Atomic resolution structural analysis of magnesium segregation at a pyramidal inversion domain in a GaN epitaxial layer

Pyramidal inversion domains (PIDs) with (0001) bases several nanometers wide are commonly observed in highly Mg-doped GaN epitaxial layers. High-angle annular dark field scanning transmission electron microscopy clarified the PID boundary structure, concluding debate on previously proposed Mg segregation models: Mg atoms segregate to form a single atomic layer at the boundary and substitute 1/4 of Ga atoms in the neighboring Ga layers. We explain that the Mg segregation produces electrically inactive Mg atoms and can be a cause of the free carrier reduction in the highly Mg-doped GaN. The PID formation process during the epitaxial growth is also discussed.