Superconducting layers by Ga implantation and short-term annealing in Si

Superconductivity in elemental group-IV semiconductors is of great interest because of both, the high potential for new microelectronic applications and its underlying physics.
To observe superconductivity at ambient pressure conditions high doping levels are needed. Sufficient doping concentrations of few at.% were achieved first for boron doped diamond [1]. Suprisingly also silicon, the basic material of todays microelectronic indurstry becomes superconducting below 0.6 K when heavily doped with boron [2]. In our previous work we used ion implantation and short-term annealing to fabricate superconducting Ga doped Ge layers with critical temperatures below 1 K [3]. The solid solubility is exceeded by far and therefore the presence of Ga clusters has to be excluded [4].
However the question arises, how superconducting precipitates influence the low- temperature transport properties. We demonstrate the possibility of embedding extrinsic superconducting nanolayers in commercial microelectronic Si wafers. Ga implantation (4x10¹⁶cm^-2) through a 30 nm SiO₂ cover layer is used because Ga itself is a superconducting element. Sturctural investigations by means of RBS/C and TEM reveal the stabilization of a Ga-rich layer at the SiO₂/Si interfae after rapid thermal annealing (RTA). At defined RTA temperatures of 600 – 700°C this interface layer becomes superconducting [5,6]. Amorphous Ga has a critical temperature of 7 K which is comparable to the value of our Ga-rich interface layers. High critical magnetic fields up to 14 T and critical current densities as high as 50 kA/cm² make the Si:Ga layers interesting for applications.
These results in combination with investigations on similar prepared Ga-rich layers at SiO₂/Ge interfaces imply that superconductivity driven by Ga clusters occurs at temperatures of 6 – 7K [7]. If in Ge the onset of superconductivity is below 1 K, it can clearly be attributed to a doping effect.

Financial support by DFG (HE 2604/7-1) is gratefully acknowledged.
[1] E. A. Ekimov et al., Nature 2004;428:542.
[2] E. Bustarret et al., Nature 2006;444:465.
[3] T. Herrmannsdörfer et al., Phys, Rev. Lett. 2009;102:217003.
[4] V. Heera et al., J. Appl. Phys. 2010;107:053508.
[5] R. Skrotzki et al., Appl. Phys. Lett. 2010;97:192505.
[6] J. Fiedler et al., Phys. Rev. B 2011;83:214504.
[7] J. Fiedler et al., Phys. Rev. B 2012;85:134530.