Structure and electron affinity of silicon and germanium terminated (001)-(2 x 1) diamond surface

Control over the chemical termination of diamond surfaces has shown great promise in the realization of field-emission applications, the selection of charge states of near-surface colour-centres such as NV, and the realisation of surface-conductive channels for electronic device applications. Experimental investigations of ultra-thin Si and Ge layers yield surface states both within the band-gap and resonant with the underlying diamond valence band. In this report, we report the results of density-functional simulations of a range of coverages of Si and Ge on diamond (001) surfaces. We have found that surface coverage with crystallogen:carbon ratios of 67% and 75% are more stable than both higher and lower coverages on the (001)-diamond surface, and that they can explain the observation of an occupied band around 1.7 eV below the valence band top. We also report geometries, adsorption energies and electron affinities of these surface structures, and show that the resonant state is made up from conventional spd-covalent sigma-bonding orbitals between the surface adsorbates.