Fig. 2: Terrace and step structure of vicinal (0001) surface of an HCP-type crystal.

For GaN, only Ga atoms are shown. a Circles show top-layer sites on each terrace, with color indicating height. Steps typically have lowest edge energy when they are normal to \([01\overline{1}0]\), \([10\overline{1}0]\), or \([1\overline{1}00]\). Steps in a sequence have alternating structures, A and B, which swap when step azimuth changes by 60°. b Cross-section of the A and B step structures in the region marked by an orange rectangle in a. Lighter and darker colors indicate atoms in different rows. c Detail of α and β terrace structures. Orientation of triangle of top-layer atoms around 6₃ screw axis shows difference between layers. d AFM height image of GaN (0001) surface typical of films grown on sapphire substrates by OMVPE, showing regions of alternating step spacings and interlacing at corners where the step azimuth changes. Step heights are c/2 = 2.6 Å.