The origin of shallow n-type conductivity in boron-doped diamond with H or S co-doping: Density functional theory study

The ionization potentials and the electron affinities of doped diamond were calculated using B3LYP hybrid density functional theory and nanocrystalline cluster models, while taking into account the quantum confinement of the charge carriers. In many cases donor and acceptor levels were created in the middle of the gap between the conduction and valence bands. A possible explanation for the n-type behavior created by co-doping diamond films with boron and sulfur is given in terms of thermally activated electron donation from an SVS (V is vacancy) donor to a BB acceptor band. Both lie deep in the band gap. It is proposed that electrons in the BB acceptor band are mobile charge carriers. It is also proposed that the conversion of boron-doped diamond from p-type conductivity, with hole charge carriers in the top of the valence band, to n-type conductivity, following treatment in a deuterium plasma, may arise from formation of interstitial hydrogen donor levels and B_nH_m acceptor levels that create an acceptor band in which electrons are mobile. Again, both are deep in the band gap of pure diamond. In a prior attempt to explain this n-type behavior, BH_n defects with unrelaxed structures were proposed to be shallow donors to the diamond conduction band. This paper shows that these defects become deep donors when their structures are optimized. Finally, defects created from vacancies with 1 to 4 H in them are shown to be deep donors to the diamond conduction band.