Imaging of defect-mediated surface morphology of GaN(0001 ) grown on sapphire by molecular beam epitaxy

In situ scanning tunneling microscopy and ex situ atomic force microscopy are used to study the surface morphology of GaN(0001 ) grown on sapphire substrates by plasma-assisted molecular beam epitaxy. Several types of defect-mediated surface morphologies are characterized, including surface terminations of screw and mixed dislocations, defect-induced grooves, and impurity-induced step bunching. In addition to the early theory of dislocation-enhanced crystal growth developed by Frank [F. C. Frank, Acta Crystallogr 4, 497 (1951)] step structures are also found to be critical to understand the formation of spiral growth hillocks around surface terminations of different types of threading dislocations. Growth experiments are also performed under different conditions, e.g., by varying III/V flux ratio and growth temperature to further investigate the relationship between the final surface morphology and the early stages of growth, such as sapphire nitridation and the transition stage from the three-dimensional buffer layer growth under nitrogen-rich conditions to the two-dimensional active layer growth under slightly Ga-rich conditions.