Reaction Pathway of Surface-Catalyzed Ammonia Decomposition and Nitrogen Incorporation in Epitaxial Growth of Gallium Nitride

We report density functional calculations that clarify atom-scale mechanisms of vapor-phase epitaxial growth of GaN with gas sources of trimethylgallium and ammonia. We identify various stable adsorption structures of a Ga atom and NH_x molecules (x = 0-3) on GaN(0001) surfaces and find that NH₂ and NH units spontaneously intervene in Ga-Ga surface bonds on Ga-rich GaN(0001) surface. We then explore the reaction in which NH₃ on the surface is decomposed and becomes an N-incorporated structure, -Ga-(NH)-Ga-, on the Ga-rich GaN(0001) and find that the reaction occurs with the energy barrier of 0.63 eV. Further exploration reveals that a reaction of H₂ desorption occurs with the energy barrier of 2 eV, leaving the N atom incorporated in the Ga-N network. This barrier can be overcome when we consider the chemical potential of an H₂ molecule in the gas phase at the growth temperature. This N incorporation on GaN is a new growth mechanism catalyzed by the growing surface.