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TEM investigation contributing to the comprehension of superconductivity in Ga-doped Si

The discovery of superconductivity in heavily boron-doped diamond [1] has demonstrated that group-IV semiconductors can become superconducting upon carrier doping even at ambient conditions. Meanwhile superconductivity has been found in further heavily doped group-IV semiconductors such as Si and SiC [2]. Compared to these semiconductors, Ge seems to be less promising for realizing superconductivity as was based upon estimates of the electron-phonon coupling strength [3]. The challenge is to achieve extremely high hole concentrations which are commonly limited by the equilibrium solid solubility of the acceptor. Nevertheless, we succeeded in making Ge superconducting as recently reported [4]. A nonequilibrium doping process consisting of 100 keV Ga+-ion implantation with a fluence of 21016cm-2 and subsequent 3 ms flash-lamp annealing (FLA) enabled hole concentrations as high as 1.41021 cm-2. The superconducting state was observed in a thin (~60 nm) Ge layer with a maximum Ga content of about 8 at.% at critical temperatures below 0.5 K. From the measured critical parameters it follows that Ga-doped Ge is a type-II superconductor with a large Ginzburg-Landau parameter (>103). The structure as well as the superconducting properties of the Ga-doped Ge layers depend sensitively on the preparation conditions as shown in Fig. 1. In search for higher transition temperatures, implantation and annealing conditions were varied in a more comprehensive study. Critical temperatures above 1 K were obtained for samples either implanted with 41016 cm-2 and flash-lamp annealed at 52 Jcm-2 or implanted with 21016 cm-2 and subjected to rapid thermal annealing (RTA) at 910°C for 60 s (Fig. 2). Critical magnetic fields perpendicular and parallel to the Ge:Ga plane up to about 0.3 and 1 T, respectively, were observed. Thus superconductivity in thin Ge:Ge layers is a robust effect and could be utilized in superconducting quantum devices. [1] E. A. Ekimov, V. A. Sidorov, E. D. Bauer, et al., Nature 428, 542 (2004) [2] K. Iakoubovskii, Physica C 469, 675 (2009) [3] L. Boeri, J. Kortus, O. K. Anderson, J. Phys. Chem. Solids 67, 552 (2006) [4] T. Herrmannsdörfer, V. Heera, O. Ignatchik, et al., Phys. Rev. Lett. 102, 217003 (2009)

Recently, superconductivity was detected in heavily boron doped group IV semiconductors like diamond (cB=2.8 at%, TC=4 K) [1] and silicon (cB=1.2 at%, TC=0.34 K) [2]. These unexpected results initiated a new debate about the possibility and the mechanism of superconductivity in doped semiconductors. Theoretical calculations, based on the classical electron-phonon coupling mechanism, demonstrated that critical temperatures in diamond can clearly exceed 1 K for acceptor concentrations higher than 5 at% [3]. However, unrealistic high doping concentrations are predicted for observable superconductivity in Si or even Ge. It was an open question whether superconductivity can be achieved in doped Ge. In order to fabricate group IV semiconductors with acceptor concentrations much higher than their equilibrium solid solubility exotic doping methods like high-pressure-high-temperature synthesis [1] or gas immersion laser doping [2] were applied. We used a more conventional doping process consisting of high dose implantation and 3 ms flash lamp or 60 s rapid thermal annealing in order to form Ge layers with Ga concentrations up to 6 at%. According to Hall effect measurements the hole concentrations are in the range between 0.3x1021 and 1.4x1021 cm-3. Superconductivity was found in the Ga-doped Ge samples below critical temperatures between 0.1 and 0.5 K in dependence on the annealing conditions. References [1] E. A. Ekimov et al., Nature 428 (2004) 542 [2] E. Bustarret et al., Nature 444 (2006) 465 [3] L. Boeri, J. Kortus, O. K. Anderson, J. Phys. Chem. Solids 67 (2006) 552

Recently, superconductivity has been discovered in heavily boron-doped group IV semiconductors like diamond [1] and silicon [2]. Because theoretical studies predict only a weak tendency to superconductivity in heavy p-type doped Ge [3] investigations of the low-temperature transport behaviour in Ge are still lacking. In order to obtain superconductivity in group IV semiconductors, heavy p-type doping above the metal-insulator-transition and low lattice damage is required. The combination of both conditions make it difficult to apply ion implantation as doping technique. The challenge is to reconstruct the damaged or even amorphized crystal lattice and to activate the acceptor atoms after implantation by annealing, avoiding at the same time long range diffusion and precipitation of the acceptors in the supersaturated semiconductor. So far only in-situ doping during growth (high-temperature-high-pressure synthesis [1] and chemical vapour deposition) for boron-doped diamond and ultra-short-time laser melting of the Si surface in BCl3 atmosphere (gas immersion laser doping [2]) have met these conditions. Here an alternative process compatible with semiconductor technology is presented. Ga implantation and flash lamp annealing in the ms range enables the production of Ga supersaturated (up to 15 at%) crystalline Ge layers which become superconducting below 0.5 K. The layer structure investigated by AES, XTEM, RBS/C and the electrical transport properties at low temperatures are reported. [1] E. A. Ekimov, V. A. Sidorov, E. D. Bauer, et al. , Nature 428 (2004) 542 [2] E. Bustarret, C. Marcenat, P. Achatz, et al., Nature 444 (2006) 465 [3] L. Boeri, J. Kortus, O. K. Anderson, J. Phys. Chem. Solids 67 (2006) 552