Coherent GdN clusters in epitaxial GaN:Gd thin films determined by transmission electron microscopy

The detection of non-uniform magnetic element distribution and phase separation has been highly challenging in dilute magnetic semiconductor alloys. Here, we present a dedicated transmission electron microscopy (TEM) investigation of epitaxial GaN:Gd thin films and unambiguously identify the occurrence of nano-scale coherent GdN clusters. The films were grown by molecular beam epitaxy with Gd concentrations varying between 10(16) and 10(19) cm(-3). The TEM results revealed the presence of nano-scale, coherently strained platelets lying parallel to the (0001) basal planes of the wurtzite GaN matrix. The platelet-shaped clusters consist of GdN as verified by chemically sensitive Z-contrast scanning TEM imaging. The cluster dimension as well as the displacement of the distorted lattice along the [0001] direction are quantitatively determined by high-resolution TEM based on geometric phase analysis. Dynamic strain contrast calculations taking the measured displacement field as an input parameter attained an excellent agreement with the experimental diffraction contrast results, which enables an estimate of average cluster size and distance. The GdN platelet clusters are of two monolayer thickness with a base diameter of about 2 nm for samples with low Gd concentrations (10(16) cm(-3)) and about 4 nm for samples with high Gd concentrations (10(19) cm(-3)). A discussion about the local stress environment establishes that the GdN platelets are incorporated in a metastable wurtzite crystal phase.