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

Abstract

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 impact of heavy Ga doping on superconductivity in germanium

“…We find that the qualitative doping dependence of Ge:Ga is in good agreement with previous observations in BDD and Si:B [7,9]. Also the critical hole concentrations of 0.5 at.% (0.9·10 21 cm -3 ) [9] in BDD and 2 at.% (1·10 21 cm -3 ) [7] in Si:B are comparable to our finding of 0.4 at.% (0.2·10 21 cm -3 ) in Ge:Ga. However, a quantitative description in terms of an universal understanding requires further knowledge about the role of the dopants.…”

“…It was found that above a certain concentration, usually tagged as critical doping level, superconductivity emerges. Close to this point, T c rises steeply and seems to saturate at higher concentrations [7,9,10].…”

“…Since nonequilibrium preparations almost inevitably introduce disorder down to the atomic scale, its impact on superconductivity -which up to now remains uncertain -must be considered. While most reports unveil critical fields in the Tesla range, thus obviously indicating type-II superconductivity, recent investigations on more homogeneously doped Si:B samples give rise to a possible intrinsic type-I character that may artificially be hidden due to mean-free path and Ginzburg-Landau coherence-length restrictions, respectively [7]. Further physical properties may change when extraordinarily high dopant concentrations of often more than 10 at.% are incorporated.…”

“…Replacing high-pressure high-temperature synthesis by chemical vapor deposition successfully increased T c of BDD from around 4 to more than 7 K [3,6]. Alternatively, improved gas-immersion laser doping (rising T c from 0.4 to 0.6 K in Si:B) [4,7] and enhanced ion implantation accompanied by flash-lamp or rapid thermal annealing (rising T c from 0.5 to 1.2 K in Ge:Ga) [5,8] have been demonstrated.…”

Thickness dependence of the superconducting critical temperature in heavily doped Si:B epilayers

“…3 In both cases, superconductivity develops in the partially unfilled valence band for boron concentrations n B ∼ 10 21 cm −3 and most probably originates from a standard electron-phonon coupling mechanism. The critical temperature reaches T c ∼ 0.6 K in Si:B 4 but rises up to ∼10 K in C:B 5 due to a better coupling potential in this lighter element (for a review see Ref. 6).…”