Ammonia is typically used as source of nitrogen in HVPE growth of GaN. The growth is routinely carried out on GaN (0001) or Ga-surface. In most cases the ammonia flux is at least two orders of magnitude higher than GaCl flux.

Ammonia based methods have not been analyzed using ab initio methods in much detail. Recently a number of investigations were carried out using Siesta DFT based methods. The obtained results included the equilibrium configuration of the surface in presence and absence of hydrogen and ammonia. It was shown that the micro-state state of the surface depends on the size of the supercell used for the simulations.

Adsorption of the active species: ammonia, gallium chloride and hydrogen was investigated. It was shown that the adsorption processes occur without any energy barrier for all the three compounds. This results indicate that the surface state is determined by the ration of the fluxes, i.e. GaN(0001) surface remains in nitrogen-rich state.

Desorption of the adatoms was also considered. It particular it was shown that the desorption of a single Ga adatom is energy costly process. This confirms that the surface diffusion length of Ga adatoms is large, even compared to large inter-step spacing of order of several hundreds of lattice constants. Therefore the growth of GaN by HVPE method is controlled by the magnitude of Ga flux at the surface, i.e. it is transport-controlled.

[1] Kempisty P., Krukowski S. Journal of Crystal Growth 303 (2007) 37

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