Talk abstract

Recent experiments have shown that diamond can become p-type by the addition of an aqueous solution, probably of carbonic acid, to a hydrogenated surface. Models for the mechanism behind the conduction suggest that an electronegative molecule close to the surface can accept an electron from the diamond valence band, leaving a hole. The subsequent conduction is stable at modest temperatures (~100 C), and is removed by annealing to >200 C. This implies that the species are relatively weakly bound to the surface, i.e., physisorbed rather than chemisorbed. We present the results of first principles calculations examining the binding energy and electronic structures of candidate molecules on various diamond surfaces. The results strongly support the principle of p-type surface conduction via the donation of valence electrons to physisorbed molecules, but indicate that a wide range of molecules should give rise to this effect, contrary to observation. It is suggested that, in contrast to carbonic acid, some other species either chemisorb to the surface or remove surface hydrogen leaving compensating sites.