Superconductivity in Ga-implanted group-IV semiconductors

Beginning in 2004, the interest in superconductivity of elemental group-IV semiconductors has been renewed because Ekimov et al. [1] showed that boron doped diamond could become superconducting at ambient pressure conditions. Besides fundamental physical background of driving a semiconductor into a superconducting state, the high potential for applications in new microelectronic devices is in the main focus.
High doping levels are needed to observe superconductivity at ambient pressure conditions in elemental group-IV semiconductors. Gas immersion laser doping is used to fabricate superconducting boron doped silicon [2]. The possibility to use Ga-ion implantation and short-time annealing for creating superconducting Ga-doped Ge layers was shown in our previous work [3, 4]. These highly doped Ge-layers show an onset of superconductivity below 1 K. All doping techniques mentioned above exceed the equilibrium solid solubility limit by far and the question arises, whether the observed superconductivity is a doping effect or related to dopant clusters [5].
Especially if the doping element itself is a superconductor, like Ga in Ge, it was not clear how superconducting precipitates influence the low-temperature transport properties. To investigate these effects, we stabilized superconducting Ga-rich layers at SiO₂/Si interfaces [6, 7]. Again, we have used ion implantation through a 30 nm thick SiO₂ cover layer and rapid thermal annealing. The critical temperature of 7 K is comparable to the values obtained for amorphous Ga. Furthermore, high critical magnetic fields of 14 T and critical current densities of 50 kA/cm² were achieved.
With the results of the investigations discussed above, we could go one step further and fabricate similar Ga-rich layers at SiO₂/Ge interfaces. Now it is possible to investigate selectively the influence of superconducting Ga-rich areas on the normal- and superconducting properties of Ga-doped Ge. It will be shown that the critical temperature changes dramatically while the critical magnetic field stays rather constant. The results of detailed microstructural investigations by means of XTEM and time-of-flight SIMS will be correlated with electrical properties. Finally, the presented results indicate that superconductivity with critical temperatures around 1 K can clearly be attributed to a doping effect.

[1] E. A. Ekimov et al., Nature (London) 428 (2004) 542.
[2] E. Bustarret et al., Nature 444 (2006) 465.
[3] T. Herrmannsdörfer et al., Phys, Rev. Lett. 102 (2009) 217003.
[4] V. Heera et al., J. Appl. Phys. 107 (2010) 053508.
[5] N. Dubrovinskaia et al., PNAS 105 (2008) 11619.
[6] R. Skrotzki et al., Appl. Phys. Lett. 97 (2010) 192505.
[7] J. Fiedler et al., Phys. Rev. B 83 (2011) 214504.