Morphology of GaN Monolayers Grown on AlN Surface During Ammonia Flow Cycling

In this paper, the surface condition of a pseudomorphic two-dimensional (2D) GaN layer, grown on the surface of an AlN/Al 2 O 3 heterostructure, before and after ammonia flow switch-off/on cycles has been investigated by analyzing it using Atomic Force Microscopy with Kelvin Probe Force Microscopy. It is experimentally shown that after 5 cycles the GaN layer morphology is a smooth surface with three dimensional (3D) GaN islands lying on it. The smooth layer is “frozen” and does not transform into 3D islands after switching off the ammonia flow. It is also found that the 2D morphology of the original GaN layer is not fully restored after ammonia turn off/on cycles. The experimental results have been explained according to the formerly proposed theoretical model. Thus, the presence of a “frozen” GaN layer is explained by the transition of ammonia fragments (excluding hydrogen) to “tightly bound” states on its surface. The incomplete reversibility is due to the inhibition of material diffusion from the 3D GaN islands by the particles in the “tightly bound” positions.