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

• The electronic band structures of GaN/AlN and InN/GaN superlattices are calculated.
• Different strain conditions and their influence on band gap values are considered.
• The built-in electric fields are determined.
• Band gap trends are discussed in terms of hybridization and electric field effects.

Short period superlattices of the form mGaN/nAlN, 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 mInN/nGaN 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.