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

Precursor molecules (NH3 and Ga compounds) along with carrier gas (H-2 or N-2) 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 (H-2) 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.