A TEM Study of A1N Interlayer Defects in AlGaN/GaN Heterostructures

Abstract

A series of Al_0.47Ga_0.53N/GaN heterostructures with a range of different AlN interlayer thicknesses has been examined. We find that when the interlayer thickness is greater than ∼5nm, it becomes possible to grow 250nm of AlGaN without cracking. The interlayers are then believed to be sufficiently relaxed to place the AlGaN under compressive strain. The mechanisms for this relaxation have been studied using high angle annular dark field (HAADF) imaging, conventional transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It is found that relaxation takes place through both the small-scale cracking of the interlayer and the generation of misfit dislocations at the GaN/AlN interface. EELS has also been used to probe the Al content of the material filling the interlayer cracks, showing it to be depleted of Al compared to the rest of the AlGaN.