An atomistic insight into reactions and free-energy profiles of NH3 and Ga on GaN surfaces during the epitaxial growth

[Display omitted] •Advanced computational methods unveil the GaN epitaxial growth mechanism.•Free-energy landscapes are obtained by effective phase-space sampling.•NH3 decomposition and dissociation generate Ga-N-Ga structures.•Atomistic clarification of reaction processes of NHx on hydrogenated GaN surface is provided.•Realistic simulations on Ga-rich GaN surface allow to quantify the mobility of Ga adatoms. Precursor molecules (NH3 and Ga compounds) along with carrier gas (H2 or N2) used to grow GaN structures bring a large amount of hydrogen atoms which affect the growing mechanism of GaN. This has a non-negligible effect of the chemistry and diffusivity of precursors and dissociation products. To encompass the experimentally difficulty in of unraveling such a complicated reaction mechanism, we resort to first principles molecular dynamics modeling, providing an atomistic insight into two major issues. The first one is the evolution of H atoms after the adsorption and dissociation of NH3 on the growing GaN surface. The second issue is to shed light on the role of passivating hydrogen at growth conditions for a typical GaN Ga-rich (0001) surface. In the first case, reaction pathways alternative to the product of molecular hydrogen (H2) can be realized, depending on the initial conditions and morphology of the surface, resulting in an adsorption of H atoms, thus contributing to its hydrogenation. In the second one, instead, we show how the presence of passivating H atoms at the surface, corresponding to a relatively high degree of hydrogenation, contribute to limit the diffusivity of Ga adatoms at the typical growth temperatures.