Surface properties and the principal processes at the growth of gallium nitride on GaN (0001) face in ammonia-based are modeled using DFT (density functional theory - SIESTA code) ab initio calculations and 2-d diffusion analysis. The GaN growth methods are: ammonia-source MBE, MOVPE, and also HVPE. The adiabatic trajectories, calculated for hydrogen-rich and hydrogen-free state of the GaN(0001) surface, include the adsorption of NH3, GaCl and HCl molecules and the desorption of Ga atoms. The adsorption of ammonia and GaCl has no energy barrier. Thus, in contrast to the results concerning Plasma-Assisted Molecular Beam Epitaxy (PA MBE), proving that the GaN(0001) surface remains in metal-rich state, these results indicate that, in the ammonia-rich environment, typical for HVPE and MOVP growth, the GaN(0001) surface remains in the nitrogen-rich state. In the case of HCl adsorption, the energy barrier depends on the surface coverage, and could reach 2.0 eV. The direct desorption of single Ga atom has the energy barrier, close to 7 eV. This indicates that Ga surface diffusion (growth controlling process) length is very large, leading to strong interaction of the step kinetics and the diffusion on the terraces. This interaction leads to double-step intertwined structures both in the case of dislocation-mediated spiral growth and in the step flow growth mode. These morphologies, proposed by the geometric arguments, are observed in the atomic force microscopy (AFM) scans of the GaN(0001) surface. Additionally we have compared the interaction energy of two hydrogen atoms obtained in the DFT SIESTA and the high precision Gaussian in coupled cluster singles, double and perturbation triples CCSD(T) approximation. Both approaches yielded virtually identical interaction energy confirming the validity of DFT analysis of ammonia-rich growth of GaN.
All Science Journal Classification (ASJC) codes
- General Chemistry
- General Materials Science
- Condensed Matter Physics