  Recently developed model allows for simulations of electric field influence
on the surface states. The results of slab simulations show considerable change
of the energy of quantum states in the electric field, i.e. Stark Effect
associated with the surface (SSSE - Surface States Stark Effect). Detailed
studies of the GaN slabs demonstrate spatial variation of the conduction and
valence band energy revealing real nature of SSSE phenomenon. It is shown that
long range variation of the electric potential is in accordance with the change
of the energy of the conduction and valence bands. However, at short distances
from GaN(0001) surface, the valence band follows the potential change while the
conduction states energy is increased due to quantum overlap repulsion by
surface states. It is also shown that at clean GaN(0001) surface Fermi level is
pinned at about 0.34 eV below the long range projection of the conduction band
bottom and varies with the field by about 0.31 eV due to electron filling of
the surface states.
