Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T(c) approximately 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.

We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by CH 4 /H 2 plasma chemical vapor deposition.Diamond thin films with a thickness below 350 nm and with boron concentration ranging from 10 17 cm -3 to 10 21 cm -3 have been investigated. UV Raman spectroscopy and AFM have been used to assess the quality and morphology of the diamond films. Hall effect measurements confirmed the expected p-type conductivity. At room temperature, the conductivity varies from 1.5x10 -8 Ω -1 cm -1 for a non-intentionally doped film up to 76 Ω -1 cm -1 for a heavily B-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 cm 2 V -1 s -1 down to 0.2 cm 2 V -1 s -1 . For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature-dependent for films with the highest boron doping, and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2K. The critical boron concentration for the metal-insulator transition is in the range from 2x10 20 cm -3 up to 3x10 20 cm -3 . We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence band transport dominates at low boron concentration and high temperatures, 2 whereas hopping between boron acceptors is the dominant transport mechanism for boron doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 -1.0 eV, where both Boron impurities and the sp 2 carbon phase in the grain boundaries govern the optical absorption.

We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the ͕100͖ direction. The system presents a metal-insulator transition ͑MIT͒ for a boron concentration ͑n B ͒ on the order of n c ϳ 4.5ϫ 10 20 cm −3 , in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for n B Ͻ n c with a characteristic hopping temperature T 0 strongly reduced due to the proximity of the MIT. All metallic samples ͑i.e., for n B Ͼ n c ͒ present a superconducting transition at low temperature. The zero-temperature conductivity 0 deduced from fits to the data above the critical temperature ͑T c ͒ using a classical quantum interference formula scales as 0 ϰ ͑n B / n c −1͒ with ϳ 1. Large T c values ͑ജ0.4 K͒ have been obtained for boron concentration down to n B / n c ϳ 1.1 and T c surprisingly mimics a ͑n B / n c −1͒ 1/2 law. Those high T c values can be explained by a slow decrease of the electron-phonon coupling parameter and a corresponding drop of the Coulomb pseudopotential * as n B → n c .

The optical properties of nanocrystalline diamond films grown from a hydrogen-rich CH4∕H2 gas phase by hot filament chemical vapor deposition, as well as from an argon-rich Ar∕CH4 gas phase by microwave plasma enhanced chemical vapor deposition, are reported. The influence of nitrogen incorporation on the optical absorption is investigated. The diamond films are characterized by photothermal deflection spectroscopy and temperature dependent spectrally resolved photoconductivity. An onset of absorption at about 0.8eV in undoped films is attributed to transitions from π to π states introduced into the band gap by the high amount of sp2 bonded carbon at the grain boundaries. Incorporation of nitrogen leads to a strong absorption in the whole energy spectrum, as a result of the increasing number of sp2 carbon atoms. The effect of surface states has been observed in the high energy region of the spectrum. Transitions to the conduction band tail and photothermal ionization processes account for the observed onset at 4.4eV. Photocurrent quenching at about 3.3eV is observed in the case of samples grown from a hydrogen-rich CH4∕H2 gas phase.

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International audienceWe report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by gas immersion laser doping. The doping concentration c(B) has been varied up to similar to 10 at. % by increasing the number of laser shots to 500. No superconductivity could be observed down to 40 mK for doping level below similar to 2 at. %. The critical temperature T(c) then increased steeply to reach similar to 0.6 K for c(B) similar to 8 at. %. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field mu(o)H(c2) (0) was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. [E. Bustarret et al., Nature (London) 444, 465 (2006)]

The experimental discovery of superconductivity in boron-doped diamond came as a major surprise to both the diamond and the superconducting materials communities. The main experimental results obtained since then on single-crystal diamond epilayers are reviewed and applied to calculations, and some open questions are identified. The critical doping of the metal-to-insulator transition (MIT) was found to coincide with that necessary for superconductivity to occur. Some of the critical exponents of the MIT were determined and superconducting diamond was found to follow a conventional type II behaviour in the dirty limit, with relatively high critical temperature values quite close to the doping-induced insulator-to-metal transition. This could indicate that on the metallic side both the electron-phonon coupling and the screening parameter depend on the boron concentration. In our view, doped diamond is a potential model system for the study of electronic phase transitions and a stimulating example for other semiconductors such as germanium and silicon.