InN/ n GaN superlattices. The strain caused by mismatch to the substrate leads to significant deformations of bonds in InN/GaN superlattices, whereas this effect is smaller in GaN/AlN superlattices. The general trends in gap behavior can to a large extend be related to the strength of the internal electric field, E , in the respective GaN and InN quantum wells. In the GaN/AlN superlattices E reaches values as high as 8 MV/cm, while in the InN/GaN superlattices E  ≈ 15 MV/cm may be reached. The strong electric fields are caused by spontaneous and piezoelectric polarizations. The latter contribution dominates in InN/GaN superlattices.