Ab initio study of the properties of GaN(0001) surface at MOVPE and HVPE growth conditions

Density functional theory (DFT) is used to simulate the properties of GaN(0001) surface employing a finite slab with additional modifications of the non-real side by termination with variously charged pseudo-hydrogen atoms. Different terminations change the electric field inside the slab and causes a relative shift of the energy of the surface and band states, which is known as Surface States Stark Effect (SSSE). It is shown that single hydrogen atom is barrierlessly adsorbed on GaN(0001) surface in the on-top position. Its adsorption energy strongly depends on the surface coverage by other hydrogen atoms, being above 3 eV and below 1.5 eV for the coverage lower and higher than 0.75 ML, respectively. This large difference is due to the difference of electronic occupation of the surface states. The adsorption energies indicate that hydrogen can be stably attached to the surface up to 0.75 ML coverage. It is shown that the adsorption energy of molecular hydrogen depends to some degree on the electric field at the surface, i.e. doping in the bulk. The dependence of hydrogen coverage on the pressure in the vapor shows that complete removal of hydrogen from GaN(0001) surface is difficult. Also chemical potential of hydrogen at the surface as a function of its coverage is determined.