Mid-gap states in the forbidden gap of silica

Green-function method is applied to study possibilities for surface states in the forbidden gap of silica. It is shown that these state differ with re spect to surface states in pure silicon. In the latter case breaking of bon ds at a surface leads to mid-gap states. One can say that vanishing hopping integrals at the surface is the main reason for this effect. On the other hand, in silica, changing Si-O hopping integrals does not result in states with energies less than the energy of the lowest Si empty level. This state ment was proven rigorously. Consequences of this result are discussed. Poss ible role of long-range interaction in formation of mid-gap states in silic a is underlined. (C) 1999 Elsevier Science B.V. All rights reserved.