Simple model of surface and chemisorption states on the (111) plane of ZnS type crystals

Abstract

The surface and chemisorption states of a simple model of a semi-infinite ZnS type (sphalerite) crystal, limited by the (111) plane were studied. The existence conditions of various states localised near the surface were studied and the classification of these states was carried out. The differences between characteristics of surfaces which are formed by more electronegative atoms and surfaces formed by more electropositive atoms were pointed out. In the case of more electronegative surfaces the energy band of Shockley surface states is placed closely above the top of the highest valence band, the formation of Tamm surface states with the energies below the valence band is facilitated and the formation of Tamm surface states with the energies above the conductivity bands becomes more difficult. As far as the more electropositive surface is concerned, the energy band of Shoekley states is placed near the bottom of the conductivity band, the formation of Tamm states with energies below the lowest valence band is made more difficult and the formation of Tamm states with the energies above the highest conductivity band facilitated. In the gaps between individual valence or conductivity energy bands of volume states, further energy bands of surface states of a new kind can originate. The earlier studied model of the diamond lattice is a special case of the model investigated in this work. From the studied model follows, that a semi-infinite crystal can become a degenerated p-or n-semi-conductor if, in the forbidden energy gap, Shockley surface states do not exist at least for one of the two possible (111) delimiting planes.