Invited Paper EM+SS-ThM9
GaN(0001) Surface at Various Conditions: Fundamental Properties and Basic Dynamic Processes - Ab Initio Study

Thursday, October 21, 2010, 10:40 am, Room Dona Ana

Due to its polar nature, gallium nitride, GaN properties strongly depend on its orientation. Therefore various GaN surfaces have different physical properties, which is related to their different atomic structure and also to the different electric fields in the subsurface layers. A new developed procedure allows to simulate these fields exactly, in a controlled fashion, within the slab model, by modification of the termination of the opposite side of the slab. These techniques will be discussed using bare Ga-terminated GaN(0001) surface as a working example. It will be shown that the field causes shift of the energy of the surface and band states, which is known as Surface States Stark Effect (SSSE). This approach will be employed also in the discussion of the hydrogen covered GaN(0001) surface. Both equilibrium properties and dynamic processes will be investigated with the account of the role of electric field, or equivalently doping in the bulk semiconductor, in shaping of the surface electronic and energetic properties. .Equilibrium properties such as hydrogen adsorption sites and energy, and also the thermodynamic and energetic stability will be discussed for various coverage. It will be shown that adsorption of molecular hydrogen on bare GaN(0001) surface leads to its dissociation and location of H atoms in the sites, above the Ga surface atoms. The interaction with adsorbed species leads to strong relaxation of the surface with Ga atoms moving upwards towards H adatoms. Generally, the hydrogen adatoms are strongly confined at the GaN(0001) surface, having their energy 1 eV below that in the GaN bulk. Thus the hydrogen coverage of GaN (0001) surface is stable, and its removal is extremely difficult. The energy of H - induced surface states strongly depend on the doping in the bulk: in p-type GaN the state is close to valence band maximum, but for n-type it is located 2 eV lower. The determined dynamic properties include the adsorption paths showing that the adsorption of molecular hydrogen depends on the coverage: for small coverage, molecular hydrogen is chemically adsorbed at the surface while for 1ML H-coverage it encounters significant energy barriers, drastically lowering the probability of this process. Adsorption of atomic hydrogen does not depend on the coverage, leading at some instances to creation of H₂ admolecules which subsequently could be desorbed form the surface.

The research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).