Oxygen segregation to dislocations in GaN

The structure and composition of threading dislocations in GaN grown by hydride vapor phase epitaxy have been examined by electron microscopy. Transmission electron microscopy showed that the core structure of screw dislocations varied widely, alternating irregularly between open core (“nanopipe”) and closed core structures, with evidence that the equilibrium structure was a closed core configuration. A combination of electron energy loss spectroscopy and atomic resolution imaging in the scanning transmission electron microscope showed that the surfaces of nanopipes had $1.7\pm 0.3$ monolayers of nitrogen substituted by oxygen, and that closed core dislocations showed little evidence of oxygen segregation. It is argued that these results support a model where nanopipe formation is controlled by the segregation of oxygen by surface diffusion to surface pits, rather than dislocations per se. The implications for understanding the electronic properties of dislocations in GaN are discussed.