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

Pawel Kempisty 1Pawel Strak 1Stanisław Krukowski 1,2

1. Polish Academy of Sciences, Institute of High Pressure Physics (UNIPRESS), Sokolowska 29/37, Warszawa 01-142, Poland
2. University of Warsaw, Interdisciplinary Centre for Mathematical and Computational Modelling (ICM), Pawinskiego 5a, Warsaw 02-106, Poland

Abstract

DFT calculations were used to investigate ammonia adsorption on GaN(0001) surface under mixed coverage composed of H adatoms, NH2 radicals and NH3 admolecules typical for the growth of GaN layers and crystals by MOVPE, HVPE and ammonothermal methods. The investigation elucidated role of electronic properties of the surface on the course of adsorption processes showing important role of electronic degrees of freedom in some cases. In the case of bare surface ammonia adsorption is dissociative: it leads to dissociation of ammonia and adsorption of NH2 radicals and single H adatoms. The adsorption energy is close to 2.9 eV/molecule. For very low hydrogen coverage the ammonia adsorption follows the pattern of clean surface dissociation to NH2 radicals and H adatoms which is caused by tendency to saturate broken Ga bonds. The increase of hydrogen coverage stabilizes ammonia molecule that is adsorbed in the molecular form at the surface with the reduction of adsorption energy to about 2 eV. This is related to tendency of GaN(0001) surface of to avoid pinning Fermi level at the surface states. In this case the adsorption of ammonia preserves its molecular structure, with only single molecular bond transformed into highly dispersive state located in the upper subband of GaN valence band. Finally for full coverage, the adsorption leads to creation of Ga-H-N bond weakly attaching the molecule with the energy. Further evolution of the system may either lead to return of the ammonia to the vapor or desorption of H2 molecule with NH2 radical attached to the surface. For higher content of ammonia admolecules or NH2 radicals the pattern is changed, the adsorption affects the bonding of ammonia changing the bonding structure of the admolecule. This is related to change of the Fermi level at the surface which is shifted up into the conduction band. This change drastically reduces the bonding energy making the ammonia adsorbed admolecule unstable at the GaN(0001) surface for relatively high NH3/NH2 ratio. 

 

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Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 9, by Pawel Kempisty
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-24 14:33
Revised:   2013-04-24 14:33