Influence of pyramidal defects on photoluminescence of Mg-doped AlGaN/GaN superlattice structures

Mg-doped AlGaN/GaN superlattice structures (SLS) grown by metalorganic chemical vapor deposition (MOCVD) are investigated by transmission electron microscopy (TEM) and photoluminescence (PL) measurements. TEM shows the good quality of moderately doped AlGaN/GaN SLS, the PL of which is dominated by the recombination of excitons confined in AlGaN/GaN quantum wells. A Mg-related near-ultraviolet emission band can be attributed to donor-acceptor pair (DAP) transition. However, for a high Mg doping level, the contrast from AlGaN/GaN SLS in TEM images completely disappears and a high density of pyramidal defects is observed in the original AlGaN/GaN SLS regions, which are identified as Mg-related inversion domains with boundary facets predominantly along the {0001} and {1123} planes. It is suggested that Al preferably aggregates around these pyramidal defects and as a result the periodic structure of AlGaN/GaN is destroyed by these defects. Consequently, the weak exciton emission peak of GaN is observed instead of the emission peak of AlGaN/GaN SLS. The PL emission is dominated by a combination of free electron-acceptor (eA) and DAP transitions.