Band Gaps in InN/GaN Superlattices: Polar and Nonpolar Growth Directions

The electronic structures of short-period superlattices (SLs) consisting of $m$InN/$n$GaN unit cells with composition ($m$,$n)$ have been calculated within the density-functional theory including corrections for the ``LDA gap error''. The variation of the gaps with SL composition and the dependence on the growth direction, the \textit{polar} (c) and \textit{nonpolar} (a,m) directions in the wurtzite structure, are compared. The band gaps calculated for the polar SLs are much smaller than those found for non-polar SLs due to the electric polarization fields in the (c) SLs. For the (\textit{1,m}) class of polar samples photoluminescence measurements yield energy-gap values, which are much larger than the calculated values. The reason for this is that the structure of the samples differs from the assumed ideal composition. Transmission electron microscopy studies of the assumed polar \textit{1}InN/$n$GaN SLs show that the real structure is 1In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN with In-content x$=$0.33. New calculations for such SLs are in perfect agreement with photoluminescence experiments.