Composition fluctuations and clustering in (Ga,In)(N,As)/GaAs(0Â 0Â 1) heterostructures studied by analytical transmission electron microscopy

In this article, we summarize our studies on the spatial element distribution in (Ga,In)(N,As) quantum wells and epilayers grown on GaAs(0 0 1) substrates by molecular beam epitaxy. Nanometer-sized composition fluctuations are detected in (Ga,In)(N,As) layers with In and N concentration above 20% and 2%, respectively, by dark-field transmission electron microscopy and spatially resolved electron energy-loss spectroscopy. The fluctuations and clustering are inherently present in these quaternary alloys due to the phase separation tendency. Morphological instabilities, such as the surface roughening due to the elastic strain relief (i.e., a 2D- to 3D-growth mode transition), are succeeding processes. The origin of the fluctuations is discussed with respect to the selected growth conditions and the post-growth annealing procedure.