Transformation of the elemental composition on the GaN surface during a 2D-3D transition

Highlights: • The change in surface energy of GaN when ammonia is cut off was calculated. • The surface energy of GaN shows non-monotonic behavior after ammonia cut-off. • Non-monotonic behavior explains the transformation of 2D GaN into QDs and vice versa. The transformation of a two-dimensional elastically strained GaN layer on the AlN surface into GaN quantum dots (forward 2D-3D transition) after switching off the ammonia flow at different temperatures was investigated by reflection high-energy electron diffraction method. It was found that after a forward 2D-3D transition, a reverse 3D-2D transition occurs, the quantum dots are transformed into an elastically strained two-dimensional layer. The change in the elemental composition and surface energy of GaN after switching off the ammonia is calculated. It is shown that the forward and reverse 2D ↔ 3D transitions are explained by the non-monotonic behavior of the surface energy due to the competition between two processes: desorption of NH₂ and H particles and the movement and fixation of NH₂, NH, and N particles to energetically more favorable positions on the GaN surface.