The physical properties of lightly doped semiconductors are well described by electronic band-structure calculations and impurity energy levels. 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 bands. 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 transition in heavily boron-doped diamond 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 bands or the intrinsic diamond bands. 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 germanium. It should also provide a foundation for the possible development of diamond-based devices.

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Superconductivity was discovered in heavily boron-doped diamond thin films deposited by the microwave plasma assisted chemical vapor deposition (MPCVD) method. Advantages of the MPCVD deposited diamond are the controllability of boron concentration in a wide range, and a high boron concentration, especially in (111) oriented films, compared to that of the high-pressure high-temperature method.The superconducting transition temperatures are determined to be 8.7K for Tc onset and 5.0K for zero resistance by transport measurements. And the upper critical field is estimated to be around 7T.2

Boron-doped diamond films can become superconducting with critical temperatures Tc well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm −1 , by also using Coherent Synchrotron Radiation. We thus determine the optical gap 2∆, the field penetration depth λ, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function α 2 F (ω). We conclude that diamond behaves as a "dirty" BCS superconductor.PACS numbers: 74.78. Db, Diamond, with its extraordinary mechanical properties, excellent thermal conductivity, and large gap between the valence and the conduction band, is potentially a semiconductor more attractive than silicon for many applications. Therefore the transport properties of diamond films, deposited by Chemical Vapor Deposition (CVD), and doped by acceptors or donors, are being extensively explored in view of a possible, future diamondbased electronics. In this framework it has been discovered recently that heavily boron-doped diamond can also become a superconductor [1] below critical temperatures T c well above the liquid helium temperature [2], if the doping level is 2.5%.Strongly covalent bonds, high concentration of impurities, and high phonon frequencies make B-doped diamond much different from the conventional metals where the Bardeen-Cooper-Schrieffer (BCS) [3] theory of superconductivity holds. Indeed, the metallic properties of heavily B-doped diamond are now the subject of an intense theoretical investigation. If many authors suggest that B-doped diamond in the doping regime above ∼ 0.5% should be a degenerate metal [4,5], with a conduction band strongly broadened by disorder, others point out that the deep 0.37 eV level of the isolated Bacceptor [6] may prevent the merging of the B-like bands with the C valence band, and propose unconventional models for the metallization of diamond [7]. One thus may wonder whether diamond is anyhow a BCS material, eventually with a high degree of disorder, or an exotic superconductor like most of those discovered in the last two decades. The study of the electron-phonon interaction in metallic diamond, a likely candidate for the Cooper pairing mechanism, has also attracted considerable attention, since the high phonon frequencies make the adiabatic limit questionable and the covalent bonds may produce a very strong coupling costant, like in MgB 2 [8, 9]. Here we approach both this problems by first measuring the reflectivity of a superconducting diamond film, in the sub-Terahertz region down to 5 cm −1 where the gaps of superconductors are observed, and in the infrared region, where the signatures of the electron-phonon coupling appear. The sub-Terahertz frequencies have been reached, with the required signal-to-noise ratio, by use of Coherent Synchrotron Radiation.A basic feature of the superconducting state is the opening, for T < T c , of a gap E g in the electronic density of states. Correspondingly, if the Cooper pairs are in a spherically symmetric s state, the reflectivity becomes R s (ω)...

Carbon-and boron-2p states of superconducting and non-superconducting boron-doped diamond samples are measured using soft X-ray emission and absorption spectroscopy. For the superconducting sample, a large density of hole states is observed in the valence band in addition to the states in the impurity band. The hole states in the valence band is located at about 1.3 eV below the valence band maximum regardless of the doping level, which cannot be interpreted within a simple rigid band model. Present experimental results, combined with the first principles calculations, suggest that superconductivity is to be attributed to the holes in the valence band.