Atomic and electronic structure of diamond(111) surfaces .1. Reconstruction and hydrogen-induced de-reconstruction of the one dangling-bond surface

We present ab-initio local-density functional calculations of the atomic and electronic structure of the clean and hydrogen-covered one dangling-bond diamond (111) surfaces. The calculations are based on a finite-temperature local-density approximation, optimized ultrasoft pseudopotentials, and an exact calculation of the electronic ground state and Hellmann-Feynman forces before each step in the geometrical optimization of the surface. We find that the clean C(111) surface reconstructs in a (2 x 1) geometry with symmetric, unbuckled pi-bonded Pandey chains. Although the surface chains are unbuckled and undimerized, we find a substantial buckling in the deeper layers. Our calculations show that there is essentially no energy barrier against reconstruction. In equilibrium, a monolayer coverage of hydrogen leads to a complete de-reconstruction with a slightly relaxed C(111)-(1 x 1):H surface. However, there is an appreciable energy barrier against de-reconstruction. A reconstructed C(111)-(2 x I):H surface can exist as a metastable configuration. For the clean reconstructed surface we predict both occupied and empty surface states in the bulk gap, in good agreement with experiments. For both the stable unreconstructed and the metastable reconstructed hydrogenated surfaces we find that the hydrogen saturation of the dangling-bond state shifts the surface state to larger binding energies. Interaction with bulk states leads to de-localization. We find that the hydrogenation induces a negative electron-affinity effect on the C(111) surface.