Origin of the metallic properties of heavily boron-doped superconducting diamond

The physical properties of lightly doped semiconductors are well described by electronic band-structure calculations and impurity energy levels ¹. Such properties form the basis of present-day semiconductor technology. If the doping concentration n exceeds a critical value n _c, the system passes through an insulator-to-metal transition and exhibits metallic behaviour; this is widely accepted to occur as a consequence of the impurity levels merging to form energy bands². However, the electronic structure of semiconductors doped beyond n_c have not been explored in detail. Therefore, the recent observation of superconductivity emerging near the insulator-to-metal transition³ in heavily boron-doped diamond^4,5 has stimulated a discussion on the fundamental origin of the metallic states responsible for the superconductivity. Two approaches have been adopted for describing this metallic state: the introduction of charge carriers into either the impurity bands⁶ or the intrinsic diamond bands^7-9. Here we show experimentally that the doping-dependent occupied electronic structures are consistent with the diamond bands, indicating that holes in the diamond bands play an essential part in determining the metallic nature of the heavily boron-doped diamond superconductor. This supports the diamond band approach and related predictions, including the possibility of achieving dopant-induced superconductivity in silicon and germanium⁷. It should also provide a foundation for the possible development of diamond-based devices¹⁰.