Critical thickness of high-temperature AlN interlayers in GaN on sapphire (0001)

We analyze by cross-sectional transmission electron microscopy the threading dislocation behavior when crossing an AlN intermediate layer in the GaN/AlN/GaN/sapphire system grown by molecular beam epitaxy. Dislocation behavior is explained calculating critical thickness by applying the Fischer model to an,AlN layer capped with GaN. Due to elastic interaction between straight misfit dislocations, the Matthews and Blakeslee model does not explain the observed dislocation behavior. The understanding of critical thickness in the studied system permits to select the AlN interlayer thickness that minimizes the threading dislocation density in the capped GaN layer.