J. Boulmer

Superconductivity in doped cubic silicon

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 Tc ≈ 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.

Realization of Si1−x−yGexCy/Si heterostructures by pulsed laser induced epitaxy of C+ implanted pseudomorphic SiGe films and of a-SiGeC:H films deposited on Si(100)

Si 1-x-y Ge x C y /Si heterostructures are realized by pulsed laser induced epitaxy (PLIE) from C + implanted pseudomorphic Si 0.84 Ge 0.16 films and from hydrogenated amorphous SiGeC films deposited on Si(100). The laser treated samples are examined by electron channelling, energy dispersive X-ray analysis, Rutherford backscattering spectroscopy and ion channelling, X-ray diffraction, secondary ion mass spectrometry, and infrared and Raman spectroscopy. We show that PLIE occurs when the laser fluence exceeds a threshold for which the liquid-solid interface reaches the crystalline substrate at each laser pulse. Above this threshold, germanium and carbon atoms diffuse inside the melted layer, and carbon incorporation in substitutional sites increases with the laser fluence and the number of pulses. The resulting SiGeC layers are pseudomorphic.

Low-temperature transition to a superconducting phase in boron-doped silicon films grown on (001)-oriented silicon wafers

We 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)].

Time of flight study of low pressure laser etching of silicon by chlorine

Abstract We have studied c-Si etching by chlorine and an XeCl laser. Cl 2 is supplied by a pulsed valve, allowing large etch rates under very low chamber pressure. The etch products are studied using a double time-of-flight measurement in which product velocities and masses are determined independently. For laser fluences ranging from 500 to 1000 mJ cm -2 and Cl 2 doses from 8 x 10 15 to 2 x 10 18 cm -2 , the etch rate is constant and nearly equal to half a Si monolayer per laser pulse. The experimental findings are compatible with a two-step mechanism, namely Cl chemisorption (with no laser required) and laser desorption of chloride species.

Laser etching of silicon by chlorine: effect of post-desorption collisions and chlorine in-diffusion on the laser desorption yield

Abstract The yield of SiCl laser induced desorption in laser chemical etching of silicon by chlorine is found to never exceed 80%, even under conditions where the desorption rate is saturated. We show that this incomplete desorption is not a consequence of SiCl redeposition due to post-desorption collisions. Instead it is accounted for by in-diffusion and segregation of chlorine during the superficial melting of silicon induced by the pulsed laser.

Subkelvin tunneling spectroscopy showing Bardeen-Cooper-Schrieffer superconductivity in heavily boron-doped silicon epilayers

Scanning tunneling spectroscopies in the subkelvin temperature range were performed on superconducting silicon epilayers doped with boron in the atomic percent range. The resulting local differential conductance behaved as expected for a homogeneous superconductor, with an energy-gap dispersion below

Laser chemical etching of copper films

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Absence of boron aggregates in superconducting silicon confirmed by atom probe tomography

. The spectral shape, the amplitude, and temperature dependence of the superconductivity gap follow the BCS model, bringing support to the hypothesis of a hole pairing mechanism mediated by phonons in the weak-coupling limit.

Raman spectroscopy of Si1−x−yGexCy layers obtained by pulsed laser induced epitaxy

Copper surface interactions with chlorine and carbon tetrachloride and a UV excimer laser are studied, in the context of applications to interconnects in microelectronic circuits. With Cl2, a highly reactive gas, the etching process is governed by bulk diffusion. With CCl4, which is less reactive, the limiting step is the creation of reactive species at the surface. The diffusion of active species in the gas phase is evidenced by a degree of nonuniformity of mm-sized etched spots.

Superconducting properties of laser annealed implanted Si:B epilayers

Superconducting boron-doped silicon films prepared by gas immersion laser doping (GILD) technique are analyzed by atom probe tomography. The resulting three-dimensional chemical composition reveals that boron atoms are incorporated into crystalline silicon in the atomic percent concentration range, well above their solubility limit, without creating clusters or precipitates at the atomic scale. The boron spatial distribution is found to be compatible with local density of states measurements performed by scanning tunneling spectroscopy. These results combined with the observations of very low impurity level and of a sharp two-dimensional interface between doped and undoped regions show that the Si:B material obtained by GILD is a well-defined random substitutional alloy endowed with promising superconducting properties.