Structure, electronics, and interaction of hydrogen and oxygen on diamond surfaces

The atomic geometry and electronic structure of diamond surfaces, both clean and with various hydrogen and oxygen terminations, have been studied using ab initio density-functional-theory calculations. Calculated ionization potentials and estimated electron affinities are presented for the different surfaces, while bulk- and surface-related effects are distinguished. Interaction between hydrogen and oxygen on the technologically important (001) surface is also examined. Structural energies indicate that a hydroxylated (001) surface is favored over an oxygenated surface plus gas-phase hydrogen molecules, although an overestimate in the strength of hydrogen bonding on the -OH-terminated surface might lend it an artificially high stability. A surface terminated with a combination of -O-, -H, and -OH groups has a structural energy part-way between the extremes of a fully oxygenated and a fully hydroxylated surface. Electronically, while the hydrogenated and oxygenated surfaces respectively show their expected negative and positive (bulk) electron affinities, the -OH-terminated surface and the surface with the aforementioned combination of groups both show small negative electron affinities. However, all surfaces except the combination surface have introduced unoccupied states into the bandgap, which correspond to positive surface electron affinity and could act as traps for electrons that would otherwise escape the material; the implications for band bending are discussed. As expected, the hydrogenated surfaces show by far the lowest ionization potentials, and are therefore the most suitable for exploiting the transfer doping effect, although -OH-terminated surfaces might be successfully transfer doped if an adsorbate molecule of very high electron affinity were to be used.