Atomic force microscopy of partially polished and epi-ready c-plane GaN substrates obtained by an ammonothermal method

In this paper, the propagation of scratches on the surfaces of c-plane GaN substrates due to slicing and polishing is studied through atomic force microscopy (AFM). For epi-ready substrates, the AFM images confirm a flat surface with atomic step roughness, while for partially polished GaN substrates, many scratches are visible in the AFM images. A Fourier analysis of the AFM images shows that the scratches propagate more easily along the {m-plane} and {a-plane} directions on a c-plane GaN surface. Most of these scratches are generated by the mechanical slicing of GaN crystals and/or non-optimal polishing conditions. A proper chemomechanical polishing process is able to remove the damaged material and obtain a flat surface with atomic step roughness. This observation is evidence for the anisotropy of mechanical properties of GaN crystals in the microscale range. (C) 2014 The Japan Society of Applied Physics