Adsorption of ammonia at GaN(0001) surface in the mixed ammonia/hydrogen ambient - a summary of ab initio data

Adsorption of ammonia at NH₃/NH₂/H-covered GaN(0001) surface was analyzed using results of ab initio calculations. The whole configuration space of partially NH₃/NH₂/H-covered GaN(0001) surface was divided into zones of differently pinned Fermi level: at the Ga broken bond state for dominantly bare surface (region I), at the valence band maximum (VBM) for NH₂ and H-covered surface (region II), and at the conduction band minimum (CBM) for NH₃-covered surface (region III). The electron counting rule (ECR) extension was formulated for the case of adsorbed molecules. The extensive ab intio calculations show the validity of the ECR in case of all mixed H-NH₂-NH₃ coverages for the determination of the borders between the three regions. The adsorption was analyzed using the recently identified dependence of the adsorption energy on the charge transfer at the surface. For region I ammonia adsorbs dissociatively, disintegrating into a H adatom and a HN₂ radical for a large fraction of vacant sites, while for region II adsorption of ammonia is molecular. The dissociative adsorption energy strongly depends on the Fermi level at the surface (pinned) and in the bulk (unpinned) while the molecular adsorption energy is determined by bonding to surface only, in accordance to the recently published theory. Adsorption of Ammonia in region III (Fermi level pinned at CBM) leads to an unstable configuration both molecular and dissociative, which is explained by the fact that broken Ga-bonds are doubly occupied by electrons. The adsorbing ammonia brings 8 electrons to the surface, necessitating the transfer of these two electrons from the Ga broken bond state to the Fermi level. This is an energetically costly process. Adsorption of ammonia at H-covered site leads to the creation of a NH₂ radical at the surface and escape of H₂ molecule. The process energy is close to 0.12 eV, thus not large, but the direct inverse process is not possible due to the escape of the hydrogen molecule