Adsorption of ammonia on hydrogen covered GaN(0001) surface - Density Functional Theory study

Density Functional Theory (DFT) simulations of ammonia adsorption at clean and H-covered surface confirmed that ammonia may dissociate into NH₂radical and H adatom or remain in the molecular form. The remaining hydrogen atoms are attached to Ga atoms where the charge transfer to the surface is possible. The calculations show that for the molecular process, the ammonia adsorption energy is close to 2.0 eV, independent of hydrogen coverage. The dissociative process is strongly H-coverage dependent, for low H-coverage the adsorption energy is close to 2.8 eV, for high coverage changes by more than 4 eV reaching negative values. Thus for low coverage the energetically preferred adsorption is dissociative, for high is molecular. The dissociation energy and preferred mode change are related to the change of the Fermi level pinning from Ga broken bond state to valence band maximum (VBM), confirming the decisive role of charge transfer in the adsorption processes.