The impact of heavy Ga doping on superconductivity in germanium
R. Skrotzki1,2, T. Herrmannsdörfer1, V. Heera1, J. Fiedler1, A. Mücklich1, M. Helm1, and J. Wosnitza1
1Dresden High Magnetic Field Laboratory (HLD) and Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), D-01314 Dresden, Germany
2Department of Chemistry and Food Chemistry, TU Dresden, D-01062 Dresden, Germany
Received April 11, 2011
We report new experimental results on how superconductivity in gallium-doped germanium (Ge:Ga) is influenced
by hole concentration and microstructure. Ion implantation and subsequent flash-lamp annealing at various temperatures have been utilized to prepare highly p-doped thin films consisting of nanocrystalline and epitaxially grown sublayers with Ga-peak concentrations of up to 8 at.%. Successive structural investigations were carried out by means of Rutherford-backscattering spectrometry in combination with ion channelling, secondaryion-
mass spectrometry, and high-resolution cross-sectional transmission electron microscopy. Hole densities of
1.8·1020 to 5.3·1020 cm–3 (0.4 to 1.2 at.%) were estimated via Hall-effect measurements revealing that only a
fraction of the incorporated gallium has been activated electrically to generate free charge carriers. The coincidence
of a sufficiently high hole and Ga concentration is required for the formation of a superconducting condensate. Our data reflect a critical hole concentration of around 0.4 at.%. Higher concentrations lead to an increase of Tc from 0.24 to 0.43 K as characterized by electrical-transport measurements. A short mean-free path indicates superconductivity in the dirty limit. In addition, small critical-current densities of max. 20 kA/m2 point
to a large impact of the microstructure.
PACS: 74.10.+v Occurrence, potential candidates; PACS: 74.78.–w Superconducting films and low-dimensional structures.