Mechanisms of dislocation reduction in an $Al0.98Ga0.02N$ layer grown using a porous AlN buffer

Greatly improved optical properties of $Al0.55Ga0.45N∕Al0.65Ga0.35N$ multiple quantum well (MQW) structures grown on the low dislocation density $Al0.98Ga0.02N$ layer grown using a porous AlN buffer have been observed by photoluminescence measurement, in comparison with a MQW grown on a normal AlN layer. The mechanism for the dislocation reduction has been explored by transmission electron microscopy. Due to an attraction towards lateral growth fronts, the threading dislocations in the AlN layer bend during the overgrowth, but mainly by small angles, which is different from the 90° bending generally observed in classic epitaxial lateral overgrowth method. The dislocation reduction is realized through two main pathways: enhanced dislocation annihilation due to the small-angle line bending and the termination of dislocations at local free surfaces introduced by the nanoscale voids, produced around the interface between the $Al0.98Ga0.02N$ and AlN layers.