Advanced transmission electron microscopy investigation of nano-clustering in Gd-doped GaN
The central goal of this dissertation is (1) to clarify the distribution of Gd atoms in GaN:Gd with Gd concentration in the range between 10^16–10^19 cm^-3 by means of advanced (scanning) transmission electron microscopy [(S)TEM]; and based on that,...
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The central goal of this dissertation is (1) to clarify the distribution of Gd atoms in GaN:Gd with Gd concentration in the range between 10^16–10^19 cm^-3 by means of advanced (scanning) transmission electron microscopy [(S)TEM]; and based on that, (2) to understand the mechanisms that control such distribution. We discuss in detail the application and limitations of (S)TEM imaging and analysis techniques and modeling methods dedicated to the study of embedded nano-clusters. Besides, two case studies of semiconductor material systems that contain apparently observable nano-clusters are considered. One is about intentionally grown InAs nano-clusters embedded in Si and the other study the formation and phase transformation of Bi-containing clusters in annealed GaAsBi epilayers. Finally, we are able to identify the occurrence of GdN clusters in GaN:Gd samples and to determine their atomic structure. Strain contrast imaging in conjunction with contrast simulation unambiguously identifies the occurrence of small, platelet-shaped GdN clusters. These clusters are nearly uniform in size with their broader face parallel to the GaN (0001) basal plane. The result is confirmed by dark-field STEM Z-contrast imaging. The strong local lattice distortion (displacement field) induced by the clusters is recorded by HRTEM images and quantitatively analyzed. By comparing the displacement fields which are analyzed experimentally with these fields that are derived from energetically favored models, we conclude that the clusters are bilayer GdN with platelet diameter of only few Gd atoms; their internal structure is close to rocksalt GdN. This atomic structure model enables our discussion about the energetics of the clusters. The results indicate that the driving force for the formation of observed platelet in specific size is a compromise between the gain in cohesive energy and the penalty from interfacial strain energy due to lattice mismatch between the GdN cluster and GaN host. ; Das zentrale Ziel der vorliegenden Arbeit besteht ...
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