SURFACE RECONSTRUCTIONS OF THE C-BN(001) N-RICH SURFACE

The structures of all III-V and II-VI semiconductor surfaces which hav e been determined up to now are consistent with the electron-hole coun ting rule, and the 2 x 4 reconstructions of (001) surfaces with missin g dimer rows occur as straightforward consequences of this condition. We perform molecular cluster calculations using the AM1 hamiltonian to study several of these models for the nitrogen-terminated c-BN(001) s urface, and compare them with structures terminated with a full covera ge of nitrogen dimers or N-N bridges. We find that the energetics is d ominated by elastic distortion effects related to the different covale nt atomic radii of boron and nitrogen atoms and, as a result, the most favourable models are those with a low nitrogen coverage and N-N brid ges rather than dimers. We also observe that the surface energies of s ome of the structures which do not comply with the electron-hole count ing rule are lower than the energies of those which are consistent wit h it. This can be explained by the fact that the latter contain a larg er amount of surface N dimers than the former. Each of these dimers ra ises the respective surface energy by introducing additional stress in to the surface, owing to the relatively short N-N bond length. The sur face stress, being the major component of the surface energy, complete ly counterbalances the energy-reducing electron mechanism of the elect ron-hole counting rule. Thus this rule is violated in the case of c-BN and cannot serve as a guiding principle for predicting the atomic str ucture of its surfaces.