Superconducting State in a Gallium-Doped Germanium Layer at Low Temperatures

We demonstrate that the third elemental group-IV semiconductor, germanium, exhibits superconductivity at ambient pressure. Using advanced doping and annealing techniques of state-of-the-art semiconductor processing, we have fabricated a highly Ga-doped Ge ($\mathrm{Ge}:\mathrm{Ga}$) layer in near-intrinsic Ge. Depending on the detailed annealing conditions, we demonstrate that superconductivity can be generated and tailored in the doped semiconducting Ge host at temperatures as high as 0.5 K. Critical-field measurements reveal the quasi-two-dimensional character of superconductivity in the $\sim 60\mathrm{nm}$ thick $\mathrm{Ge}:\mathrm{Ga}$ layer. The Cooper-pair density in $\mathrm{Ge}:\mathrm{Ga}$ appears to be exceptionally low.

Figure 1

(a) Ga depth distribution obtained by secondary ion mass spectrometry (SIMS) of a $\mathrm{Ge}:\mathrm{Ga}$ sample before and after annealing ($\Phi =50.8\mathrm{J}/{\mathrm{cm}}^2$) compared to a Ga profile simulated for an as-implanted $\mathrm{Ge}:\mathrm{Ga}$ sample using the SRIM code [22]. (b) The cross-sectional transmission electron micrograph and the diffraction pattern (inset) from the same region demonstrate that the implanted layer is completely (poly)recrystallized after flashlamp annealing down to about 50–60 nm, followed by $\approx 10\mathrm{nm}$ thin wavy interface (indicated by the left white line) between the polycrystalline layer and the single-crystalline region. The initial amorphous-crystalline interface is indicated by the right white line. (c) Results of a channeled-ion Rutherford backscattering (RBS/C) analysis confirm the random orientation of the Ge grains. The backscattering signal for the annealed sample reaches the same value as in random orientation. For comparison the RBS/C results of unimplanted (virgin) and as-implanted Ge are shown.

Figure 2

Temperature dependence of the electrical resistance of unimplanted Ge and as-implanted as well as annealed $\mathrm{Ge}:\mathrm{Ga}$ ($\Phi =53.6\mathrm{J}/{\mathrm{cm}}^2$, $t=3\mathrm{ms}$) taken in $B=0$. Only annealed $\mathrm{Ge}:\mathrm{Ga}$ exhibits superconductivity below 0.5 K, depending on the flashlamp fluence $\Phi$ (see inset).

Figure 3

Electrical-transport data of a $\mathrm{Ge}:\mathrm{Ga}$ sample (flashlamp annealed at $\Phi =53.6\mathrm{J}/{\mathrm{cm}}^2$) as function of the magnetic field aligned in plane $B_{\parallel }$ (green), and perpendicular $B_{\perp }$ (blue). The offset of $B$ (dashed line at $\sim 27\mathrm{mT}$) results from frozen flux in the superconducting 20 T magnet after Hall-effect measurements (red data in inset).

Figure 4

Field-temperature phase diagram of the $\mathrm{Ge}:\mathrm{Ga}$ sample (flash lamp annealed at $\Phi =50.8\mathrm{J}/{\mathrm{cm}}^2$) with the highest $T_c$ at 0.45 K for two magnetic-field orientations relative to the $\mathrm{Ge}:\mathrm{Ga}$ plane (see illustrations).